Superhydrophilic–superhydrophobic patterned surfaces constitute a branch of surface chemistry involving the two extreme states of superhydrophilicity and superhydrophobicity combined on the same surface in precise patterns. Such surfaces have many advantages, including controllable wettability, enrichment ability, accessibility, and the ability to manipulate and pattern water droplets, and they offer new functionalities and possibilities for a wide variety of emerging applications, such as microarrays, biomedical assays, microfluidics, and environmental protection. This review presents the basic theory, simplified fabrication, and emerging applications of superhydrophilic–superhydrophobic patterned surfaces. First, the fundamental theories of wettability that explain the spreading of a droplet on a solid surface are described. Then, the fabrication methods for preparing superhydrophilic–superhydrophobic patterned surfaces are introduced, and the emerging applications of such surfaces that are currently being explored are highlighted. Finally, the remaining challenges of constructing such surfaces and future applications that would benefit from their use are discussed.

  • •The fundamental theories of surface wettability are introduced to explain the spreading of a droplet on a solid surface.

  • •The methods for constructing superhydrophilic–superhydrophobic patterned surfaces are described regarding hierarchical structure fabrication, chemical modification, and wettability patterning.

  • •The emerging applications of such surfaces are highlighted, including droplet arrays, cell arrays, biosensors, microfluidic systems, and environmental protection.

  • The remaining challenges of preparing such surfaces and future applications that would benefit from their use are discussed.

The superwetting behavior of a solid surface by a liquid plays a vital role in liquid–solid–gas systems and has elicited increasing research interest in numerous fields, including anti-icing, self-cleaning, energy conversion, and sensors.1–5 For example, superhydrophobic surfaces are used to improve the detection performance of surface-enhanced Raman scattering (SERS) sensors,6 and superhydrophilic electrospun nanofibers have ultrahigh efficiency in water–oil separation.7 Research on the interface between a solid and a liquid can be traced back to the proposition of Young’s equation in 1805,8 which provided a basis for the further study of wettability. In that equation, the contact angle was explained as representing the total molecular interactions of a liquid phase and solid surface, and it is now widely accepted and used to characterize the state of a solid surface when contacting a liquid. Specifically, superhydrophobic surfaces are described as having a water contact angle (WCA) larger than 150°, and the difference between the advancing and receding WCAs—which is termed WCA hysteresis—is usually less than 10°, by contrast, superhydrophilic surfaces have a WCA of 0°–10°, and both the advancing and receding WCA is close to 0°. Therefore, water droplets can freely roll off a superhydrophobic surface and remove dust, thereby making the surface a self-cleaning one,9 and a water drop placed on a superhydrophilic surface forms a liquid puddle, with the solutes in the liquid finally deposited on the surface after drying, this can be used for printing and lithography techniques.10–13 Superhydrophilic and superhydrophobic surfaces are prevalent in nature and inspire biomimetic designs for controlling surface wettability. One of the best-known examples of a natural superhydrophobic surface is the lotus leaf, Barthlott and Neinhuis came up with the now-famous theory of the “lotus effect” and attributed the superhydrophobicity of lotus leaves to the epicuticular wax and microscale papillae comprising nanoscale nipples on their surfaces [Figs. 1(a), 1(d), and 1(g)].14,15 Other biological surfaces that exhibit superhydrophobicity include butterfly wings,16 red rose petals,17 rice leaves,18 the legs of water striders,19 and the compound eyes of mosquitos.20 Superhydrophilic surfaces found in nature are important for the uptake of water and nutrition as well as predation. For example, the superhydrophilic surface of the pitcher plant causes insects aquaplaning on this slippery surface to fall into the pitcher, whereupon they are digested by the inner digestive fluid [Figs. 1(b), 1(e), and 1(h)].21 There have been several excellent reviews of superhydrophilic22–24 and superhydrophobic3,25–29 biomimetic surfaces from fundamental research to practical applications. However, with surfaces with a desired single type of superwettability feature, including superhydrophobic water repellency and superhydrophilic water affinity, it is usually challenging to manipulate liquids with high selectivity and accuracy. Furthermore, surfaces with a single type of superwettability often fail to meet the rigorous requirements of complex and diverse environments, such as high-resolution functional material patterning and enrichment for parallel detection.

FIG. 1.

Natural surfaces with superwettability and their multiscale structures: (a) superhydrophobic surface of a lotus leaf;42 (b) superhydrophilic surface of a pitcher plant;21 (c) superhydrophilic–superhydrophobic patterned surface of a desert beetle.30 The SEM images in (d)–(f) show the specific microstructural features of the respective surfaces, and the optical images in (g)–(i) show the synthetic equivalents of the respective surfaces created by different researchers.21,58,61

FIG. 1.

Natural surfaces with superwettability and their multiscale structures: (a) superhydrophobic surface of a lotus leaf;42 (b) superhydrophilic surface of a pitcher plant;21 (c) superhydrophilic–superhydrophobic patterned surface of a desert beetle.30 The SEM images in (d)–(f) show the specific microstructural features of the respective surfaces, and the optical images in (g)–(i) show the synthetic equivalents of the respective surfaces created by different researchers.21,58,61

Close modal

A superhydrophilic–superhydrophobic patterned surface is defined as an integrated system combining the two extreme water-repellent and water-loving properties on a single substrate. Such surfaces exhibit excellent capability in regulating solid–liquid interactions precisely and so are considered candidates for meeting the aforementioned stringent requirements. An example of a superhydrophilic–superhydrophobic patterned surface found in nature is the desert beetle Stenocara living in the Namib Desert [Figs. 1(c), 1(f), and 1(i)]. It can capture fog in the moist morning and condense it into large water droplets because of the non-waxy hydrophilic peaks and waxy hydrophobic sides and troughs of the bumps on its elytra.30 Such superhydrophilic–superhydrophobic patterned surfaces have several advantages. First, they offer diverse control of droplets. The superhydrophilic patterns on a superhydrophobic substrate limit the outward flow of droplets because of the gradient wettability in adjacent regions, and so such surfaces offer excellent control of droplets—including liquid collection, adhesion, rolling, and transportation—by changing the geometry and position of the patterns.31,32 Second, they offer precise and passive dispensing of solutions. The extreme difference in WCA between the superhydrophilic and superhydrophobic regions allows an aqueous solution to be dispensed on a patterned surface with no external driving force. In particular, high-precision and high-resolution gradient wettability can be achieved with the developed microfabrication techniques to position the droplets extremely close to each other.33 Third, they offer high-throughput applications. Micro-patterned or even nano-patterned droplets without merging on a planar surface are essential for increasing the density of arrays and hence for miniaturization and high-throughput applications. Fourth, they offer open and direct platforms. A platform based on superhydrophilic–superhydrophobic patterned surfaces makes droplets easily accessible and addressable, enabling them to be analyzed directly on the surface using various techniques. Figure 2 shows the benefits of combining superhydrophilicity and superhydrophobicity on the same surface. These findings greatly boost the development of related research and applications, and several reviews have focused on the processes for designing and using superhydrophilic–superhydrophobic patterned surfaces for cell arrays, droplet arrays, microfluidics, and biosensors.34–41 

FIG. 2.

Benefits of combining superhydrophilicity and superhydrophobicity on the same surface.

FIG. 2.

Benefits of combining superhydrophilicity and superhydrophobicity on the same surface.

Close modal

In this review, we present a comprehensive overview of the fabrication techniques and emerging applications of superhydrophilic–superhydrophobic patterned surfaces. First, we briefly introduce the principle of surface wettability, followed by the construction methods of superhydrophilic–superhydrophobic patterned surfaces regarding hierarchical-structure fabrication, chemical modification, and surface patterning. Then, we highlight emerging applications for such patterned surfaces in the areas of droplet arrays, cell arrays, biosensors, microfluidic systems, and environmental protection. Finally, we provide a summary, with challenges and opportunities for the fabrication and application of superhydrophilic–superhydrophobic patterned surfaces.

In general, the superwettability of a surface is characterized by the contact angle of water droplets resting on it. Experimentally, contact angles are measured using the sessile-drop method, in which a small volume of liquid is placed on a surface and an optical image is taken.43 Then a contact line is traced on the solid–liquid–gas three-phase interface and is used to calculate the contact angle of the drop. The measured value of a contact angle is related to several intrinsic factors, e.g., roughness, surface chemical composition, and impurity of the solid surface. Besides, several theoretical models are used to explain wetting phenomena, such as Young’s equation, the Wenzel model, and the Cassie–Baxter model.8,44,45 We introduce briefly the three classical contact models for a fundamental understanding of superwetting.

The intrinsic contact angle (θ) of a water droplet placed on an ideal smooth solid surface in air is calculated by Young’s equation, i.e.,

cosθ=γSGγSL/γGL,

where γSG, γSL, and γGL are the surface tensions between solid and gas, solid and liquid, and gas and liquid, respectively [Fig. 3(a)]. Note that surface tension is also called surface stress and surface free energy in materials science. Therefore, the contact angle can be adjusted by changing the surface energy of the solid surface according to Young’s equation. The latter is valid for an ideally flat substrate and assumes that the bulk phase, the surface of the droplet, and the substrate are consistent. However, most surfaces are rough, which has a crucial influence on the wetting behavior of liquid on the substrate. To explain the impact of a textured structure on surface wettability, the Wenzel and Cassie–Baxter equations have been derived from experimental data.

