Interfacial Tension Modulation of Liquid Metal via Electrochemical Oxidation

Abstract

Among all of the elements in the periodic table, there are only five metals that are liquid near room temperature: francium (Fr), cesium (Cs), rubidium (Rb), mercury (Hg), and gallium (Ga).The first three elements, however, are not practical for applications because they are either radioactive (Fr, Cs) or explosively reactive with air (Rb). Mercury was widely used in the past but is now avoided due to its toxicity. [1,2] By process of elimination, therefore, gallium and gallium-based alloys are the safest liquid metals at or near room temperature. [3] Gallium, discovered in 1875, has an extremely low vapor pressure even at high temperatures (effectively zero at room temperature and only 1 kPa at 1037 C; [4] by comparison, water has a vapor pressure of 1 kPa at 7 C). This low vapor pressure keeps the liquid from evaporating and eliminates the danger of vapor inhalation. While gallium should be found as solid at room temperature, because its melting point is 30 C, it is often liquid due to its ability to supercool. To assure a roomtemperature liquid phase, eutectic alloys containing at least one additional metal are commonly used to lower the melting point below room temperature. Two popular commercial alloys are Galinstan (68.5 wt% Ga, 21.5 wt% In, and 10.0 wt% Sn) and eutectic gallium indium (EGaIn, 75.5 wt% Ga and 24.5 wt% In), for which the melting point is 10.7 and 15.5 C, respectively. [5] In air, these alloys all form surface oxides within microseconds due to a high reactivity of gallium with oxygen. Because Ga oxidizes more readily than other components in these alloys, the surface oxides are dominated by gallium oxide species. These alloys thereby exhibit similar interfacial behavior to gallium in air. [6] Thus, we discuss these gallium-based alloys without distinguishing them by their bulk composition, except where warranted. Interfacial Tension ModulationMetals have significantly larger surface tension (>400 mN m À1 depending on the liquid metal [7] ) than common fluids such as water (72 mN m À1 ), as shown in Figure 1a. The large surface tension of metals is due to the metallic bonds. [4] Gallium is notable for having the highest surface tension (708 mN m À1 ) of any liquid in air near room temperature [8,9] and alloys of Ga, such as EGaIn, also have enormous surface tension (624 mN m À1 ). [10] We note the term "surface tension" refers to the tension of fluid interfaces in contact only with a vapor phase, whereas "interfacial tension" refers more broadly to the tension of fluids

Superelongation of Liquid Metal

“…The unidirectional elongation is probably due to the existence of Marangoni forces, which the effective interfacial tension at the region closer to the copper electrode being lower than the region further away. [32,34] We verify this hypothesis by tracking the flow of the surrounding liquid along the elongated LM using food dye, as shown in Figure 2b (see also Movie S2 in the Supporting Information), in which a flow toward the anode was observed for the NaOH solution, indicating the generation of Marangoni flows. For bare LM droplets, nonuniform electrochemical growth of an oxide layer causes an inhomogeneous distribution of stress on the LM surface; such an inhomogeneities can then be amplified to cause perturbations to result in a finger-like pattern.…”

“…However, the Marangoni instabilities due to gradients in effective interfacial tension makes the spreading process unreliable, and eventually causes fragmentation (see also Movie S7, Part 1, in the Supporting Information). [32] In contrast, uniform and rapid spread-ing of LM without fragmentation was observed for the LM-Fe mixture (1 mL), replicating the heart-shaped pattern of the mold, as shown in Figure 5c (see also Movie S7, Part 2, in the Supporting Information). To further present the capability of this unique property, we achieved the spreading of the LM-Fe mixture (1 mL) in molds of various shapes with small and complex features, such as maple leaf (smallest feature of ≈300 μm), pine tree, and snowflake, as shown in Figure 5d (see also Movie S8 in the Supporting Information).…”

“…We believe the key factor for inducing such a large elongation of the LM-Fe mixture is due to the formation of a stable oxide on the elongated LM surface with the presence of the LM-Fe shell (tail), which can improve the deformability of the droplet by significantly reducing the surface tension. 32 The LM-Fe mixture is bipolarized upon applying a DC potential, and Figure 2a depicts the proposed electrochemical reaction on the cathodic and the anodic poles of the LM-Fe droplet when lengthened. We ob-served the formation of gas bubbles on the surface of the LM-Fe shell (cathodic pole) during elongation, which is due to the electrochemical reduction of protons to form hydrogen gas (H 2 O + 2e − → 2OH − + H 2 ↑).…”

“…However, the electrochemical oxidation of LM is highly dynamic, often accompanied with the formation of fractal shapes for an oxidized LM droplet, and the fractals may eventually break away from the electrode due to Marangoni instabilities. [32,33] This makes it difficult to directly use the electrochemical method to precisely and stably manipulate the morphology of LM, particularly in an unconstrained 2D plane. [34] Elongating LM droplets can form conductive wires or patterns with semicircular cross sections, enabling innovative applications in soft electronics and microfluidics.…”

An On‐Chip Liquid Metal Plug Generator

“…It is based on the application of an oxidative potential on the liquid metal, resulting in a sharp decrease in its surface tension owing to the formation of an oxide skin allowing it to rapidly fill microchannels. [42][43][44] While electrochemical oxidation enables the liquid metal to rapidly fill microchannels, it would be necessary to trap the liquid metal by designing constrictions since on switching off the oxidative potential, the liquid metal will retract back to its source without generating any plugs. These constrictions could be compared to Laplace barriers, as liquid metal plugs will coalesce owing to inherent Laplace pressure imbalance inside the microchannels.…”