By combining metallic electrical conductivity with low viscosity, liquid metals and liquid metal alloys offer new and exciting opportunities to serve as reconfigurable components of electronic, microfluidic, and electromagnetic devices. Here, we review the physics and applications of techniques that utilize voltage to manipulate the interfacial tension of liquid metals; such techniques include electrocapillarity, continuous electrowetting, electrowetting-on-dielectric, and electrochemistry. These techniques lower the interfacial tension between liquid metals and a surrounding electrolyte by driving charged species (or in the case of electrochemistry, chemical species) to the interface. The techniques are useful for manipulating and actuating liquid metals at sub-mm length scales where interfacial forces dominate. We focus on metals and alloys that are liquid near or below room temperature (mercury, gallium, and gallium-based alloys). The review includes discussion of mercury—despite its toxicity—because it has been utilized in numerous applications and it offers a way of introducing several phenomena without the complications associated with the oxide layer that forms on gallium and its alloys. The review focuses on the advantages, applications, opportunities, challenges, and limitations of utilizing voltage to control interfacial tension as a method to manipulate liquid metals.

Liquid metals combine uniquely the low viscosity of liquids with the desirable properties of metals (e.g., high thermal and electrical conductivity), making them suitable for use in soft, stretchable devices such as soft robotics,1,2 e-skin,3 wearables,4 and stretchable electronics.5 Furthermore, they can be patterned at room temperature in unique ways6 and incorporate readily into microfluidic networks as pumps, electrodes, and valves. Perhaps the most promising and distinct property of liquid metals is their ability to be reversibly shape-reconfigured at room temperature; this opens up the possibility for devices that can change their function based on the switchable shape or position of their liquid metal components. This review focuses on methods to control the shape, position, and flow of liquid metals by manipulating the interfacial tension utilizing voltage.

There are five known metallic elements that are in the liquid state at or near room temperature: francium, cesium, rubidium, mercury, and gallium. However, francium and cesium are radioactive, while rubidium is explosively reactive when contacted with air, rendering these materials unsuitable for practical applications. Mercury, on the other hand, has been used in a wide variety of applications such as measurement equipment (e.g., barometers, thermometers), lamps, diffusion pumps, dental amalgams, and as the working electrode in its own field of electrochemistry (polarography). In spite of its long history of use, mercury is limited by its toxicity to humans.7 A safer alternative to mercury is gallium.8 Although gallium itself melts just above room temperature (30 °C), it can be combined with various other metals (e.g., indium and tin) to form alloys that will melt at lower temperatures.9 Unlike mercury, gallium also forms spontaneously a thin (1–3 nm),10 passivating oxide layer on its surface. This oxide layer has traditionally restricted the use of gallium; the formation of the oxide complicates its use as an electrode in electrochemical experiments,11 and its tendency to stick to many surfaces makes it difficult to actuate droplets.12 The oxide, however, does provide gallium with many unique advantages. Although the interfacial tension of liquid metals is very large (>400 mN/m),13 the oxide skin allows the liquid metal to maintain shapes that are otherwise prohibited by surface tension.14 This property enables simple methods of patterning gallium;6,15–18 it also allows gallium to remain stable after injection into microfluidic channels, providing an effective means of incorporating these metals into soft devices.14 

This review focuses on ways to move and manipulate liquid metals into new shapes and positions using voltage. With regard to the organization, content, and focus, we offer four clarifying comments. First, we focus primarily on gallium and its alloys. However, we also include mercury—despite its toxicity—in this review because it has been utilized in numerous applications and it offers a way of introducing several phenomena without the complications associated with the oxide layer that forms on gallium and its alloys. Second, we limit discussion primarily to methods to manipulate interfacial tension because these forces dominate on sub-mm length scales. Third, we focus primarily on methods that offer reversible actuation such that the metal can cycle back to its original shape or position (e.g., conventional surfactants can lower interfacial tension, but not in a way that is useful for actuation). Fourth, we recognize mechanical manipulation (e.g., pumping, physical prodding) as a method for moving metals, but choose to focus on voltage-driven phenomena.

Voltage-driven actuation has the appeal of being easy to implement, control, and miniaturize. It does not require bulky pumps, nor does it necessarily require direct contact with the liquid metal. We review electrocapillarity, electrowetting, continuous electrowetting, and electrochemistry as methods to move and manipulate liquid metals (as summarized in Figure 1). We detail the underlying physical mechanisms of these techniques, the ways in which they have been used, and their advantages and challenges after first briefly motivating applications of liquid metals.

FIG. 1.

Summary of primary methods for liquid metal actuation. (a) Electrocapillarity utilizes charges in the electrical double layer to realize modest changes in surface tension. (b) Continuous electrowetting creates surface tension gradients to actuate liquid metal within channels. (c) Electrowetting-on-dielectric uses large voltages to achieve modest changes in wetting behavior on a substrate. (d) Electrochemically controlled capillarity utilizes interfacial reactions to achieve enormous changes in surface tension.

FIG. 1.

Summary of primary methods for liquid metal actuation. (a) Electrocapillarity utilizes charges in the electrical double layer to realize modest changes in surface tension. (b) Continuous electrowetting creates surface tension gradients to actuate liquid metal within channels. (c) Electrowetting-on-dielectric uses large voltages to achieve modest changes in wetting behavior on a substrate. (d) Electrochemically controlled capillarity utilizes interfacial reactions to achieve enormous changes in surface tension.

