Different Shades of Oxide: From Nanoscale Wetting Mechanisms to Contact Printing of Gallium-Based Liquid Metals

Abstract

Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because, despite having surface tension 10 times higher than water, they strongly adhere to a majority of substrates. This unusually high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In this work, we demonstrate that, dependent on dynamics of formation and resulting morphology of the liquid metal-substrate interface, GaInSn adhesion can occur in two modes. The first mode occurs when the oxide shell is not ruptured as it makes contact with the substrate. Because of the nanoscale topology of the oxide surface, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate's surface energy or texture. In the second mode, the formation of the GaInSn-substrate interface involves rupturing of the original oxide skin and formation of a composite interface that includes contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide. We demonstrate that in this latter mode GaInSn adhesion is dominated by the intimate contact between new oxide and substrate. We also show that by varying the pinned contact line length using varied degrees of surface texturing, the adhesion of GaInSn in this mode can be either decreased or increased. Lastly, we demonstrate how these two adhesion modes limit microcontact printing of GaInSn patterns but can be exploited to repeatedly print individual sub-200 nm liquid metal drops.

A skin-attachable, stretchable integrated system based on liquid GaInSn for wireless human motion monitoring with multi-site sensing capabilities

“…The gallium alloy forms a thin oxide layer on its surface in the presence of oxygen at a concentration of over 1 p.p.m. 30 This oxide shell easily adheres to the surface of almost any material, in contrast to the bare liquid metal, making it difficult to treat and pattern. 30,31 Various techniques have been developed to pattern liquid metals, including the injection method, stencil lithography, inkjet printing, micro-textured surface enabled patterning and selective wetting.…”

“…30 This oxide shell easily adheres to the surface of almost any material, in contrast to the bare liquid metal, making it difficult to treat and pattern. 30,31 Various techniques have been developed to pattern liquid metals, including the injection method, stencil lithography, inkjet printing, micro-textured surface enabled patterning and selective wetting. [31][32][33][34][35] Figure 2 (a-c) Contact angle images of the GaInSn droplet on various substrates of (a) PDMS, (b) Au deposited glass substrate and (c) Au deposited PDMS substrate.…”

Giant and switchable surface activity of liquid metal via surface oxidation

“…The presence of the oxide pins the drop to the dry substrate and allows the drop to be physically manipulated to a low initial contact angle before applying potential. The ability to mechanically manipulate the contact angle is indicative of the hysteretic nature of the wetting of the oxidecoated metal on dry surfaces (33). Removing the oxide allows the metal to dewet the substrate and return to an equilibrium contact angle.…”

Polyurethane foam coated with a multi-walled carbon nanotube/polyaniline nanocomposite for a skin-like stretchable array of multi-functional sensors

“…This printing method exploits the good wettability of Galinstan with the surface of mixed thin Silbione and PDMS film, which is a silicon-based substrate. 29,30 The sticky property of the film enhances the wetting of the liquid metal on the substrate surface. 31 Furthermore, formation of the gallium-oxide skin during the direct printing process allows the written pattern to maintain its printed shape, even after the encapsulation process with elastomer.…”