FIG. 3.

Droplet on a surface described by (a) Young’s model, (b) Wenzel’s model, and (c) the Cassie–Baxter model. (d) Rough surface with hierarchical structures for improved hydrophilicity or hydrophobicity.

FIG. 3.

Droplet on a surface described by (a) Young’s model, (b) Wenzel’s model, and (c) the Cassie–Baxter model. (d) Rough surface with hierarchical structures for improved hydrophilicity or hydrophobicity.

Close modal

There are two wetting states on the surface of a rough solid, i.e., uniform wetting and heterogeneous composite wetting. The Wenzel model [Fig. 3(b)] describes the uniformly wet state by the Wenzel equation, i.e.,

cosθW=rcosθ,

where r is the roughness factor, which is defined as the ratio of the actual surface area to the geometric area, and θW and θ are Wenzel’s contact angle and Young’s contact angle, respectively. In the Wenzel state, the rough surface is entirely wetted by the liquid where they contact.26,46 Nevertheless, the weakness of the Wenzel model is that it is only suitable for a homogenous surface, which is why the Cassie–Baxter model was developed. In this model, air pockets at the liquid–solid interface are considered and are assumed to be trapped underneath the liquid, which gives a composite surface [Fig. 3(c)]. It is predicted that the trapped air pushes the fluid off the surface and reduces the surface wettability. In the Cassie–Baxter model, the contact angle is derived as

cosθC=f1+cosθ1,

where f is the fraction of the surface that is in contact with the liquid.47 From the Cassie–Baxter equation, it can be found that for a hydrophobic surface (θ > 90°), the surface roughness increases the apparent angle. This is unlike the Wenzel case because even when the intrinsic contact angle of a liquid on a smooth surface is less than 90°, it can still be increased by the trapped superhydrophobic vapor pockets. The contact angle of a liquid in the three different regimes usually follows the trend θ < θW < θC.

Note that 90° is usually considered as the boundary between hydrophilic and hydrophobic wettability according to Young’s equation. However, considering the effects of surface chemicals and structures,48,49 which cannot be ignored in reality, a lower wetting threshold of 65° is proposed as the boundary between hydrophobicity and hydrophilicity;50,51 in other words, if a WCA is less than 65°, it denotes a hydrophilic surface, and the substrate becomes superhydrophilic upon increasing the roughness. Specifically, a superhydrophilic surface has a WCA of less than 10°, a hydrophobic surface has a WCA of 65°–150°, and a superhydrophobic surface has a WCA of greater than 150° [Fig. 3(d)]. For a rough surface, hierarchical structures can amplify the hydrophobicity or hydrophilicity depending on the intrinsic nature of the surface. For flat substrates, the wettability is determined primarily by the surface modification, and consequently the wettability of a solid surface is determined by both surface chemistry (determination of intrinsic contact angle) and microstructures (affecting the apparent contact angle). These guidelines are essential for predicting the surface’s behavior and for further designing superwetting surfaces. Specifically, to prepare a functional surface with both superhydrophilicity and superhydrophobicity, it is necessary to construct a nonuniform chemical component and hierarchical structures on the material surface.

Fabricating a superhydrophilic–superhydrophobic patterned surface generally involves three main procedures, i.e., (i) fabrication of hierarchical structures on the surface, (ii) surface modification, and (iii) surface patterning. Various physical or chemical approaches have been attempted to prepare superhydrophilic–superhydrophobic patterned surfaces in a simple and environmentally friendly way. Here, we briefly introduce the main existing fabrication methods for the preparation of hierarchical structures, surface modification, and surface patterning.

Both the Wenzel and Cassie–Baxter theories show that a rough surface is essential for controlling wettability. For a hydrophilic surface, increasing the surface roughness usually increases the hydrophilicity of the substrate, while for a hydrophobic surface, increasing the roughness can also improve the hydrophobicity of the surface. Therefore, much effort has gone into fabricating hierarchical structures with higher surface roughness.32,52–56 In the following, several approaches are addressed in detail.

1. Template replication

Template replication is used widely for generating well-ordered hierarchical structures and allows for massive production.57,63,64 Usually, by depositing the relevant material physically or chemically into the pores or surface of the template and removing the template after solidification, a surface with a complementary structure to that of the template can be obtained, and the surface roughness can be adjusted by using multiple templates.65–69 For example, Dai et al.57 duplicated the surface structures of a fresh rose petal to develop hierarchical and flexible polydimethylsiloxane (PDMS) substrates for growing functional nanostructures [Fig. 4(a)]. In their process, a PDMS pre-polymer was first poured onto the petal surface, cured under mild conditions, and then peeled off to give a PDMS replica with negative rose-petal surface structures. A positive PDMS replica was obtained from the negative replica by the same process and was observed by SEM to have a rose-petal-like surface structure [Fig. 4(b)]. Moreover, the positive PDMS replica had a superhydrophobic surface with a WCA of 153°, thereby preventing water droplets from rolling off it even if it was turned upside down. Other researchers have also used biological surfaces with superhydrophobic properties to copy the surface morphology onto a polymer directly. Wan et al.58 constructed a hierarchical structure of micropatterned PDMS film to improve the performance of a pressure sensor based on bio-inspiration from the lotus leaf [Fig. 4(c)]. Instead of red rose petals or lotus leaves, Peng et al.59 selected taro leaves as biological surfaces and then built the surface structure with fine cavities by the modified-template method [Fig. 4(d)]. Recently, air microbubbles were used in a template process to mimic the dome-shaped protuberances found in the suction cups of octopi [Fig. 4(e)].60 The octopus-inspired system exhibited strong, reversible, repeatable adhesion to silicon wafers, glass, and rough skin surfaces under various conditions, and it was used to transport a large silicon wafer in air and underwater without any resulting surface contamination.

FIG. 4.

Schematics of process for fabricating hierarchical structures by template replication and photolithography: (a) process for preparing superhydrophobic ZnO nanostructures on a polydimethylsiloxane (PDMS) substrate copied from a rose petal;57 (b) SEM image and water contact angle (WCA) measurement of positive PDMS replica;57 (c) SEM image of micropatterned PDMS film copied from a lotus leaf;58 (d) SEM image of negative PDMS replica obtained from a taro leaf (inset is WCA image);59 (e) photograph and SEM image of microcavities with protuberance-like dome-shaped structures;60 (f) schematic of procedure for producing hydrophilic circular bump patterns on a hydrophobic polypropylene membranes;61 (g) SEM image of hydrophilic bump surface and hydrophobic background showing structure of sample at different magnfications;61 (h) surface topography of polydopamine-coated SU-8 studied by atomic force microscopy;61 (i) schematic of procedure for producing array of overhanging microdisks via two-step photolithography;62 (j) SEM image of overhanging microdisk array (inset shows a magnified view).62 

FIG. 4.

Schematics of process for fabricating hierarchical structures by template replication and photolithography: (a) process for preparing superhydrophobic ZnO nanostructures on a polydimethylsiloxane (PDMS) substrate copied from a rose petal;57 (b) SEM image and water contact angle (WCA) measurement of positive PDMS replica;57 (c) SEM image of micropatterned PDMS film copied from a lotus leaf;58 (d) SEM image of negative PDMS replica obtained from a taro leaf (inset is WCA image);59 (e) photograph and SEM image of microcavities with protuberance-like dome-shaped structures;60 (f) schematic of procedure for producing hydrophilic circular bump patterns on a hydrophobic polypropylene membranes;61 (g) SEM image of hydrophilic bump surface and hydrophobic background showing structure of sample at different magnfications;61 (h) surface topography of polydopamine-coated SU-8 studied by atomic force microscopy;61 (i) schematic of procedure for producing array of overhanging microdisks via two-step photolithography;62 (j) SEM image of overhanging microdisk array (inset shows a magnified view).62 

Close modal

2. Photolithography

Photolithography was initially developed for microelectronic processes and is now used widely for preparing microarray-like surfaces with previously designed hierarchical structures. In this technique, a mask with predefined geometries is placed on the positive photoresist coated on the surface of a substrate so that the exposed areas of the photoresist can be irradiated by light, which results in their degradation. Finally, the unwanted part of the photoresist is removed by dissolving it in solvent. Photolithography can be divided into UV lithography and x-ray lithography based on the light source,29 and the linewidth of the pattern ranges from thousands of microns down to 5 nm based on the mask type and the light wavelength. Photolithography enables the fabrication of hierarchical structures essential to superhydrophilic–superhydrophobic patterned surfaces. For instance, Moazzam et al.61 used negative photolithography and the biopolymer polydopamine (PDA) to produce a porous membrane surface with contrast wettability by creating hydrophilic patterns (nanoscale PDA-coated SU-8 bumps) on the hydrophobic background of polypropylene membranes [Fig. 4(f)]. The diameters of the circular bumps were ∼200 μm, which could be changed easily during the microfabrication process [Fig. 4(g)]. Atomic force microscopy images show that the PDA coating increased the roughness of the bumps significantly from ∼2 to 19 nm [Fig. 4(h)], which according to Wenzel’s theory changes the wettability of the surface toward the extreme wettability of either superhydrophilicity or superhydrophobicity. Also, microarrays of overhanging disks [Fig. 4(i)] can be fabricated by two-step photolithography:62 first, round microdisks are prepared by photolithography, then the pillar from which each microdisk hangs is formed at its center by reaction–diffusion photolithography. Further study showed that the contact area is sufficient to provide high adhesion when the pillar diameter exceeds 30 µm, thereby making it possible to invert the pillar-on-disk structure to obtain overhanging microdisks [Fig. 4(j)].