Close modal

In addition to the aforementioned historical applications of mercury, liquid metals are being investigated for use in a variety of other technologies.19 These applications help motivate the use of voltage to manipulate the shape and position of liquid metals.

Many of the recent advances in liquid metals have been enabled by microfluidics, a field designed to study and manipulate fluids at sub-mm length scales. Microfluidic channels can be easily fabricated (e.g., through soft lithography20,21) from a wide variety of materials. The low viscosity of liquid metals allows them to be injected into these microchannels at relatively low pressures and temperatures. While mercury adopts shapes in microchannels that minimize surface energy (including spontaneous withdrawal), the surface oxide allows gallium to remain stable within microchannels after injection.14 This property offers many key benefits.

Unlike their solid counterparts, liquid metals are able to withstand significant bending, stretching, and deformation of the polymer in which they are embedded, without significant loss of electrical conductivity. This feature has been utilized to create stretchable circuitry and interconnects for artificial skin,22,23 stretchable conductors,24–27 inherently aligned microfluidic electrodes,28 compliant MEMS components,29,30 microfluidic heat sinks,31 and antennas32–40 (Figure 2).

FIG. 2.

Emerging applications for liquid metals. (a) Ultra-stretchable wire made by injecting liquid metal into a hollow elastomeric fiber.26 Reproduced with permission from Zhu et al., Adv. Funct. Mater. 23, 2308 (2013). Copyright 2013 John Wiley and Sons. (b) Liquid metal stretchable RFID tag placed on skin.41 Reproduced with permission from Jeong et al., Lab Chip 12, 4657 (2012). Copyright 2012 Royal Society of Chemistry. (c) A nitrile glove functionalized by inkjet-printed liquid metal nanoparticles.27 Reproduced with permission from Boley et al., Adv. Mater. 27, 2355 (2015). Copyright 2015 John Wiley and Sons. (d) LED integrated into a stretchable polymer with liquid metal interconnects.24 Reproduced with permission from Appl. Phys. Lett. 92, 11904 (2008). Copyright 2008 AIP Publishing LLC. (e) Liquid metal RF sensor under strain.35 Reproduced with permission from S. Cheng and Z. Wu, Lab Chip 10, 3227 (2010). Copyright 2010 Royal Society of Chemistry.

FIG. 2.

Emerging applications for liquid metals. (a) Ultra-stretchable wire made by injecting liquid metal into a hollow elastomeric fiber.26 Reproduced with permission from Zhu et al., Adv. Funct. Mater. 23, 2308 (2013). Copyright 2013 John Wiley and Sons. (b) Liquid metal stretchable RFID tag placed on skin.41 Reproduced with permission from Jeong et al., Lab Chip 12, 4657 (2012). Copyright 2012 Royal Society of Chemistry. (c) A nitrile glove functionalized by inkjet-printed liquid metal nanoparticles.27 Reproduced with permission from Boley et al., Adv. Mater. 27, 2355 (2015). Copyright 2015 John Wiley and Sons. (d) LED integrated into a stretchable polymer with liquid metal interconnects.24 Reproduced with permission from Appl. Phys. Lett. 92, 11904 (2008). Copyright 2008 AIP Publishing LLC. (e) Liquid metal RF sensor under strain.35 Reproduced with permission from S. Cheng and Z. Wu, Lab Chip 10, 3227 (2010). Copyright 2010 Royal Society of Chemistry.

Close modal

Liquid metals offer the unique opportunity for creating shape reconfigurable conductors for reconfigurable circuits, switches, antennas, and optofluidic devices. Antennas are a particularly compelling application of liquid metals because the spectral properties (frequency, bandwidth, etc.) depend on the shape of the conductors that comprise them. Thus, the ability to change the shape of liquid metals offers the opportunity to create reconfigurable antennas. Liquid metal antennas that change shape (and thus, function) via external stretching have been demonstrated,33,36,42 but mechanical tuning of antennas offers limited utility due to the reliance on mechanical mechanisms.

An array of techniques has been developed to actuate liquid metals in-situ, but none of these techniques are without drawbacks. Pneumatic pressure has been used previously to dynamically move liquid metals, both to eject liquid metal droplets onto surfaces16,43,44 and to inject them into microfluidic channels.45 However, pneumatic pressure requires bulky and rigid mechanical components. Furthermore, microfluidic injection of gallium and its alloys is effectively irreversible; while the pneumatic pressure can be reversed, the oxide layer tends to stick to the walls and leave behind a residue,46 though the use of acids or fluid slip layers helps prevent this issue.40,46–48 Chemical driving forces have also successfully moved liquid metal drops, but these sources are quickly exhausted.49,50 Though some recent work has focused on electromagnetic51 and photochemical52 methods for actuation, these are outside the scope of this review.

Utilizing electrical signals to manipulate liquid metal provides a key advantages over other techniques. Electrical approaches do not require moving parts or large power consumption, they are scalable to microsystems, and they offer control over both position and magnitude. Recent advances in the field have been achieved through an electrical technique that has existed for over a century: electrocapillarity.

