Superhydrophobic surfaces: Fundamentals, manufacture, and applications

Inspired by the “lotus effect,” superhydrophobic surfaces have been engineered to repel water with unparalleled efficiency.¹ More specifically, low adhesion superhydrophobic surfaces, defined by a large water contact angle and low contact angle hysteresis, ease self-removal of liquids and particles from the surface through droplet rolling, jumping, or bouncing, thereby holding great prospects for diverse applications such as anti-icing, anti-fogging, self-cleaning, heat transfer enhancement, liquid manipulation, and energy harvesting^2,3 In addition, the better understanding of superhydrophobicity has led to the development of new liquid repellency concepts such as superoleophobic surfaces,⁴ liquid marbles,⁵ or lubricant infused surfaces.^6–8 In recent decades, the demand for liquid repellent properties has seen a marked increase, involving materials science, physics, device design, applications, etc.^9,10 For example, multifunctional superhydrophobic surfaces integrating synergies of photothermal materials, phase change materials, structural, and chemical patterned wettability, etc., possess great potential in the fields of anti-icing and deicing. Additionally, droplet manipulation techniques are evolving for biochemical analysis and materials synthesis, among others. The development of superhydrophobic surfaces, alongside novel insight into droplet behavior mechanisms, and manipulation strategies of droplets on these surfaces are poised to widen their future applicability.

This Special Topic on superhydrophobic surfaces encompasses a range of current state-of-the-art investigations and interest, including, but not limited to, fundamental and applied research work encompassing both experimental, theoretical, and simulations in the field, as depicted in Fig. 1. Our desire is to present diverse and stimulating areas within this Special Topic, fostering the foundations as well as inspiration for future scientific exploration and innovation of new devices.

For decades, research into superhydrophobic surfaces has advanced our understanding of how surface chemical composition and morphology significantly influence the intimate interactions between droplets and solid surfaces exemplified by their adopted contact angle. To this end, researchers have proposed a variety of theoretical models and semiempirical formulas under special conditions. In this Special Topic, different timely perspectives on several fundamental and applied open questions have been included aiming to summarize current specific topics on superhydrophobicity as well as to provide future outlook and directions. To this end, Coux et al. have observed how superhydrophobic materials, which can also be regarded as superaerophilic, can capture a plastron of air when immersed in water. They measured the thickness of these plastrons and applied a model of the process of withdrawing plates from a liquid by modifying the Landau–Levich–Derjaguin theory.¹¹ Additionally, Erbil and McHale have synthesized the research on droplet evaporation on superhydrophobic surfaces to date, focusing on the theory of droplet evaporation on smooth and patterned superhydrophobic surfaces and the practical applications by exploiting the dynamics of droplet evaporation.¹² Hoque et al. have discussed important recent advancements and challenges to the development of durable superhydrophobic surfaces, including scalable fabrication, efficacy with low surface tension fluids, and the absence of standardized durability testing methods. They have also presented a roadmap to underscore the areas in both foundational and applied research.¹³ Chu et al. have reviewed two strategies to expand the functionality of superhydrophobic surfaces via building macrostructures and designing surface wettability patterns. They have also summarized the investigation of droplet dynamics on moving superhydrophobic surfaces, along with a look toward future perspectives.¹⁴ 

In addition to the superhydrophobic immersion reported by Coux et al., other novelties have been reported in this Special Topic, including the effects on the vortex-induced vibrations of superhydrophobic cylinders by Zhao et al.¹⁵ and the wetting transitions on deformable superhydrophobic materials with auxetic properties by Armstrong et al.¹⁶ Zhao et al. have studied the mechanisms of ice adhesion on graphene-patterned aluminum substrates, and Tenjimbayashi has proposed a new way to mass produce stable micrometric-scale liquid marbles by using a superhydrophobic mesh coated with a hydrophobic powder.^17,18

The dynamic interactions between liquid droplets and solid surfaces are also of great interest to the scientific community. Since the complete rebound of droplets on flat superhydrophobic surfaces has been observed, the maximum spreading coefficient and contact time have emerged as crucial parameters in the study of droplet impact dynamics. The control of droplet impact behavior is a prominent research area with emerging practical applications. In this Special Topic, various behaviors of droplet dynamics on superhydrophobic surfaces have been explored, such as droplet re-spreading behavior under low We,¹⁹ droplet impacting on superhydrophobic surfaces (cylindrical, high contact angles, and vibrating surface),^20–22 and suppressing the pancake bouncing induced secondary contact on superhydrophobic surfaces via jet splash.²³ The coalescence induced droplet jumping on superhydrophobic surfaces has also enormous potential in various applications, and to this end, Hou et al. have investigated the condensation and coalescence behavior of droplets on an inclined superhydrophobic surface and have proposed an equation describing the multi-hop jumping transport motion and distance.²⁴ 

Additionally, superior water repellency plays an essential role in the development of flexible sensors,^25,26 nanofluidic technologies,²⁷ droplet transport,²⁸ and condensation and heat transfer.^29,30 The stability and durability of superhydrophobicity are a topic of interest; technological advancements need materials that not only exhibit superhydrophobicity, but also embody multifunctionality as well as stability and durability, such as abrasion resistance and anti-corrosion.^31–33 Novel efforts to assess stability and durability of superhydrophobic coatings have further been reported in this collection. To this end, Wang et al. have introduced a method for numerically analyzing dry friction on superhydrophobic nanocomposite coatings,³¹ while Zhang et al. have devised a real-time method for the monitoring and evaluation of abrasion resistance of superhydrophobic surfaces.³² 

In practical applications, the limitations of single-functionality surfaces necessitate an evolution toward multifunctionality to fit complex environment applications. The integration of superhydrophobic properties, combined with optical, electromagnetic, and acoustic effects, has also opened up the potential to create superhydrophobic surfaces with unique multi-functionalities such as photothermal and magneto-responsive surfaces,³⁴ which also stimulate greatly multiple potential applications.

In summary, this Special Issue provides an opportunity for readers to learn about superhydrophobic surfaces and their diverse potential functionalities by deepening into four state-of-the-art and timely perspectives on important topics surrounding superhydrophobicity as well as on dozens of recent research advances including new functionalities, interactions, coating stability, etc. We would like to truly thank all the authors that have contributed to this Special Topic as well as the journal editors, editorial team, and staff who helped to put this great collection together.