3. Electrospinning

Electrospinning is a simple and efficient method of generating fibers with diameters ranging from a few nanometers to several micrometers by stretching viscous polymeric solutions under high-voltage electrostatic force.74,75 The dimensions and shapes of the electrospun fibers can be tailored well by controlling the feed rate of the spinning solution, the applied voltage, the spinneret tip-to-collector distance, and the relative humidity of the surroundings, among other aspects.76–78 Using this strategy, Ding et al. fabricated superhydrophobic fibrous mats by electrospinning a polystyrene solution in the presence of silica nanoparticles (SiO2 NPs), which resulted in a structure that combined nanoprotrusions and multiple grooves.79 Tijing et al. fabricated tourmaline NP-decorated polyurethane composite nanofibers with superhydrophilic properties by one-step electrospinning.80 Electrospraying has been deemed an efficient method of fabricating nanospheres or microspheres,81,82 and recently some researchers combined it with electrospinning, this hybrid electrospinning–electrospraying technique is more effective for manufacturing superhydrophobic membranes with suitable surface wettability and mechanical and chemical durability [Fig. 5(a)]. Ge et al.70 prepared a novel membrane with a hierarchically structured skin via electrospinning and electrospraying, first, a uniform polyacrylonitrile (PAN) nanofibrous membrane (pristine NFM) was prepared via conventional electrospinning [Fig. 5(b)], then dilute PAN solutions incorporating SiO2 NPs were prepared as precursor solutions to construct the hierarchically structured skin layers on the pristine NFM through electrospraying [Fig. 5(c)]. SEM images of the obtained composite membranes showed them with microsphere layers containing various amounts of SiO2 NPs, which contributed to the formation of hierarchical structures on their surfaces. However, the deposition and alignment of electrospun nanofibers are often influenced by electrical instability, so electrospinning is better for fabricating surfaces with random roughness rather than ones with exquisitely controlled structures.83–85 

FIG. 5.

Schematics of process for fabricating hierarchical structures by electrospinning, chemical deposition, and sol-gel processing: (a) schematic of hybrid electrospinning–electrospraying method;70 (b) SEM image of pristine nanofibrous membrane;70 (c) SEM image of composite membrane with microspheres incorporating a certain amount of SiO2 nanoparticles;70 (d)–(g) SEM images of ZnO microstructured surface by chemical etching;71 (h) schematic of sol-gel processing;72 (i)–(n) SEM images of modified cellulose membrane by sol-gel processing;72 (o) SEM image of nanorod film of Cu-ferrite by sol-gel processing, and (p) magnified view of Cu-ferrite film.73 

FIG. 5.

Schematics of process for fabricating hierarchical structures by electrospinning, chemical deposition, and sol-gel processing: (a) schematic of hybrid electrospinning–electrospraying method;70 (b) SEM image of pristine nanofibrous membrane;70 (c) SEM image of composite membrane with microspheres incorporating a certain amount of SiO2 nanoparticles;70 (d)–(g) SEM images of ZnO microstructured surface by chemical etching;71 (h) schematic of sol-gel processing;72 (i)–(n) SEM images of modified cellulose membrane by sol-gel processing;72 (o) SEM image of nanorod film of Cu-ferrite by sol-gel processing, and (p) magnified view of Cu-ferrite film.73 

Close modal

4. Etching

Etching is another relatively simple, fast, and reproducible technique for fabricating hierarchical structures, and it can be divided into chemical etching and physical etching according to its conditions. Chemical etching is a cheap and simple way to mass-produce hierarchical structures, and it is usually done in an acidic or alkaline bath.23 For example, Wu et al.71 used the growth of zinc oxide nanorods in a basic solution of Zn2+ to prepare a microstructured surface, and SEM images of the prepared surface show that the ZnO nanorods formed a uniform and dense film at a large scale on the substrate [Figs. 5(d) and 5(e)], these rods had diameters of 400–600 nm and lengths of 3–4 µm with a hexagonal structure [Figs. 5(f) and 5(g)]. As means of chemical etching, electrochemical methods are also usually used to fabricate hierarchical coatings of metals, metal oxides, and conducting polymers.86,87 For example, Shibuichi et al.88 anodized the surface of an aluminum plate and then placed it in a solution of sulfuric acid at room temperature for three hours, after which they found a fractal structure (e.g., microscopic crakes) on an alumina surface. Later, similar deposition methods were used to create superhydrophobic gold, silver, and copper surfaces.89–91 Unlike chemical etching, physical etching offers precise control of surface morphology, especially for miniature structures. For example, Baldacchini et al.92 used a femtosecond laser to irradiate a silicon surface for preparing hierarchical structures thereon, Infante et al.93 fabricated multifunctional nanostructures on glass by reactive ion etching through a nano-mask, and the surface exhibited superhydrophobicity and low haze after rapid thermal annealing.

5. Sol-gel processing

In general, the sol-gel method is used to synthesize porous network structures. It is a low-temperature technique that is simple and affordable for controlling the surface roughness by adjusting the composition of the starting materials and the operation process. Sol-gel methods have been used widely for fabricating hierarchical structures.94–96 For example, Xie et al.72 used a one-step sol-gel strategy and immersed a cellulose membrane in an ethanol solution of tetraethyl orthosilicate and cetyltrimethoxysilane to obtain a sustainable superhydrophobic cellulose membrane possessing micro/nano hierarchical structures on its surface [Fig. 5(h)], and they observed many NPs forming micro/nano hierarchical structures on the cellulose fiber surfaces after one-step TEOS and hexadecyltrimethoxysilane hydrolysis and polycondensation [Figs. 5(i)5(n)], they then used the superhydrophobic cellulose membrane for oil–water separation and found a WCA of 161.8°. Shang et al.97 prepared roughly structured films including nanoclusters and NPs on glass plates through a sol-gel approach using different components of SiO2 sols acting as precursors, and the resulting surface structures could be adjusted by controlling the hydrolysis and condensation reactions. Huang et al.73 obtained lotus-leaf-like copper ferrite nanorods on a copper substrate through the sol-gel technique, they synthesized copper ferrite nanorods and their flower-like papillae with a dilute nanorod suspension for fabricating superhydrophobic surfaces, and the nanorods grew like flowers from the substrate and self-assembled into hierarchical structures that are expected to show superwettability after chemical modification [Figs. 5(o) and 5(p)]. Yeh et al.98 formed a hierarchical structure of SiO2 NPs with nanopillar-like patterns on a glass substrate, after obtaining the nanopattern structure, they modified the surface with a fluorosilanated monolayer to reduce the surface energy, which led to the formation of a hierarchical structure with low surface energy for obtaining superhydrophobicity. However, the disadvantage of sol-gel processing is the long gelation time.99 

6. Other methods

As well as the methods listed above, also available are plasma treatment,100 3D printing,101–103 laser processing,104–106 hydrothermal processing,107,108 and layer-by-layer assembly methods.109 These methods are also used widely as efficient and effective strategies for fabricating hierarchical structures with special wettability. Based on the present discussion, multiple factors must be considered when choosing the fabrication approach. Notably, combining two processes with different methods has been used. Milionis et al.110 fabricated substrates with multiscale roughness by combining photolithography and spray deposition of NPs, and Xu et al.111 obtained superwettable electrochemical sensors with patterned superhydrophobic and superhydrophilic microarrays by combining electrochemical deposition with template oxygen plasma technology.

The fabrication of hierarchical structures is followed by surface modification, which selectively alters the surface’s wettability. In the case of superhydrophobicity, the surface energy of the solid surface should be lower than that of the given liquid to avoid the surface getting wet.112 Numerous low-surface-energy reagents (e.g., fluoroalkanes) have been used to decrease surface tension, and similarly, many reagents (e.g., silane coupling agents ending with hydrophilic groups) can be used to enhance the hydrophilicity of surfaces.113 

1. Chemical modification

Chemical methods provide a robust and universal way to modify the surfaces of various materials. The chemical modification method used most widely is silanization, in which the hydroxyl groups on a substrate surface react with silane coupling agents to form covalent Si-O-Si bonds.114 For example, Zhang et al.115 obtained superhydrophobic filter paper that exhibited high WCA (>150°) and low wetting hysteresis (∼10°) via a rapid and straightforward binary silanization reaction. The hydrophobic modification was carried out using a binary hexane solution of methyltrichlorosilane and octadecyltrichlorosilane (OTS). Other than OTS, various silane reagents with hydrophobic moieties (e.g., 1H,1H,2H,2H-perfluorooctyltrichloro silane,116 hexamethyldisilazane,117 and trimethoxyoctadecylsilane118) have also been used to modify substrates chemically. Guo et al.119 modified electrospun silica nanofibrous mats using (fluoroalkyl)silane to realize the amphiphobic property, comprising different fractions of silica, the fabricated superhydrophilic mats showed an obvious difference in hydrophobicity, which was attributed to the difference in the amount of silicon hydroxyl groups. Besides silanization, polymerization is another frequently used surface-modification method: polymers with specific functional groups are introduced onto the substrate surfaces to alter the surface wettability. For example, Li et al. reported a switchable, CO2-responsive oil–water separation material based on aerogels of cellulose nanofibers. The material was prepared by grafting poly(N, N-dimethylamino-2-ethyl methacrylate) (PDMAEMA) polymer brushes via surface-initiated atom-transfer radical polymerization [Fig. 6(a)].120 In further work, they used Fourier-transform–infrared and 13C nuclear magnetic resonance to confirm the presence of PDMAEMA polymer brushes [Figs. 6(b) and 6(c)]. The high density of amino and hydroxyl groups on the surface of the aerogels offers a versatile platform for secondary reactions to impart the desired functionality. Moreover, the hydrophobic surface of aerogel grafted with PDMAEMA polymer changes to hydrophilic in the presence of CO2, and this can be used to separate oil from an oil–water mixture [Fig. 6(d)].

