Oxide-Free Actuation of Gallium Liquid Metal Alloys Enabled by Novel Acidified Siloxane Oils

Abstract

Electrowetting and electrocapillarity of liquid metals have a long history, and a recent explosion of renewed interest. Liquid metals have electromagnetic properties and surface tensions (>500 mN/m) that enable new forms of reconfigurable devices. However, the only nontoxic option, gallium alloys, suffer from immediate formation of a semirigid surface oxide. Although acids or electrochemical reduction can remove this oxide, these approaches surround the gallium alloy in a fluid that is also electrically conducting, diminishing electromagnetic effectiveness and precluding electrowetting actuation. Reported here are acidified siloxanes that remove and prevent oxide formation. Importantly, the siloxane oil associatively incorporates hydrochloric or hydrobromic acids, is electrically insulating, is chemically stable, removes etching byproducts (including water), and allows robust electrowetting. This work opens up new opportunities for liquid metal reconfiguration, and is of fundamental interest due to the unexpected chemical stability of the acidified siloxanes and their application to other materials and surfaces.

Recent progress on liquid metals and their applications

“…This approach provides a novel method for soft device manufacture and offers a way to explore electrochemical performance of liquid metal [55]. In addition to using conductive electrolyte, Holcomb et al introduce a technique to actuate liquid metal that uses acidification of nonconductive siloxanes to remove and prevent oxidation of liquid metal [56]. The siloxane oil [55].…”

“…The liquid metal is in the form of droplets when no electric potential is applied. When the bias is applied, the liquid metal changes to line shapes [56].…”

“…The contact angle was expected to be 180° without voltage, but it was measured to be 160° due to the influence of gravity and other factors. When bias was applied, the contact angle changed to nearly 120° at 300 V [56]. Figure 11(d) shows a switchable wire-grid polariser system, which is one application for this concept.…”

“…Liquid metal is in the form of droplets with no applied voltage. When a bias is applied, the liquid metal changes to a line shape [56]. EWOD is a novel method that provides many benefits, including easy fabrication and no moving parts.…”

Gallium-Based Room-Temperature Liquid Metals: Actuation and Manipulation of Droplets and Flows

“…The method was applied toward radio-frequency (RF) electronics and used acidic siloxanes to remove the oxide skin out of liquid metal surface. The advantage of siloxanes is that they are electrically insulating fluids which do not participate in the chemical reactions in the absence of water (Holcomb et al, 2016).…”