Electrocapillarity is one of the earliest reported methods for tuning the interfacial tension of liquid metals. Initially discovered by Lippmann in the 1870s,53 electrocapillarity is the change in the effective interfacial tension of a liquid metal in an inert electrolyte upon application of an electrical potential to the metal relative to a counter electrode. The density of charge in the electrical double layer at the metal-solution interface changes in response to voltage. The double layer is effectively a capacitor; in order to lower the capacitive energy at the surface, the surface area of the metal increases, thereby changing the effective interfacial tension. For an ideally polarized electrode at constant composition, this change is described by the Lippmann equation (Equation (1))54 

q=dγdV,
(1)

where γ is the interfacial tension of the liquid metal, V is the electrode potential, and q is the charge density at the interface. One of the implications of this equation is that any increase in charge density—whether positive or negative—will result in a decrease in the interfacial tension of the liquid metal. This change can be characterized by an electrocapillary curve (Figure 3), which shows the change in interfacial tension as a function of potential.55 The peak of the electrocapillary curve represents the maximum interfacial tension of the metal, which occurs at the so-called “potential of zero charge.” The potential of zero charge depends on the metal and the electrolyte. Integration of the Lippmann equation yields

γ(V)=γ012C(VV0)2,
(2)

where γ0 is the interfacial tension at the potential of zero charge, C is the capacitance at the double layer, and V0 is the potential of zero charge. These curves can be used to characterize the interfacial properties of mercury56,57 and oxide-free liquid gallium58 in solution. Because of adsorption at the interface and the nature of the double layer, these curves are dependent on the concentration and type of electrolyte chosen.54 

FIG. 3.

Electrocapillary behavior of liquid metals in electrolyte. (a) Electrocapillary curve showing the surface tension of a liquid metal drop as a function of voltage. The apex of the curve represents the potential of zero charge; any change in voltage, whether positive or negative relative to the PZC, results in a decrease in interfacial tension and a relative flattening of the drop due to gravity. The lowered interfacial tension is caused by the capacitance at the electrical double layer. Data reproduced with permission from Frumkin et al., Electrochim. Acta 10, 793–802 (1965). Copyright 1965 Elsevier.55 (b) and (c) The lowering of interfacial tension by electrocapillarity can induce the capillary rise of the metal through a small tube.

FIG. 3.

Electrocapillary behavior of liquid metals in electrolyte. (a) Electrocapillary curve showing the surface tension of a liquid metal drop as a function of voltage. The apex of the curve represents the potential of zero charge; any change in voltage, whether positive or negative relative to the PZC, results in a decrease in interfacial tension and a relative flattening of the drop due to gravity. The lowered interfacial tension is caused by the capacitance at the electrical double layer. Data reproduced with permission from Frumkin et al., Electrochim. Acta 10, 793–802 (1965). Copyright 1965 Elsevier.55 (b) and (c) The lowering of interfacial tension by electrocapillarity can induce the capillary rise of the metal through a small tube.

Close modal

The mercury “beating heart” experiment is a classic example of the effects of electrocapillarity.59 A drop of mercury in solution, when contacted from above by an iron or aluminum wire, develops an electrochemical potential that causes the interfacial tension of the mercury to decrease (and a corrosion reaction at the surface of the wire). Gravity causes the drop to flatten out, detaching the mercury from the wire, at which point the surface tension increases, causing the drop to bead up and once again contact the wire. This cycle repeats itself, such that the mercury “beats” at a certain frequency.

Electrocapillarity can create changes (>100 mN/m) to the interfacial tension of a liquid metal drop at relatively small potentials (≈1 V). The ability to affect the liquid metal pressure with voltage has made electrocapillarity an effective tool for pumping at the microscale,60–62 including the capillary rise (Figure 3(b)). For example, electrocapillarity can create a microfluidic pump for aqueous electrolytes. Mercury confined to a vertically oriented microchannel involves two primary forces: gravity and surface tension (cf. Figure 3(b)). An alternating potential changes the surface tension, causing the height of the mercury column to oscillate continuously.60 A similar method was also utilized to create a microfluidic check valve using liquid metal.63 This technique has also been applied to the nanoscale to pump mercury into single-walled carbon nanotubes at less than 2.5 V, a technique that could lead to fabrication of continuous, uni-directional nanowires of liquid metal.61 A similar method was recently used to steer the flow of liquid metals into select pathways through complex microchannels by electrically controlling the interfacial tension of the leading interface of the metal.64 

The terms “electrocapillarity” and “electrowetting” are often used interchangeably in the literature, but the two terms are distinct. Here, we differentiate the two phrases following the distinction offered by Jackel et al.65 While electrocapillarity specifically refers to the change in interfacial tension induced by an electrical potential at the boundary between two fluids, electrowetting refers to the change in wetting properties between a fluid and a separate material caused by electrocapillarity. This material can be a fluid, causing a continuous change in the wetting properties (as in continuous electrowetting) or a solid (as in electrowetting-on-dielectric).

Applying an external electric field to discrete drops of the liquid metal in aqueous solution can create a surface tension gradient across the liquid metal surface. This gradient is caused by a potential drop through the electrolyte surrounding the metal and, according to the principles of electrocapillarity, can drive fluid motion inside of the channel without directly contacting the liquid metal with an electrode. Figure 4 shows an example of a mercury plug placed in a microfluidic channel filled with electrolyte.66 In the absence of an applied potential, the electrical double layer is distributed equally across both sides of the drop. However, the application of a potential across the ends of the microchannel results in a potential drop through the thin layer of liquid between the metal and the capillary walls. This potential drop induces an asymmetry in the electrical double layer across the drop, which causes a differential in surface tension that forces the drop to move.

FIG. 4.

Mercury confined to a 500 μm wide microfluidic channel.66 Electrodes are present on both ends of the microchannel. (a) In the absence of an applied voltage, the mercury plug does not move in the channel. (b) An applied potential between the electrodes causes a gradient in interfacial tension and thus actuation of the metal plug. Reproduced with permission from J. Lee and C. J. Kim, J. Microelectromech. Syst. 9, 171 (2000). Copyright 2000 IEEE.