FIG. 6.

Schematics of surface modification by chemical modification, physical absorption, and surface coating: (a) schematic of the preparation of a poly(N, N-dimethylamino-2-ethyl methacrylate) (PDMAEMA)-grafted surface made hydrophobic by chemical modification,120 (b) Fourier-transform–infrared spectra and (c) solid-state 13C nuclear magnetic resonance spectra of nanocellulose-based aerogels,120 (d) wettability of CNF-g-PDMAEMA in the presence of CO2,120 (e) photographs of 20 μl hexane droplets sliding on antifogging film-covered lens (left) and bare lens (right),124 time-sequence images of 20 μl hexane droplets sliding on plasma-etched film (f) before and (g) after healing in ambient conditions,124 (h) schematic of spin-coating process,125 (i) schematic of dip coating and spray coating to modify a mussel-inspired dendritic polyglycerol-coated surface.126 

FIG. 6.

Schematics of surface modification by chemical modification, physical absorption, and surface coating: (a) schematic of the preparation of a poly(N, N-dimethylamino-2-ethyl methacrylate) (PDMAEMA)-grafted surface made hydrophobic by chemical modification,120 (b) Fourier-transform–infrared spectra and (c) solid-state 13C nuclear magnetic resonance spectra of nanocellulose-based aerogels,120 (d) wettability of CNF-g-PDMAEMA in the presence of CO2,120 (e) photographs of 20 μl hexane droplets sliding on antifogging film-covered lens (left) and bare lens (right),124 time-sequence images of 20 μl hexane droplets sliding on plasma-etched film (f) before and (g) after healing in ambient conditions,124 (h) schematic of spin-coating process,125 (i) schematic of dip coating and spray coating to modify a mussel-inspired dendritic polyglycerol-coated surface.126 

Close modal

2. Physical adsorption

Physical adsorption based on electrostatic interaction between opposite charges provides a simple method for hierarchical-structure modification.121 Liu et al.122 investigated the surface wetting on polyelectrolyte multilayers (PEMs) that were fabricated by alternative deposition of polydiallyldimethylammonium chloride and poly(styrene sulfonate), they found that the wetting behavior on the surfaces of the as-prepared PEMs was correlated with the molecular structures of the uncompensated ionic groups thereon. Manabe et al.123 prepared multifunctional surfaces by alternative deposition of a mixture of polycyclic aromatic hydrocarbon, polyvinyl alcohol (PVA), and poly(acrylic acid) (PAA) (as the cationic solution) and a PVA–PAA mixture (as the anionic solution). Recently, Xu et al.124 fabricated a healable oil-repellent antifogging film by alternative deposition of hyaluronic acid and branched poly (ethylenimine), followed by perfluorooctanesulfonic acid (PFOS) modification: the resulting films exhibited antifogging ability, and various organic liquids could slide down them easily when tilted slightly (<10°) [Fig. 6(e)], moreover, impregnating the antifogging films with PFOS gave them oil-repellency healing ability after O2 plasma etching [Figs. 6(f) and 6(g)], thereby suggesting an application as anti-fingerprint coatings.

3. Surface coating

Instead of undergoing chemical reactions or electrostatic interaction, surface coating is a simple, cheap, and faster method that can be achieved just by deposition. The resultant surface wettability depends on the chemical properties and homogeneity of the deposition species.127–129 For example, Abe et al.125 fabricated hydrophilic dots on a hydrophobic resin-coated surface and then used the resulting substrate to spin-coat a perovskite precursor solution [Fig. 6(h)], well-defined perovskite microdroplets were preserved on the hydrophilic dots at a desired rotational speed and in a specific precursor solution. However, spin-coating provides only a facile way to fabricate superhydrophobic surfaces on a flat substrate, especially for inorganic and polymer composite coatings, and it is unsuitable for a curved surface.130–133 Schlaich et al.126 developed a mussel-inspired dendritic polyglycerol (MI-dPG)-based spray-coating strategy for surface modification under extremely mild conditions with a spray coater [Fig. 6(i)], they rendered the MI-dPG-coated substrates either hydrophobic or hydrophilic using different modification strategies, such as spray-coating using hydrophobic SiO2 NPs and dip coating with amines, thiols, or acid chlorides. This process is applicable for various substrates and is not limited to small areas and flat substrates.

Among the aforementioned surface-modification methods, silanization, polymerization, and electrostatic interaction are relatively complex and so offer researchers abundant choices for modifying surfaces with desired wettability under controllable conditions. In comparison, coating-based methods are simpler but easier to use, but modification by coating lacks durability. Therefore, an appropriate modification method should be used to fabricate surface wettability.

After hierarchical-structure fabrication and surface modification, superhydrophobic or superhydrophilic surfaces can be fabricated successfully. Finally, surface patterning is needed to manufacture micropatterned superwettable surfaces. There are several emerging methods for preparing various patterns with different constructions such as arrays and channels.41,134–137 In the following, we introduce some of the most widely used methods for fabricating superhydrophilic–superhydrophobic patterned surfaces, including plasma etching or UV irradiation through masks, printing, and particle assembly.

1. Mask-based methods

A mask used for pattern fabrication is a thin film with hollowed-out patterns on it and can be made of stainless steel, resin, or other materials. After being covered by a mask, active reactions such as surface modification occur at the positions corresponding to the pattern on the substrate surface,138,139 after which the wettability of the patterned areas differs from that elsewhere. For example, Yin et al.140 fabricated four types of superhydrophilic–superhydrophobic patterned surface by O2 plasma with a mask [Fig. 7(a)]: in their experiments, anodic aluminum oxide was modified with fluoroalkylsilane to become superhydrophobic and could be removed locally under the treatment of O2 plasma, they found that ring deposits could be suppressed considerably on the fabricated surfaces because of the differences in surface free energy between the superhydrophilic and superhydrophobic regions. Xu et al.111 obtained electrochemical sensors with patterned superhydrophobic and superhydrophilic microarrays by combining electrochemical deposition with template oxygen plasma technology, in their approach, oxygen plasma was irradiated for 120 s onto the dodecanethiol-modified substrate with a photomask so that the irradiated region became superhydrophilic while the non-irradiated region remained superhydrophobic.

FIG. 7.

Schematics of surface patterning processes: (a) schematic of preparation of patterned surface under O2 plasma treatment;140 (b) schematic of method for making superhydrophilic micropatterns on superhydrophobic porous polymer by UV irradiation;141 (c) programmable wettability pathways on surface of paraffin-infused porous graphene film formed with integration of near-infrared masks;142 (d) schematic of inkjet printing method for micropatterning superhydrophobicity on superhydrophilic surfaces;143 (e) schematic of inkjet printing method for micropatterning superhydrophilicity on superhydrophobic surfaces;40 (f) schematic of fabrication of micropatterned photonic nitrocellulose arrays;144 (g) schematic of formation of microdroplets on hydrophilic–superhydrophobic substrate.144 

FIG. 7.

Schematics of surface patterning processes: (a) schematic of preparation of patterned surface under O2 plasma treatment;140 (b) schematic of method for making superhydrophilic micropatterns on superhydrophobic porous polymer by UV irradiation;141 (c) programmable wettability pathways on surface of paraffin-infused porous graphene film formed with integration of near-infrared masks;142 (d) schematic of inkjet printing method for micropatterning superhydrophobicity on superhydrophilic surfaces;143 (e) schematic of inkjet printing method for micropatterning superhydrophilicity on superhydrophobic surfaces;40 (f) schematic of fabrication of micropatterned photonic nitrocellulose arrays;144 (g) schematic of formation of microdroplets on hydrophilic–superhydrophobic substrate.144 

Close modal

As well as plasma treatment, UV irradiation is also an effective and straightforward way to obtain superhydrophobic–superhydrophilic patterned surfaces. Zahner et al.141 demonstrated the preparation of superhydrophilic patterns on superhydrophobic porous thin films on glass support via UV-initiated surface photografting, where a mask determined the geometry [Fig. 7(b)], UV irradiation through the mask after wetting with a photografting mixture led to a superhydrophilic region with a static WCA of 0°. In another study, Huang et al.145 fabricated a superhydrophobic–superhydrophilic surface on a three-layer heterostructure of a TiO2/PDMS/Cu superhydrophilic surface, they used a UV LED (3 W) flashlight to irradiate the TiO2/PDMS/Cu at an intensity of 10 mW cm−2 for two minutes so that the irradiated region exhibited a WCA of 153° as opposed to 6° for the non-irradiated part. Recently, Wang et al.142 used the photothermal property of graphene to present a novel paraffin-infused porous graphene film (PIPGF) with programmable wettability [Fig. 7(c)], with the integration of near-infrared masks, the paraffin could melt in corresponding patterns on the PIPGF, thus providing programmable wettability pathways for droplet manipulation.

