We present a method to control the interfacial tension of a liquid alloy of gallium via electrochemical deposition (or removal) of the oxide layer on its surface. In sharp contrast with conventional surfactants, this method provides unprecedented lowering of surface tension (∼500 mJ/m 2 to near zero) using very low voltage, and the change is completely reversible. This dramatic change in the interfacial tension enables a variety of electrohydrodynamic phenomena. The ability to manipulate the interfacial properties of the metal promises rich opportunities in shape-reconfigurable metallic components in electronic, electromagnetic, and microfluidic devices without the use of toxic mercury. This work suggests that the wetting properties of surface oxides-which are ubiquitous on most metals and semiconductors-are intrinsic "surfactants." The inherent asymmetric nature of the surface coupled with the ability to actively manipulate its energetics is expected to have important applications in electrohydrodynamics, composites, and melt processing of oxideforming materials.EGaIn | electrocapillarity | electrorheology | dewetting | spreading T he ability to control interfacial energy is an effective approach for manipulating fluids at submillimeter length scales due to the dominance of these forces at these small length scales and can be accomplished using a wide variety of methods including temperature (1, 2), light (3), surface chemistry (4-6), or electrostatics (7). These techniques are effective for many organic and aqueous solutions, but they have limited utility for manipulating high interfacial tension liquids, such as liquid metals. Liquid metals offer new opportunities for soft, stretchable, and shape-reconfigurable electronic and electromagnetic components (8-12). Although it is possible to mechanically manipulate these fluids at submillimeter length scales (13), electrical methods (14, 15) are preferable due to the ease of miniaturization, control, and integration. Existing electrohydrodynamic techniques can modestly tune the interfacial tension of metals but either limit the shape of liquid metals to plugs (e.g., continuous electrowetting) (16) or necessitate excessive potentials to achieve actuation on a limited scale (e.g., electrowetting) (17). Here, we demonstrate that the surface oxide on a liquid metal can be formed or removed in situ using low voltages (<1 V) and behaves like a surfactant that can significantly lower its interfacial tension from ∼500 mJ/m 2 to near zero. In contrast, conventional molecular surfactants effect only modest changes in interfacial tension (changes of ∼20-50 mJ/m 2 ) and are difficult to remove rapidly on demand (18). Our approach relies on the electrical control of surface oxidation, which is simple, requires minimal energy, and provides rapid and reversible control of interfacial tension over an enormous range, independent of the properties of the substrate upon which it rests. Furthermore, this method avoids the use of toxic mercury and the ensuing modulation of surface ten...

1605630(1 of 8) temperature, and low viscosity. The latter property allows LM to flow in response to deformation, whereas solid metals are stiff and prone to fail at small strains. Embedding LM in elastomer decouples the electrical and mechanical properties; that is, these composites have the electrical properties of the metal and the mechanical properties of the elastomer. Incorporating the LM into the hollow core of an elastomeric fiber results in a useful form for sensors because fibers may be integrated into clothing and fabrics. [12][13][14][15] Furthermore, fibers are inherently flexible, compliant, and conformal due to their narrow cross-section. Thus, fibers can readily wrap onto and conform to surfaces with Gaussian curvature whereas 2D sheets cannot without significant deformation. Fibers can also be mass produced at high speeds with small diameters (hundreds of microns) and produced by hand in a laboratory environment at room temperature. [16] The fibers described in this work have the additional advantage of being built from stretchable and soft materials. Fibers with LM cores have previously been used to make light-emitting structures [17] and stretchable wires that retain metallic conductance up to ≈800% strain. [18] We reasoned that elastomeric fibers filled with LM could also be used for capacitive sensing of torsion, strain, and touch.Here, we intertwined two fibers into a double-helix to create sensors of both torsion and strain since twisting or stretching the fibers increases the contact area between them, and therefore changes the capacitance. The complexity of torsion, which causes both normal and shear strain, has previously precluded the development of a simple sensor capable of measuring a large range of torsion. Existing torsion sensors measure changes in normalized resistance, [2,8,19,20] pressure, [9] and optical properties, [21] or utilize surface acoustic waves [22] or the inverse magnetostrictive effect. [23] Some of these sensors can detect changes as small as 0.3 rad m −1 and can measure torsion up to 800 rad m −1 before failure. Most existing torsion sensors, however, are rigid, cumbersome, expensive, and complex. The soft and stretchable sensor developed here offers a simple mechanism to measure large changes in torsion which may be useful for unconventional robotics [24,25] or artificial muscles. [26] In addition to sensing torsion, intertwined fibers increase capacitance in response to strain due to the increase in contact Soft and stretchable sensors have the potential to be incorporated into soft robotics and conformal electronics. Liquid metals represent a promising class of materials for creating these sensors because they can undergo large deformations while retaining electrical continuity. Incorporating liquid metal into hollow elastomeric capillaries results in fibers that can integrate with textiles, comply with complex surfaces, and be mass produced at high speeds. Liquid metal is injected into the core of hollow and extremely stretchable elastomeric fibers and the re...