FIG. 4.

Mercury confined to a 500 μm wide microfluidic channel.66 Electrodes are present on both ends of the microchannel. (a) In the absence of an applied voltage, the mercury plug does not move in the channel. (b) An applied potential between the electrodes causes a gradient in interfacial tension and thus actuation of the metal plug. Reproduced with permission from J. Lee and C. J. Kim, J. Microelectromech. Syst. 9, 171 (2000). Copyright 2000 IEEE.

Close modal

This effect is known as continuous electrowetting (CEW), and it is a direct result of electrocapillarity. To model CEW for an incompressible Newtonian fluid confined to a capillary, the Navier-Stokes equation for interfacial forces reduces to

dpdx=2Ddγdx,
(3)

where p is pressure, x is the dimension along the length of the capillary, and D is the inner diameter. Solving for the average velocity yields

v=D6μ{dγdx},
(4)

where v is the average velocity of the fluid, μ is the viscosity of the liquid metal, and {dγ/dx} represents the average interfacial tension change normalized over the length of the drop. The voltage drop across the liquid metal from the applied potential difference dictates the average interfacial tension difference. Substituting the potential gradient yields Equation (5), an estimate for the average velocity

v=qD6μLΔϕ,
(5)

where L is the length of the drop, q is the charge in the electrical double layer at the interface between the electrolyte and liquid metal, and Δφ is the potential difference across it. This equation implies that the velocity—and the direction of the velocity—is dictated by the external applied potential, which can be easily controlled. In the absence of a potential, there is no gradient across the drop to drive the fluid flow. Substituting typical values for a liquid metal in aqueous solution indicates that the droplet velocity can exceed 100 mm/s. A more detailed derivation can be found in Beni et al.67 

CEW makes it relatively easy to move plugs of mercury inside of a channel filled with an electrolyte; the same is true for gallium-based alloys as long as precautions are taken to avoid adhesion of the oxide layer. There are at least two options to avoid adhesion in the presence of electrolyte: utilize channels filled with acid or base to chemically remove the oxide or pre-fill the channels with electrolyte (prior to injecting the metal plug) so that a slip layer of electrolyte forms between the metal plug and capillary walls.46,48 Surfaces that are rough can also help minimize adhesion.

CEW has been utilized to create a wide array of devices including pumps,68 optical components,69 valves,70 and RF devices.71 However, the low friction environment found during CEW makes it difficult to control the final position of the liquid metal plug. A microfluidic channel with a variable diameter, as shown in Figure 5(a), was designed to overcome this issue. The gallium alloy settled into local energy minima in the absence of a voltage, thereby preventing “drift” of the plug of metal.71 

FIG. 5.

Devices based on continuous electrowetting. (a) Simulation and experimental results of a plug of liquid metal moving to the right by continuous electrowetting for RF applications (electrodes not shown).71 Reproduced with permission from Gough et al., IEEE Access 2, 874 (2014). Copyright 2014 IEEE. (b) Schematic for a micromotor based on continuous electrowetting of a slug of Hg.66 (c) Liquid metal micromotor. 14 μm “filters” connect the main track to the electrode while preventing the liquid metal from exiting. Reproduced with permission from J. Lee and C. J. Kim, J. Microelectromech. Syst. 9, 171 (2000). Copyright 2000 IEEE.

FIG. 5.

Devices based on continuous electrowetting. (a) Simulation and experimental results of a plug of liquid metal moving to the right by continuous electrowetting for RF applications (electrodes not shown).71 Reproduced with permission from Gough et al., IEEE Access 2, 874 (2014). Copyright 2014 IEEE. (b) Schematic for a micromotor based on continuous electrowetting of a slug of Hg.66 (c) Liquid metal micromotor. 14 μm “filters” connect the main track to the electrode while preventing the liquid metal from exiting. Reproduced with permission from J. Lee and C. J. Kim, J. Microelectromech. Syst. 9, 171 (2000). Copyright 2000 IEEE.

Close modal

The principles of CEW have been applied in ways that ensure a continuous flow of the liquid metal for various applications. One method for achieving continuous flow is the use of a microchannel loop to enable liquid metal micromotors66 (Figures 5(b) and 5(c)). A sequential voltage was applied to electrodes placed in regular intervals around the perimeter of the channel; when the liquid metal reached an electrode, the polarity switched and an electrode was activated further along the channel, causing uninterrupted actuation of the drop. This method can move metal droplets with a velocity of 44 mm/s, with low voltage and low current.

The optical properties of liquid metals make them ideal candidates to prevent light transmission or serve as fluidic mirrors. These properties were utilized to create an optical switch based on CEW.65 Low voltages were applied (from −1 V to 1 V) to a mercury plug in a rectangular capillary. Without an applied potential, light entering through a fiber refracts through the solution in the microchannel and is guided to a fiber; applying a voltage causes the drop to move, thereby reflecting the light back into a secondary fiber above the drop. This method for optical switching is advantageous, due to the low voltage and power consumption (≈1 μW) and reasonable switching times (≈20 μs).