2. Printing methods

Printing is a direct and efficient method for depositing various materials on different substrates, including inkjet printing,146,147 3D printing,148 and microcontact printing.149 By choosing functional materials with specific wettabilities and designing preset regions of the substrate, patterned coating thereon can be controlled with different wettability from that of the unprinted areas.10,150 Superhydrophilic–superhydrophobic patterned surfaces can be fabricated by printing superhydrophobic materials on superhydrophilic surfaces or printing superhydrophilic materials on superhydrophobic surfaces. For example, Sun et al.143 fabricated various superhydrophilic–superhydrophobic patterned surfaces with different water-soluble-polymer templates that were inkjet-printed on a fabricated superhydrophilic titanate layer [Fig. 7(d)], they then studied the separation of water droplets and deposition of functional materials on the fabricated surfaces with different wetting abilities. Zhang et al.40 reported a bio-inspired method for preparing stable superhydrophilic micropatterns on a superhydrophobic surface by directly inkjet-printing a mussel-inspired ink of dopamine solution [Fig. 7(e)], they achieved a well-controlled pattern dimension of the superhydrophobic surfaces upon the formation of polydopamine via in situ polymerization. Besides inkjet printing, conventional laser printing can also achieve wettability patterns. Chi et al.144 proposed an easy-to-operate method for fabricating hydrophilic–superhydrophobic patterns by laser printing: in their approach, well-designed patterns with superhydrophobicity were printed on a prepared hydrophilic photonic crystal membrane to generate droplet arrays [Figs. 7(e) and 7(f)], the generated microdroplets were uniform in size, benefiting from the precise patterns generated by high print resolution. In summary, printing technologies are easy and scalable for fabricating superhydrophilic–superhydrophobic micropatterns on various substrates, but the resolution and precision of the fabricated wettability patterns are usually at only the microscale. Innovative printing methods are highly desirable for fabricating superhydrophilic–superhydrophobic patterned surfaces efficiently.

Superhydrophilic–superhydrophobic patterned surfaces have shown outstanding ability to control and pattern droplets and are emerging as valuable platforms in numerous applications, ranging from self-cleaning surfaces to biomedical sciences. There have already been several high-quality reviews of superhydrophilic–superhydrophobic patterned surfaces for water–oil separation,151 cell culture and analysis,37 and lab-on-a-chip devices,152 but only for those particular fields. In this section, we give a comprehensive review of the literature on superhydrophilic–superhydrophobic patterned surfaces organized by field, e.g., droplet arrays, cell arrays, biosensors, etc.

Droplets are suited to compartmentalizing and isolating reactants and can mimic diverse conditions similar to those of a macroscale reactor, making them ideal candidates for various miniaturized applications.153–155 Conventional devices that can generate droplet arrays either use the oil phase in microfluidics or build a physical wall in a microtiter plate.156,157 For example, droplet microfluidics—which enables work with extremely small reagent amounts in physically isolated droplets—is a well-established droplet-generating platform and has been used widely in digital polymerase chain reaction, however, the size and homogeneity of the droplets depend severely on the fluid flow rate, and mass monodisperse droplet production is limited because slight fluctuations in the flow rate will result in polydisperse droplets. Therefore, a straightforward technique that is more suitable for fabricating droplet arrays is urgently needed. Reported recently was a superhydrophilic intelligent and flexible design of substrates without the use of pipetting [Fig. 8(a)]. For example, the droplet size was reported as being as low as several micrometers or even nanometers,159 and the reservoir geometry can be tuned for synthesizing materials with specific shapes.160 By passively smearing an aqueous solution on a patterned superhydrophilic–superhydrophobic surface, Feng et al.41 fabricated high-density hexadecane droplet arrays based on the surface modification of a glass slide: chloro (dimethyl)vinylsilane-coated flat glass was modified with 1H,1H,2H,2H-perfluorodecanethiol (PFDT) via the UV-induced thiolene click reaction, and when liquid was moved along a PFDT–cysteamine-patterned glass surface, the solvent dewetted discontinuously at the superhydrophobic PFDT areas and spontaneously formed an array of separated droplets located on the cysteamine-modified superhydrophilic areas, microdroplets with complex geometries such as squares, triangles, or even stars were formed on such patterns [Fig. 8(b)]. Similarly, Wu et al.161 fabricated controllable 3D microstructures from an NP suspension on hydrophilic pinning points patterned on a silicon wafer via inkjet printing, by designing the hydrophilic pinning patterns, they prepared different 3D microcolloidal crystals with precise morphologies, including line, triangle, square, star, hexagon, and octagon. Hancock et al.162 used hydrophilic–hydrophobic patterns to shape liquids into more geometries to control the deposition of microparticles and create shaped hydrogels.

FIG. 8.

Superhydrophilic–superhydrophobic patterned surfaces for preparing droplet arrays: (a) schematic of droplet array slide and images of droplets formed in a superhydrophobic–superhydrophilic pattern;158 (b) bright-field microscopy images of hexadecane microdroplets of different geometries (scale bar: 200 µm);41 (c) influence of superhydrophilic spot area (A) on average water droplet volume (V) confined in a single spot of square geometry;33 (d) example of a selected region to quantify droplet reproducibility (scale bar: 500 µm), the grayscale fluorescence image shows 0.1 mg ml−1 Rhodamine 6G deposited on an array of superhydrophilic squares (500 µm side length, 62.5 µm barrier) after drying in air;33 (e) fluorescence intensity profile of six squares from a representative sample for four different concentrations of Rhodamine 6G: 0.1 mg ml−1 (red), 0.05 mg ml−1 (green), 0.025 mg ml−1 (blue), and zero (black), each colored horizontal line is the mean fluorescent intensity across the triplicates analyzed for the corresponding Rhodamine 6G concentration.33 

FIG. 8.

Superhydrophilic–superhydrophobic patterned surfaces for preparing droplet arrays: (a) schematic of droplet array slide and images of droplets formed in a superhydrophobic–superhydrophilic pattern;158 (b) bright-field microscopy images of hexadecane microdroplets of different geometries (scale bar: 200 µm);41 (c) influence of superhydrophilic spot area (A) on average water droplet volume (V) confined in a single spot of square geometry;33 (d) example of a selected region to quantify droplet reproducibility (scale bar: 500 µm), the grayscale fluorescence image shows 0.1 mg ml−1 Rhodamine 6G deposited on an array of superhydrophilic squares (500 µm side length, 62.5 µm barrier) after drying in air;33 (e) fluorescence intensity profile of six squares from a representative sample for four different concentrations of Rhodamine 6G: 0.1 mg ml−1 (red), 0.05 mg ml−1 (green), 0.025 mg ml−1 (blue), and zero (black), each colored horizontal line is the mean fluorescent intensity across the triplicates analyzed for the corresponding Rhodamine 6G concentration.33 

Close modal

Apart from their position and geometry, which can be adjusted by the flexible design of the patterns, the volume of the produced droplets can be made uniform by controlling related parameters, including the surface tension of the solution, the way that it is applied, the size of the superhydrophobic gap between spots, the surface properties, and the temperature and humidity.137,163 Thus, Ueda et al.33 designed differently shaped arrays of superhydrophilic squares with 500 μm side length using finite-element simulations to predict the fluid shapes: they found that the average volume of droplets confined in superhydrophobic barriers increased with a larger superhydrophilic spot area [Fig. 8(c)], also, the prepared droplet arrays showed the ability to control droplet volumes from picoliters to hundreds of nanoliters and demonstrated good variability of droplet volumes measured by fluorescence intensity across the droplet arrays [Figs. 8(d) and 8(e)]. Chang et al.164 investigated the relationship between the volume of deposited droplets and the size of the gap between the pads: in their experiment, the droplets were formed by sliding a droplet on a patterned hydrophilic–superhydrophobic surface tilted at 10°, the results showed that water droplets with a volume of 11.5 ± 0.5 nl could be achieved on patterned hydrophilic–superhydrophobic surfaces, and the deposited volume increased slightly with larger gap sizes.