This paper describes the utilization of vacuum to fill complex microchannels with liquid metal. Microchannels filled with liquid metal are useful as conductors for soft and stretchable electronics, as well as for microfluidic components such as electrodes, antennas, pumps, or heaters. Liquid metals are often injected manually into the inlet of a microchannel using a syringe. Injection can only occur if displaced air in the channels has a pathway to escape, which is usually accomplished using outlets. The positive pressure (relative to atmosphere) needed to inject fluids can also cause leaks or delamination of the channels during injection. Here we show a simple and hands-free method to fill microchannels with liquid metal that addresses these issues. The process begins by covering a single inlet with liquid metal. Placing the entire structure in a vacuum chamber removes the air from the channels and the surrounding elastomer. Restoring atmospheric pressure in the chamber creates a positive pressure differential that pushes the metal into the channels. Experiments and a simple model of the filling process both suggest that the elastomeric channel walls absorb residual air displaced by the metal as it fills the channels. Thus, the metal can fill dead-ends with features as small as several microns and branched structures within seconds without the need for any outlets. The method can also fill completely serpentine microchannels up to a few meters in length. The ability to fill dense and complex geometries with liquid metal in this manner may enable broader application of liquid metals in electronic and microfluidic applications.

Eutectic gallium indium (EGaIn) is a promising liquid metal for a variety of electrical and optical applications that take advantage of its soft and fluid properties. The presence of a rapidly forming oxide skin on the surface of the metal causes it to stick to many surfaces, which limits the ability to easily reconfigure its shape on demand. This paper shows that water can provide an interfacial slip layer between EGaIn and other surfaces, which allows the metal to flow smoothly through capillaries and across surfaces without sticking. Rheological and surface characterization shows that the presence of water also changes the chemical composition of the oxide skin and weakens its mechanical strength, although not enough to allow the metal to flow freely in microchannels without the slip layer. The slip layer provides new opportunities to control and actuate liquid metal plugs in microchannels-including the use of continuous electrowetting-enabling new possibilities for shape reconfigurable electronics, sensors, actuators, and antennas.

We describe a new electrochemical method for reversible, pump-free control of liquid eutectic gallium and indium (EGaIn) in a capillary. Electrochemical deposition (or removal) of a surface oxide on the EGaIn significantly lowers (or increases) its interfacial tension as a means to induce the liquid metal in (or out) of the capillary. A fabricated prototype demonstrates this method in a reconfigurable antenna application in which EGaIn forms the radiating element. By inducing a change in the physical length of the EGaIn, the operating frequency of the antenna tunes over a large bandwidth. This purely electrochemical mechanism uses low, DC voltages to tune the antenna continuously and reversibly between 0.66 GHz and 3.4 GHz resulting in a 5:1 tuning range. Gain and radiation pattern measurements agree with electromagnetic simulations of the device, and its measured radiation efficiency varies from 41% to 70% over its tuning range.