CEW can also be applied to droplets outside of microchannels. When a droplet of a liquid metal is placed in a conductive electrolyte solution between two electrodes, a sufficient electrical potential applied between the electrodes can cause actuation of the drop in the fluid. Figure 6 shows an example of liquid metal “marbles” comprised of Galinstan (an alloy of gallium, indium, and tin) placed in a solution of NaOH with a pH of 13.5. Applying a DC potential between 2 and 15 V between the electrodes develops an asymmetry in the electrical double layer across the drop; this potential gradient leads to a differential surface tension that causes the drop to move toward the anode (due to the negative charge in basic solution).59 The effect has also been demonstrated in a pure water solution, though larger voltages were necessary.73 

FIG. 6.

Continuous electrowetting of liquid metal marbles in an open solution.72 (a) Force balance of the Galinstan drop in a solution of NaOH. The surface tension gradient overcomes the viscous drag and substrate friction to drive movement of the drop. (b) Time lapse of a Galinstan marble traversing between electrodes. Reproduced with permission from Tang et al., Nanoscale 5, 5949 (2013). Copyright 2013 Royal Society of Chemistry.

FIG. 6.

Continuous electrowetting of liquid metal marbles in an open solution.72 (a) Force balance of the Galinstan drop in a solution of NaOH. The surface tension gradient overcomes the viscous drag and substrate friction to drive movement of the drop. (b) Time lapse of a Galinstan marble traversing between electrodes. Reproduced with permission from Tang et al., Nanoscale 5, 5949 (2013). Copyright 2013 Royal Society of Chemistry.

Close modal

CEW can also be utilized for pumping electrolyte.74 Confining a drop of liquid metal in solution and constantly switching the polarity of the electrodes causes pumping, as seen in Figure 7. Confining the droplet prevents it from moving. Instead, the electrolyte moves. This type of pump is simple to fabricate and creates large flow rates (5 ml/min) with limited power consumption (<15 mW). A similar AC approach was used to create a micromixer, where the liquid metal was fixed to the substrate. Rather than moving the drop itself, AC potentials caused undulations on the drop surface that leads to rapid mixing of oil droplets in the immersion solution due to Marangoni flow (i.e., flow due to gradients in surface tension).75 

FIG. 7.

(a) Design of a liquid metal pump in an open channel.74 (b) Galinstan drop in confined channel. Polarity of graphite electrodes drives liquid metal in one direction, inducing electrolyte flow in the opposite direction. (c) Time lapse of the dyed solution pumped completely through the channel. Reproduced with permission from Tang et al., Proc. Natl. Acad. Sci. 111, 3304 (2014). Copyright 2014 PNAS.

FIG. 7.

(a) Design of a liquid metal pump in an open channel.74 (b) Galinstan drop in confined channel. Polarity of graphite electrodes drives liquid metal in one direction, inducing electrolyte flow in the opposite direction. (c) Time lapse of the dyed solution pumped completely through the channel. Reproduced with permission from Tang et al., Proc. Natl. Acad. Sci. 111, 3304 (2014). Copyright 2014 PNAS.

Close modal

In spite of its ability to create liquid metal actuation with low power consumption, there are several drawbacks associated with CEW. Excessive voltages can lead to the formation of hydrogen at the surface of the cathode via electrolysis; while this is a negligible issue in open channels, the hydrogen bubbles can electrically isolate the electrode from the solution in microchannels, permanently interfering with the operation of continuous electrowetting. The use of AC voltages at higher frequencies can mitigate bubbles, but typically some DC bias is necessary to achieve asymmetric motion of fluid. Excessive potentials can also cause the liquid metal to split up inside of the microchannel. To avoid these shortfalls, another method for liquid actuation can also be used: electrowetting-on-dielectric.

The ability to modulate and actuate fluids is not solely restricted to liquid metals immersed in electrolyte solutions. Charges at interfaces can overcome the interfacial tension of any conductive solution, including organics materials with dissolved electrolytes.76 This principle can be best illustrated by a droplet of water sitting on a conductive metal electrode. If a second electrode is inserted into the water, and a potential is applied between the electrode in the water and the metal substrate, charge rearrangement at the surface of the droplet will cause the droplet to change its apparent contact angle at the surface of the electrode. Mugele and Baret provide an excellent review on this topic.53 

This change in contact angle arises due to an imbalance at the three-point contact line between the vapor, liquid, and solid phase. In the absence of applied potential, the equilibrium contact angle is described by Young's Equation, which is derived from a force balance at the three-point contact line

γLVcosθY=γSVγSL,
(6)

where γLV is the interfacial tension between the liquid and vapor phase, γSV is the interfacial tension between the solid and vapor phase, γSL is the interfacial tension between the solid and liquid phase, and θY represents the Young's contact angle. An applied voltage causes the liquid to increase its surface area against both the vapor and solid phases, thereby lowering its capacitive energy and decreasing the apparent contact angle, as described by the electrowetting equation

cosθ=cosθYεε02dHγLV(VV0)2,
(7)

where θ represents the apparent contact angle at a macroscopic scale, ε is the dielectric constant, ε0 is the permittivity of free space, dH is the thickness of the Helmholtz layer, V is the applied potential, and V0 is the open circuit potential. A more extensive derivation can be found in the literature.53 The contact angle θ is dictated by the initial contact angle at equilibrium and the ratio of the capacitive energy (driving the droplet toward the substrate) to the surface tension of the liquid. It should be noted that although the apparent (macroscopic) contact angle changes during electrowetting, the Young's angle is thought to remain constant at the microscopic level, as does the interfacial tension between the liquid and vapor phases.