Cell arrays have been proposed as a new platform that allows thousands of genetic and chemical probes to be screened simultaneously, and they are being used increasingly in various fields, including gene expression, drug screening, and cell function studies.165–170 Although significant effort has been made to develop methods for cell arrays, most of the existing techniques rely on the physical confinement of cells to specific regions, such as microfluidic devices,171,172 micromechanical devices,173 and microwells.174 For example, most cell-based high-throughput screening is performed in commercial 384- or 1536-well cell culture plates. However, as the number of wells increases, it becomes increasingly difficult to pipette sample solutions into the open wells in parallel manually, and expensive liquid-handling robots with complex instrumental setups are usually essential. Also, the number and type of cell lines that can be patterned are limited. As an emerging arraying technique, superhydrophilic–superhydrophobic patterned surfaces are considered an ideal alternative because of their simple, power-free, and equipment-independent nature,175 and it has been shown that such surfaces allow the creation of arrays with cells encapsulated in droplets for screening applications.176–182 In a cell array in which superhydrophilic and cell-adhesive spots are patterned on a superhydrophobic and cell-repellent surface, each spot can be regarded as an independent reservoir, and different cells can be trapped in these fully isolated compartments to perform diverse biological reactions without cross-contamination. For example, Efremov et al.178 showed that superhydrophilic–superhydrophobic micropatterns could be used to confine cell-containing solutions to superhydrophilic regions, thereby creating isolated culture microreservoirs with predefined geometries on the same surface [Fig. 9(a)]: their experiments showed that 50 µm thin superhydrophobic barriers can effectively prevent the merging of neighboring droplets and cell migration across the barriers despite the high initial cell density, however, a limitation of their method is the maximum volume of cell-containing reservoirs, which is limited by the possible overlapping of neighboring droplets. Therefore, Shao et al.183 developed a templated substrate with hydrogel arrays for culturing cell spheroids: the templates were Morpho butterfly wings with chitin and protein components, which offered a natural superhydrophobic surface without modification, and because of the 3D periodically stacked nanostructures of the wings, hydrogel pre-polymer solutions could be photopolymerized as specific patterns with strong adhesion on the wing substrates [Fig. 9(b)], the as-prepared patterns could be used directly to culture spheroids in hanging drops and had the advantages of controllable volume and uniform size for the formation of 3D cell spheroids. Neto et al.184 also arranged arrays of hanging cell culture droplets to build 3D spheroids, after placing droplets of cell suspensions over the microindentations, the substrates were inverted by 180° so that the liquid droplets could hang in place.

FIG. 9.

Superhydrophilic–superhydrophobic patterned surface for preparing cell arrays: (a) schematic of cell culture in droplets on hydrophilic–superhydrophobic patterned polymer substrate;178 (b) micrograph of hydrogel pattern for 3D cell spheroid preparation (scale bar: 100 μm);183 (c) conceptual illustration of creation of cell array under various reagent concentrations;185 (d) fluorescent images of 12 × 8 array of E. coli at six different arabinose concentrations;185 (e) schematic of reverse cell transfection using a droplet microarray.177 

FIG. 9.

Superhydrophilic–superhydrophobic patterned surface for preparing cell arrays: (a) schematic of cell culture in droplets on hydrophilic–superhydrophobic patterned polymer substrate;178 (b) micrograph of hydrogel pattern for 3D cell spheroid preparation (scale bar: 100 μm);183 (c) conceptual illustration of creation of cell array under various reagent concentrations;185 (d) fluorescent images of 12 × 8 array of E. coli at six different arabinose concentrations;185 (e) schematic of reverse cell transfection using a droplet microarray.177 

Close modal

Stable and controllable cell arrays on superhydrophilic–superhydrophobic micropatterns make it possible to perform cell-based high-throughput screening. For example, Xu et al.185 reported wall-less cell arrays for mapping culturing conditions using superhydrophilic patterns on the surface of hydrophobically coated glass through CO2 laser cleaning, the laser-cleaned glass surface enables the self-partitioning of liquid into droplet arrays with quantitative volume control. A glass-poly(dimethylsiloxane) manifold and poly-(methyl methacrylate) aligner were used to facilitate the alignment of the arrays during the construction of the cell array [Fig. 9(c)]. To show the feasibility of the laser-patterned superhydrophilic arrays, Xu et al.185 cultured a green fluorescent protein (GFP) reporter-gene encoded E. coli strain (HCB1) that used L−(+)-arabinose with varying concentrations as the inducing reagent for expressing the encoded gene. The fluorescence intensity of GFP was used as the indicator of the expression level, and the E. coli array after culturing overnight under different L−(+)-arabinose concentrations showed a gradient of fluorescence intensity [Fig. 9(d)]. In another study, Popova et al.177 explored the potential of cell arrays as a miniaturized platform for high-throughput cell-based screens: they demonstrated the suitability of the platform for such applications and established the methods of reverse transfection and reverse drug screening in individual nanoliter-sized droplets by printing transfection mixtures or drug molecules directly onto superhydrophilic spots before cell seeding [Fig. 9(e)], HEK293 cells were shown to be transfected successfully with plasmids expressing H2B-YFP and H2B-RFP with a transfection efficiency of 50%–80%.

Superhydrophilic–superhydrophobic micropatterns have also attracted substantial attention as progressive strategies for biosensors. The wettability difference between the superhydrophilic and superhydrophobic patterns limits the spread of droplets,186–188 and so the solute gradually becomes concentrated with no loss on the hydrophilic spots as the droplets evaporate until they reach the hydrophobic spots, which can effectively enrich analytes from a highly diluted sample.189 With the help of such patterned surfaces, much effort has been made to enhance the sensitivity and achieve a lower detection limit of existing analytical methods, including colorimetric detection,183,190,191 SERS detection,6,192,193 fluorescent detection,194–196 and electrochemical detection.111,197 He et al.198 demonstrated a portable tape-based superhydrophilic–superhydrophobic microchip aimed at on-site colorimetric detection of heavy metals, they chose a commercial adhesive tape as the substrate and modified it with a nanodendritic silica coating to form superhydrophilic microwells on the superhydrophobic background. The superhydrophilic microwells served as independent “micro-containers” for indicator immobilization and downstream droplet-based analysis. Further experiments by He et al.198 showed that these indicator-modified superwettable tapes could be coupled with a wearable glove, and on-site sample collection could be performed by direct dip-pull from an original solution, leading to the proof-of-concept analysis of common heavy metals including chromium, copper, and nickel. The droplet-based sensors have fine interference tolerance for analyzing target ions and are capable of discriminating among the heavy-metal contents of different water samples [Fig. 10(a)]. Huang et al.199 created a superhydrophilic–superhydrophobic patterned surface with highly ordered tip-capped nanopore arrays that can be used as an intelligent platform to detect different analyte solutions with various concentrations simultaneously based on SERS, the hybrid superhydrophilicity–superhydrophobicity realizes the homogeneous distribution of the concentrated analyte in droplets at fixed places, thereby avoiding the diffusion-limit problem and further enhancing the Raman signal. Huang et al.199 tested different concentrations of Rhodamine 6G or thiram after drying on the patterned surface, and the results showed that the detection limits of Rhodamine 6G and thiram on the superhydrophilic–superhydrophobic patterned surface were 10−10 and 10−7 M, respectively, in 50 μl droplets [Fig. 10(b)]. Inspired by the water-collecting behavior of cactus spines, Chen et al.200 reported a superhydrophilic–superhydrophobic patterned surface with the capability of directional droplet self-transportation by integrating the surface superhydrophilicity and superhydrophobicity of a nanomaterial with a geometrically asymmetric surface, such a surface provides a simple way to realize accurate fluorescent detection of prostate-specific antigen in serum from prostate-cancer patients with a concentration as low as 10−12 g ml−1 [Fig. 10(c)]. Moreover, Hou et al.201 printed a polystyrene NP solution on a hydrophobic polymer surface to fabricate a patterned-wettability substrate for ultratrace detection, coupled with the fluorescent enhancement effect from photonic crystals, the superhydrophilic–superhydrophobic patterned microchip achieved detection of Rhodamine 6G with a concentration as low as 10−16 M.

FIG. 10.

Superhydrophilic–superhydrophobic patterned surface for biosensors: (a) flexible superhydrophilic–superhydrophobic tape aimed at on-site heavy-metal content discrimination among different water samples, the heavy-metal abundance was higher in the samples of artificial liquid [raw water spiked with 0.5-mg/l Cr(VI), 1.0-mg/l Cu(II), or 1.0-mg/l Ni(II)] than in those of raw water (n = 6, *, p < 0.05, **, p < 0.01, ***, p < 0.001, unpaired t-test);198 (b) surface-enhanced Raman scattering spectroscopy detection of thiram with different concentrations on the superhydrophilic points of a superhydrophilic–superhydrophobic patterned surface;199 (c) superhydrophilic–superhydrophobic patterned surface as a platform for fluorescent detection, relationship between fluorescence intensity and concentration of prostate-specific antigen based on superhydrophilic–superhydrophobic patterned surface [inset shows fluorescence images of parallel detection zones at various concentrations (zero and 10−12–10−7 g ml−1)];200 (d) electrochemical detection of target E. coli O157:H7 single-stranded DNA in food samples [PBS buffer (label a), peach juice (label b), and milk (label c)] based on superhydrophilic–superhydrophobic patterned surface.197 

FIG. 10.

