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...
We identify and characterize a new class of fingering instabilities in liquid metals; these instabilities are unexpected due to the large interfacial tension of metals. Electrochemical oxidation lowers the effective interfacial tension of a gallium-based liquid metal alloy to values approaching zero, thereby inducing drastic shape changes, including the formation of fractals. The measured fractal dimension (D=1.3±0.05) places the instability in a different universality class than other fingering instabilities. By characterizing changes in morphology and dynamics as a function of droplet volume and applied electric potential, we identify the three main forces involved in this process: interfacial tension, gravity, and oxidative stress. Importantly, we find that electrochemical oxidation can generate compressive interfacial forces that oppose the tensile forces at a liquid interface. The surface oxide layer ultimately provides a physical and electrochemical barrier that halts the instabilities at larger positive potentials. Controlling the competition between interfacial tension and oxidative (compressive) stresses at the interface is important for the development of reconfigurable electronic, electromagnetic, and optical devices that take advantage of the metallic properties of liquid metals.
Liquids typically form droplets when exiting a nozzle. Jets––cylindrical streams of fluid—can form transiently at higher fluid velocities, yet interfacial tension rapidly drives jet breakup into droplets via the Rayleigh–Plateau instability. Liquid metal is an unlikely candidate to form stable jets since it has enormous interfacial tension and low viscosity. We report that electrochemical anodization significantly lowers the effective tension of a stream of metal, transitioning it from droplets to long (long lifetime and length) wires with 100-μm diameters without the need for high velocities. Whereas surface minimization drives Rayleigh–Plateau instabilities, these streams of metal increase in surface area when laid flat upon a surface due to the low tension. The ability to tune interfacial tension over at least three orders of magnitude using modest potential (<1 V) enables new approaches for production of metallic structures at room temperature, on-demand fluid-in-fluid structuring, and new tools for studying and controlling fluid behavior.
Electrowetting uses voltage to manipulate small volumes of fluid for applications including lab-on-a-chip and optical devices. To avoid electrochemical reactions, a dielectric often separates the fluid from the electrode, which has the undesired effect of adding processing steps while increasing the voltage necessary for electrowetting. We present a new method to dramatically reduce the complexity of electrode and dielectric fabrication while enabling multiple performance advances. This method relies on a self-oxidizing paint-on liquid-metal electrode that can be fabricated in minutes on rigid, rough, or even elastic substrates, enabling low operation voltages (<1 V), and self-healing upon dielectric breakdown. Furthermore, due to the non-negligible 'potential of zero charge', electrowetting occurs by simply short circuiting the electrodes. This work opens up new application spaces for electrowetting (e.g. stretchable substrates, soft and injectable electrodes) while achieving large changes in contact angle without the need for an external power supply.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.