An insulating layer can also be added between the liquid and the conductive substrate,77 as illustrated in Figure 8. Although this layer increases the necessary voltage to move the droplet, it also prevents unwanted chemical reactions (provided that the voltage is kept below the breakdown voltage of the insulating layer).78 

FIG. 8.

Electrowetting-on-dielectric.77 (a) A conductive electrode is coated with a thin dielectric and hydrophobic coating, with a water droplet placed on top. The water reaches an equilibrium contact angle. (b) A voltage is applied between the electrode and the droplet, causing charges to migrate to the interface and thereby decrease the effective contact angle. Reproduced with permission from Shamai et al., Soft Matter 4, 38 (2007). Copyright 2007 Royal Society of Chemistry.

FIG. 8.

Electrowetting-on-dielectric.77 (a) A conductive electrode is coated with a thin dielectric and hydrophobic coating, with a water droplet placed on top. The water reaches an equilibrium contact angle. (b) A voltage is applied between the electrode and the droplet, causing charges to migrate to the interface and thereby decrease the effective contact angle. Reproduced with permission from Shamai et al., Soft Matter 4, 38 (2007). Copyright 2007 Royal Society of Chemistry.

Close modal

This method is called electrowetting-on-dielectric (EWOD) and can be described by

cosθ=cosθYεε02dγLV(VV0)2,
(8)

where d, in this case, is the thickness of the insulating dielectric layer. For water, a thin hydrophobic coating is often added to increase the initial contact angle of the drop. For the most effective operation, it is desirable to use an insulating layer that is thin, has a large dielectric constant, and a large breakdown field. The change in apparent contact angle upon applying a voltage increases with lower interfacial tension, which can be accomplished by including surfactants that reduce the interfacial tension of the liquid phase.79 

EWOD carries many advantages for moving liquids: it is simple to fabricate, scalable, and requires no moving parts. EWOD has been utilized to drive fluids through capillaries,80 for aqueous droplet actuation81 and mixing,82 for display technologies,83 and as a tool for biological diagnostics.84 EWOD's ability to function at small length scales is a particularly useful feature, as most lab-on-a-chip systems require large supporting components (e.g., mechanical pumps).

Although EWOD has conventionally been utilized for actuation of aqueous droplets, the same principles apply to liquid metals.85–87 Figure 9 shows an electrowetting curve, which measures the change in contact angle as a function of voltage, for mercury on a 600 nm thick parylene film88 (parylene is a popular dielectric since it can be vacuum-deposited to create conformal, pin-hole free films). Similar results have been achieved for electrowetting of Galinstan,13 but these took place under conditions that prevented the formation of the oxide layer, which adds a significant experimental complication. Achieving a significant change in contact angle requires a large voltage (>100s of volts) due to the large interfacial tension of liquid metals, which is a notable limitation. Placing liquid metal droplets on semiconducting substrates covered with oxides results in metal-oxide-semiconductor junctions that create asymmetry in the electrowetting curve.86,87

FIG. 9.

Electrowetting curve of a mercury drop. At larger voltages, the contact angle begins to saturate and deviate from the theoretical curve. Dielectric layer is a 600 nm parylene film.88 Reproduced with permission from Wan et al., J. Fluids Eng. 129, 388 (2007). Copyright 2007 ASME.

FIG. 9.

Electrowetting curve of a mercury drop. At larger voltages, the contact angle begins to saturate and deviate from the theoretical curve. Dielectric layer is a 600 nm parylene film.88 Reproduced with permission from Wan et al., J. Fluids Eng. 129, 388 (2007). Copyright 2007 ASME.

Close modal

The principles of EWOD have been utilized to fabricate devices that leverage the properties of liquid metals.89 Much like the optical switch using CEW, EWOD can actuate mercury to guide light, including the use of a mercury drop as a tunable light reflector.90 In this case, the dielectric-coated electrode was positioned above the drop and composed of transparent materials. Without an applied voltage, the curvature of the drop allows impinging light to be reflected in all directions. Turning on the potential causes the drop to wet the substrate, and direct light back toward the source. Mercury can also be utilized as a piston underneath a mirror.91,92 Controlling electrically the shape of the mercury directs the reflection of the light.

The fast switching properties of EWOD have also been used to create thermal93 and electrical MEMS switches.94–97 An effective liquid metal switch should have minimal latency time; this is the amount of time necessary to activate the switch after the actuation signal. This was demonstrated by with a mercury microswitch by fabricating a microframe to hold the droplet in place and reduce the distance that the droplet had to travel. This allows for on-off latency times of 60 and 150 μs, respectively, an order of magnitude improvement over previous mercury MEMS switches.

In addition to actuating droplets, electrowetting has been utilized to convert mechanical energy to electrical energy.98 Applying a voltage to a liquid metal in an electrowetting setup causes the metal to spread to a new equilibrium shape; mechanically pushing the drop beyond its equilibrium generates a current due to the resulting change in capacitance (arising from the increase in area between the liquid and substrate). In principle, this allows for power generations of up to 1 kW/m2; using multiple drops in series on a plate or in a capillary enables an increase in the current and therefore power.