Superhydrophilic–superhydrophobic patterned surface for biosensors: (a) flexible superhydrophilic–superhydrophobic tape aimed at on-site heavy-metal content discrimination among different water samples, the heavy-metal abundance was higher in the samples of artificial liquid [raw water spiked with 0.5-mg/l Cr(VI), 1.0-mg/l Cu(II), or 1.0-mg/l Ni(II)] than in those of raw water (n = 6, *, p < 0.05, **, p < 0.01, ***, p < 0.001, unpaired t-test);198 (b) surface-enhanced Raman scattering spectroscopy detection of thiram with different concentrations on the superhydrophilic points of a superhydrophilic–superhydrophobic patterned surface;199 (c) superhydrophilic–superhydrophobic patterned surface as a platform for fluorescent detection, relationship between fluorescence intensity and concentration of prostate-specific antigen based on superhydrophilic–superhydrophobic patterned surface [inset shows fluorescence images of parallel detection zones at various concentrations (zero and 10−12–10−7 g ml−1)];200 (d) electrochemical detection of target E. coli O157:H7 single-stranded DNA in food samples [PBS buffer (label a), peach juice (label b), and milk (label c)] based on superhydrophilic–superhydrophobic patterned surface.197 

Close modal

Recently, superhydrophilic–superhydrophobic patterned surfaces have also been used in electrochemical assays. For example, Zhang et al.197 presented a novel electrochemical biosensor based on a superhydrophilic–superhydrophobic patterned surface for the attomolar detection of the single-stranded DNA (ssDNA) of the food-borne microorganism E. coli O157:H7 in microdroplets [Fig. 10(d)]. To prepare the functional patterned surface, first a nanostructured Au electrode was prepared by electrodeposition on ITO glass, followed by being immersed in an ethanol solution of octadecytrichlorosilane to obtain superhydrophobic properties, finally, the superhydrophobic Au surface was irradiated with UV light through a photomask. The prepared surface exhibited an excellent ability to anchor microdroplets and was used as the detection platform for E. coli O157:H7 ssDNA sensing, lowering the sample usage dramatically, a detection limit of 30 aM was achieved, which is much lower than that without a DNA walker. Recently, different detection methods have been integrated on a superhydrophilic–superhydrophobic patterned surface to achieve versatile and robust detection in different situations. For example, a tri-modal microRNA biosensor was fabricated on a superhydrophilic–superhydrophobic patterned nanodendritic Au/graphene surface;192 the superhydrophilic detection spots are surrounded by a superhydrophobic background, and the significant contrast in wettability causes microliter droplets to be anchored on the former. Benefiting from the large surface area and high-density hot spots of nanodendritic Au, the microwells can be used to carry out sensitive electrochemical sensing and SERS detection, and the introduced graphene exhibits different absorption behavior with single and double nucleic acid strands and provides the fluorescence signal response. Tri-modal detection of microRNA-375 was achieved successfully, showing an improvement in the detection accuracy of biomarkers via the coupling of multiple sets of data, thereby demonstrating the potential applications of this material in precise early diagnosis and real-time monitoring.

Used to process small amounts of fluids through microchannels, microfluidic chips have developed rapidly for biological and chemical analysis.154,202 However, the manipulation of droplet motions in a microfluidic system usually depends on extra energy input (e.g., electrical, magnetic, centrifugal, optical, thermal, and mechanical forces), which can make the system’s construction more complex and interfere with the activity of biomolecules.203–208 As described in Sec. IV C, the wettability contrast between superhydrophilic and superhydrophobic surfaces confines a fluid to certain regions, which offers an excellent opportunity to manipulate specific fluids on open surfaces without extra energy supplied. Inspired by the microstructure of plant stomata, Sun et al.142 presented a superhydrophilic–superhydrophobic surface for droplet manipulation [Fig. 11(a)]: the desired surface was fabricated by using a coaxially assembled capillary microfluidic device to emulsify a hybrid solution of graphene oxide and N-isopropylacrylamide hydrogel in ethoxylated trimethylolpropane triacrylate with dispersed SiO2 NPs, and the outer surface of the film acquired favorable hydrophobic properties under fluorosilane evaporation without affecting the hydrophilic hydrogel array, using such a surface, Sun et al.142 demonstrated controllable droplet sliding as well as effective droplet transfer for printing. Xing et al.209 reported a microfluidic device based on a superhydrophobic-patterned surface: a hydrophilic channel was fabricated by laser irradiation, and through it mass was transferred between two droplets, by controlling the geometry and volume of the droplets, they were able to determine the radius of curvature and the internal pressure of any droplet, and so they could predict mathematically the flow characteristics of the surface-tension-driven transport [Fig. 11(b)].

FIG. 11.

Superhydrophilic–superhydrophobic patterned surfaces for microfluidic systems: (a) optical images and corresponding fluorescence images of released droplets in different patterns;142 (b) three parallel droplet-driven micropumps with channel widths of 150, 300, and 500 μm, respectively (scale bar: 2 mm);209 (c) wax-printed microfluidic paper-based analytical device after wicking a blue dye to show the integrity of the hydrophilic channel;148 (d) optical micrographs showing the hydrophobic (black dashed line) and hydrophilic (yellow dashed line) regions on wax-patterned filter paper (upper) and the static contact angle of a water drop on the wax-printed area (i.e., hydrophobic barrier) (lower);148 (e) fabrication process and system setup of liquid-template method;210 (f)–(h) applications of microfluidic devices fabricated with liquid-template method.210 

FIG. 11.

Superhydrophilic–superhydrophobic patterned surfaces for microfluidic systems: (a) optical images and corresponding fluorescence images of released droplets in different patterns;142 (b) three parallel droplet-driven micropumps with channel widths of 150, 300, and 500 μm, respectively (scale bar: 2 mm);209 (c) wax-printed microfluidic paper-based analytical device after wicking a blue dye to show the integrity of the hydrophilic channel;148 (d) optical micrographs showing the hydrophobic (black dashed line) and hydrophilic (yellow dashed line) regions on wax-patterned filter paper (upper) and the static contact angle of a water drop on the wax-printed area (i.e., hydrophobic barrier) (lower);148 (e) fabrication process and system setup of liquid-template method;210 (f)–(h) applications of microfluidic devices fabricated with liquid-template method.210 

Close modal

As a branch of microfluidics, paper-based microfluidic devices are effective at transporting aqueous liquid in paper channels with hydrophobic barriers. For example, Chiang et al.148 reported a laboratory paper-based analytical device with solid wax patterns generated by a 3D printer: the wax-patterned area had a hydrophobic surface with a WCA of 104 ± 8°, which prevented liquid wicking through the paper [Figs. 11(c) and 11(d)], while the area without wax (cellulose paper) remained hydrophilic and thus served as a channel that wicked aqueous samples into the detection zones, the fabricated device was used for quantitative colorimetric analysis of nitrite and glucose across the relevant clinical concentration range with satisfactory accuracy and reproducibility. Songok et al.211 also constructed a paper-based microfluidic device on titanium dioxide-coated hydrophobic paper and verified a capillary-driven surface flow using the photo-switchable wettability of titanium dioxide. In a recent exciting study, Lai et al.210 used superhydrophilic–superhydrophobic patterns for rapid and simple prototyping microfluidic devices:210 in the process, they defined a liquid template to be cast in PDMS with inkjet-printed hydrophilic patterns on superhydrophobically coated PDMS substrates [Figs. 11(e)11(h)], as a proof of concept, different microfluidic devices were created for various applications, such as microfluidic mixers, gradient generators, and pneumatic fluid control.

The severe pollution caused by oily wastewater and oil leakage has become a worldwide environmental problem,212–214 and so many cleanup techniques have been suggested, such as in situ burning, oil skimmers, and absorbents. However, those methods have many intrinsic limitations, such as high cost, poor recyclability, and low flux. Recently, materials that enable efficient and low-cost oil–water separation have been studied widely to solve this problem, and superhydrophilic–superhydrophobic patterned surfaces aimed at oil and water are ideal candidates for oil–water separation without external energy, given that they can achieve efficient and selective separation of the two immiscible liquids.151,215–217 The surface energy of most oils is lower than that of water, so when the surface energy of the solid surface is between those of oil and water, the solid exhibits hydrophobic and lipophilic properties.218 Based on these principles, oil–water separators can be classified as either oil-removing materials (superhydrophobic–superoleophilic) or water-removing materials (superhydrophilic–superoleophobic). For example, Song et al.192 fabricated a superhydrophobic–superoleophilic material by forming a hierarchical structure of ZnO–Co3O4 on nickel foam, using an easy and environmentally friendly one-pot hydrothermal method combined with a straightforward calcination process and superhydrophobic modification with hexadecyltrimethoxysilan, the as-prepared Ni foam was used for oil–water separation with high efficiency and exhibited good chemical stability and long-term durability by measuring contact angles over an extensive range of pH values and in an immersion test. Water is denser than oil in most cases, and oleophilic materials can be easily fouled by oil, so superhydrophilic–superoleophobic materials are more desirable.219 Xue et al.220 fabricated a novel underwater mesh coated with superhydrophilic and superoleophobic hydrogel, which can selectively separate water from oil–water mixtures [Fig. 12(a)], they tested various aqueous mixtures (e.g., vegetable oil, gasoline, diesel, and even crude oil) with no extra power [Fig. 12(b)], and during the separation process, the underwater superoleophobic interface with low affinity for oil drops prevented the coated mesh from being fouled by oil, thereby making oil recycling easy.

FIG. 12.

Superhydrophilic–superhydrophobic patterned surfaces for environmental protection: (a) water permeates selectively through a coated mesh, while oil is repelled and remains in the upper glass;220 (b) separation efficiency of mesh coated with polyacrylamide hydrogel for oil selection;220 (c) schematic of beetle-inspired fog-harvesting device and fog-collecting process;221 (d) hybrid-membrane fog-harvesting system for irrigating plants (mint);34 (e) water collection mechanism of superhydrophilic–superhydrophobic film.222 

FIG. 12.