even at ppm O 2 levels. [ 24,25 ] Though this oxide has historically been considered a nuisance, [ 26,27 ] it provides unique opportunities to control the shape of the metal. [ 28 ] This oxide "skin" envelopes the liquid and provides mechanical stability that allows EGaIn to be molded into non-equilibrium shapes that would usually be disallowed by surface tension. [ 29,30 ] The skin has been harnessed to print the metal in 3-D [ 30 ] and 2-D [ 31 ] and to form stretchable interconnects, [ 32 ] wires, [ 33 ] antennas, [ 13,29 ] sensors, [ 34,35 ] and plasmonic structures [ 36 ] fabricated by injecting the metal into microchannels.Because EGaIn is a liquid, its shape and position can be controlled by inducing it to fl ow. Injecting the metal into a microchannel is straightforward by using pressure differentials. This injection technique has been used to create shape-reconfi gurable antennas and fi lters composed of alloys of gallium that change length in response to pressure. [37][38][39] Inducing the metal to fl ow out of a microchannel, however, is more challenging. The presence of the oxide skin can cause the metal to leave residue on the channel walls ( Figure 1 a), like wet paint fl owing through a tube. [ 40 ] It is possible to use Tefl onlike surfaces or roughened surfaces [40][41][42][43] to reduce the adhesion of the metal oxide, but these approaches limit the materials of construction and increase the complexity of fabrication. It is also possible to use acid or base to remove the oxide skin, but this approach lacks external control and involves the use of possibly hazardous or corrosive materials. [ 44 ] For these reasons, most studies involving the actuation of liquid metals in microchannels focus on Hg despite its toxicity. [45][46][47] The Pourbaix diagram predicts that reductive electrochemical potentials can remove the oxide skin on gallium. [ 48 ] Without the stabilizing presence of the skin, the metal undergoes capillary action to minimize its surface energy. Figure 1 b (and Supporting Information Movie S1) illustrates this concept: A puddle of the metal beads up in response to a reductive potential. Although an applied bias likely lowers the interfacial tension of the metal (relative to bare metal) via electrocapillarity, [ 49 ] the tension is still large enough to induce capillary phenomena. This phenomenon can be exploited to induce withdrawal of the metal in microchannels (Figure 1 c and Supporting Information Movie S2) by capillary action toward a reservoir where the metal may lower its interfacial energy by adopting a larger radius of curvature. [ 50 ] Importantly, in the absence of applied potential, This paper describes the mechanistic details of an electrochemical method to control the withdrawal of a liquid metal alloy, eutectic gallium indium (EGaIn), from microfl uidic channels. EGaIn is one of several alloys of gallium that are liquid at room temperature and form a thin (nm scale) surface oxide that stabilizes the shape of the metal in microchannels. Applying a reductive potential to ...

Scanning electron micrographs of hollow elastic fibers. Injecting the core with liquid metal renders the fibers conductive. As Michael D. Dickey and co‐workers present in article number https://doi.org/10.1002/adfm.201605630, two or more fibers twisted together can sense large amounts of torsion due to changes in geometry, and therefore capacitance, between the fibers. Self‐capacitance between fingers and the metal in the fibers allows for touch sensing.

This letter describes the fabrication and characterization of a shape shifting antenna that changes electrical length and therefore, frequency, in a controlled and rapid response to pressure. The antenna is composed of a liquid metal alloy (eutectic gallium indium) injected into microfluidic channels that feature rows of posts that separate adjacent segments of the metal. The initial shape of the antenna is stabilized mechanically by a thin oxide skin that forms on the liquid metal. Rupturing the skin merges distinct segments of the metal, which rapidly changes the length, and therefore frequency, of the antenna. A high speed camera elucidates the mechanism of merging and simulations model accurately the spectral properties of the antennas.