There are two primary disadvantages of using EWOD systems for manipulating liquid metals: the ineffectiveness of EWOD on liquid metals that develop surface oxides, and the necessity of large voltages. Oxide layers provide mechanical impediments to actuation; aqueous acids and bases, which can remove the oxide, are conductive and therefore cannot be used in conjunction with EWOD. Insulating fluids that remove the oxide from gallium alloys would help address this limitation. Liquid metals also have large interfacial tensions that must be overcome to induce deformation. For water, it is possible to simply decrease the surface tension by including surfactants; while this is also possible for liquid metals (via self-assembled monolayers), these surfactants cannot affect a comparable change in surface tension.99,100 In the absence of these self-assembled monolayers, the liquid metal will have a surface tension approximately two orders of magnitude larger than an aqueous drop with a surfactant. This large surface tension necessitates the application of large voltages to achieve electrowetting, yet dielectric breakdown limits the maximum electric field (i.e., voltage divided by distance) that can be applied.

Electrochemical reactions at surfaces can, in principle, deposit species that help lower surface tension. Electrocapillarity experiments are designed typically to avoid electrochemical reactions such that changes in surface tension arise purely from electrical double layer effects. At excessive cathodic or anodic potentials, bubbles or other reactive by-products can form on the surface of the metal. This limits the voltage that can be applied in the context of electrocapillarity (cf. Figure 3(a)). A chemical reaction at the surface, however, does not necessarily preclude a change in surface tension.

It has been demonstrated that oxidation on a liquid aluminum surface causes a decline in the surface tension.101 However, these experiments were performed under carefully controlled oxygen concentrations in vacuum; furthermore, the growing oxide layer eventually acts as a mechanical impediment to further shape change. Other metals, such as gold, also show reduced surface tension under oxidation conditions.102 We believe Tsai et al. were the first to show that, on liquid gallium, the surface tension could be decreased significantly by applying large oxidative potentials in acid.103 This surface tension decrease caused gravity-driven spreading of the droplet, allowing it to increase its surface area against the substrate. The researchers used this spreading to create a light valve (Figures 10(a) and 10(b)); by applying less than 1 V into a solution of 5 M HCl, the metal blocked 96% of light transmission at fast switching speed (≈49 s) with complete reversibility. Turning off the voltage allowed the oxide layer on the surface to dissolve in the highly acidic solution, thereby increasing the surface tension and causing the gallium to bead up.

FIG. 10.

Electrochemically driven spreading of gallium-based alloys. (a) A drop of liquid gallium sits on a conductor over a backlight. (b) Applying a voltage to the drop causes spreading that blocks the back light.103 Reproduced with permission from Appl. Phys. Lett. 95, 251110 (2009). Copyright 2009 AIP Publishing LLC. (c) A drop of liquid metal being oxidized electrochemically in 1 M NaOH. (d) Surface tension of a eutectic gallium indium drop in 1 M NaOH. The vertical dotted line represents the electrochemical formation of the oxide layer.104 Reproduced with permission from Khan et al., Proc. Natl. Acad. Sci. 111, 14047 (2014). Copyright 2014 PNAS.

FIG. 10.

Electrochemically driven spreading of gallium-based alloys. (a) A drop of liquid gallium sits on a conductor over a backlight. (b) Applying a voltage to the drop causes spreading that blocks the back light.103 Reproduced with permission from Appl. Phys. Lett. 95, 251110 (2009). Copyright 2009 AIP Publishing LLC. (c) A drop of liquid metal being oxidized electrochemically in 1 M NaOH. (d) Surface tension of a eutectic gallium indium drop in 1 M NaOH. The vertical dotted line represents the electrochemical formation of the oxide layer.104 Reproduced with permission from Khan et al., Proc. Natl. Acad. Sci. 111, 14047 (2014). Copyright 2014 PNAS.

Close modal

It was later shown that the formation of the oxide was actually the cause of the spreading, rather than standard electrocapillary effects.104 Figure 10(c) shows a 2 V potential applied to eutectic gallium indium (EGaIn), in a solution of 1 M NaOH. The voltage oxidizes the surface of the metal, which lowers the interfacial tension. As gravity flattens the drop, the oxide stabilizes the cylindrical filament that connects the drop to the syringe-needle electrode above. Figure 11(d) shows interfacial tension measurements of a sessile droplet of the liquid metal in 1 M NaOH as a function of potential. In classic electrocapillarity, the surface tension should vary parabolically with respect to voltage (cf. Equation (2)). In the absence of the oxide layer, the droplet behaves in a manner consistent with traditional electrocapillarity; the surface tension decreases parabolically at negative potentials, until hydrogen bubbles form on the liquid metal at approximately −2 V. However, increasing the potential from −1.3 to −1.2 V causes the oxide to form and the interfacial tension drops sharply (by ≈150 mN/m). Increasingly positive potentials drop the tension even more, approaching near-zero interfacial tension. At these potentials, the surface area of the droplet increased until it broke free of the electrical connection to the working electrode. It is reasoned that the oxide layer behaves as a surfactant between the liquid metal and electrolyte since the largest decrease in surface tension occurs when the oxide begins forming on the metal and deviation from classic electrocapillarity occurs at the oxidation potential (i.e., at the dotted line in Figure 10(d)). Although surface oxides typically provide physical barriers to flow, the use of electrolytes (such as 1 M NaOH) that continuously remove the oxide allows the metal to flow despite being covered with a film. That is, the elecrochemical deposition of the oxide layer competes with dissolution by the basic electrolyte. Figure 10(d) suggests that the effective interfacial tension of the metal can change from ∼500 mN/m to near zero using approximately one volt (from the open circuit voltage of −1.5 V to −0.5 V) and the change is completely reversibly depending on the applied voltage. It is also possible to lower the interfacial tension utilizing the potential from redox chemistry, rather than applying an external potential.105 

FIG. 11.