Superhydrophilic–superhydrophobic patterned surfaces for environmental protection: (a) water permeates selectively through a coated mesh, while oil is repelled and remains in the upper glass;220 (b) separation efficiency of mesh coated with polyacrylamide hydrogel for oil selection;220 (c) schematic of beetle-inspired fog-harvesting device and fog-collecting process;221 (d) hybrid-membrane fog-harvesting system for irrigating plants (mint);34 (e) water collection mechanism of superhydrophilic–superhydrophobic film.222 

Close modal

Water collection is very important for alleviating water shortage and pollution, especially in arid and semiarid locations.223,224 Therefore, cost-effective and easy approaches to collecting water are becoming of interest to the scientific community. Most such studies of using structural modification of superhydrophobic–superhydrophilic patterns are inspired by the wettability pattern on the back of the desert beetle, which has hydrophilic–hydrophobic contrast for water harvesting by mist condensation in desert environments. For example, Parker and Lawrence30 mimicked the structure of the beetle’s bumpy back—which comprises alternating wax-coated hydrophobic and non-waxy hydrophilic regions—and found applications in water condensers and engines. Recently, Wen et al.221 fabricated a beetle-inspired hierarchical fog-collecting interface based on an antibacterial needle array and a hydrophilic–hydrophobic cooperative structure [Fig. 12(c)], after being transported to the connected hydrophobic sheet, the collected droplets are detached rapidly and stored in a container, achieving a high fog-harvesting rate.

Furthermore, Hu et al.34 designed a hybrid membrane with asymmetric microtopology and anisotropic wettability based on the asymmetric microgeometry of cactus spines and the alternate hydrophilicity and hydrophobicity of a desert beetle’s back: this anisotropic hybrid membrane comprised a hydrophilic nanoneedle layer and a hydrophobic nanofiber layer, leading to highly efficient fog collection, moreover, they constructed a simulated greenhouse to test the fog-collecting and irrigating applicability of the as-fabricated hybrid membrane [Fig. 12(d)], and the results showed that the hybrid membrane could collect sufficient fog water for typical irrigation requirements of plants in foggy areas. Liu et al.222 created controllably hydrophilic–superhydrophobic patterns in favor of efficient water harvesting from fog by adjusting the precursor concentration as well as the crystal growth time [Fig. 12(e)], they investigated the effects of the as-prepared patterns on the water condensation behavior and achieved a maximum water collection efficiency of 917.6 mg cm−2 h−1.

Herein, we have provided a comprehensive review of recent developments in superhydrophilic–superhydrophobic patterned surfaces, from the fundamentals of wettability for regulating liquid behaviors on solid surfaces to fabrication strategies for typical applications of superhydrophilic–superhydrophobic patterned surfaces. Taking advantage of different surface energies in different regions, a superhydrophilic–superhydrophobic patterned surface with a rational design exhibits outstanding capability in the passive dispensing of an aqueous solution, which can be implemented to advance the performance and potential of existing technologies. Using superhydrophilic–superhydrophobic patterned surfaces has improved remarkably the development of advanced droplet/cell-array fabrication in terms of resolution, quality, and scale-up. Also, liquid control in microfluidics and arrayed analysis of ultratrace biomolecules are achieved on superhydrophilic–superhydrophobic patterned surfaces. However, although considerable breakthroughs and achievements have been made in this field, several critical challenges remain, leaving a gap between proof-of-concept laboratory research and actual applications. Here, we address a wide range of remarkable opportunities that lie ahead, from basic to applied research.

From the perspective of scientific research, surface wettability is a complex phenomenon involving many parameters on surfaces, and so much experimental exploration is needed for practical applications, including the following three aspects. (1) The mechanisms of biological surfaces. The underlying mechanisms of many evolved biological surfaces—which have complex hierarchical structural features and unique wettability—remain to be revealed. Therefore, further investigations are needed to pave the way for fabricating artificial interfaces with integrated functions. (2) Geometrical wettability patterns. The geometrical parameters of wettability patterns are indispensable for manipulating fluid behavior and so are important to investigate. The current widely reported diameter range for micropatterns is 10–1000 μm, but nano-patterned surfaces may also be attractive for controlling tiny droplets and even providing new functions. (3) Fluid behavior. Differentiated or gradient wettability patterns provided elaborate control over wetting behavior and liquid adhesion, which occur in the cells, tissues, and organs of living organisms.200,225,226 A deeper understanding of fluid behavior—particularly tiny-droplet generation and collection—should be obtained from microscopic observations to determine the functions of wettability difference in order to control precisely the movement direction and adhesion behavior of droplets.

Considering their practical applications, cost-effective and massive fabrication of superhydrophilic–superhydrophobic micropatterns is highly desirable. On one hand, the materials should be commercially available and cheap, natural substrates such as cellulose, sodium alginate, and chitosan might be a good choice. Moreover, researchers have also become interested in renewable materials,227 self-healing polymers,228,229 and some elastic polymers,230,231 which are also good candidates for superhydrophilic–superhydrophobic micropatterns because of their unique and excellent properties, such as being lightweight, having an extensive range of flexibility, and being easily fabricated. On the other hand, most of the current approaches (photolithography, sol-gel processing, chemical modification, and plasma treatment) to fabricating superhydrophilic–superhydrophobic micropatterns are limited to the laboratory and are usually expensive, time-consuming, and complicated. Therefore, in-depth collaborative efforts should be made to bridge the gap between low-cost and large-scale production and high-quality processing. Conventional methods of mass production (e.g., spray coating, stamping, 3D printing) can probably promote this area to allow superhydrophilic–superhydrophobic patterned surfaces to be industrialized and commercialized.

The robustness and stability of superhydrophilic–superhydrophobic patterned surfaces—including their chemical stability and mechanical robustness—remain significant challenges for practical applications, especially in harsh environments with high humidity, mechanical friction, and corrosive chemicals.232,233 For example, sunlight, temperature, and wind can affect the performance of many water-collection systems, and the adsorption of the secretions of cells and bacteria and Marangoni-flow-induced coagulation can influence the wettability of substrates and result in poor sustainability and credibility of superhydrophilic–superhydrophobic micropattern-based biosensors. Therefore, superhydrophilic–superhydrophobic micropatterns with the ability to survive for a long time under extreme and complex conditions may become the next topic of intense research interest. Self-healing and stimuli-responsive composites that enable damaged surfaces to repair could be promising candidates for realizing platforms with long-term stability.234–236 As well as innovations in methods and materials, further discussion is needed on benchmarks and standards of functional interfaces in different application scenarios.

The development of superhydrophilic–superhydrophobic patterned surfaces requires the ability to control the geometry or size of the produced patterns, which directly impacts the precise controllability of liquids. However, surfaces based on microwell arrays cannot meet the practical requirements of some specific applications. Future studies should consider patterned surfaces with different geometries for particular functions, such as bubble manipulation, liquid transport, or material exchange.31 Furthermore, evaporation of liquid—which is a common problem in handling small volumes of liquids with decreased size194,237—should also be considered. The drying process of a droplet can be controlled by controlling the liquid and substrate properties, the interactions at solid–liquid or liquid–gas interfaces, and the environmental conditions.238 Another promising research field based on superhydrophilic–superhydrophobic patterned surfaces is the arrayed analysis of actual ultratrace samples by avoiding the coffee-ring effect, which would contribute to more-sensitive bio-assays. Furthermore, future research should diversify the format of signal outputs and detect multiple biomarkers in a single chip to achieve high-throughput sample sensing.

Although these unsolved critical issues should be addressed, their high performance in actual applications has encouraged researchers worldwide to devote more efforts to accelerate the commercialization of such patterned surfaces. In the future, the remarkable development of multifunctional and straightforward arrays is expected to revolutionize different fields, including biology, chemistry, physics, engineering, and information. We hope that this review provides readers in different fields with a comprehensive understanding of superhydrophilic–superhydrophobic patterned surfaces, and that it inspires them to address the existing issues and bring better insights to discovering new applications.

This work was supported by the Independent Innovation Fund of Tianjin University (Grant No. 2022XJS-0003) and the National Key Research and Development Program of China (Grant No. 2019YFA0905804).

The authors have no conflicts to disclose.

Data sharing is not applicable as no new data were created or analyzed in this paper.

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Hao Chen received the B.S. degree in mechanical engineering from Northwestern Polytechnical University, Xi’an, China in 2019. He is currently pursuing the M.S. degree in Instrument Science and Technology at Tianjin University, China. His research interests focus on exploring surface modification methods for high-throughput DNA synthesis chips.

Xiaoping Li received the B.S. and Ph.D. degrees in mechanical engineering from Northwestern Polytechnical University, Xi’an, China, in 2011, and 2020, respectively. He is currently a research associate in School of Precision Instruments and Optoelectronics Engineering, Tianjin University. His research interests include microfluidics, DNA synthesis chips and instruments.

Dachao Li received his Ph.D. degree in precision instruments and mechanics from Tianjin University, Tianjin, China, in 2004. Li was previously a research associate at the Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, Ohio, USA. Currently, he is a professor and Ph.D. supervisor in School of Precision Instruments and Optoelectronics Engineering, Tianjin University. His research is on microfluidics, flexible sensors, ultrasonic devices, inkjet printing, and their applications in bioinformation measuring and biomanufacturing.