A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension.

Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries.

Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( ̴500 mN/m to near zero). Furthermore, this method requires only a very modest potential (< 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

This method provides a simple way to control the surface tension of liquid metals containing gallium. The method uses modest voltages (~1 V) applied directly to the liquid metal (relative to a counter electrode in the presence of electrolyte) to achieve enormous and reversible changes to the surface tension of the metal¹.

Surface tension is a dominant force for liquids at small length scales and is important for a number of capillary phenomena including wetting, spreading, and surface-tension driven flow. Consequently, the ability to control surface tension is a sensible way to manipulate the shape, position, and flow of liquids....

1. Manipulation of the Interfacial Tension of Liquid Metal in Electrolyte

1. Oxidation
   1. Pour an aqueous electrolyte (acidic or basic) into a Petri dish. To ensure that the oxide is completely removed, use an acid or base with a concentration greater than 0.1 M²⁴ (e.g. 1 M NaOH or 1 M HCl). Use a volume that will fill the dish to a depth of approximately 1-3 mm. Avoid contacting the skin with these solutions.
   2. Use a syringe to place a drop (optimally between 10-500 µl.......

Figure 1A shows an example of the simple two-electrode technique for oxidation and reduction. In this instance, a 70 µl drop of the liquid metal placed in a 1 M NaOH solution contacts a copper wire to establish an electrical connection. The 1 M NaOH removes the surface oxide from the metal and allows the metal to bead up due to its interfacial tension. Applying a 2.5 V potential between the drop and a platinum mesh counter-elect.......

This method controls the surface tension of gallium-based liquid metals using small voltages to drive the deposition and removal of a surface oxide. Although the method only works in electrolyte solutions, it is simple, and works in a wide variety of different conditions, but there are subtleties worth noting. In the absence of electrical potential, both acidic and basic solutions etch away the oxide²⁷. The application of an oxidative potential drives the formation of surface oxide in all aqueous electrolytes,.......

The authors acknowledge support from Samsung, the NC State Chancellors Innovation Funds, NSF (CAREER CMMI-0954321 and Triangle MRSEC DMR-1121107), and Air Force Research Labs.