Electrochemical injection (ECC) and withdrawal (recapillarity) of a liquid metal. (a) Galinstan is injected into a NaOH-filled microchannel by ECC (interfacial oxidation).105 Reproduced with permission from Gough et al., ACS Appl. Mater. Interfaces 8, 6 (2016). Copyright 2016 John Wiley and Sons. (b) Reduction of the oxide layer at the interface of the channel increases the interfacial tension of the metal, causing rapid withdrawal. (c) The same mechanism allows the liquid metal to withdraw from the shortest path of a maze.106 Reproduced with permission from Khan et al., Adv. Funct. Mater. 25, 671 (2015). Copyright 2015 John Wiley and Sons. EGaIn refers to eutectic gallium indium liquid metal.

FIG. 11.

Electrochemical injection (ECC) and withdrawal (recapillarity) of a liquid metal. (a) Galinstan is injected into a NaOH-filled microchannel by ECC (interfacial oxidation).105 Reproduced with permission from Gough et al., ACS Appl. Mater. Interfaces 8, 6 (2016). Copyright 2016 John Wiley and Sons. (b) Reduction of the oxide layer at the interface of the channel increases the interfacial tension of the metal, causing rapid withdrawal. (c) The same mechanism allows the liquid metal to withdraw from the shortest path of a maze.106 Reproduced with permission from Khan et al., Adv. Funct. Mater. 25, 671 (2015). Copyright 2015 John Wiley and Sons. EGaIn refers to eutectic gallium indium liquid metal.

Close modal

The ability to selectively grow and remove the oxide “surfactant” allows simple injection and withdrawal from electrolyte-filled capillaries (Figure 11), an approach referred to as “electrochemically controlled capillarity” (ECC). ECC creates a substantial decline in surface tension, which drives the liquid metal into the channels,62,105 as shown in Figure 11(a). In this example, the application of an oxidative potential between the liquid metal and the electrolyte (0.25 M NaOH) injected the liquid metal into a 600 μm channel at up to 2.5 cm/s. These high speeds are made possible by confining the metal in a shallow reservoir to increase the Laplace pressure to help push the metal into the channel. The build-up of excessive oxide at the interface ultimately hinders the injection rate; this can also be utilized as a feature to stabilize the shape of the liquid metal.62 Removal of the oxide by a reducing potential can be used to increase the interfacial tension, causing withdrawal of the liquid metal from the channel at >10 cm/s (Ref. 106) (Figure 11(b)). The reducing potential only removes the oxide at the interface between the electrolyte and the metal. The withdrawal stops immediately in the absence of potential. Channels not in the direct path between the electrolyte and negative electrode remain stable. This can be seen in Figure 11(c), where the liquid metal can find the shortest path of a maze, leaving the rest of the metal in the channel. This phenomenon is called “recapillarity” because it uses reductive potentials to induce capillary motion.

ECC was utilized recently to create reconfigurable reflectors107 and antennas.108 An electric potential was applied to the liquid metal in a microchannel filled with NaOH to elongate the metal. Varying the potential by <8 V allowed the antenna to be tuned continuously and reversibly between 0.66 GHz and 3.4 GHz by changing the length of the liquid metal in a capillary connected to a reservoir. Voltages larger than 1 V are needed presumably due to the potential drop through the electrolyte. It is possible to freeze the metal to stabilize it in a temporary shape107 or use Laplace barriers to trap it in a metastable shape within channels.

This electrochemical method is promising, but it is not without shortcomings. Injection of the liquid metal results necessarily in a formation of the oxide, meaning that strong acids or bases must be used to continually remove excessive oxide layer. Neutral electrolytes cause the oxide to build-up and hinder mechanically the movement of the metal. There may be opportunities to explore other electrochemically active species to lower surface tension that do not present mechanical impediments to flow. In addition, electrochemical reactions at the surface of the metal are coupled with half reactions at counter electrodes that can create undesirable bubbles.

Table I shows a comparison of the advantages and disadvantages of using each of these techniques. In general, the methods that involve direct contact of the liquid metal with an electrolyte require low voltages and low power to operate.

TABLE I.

Comparison of liquid metal actuation techniques.

MethodVoltage requirementElectrolyteChemical reaction
Electrocapillarity Low Yes No 
Continuous electrowetting Low Yes No 
Electrowetting-on-dielectric High No No 
Electrochemically controlled capillarity Low Yes Yes 
MethodVoltage requirementElectrolyteChemical reaction
Electrocapillarity Low Yes No 
Continuous electrowetting Low Yes No 
Electrowetting-on-dielectric High No No 
Electrochemically controlled capillarity Low Yes Yes 

Liquid metals have already shown promise in a variety of devices due to their combination of fluidity and metallic properties. This review highlights several ways that liquid metals can reconfigure their shape through electrical potentials that alter interfacial tension or contact angle. There are still challenges with each of these methods, including the necessity of a supporting electrolyte (electrocapillarity, CEW, and ECC), as well as large voltages and oxide free surfaces in an electrically insulating ambient (EWOD). Future work should focus on addressing some of these practical limitations, increasing actuation speeds, providing routes to meta-stable shapes (so the metal can temporarily get “trapped” in shapes that remain without voltage), improving the complexity of shapes, as well as attempting to expand further into the nanoscale, with the goal of simple, low power, reconfigurable circuitry.

The authors acknowledge support from the NSF CBET-1510772, UNCGA Award 9012351-06, and AFRL. We are also grateful for insights and feedback from Professor Jason Heikenfeld.

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