Localized Instabilities of Liquid Metal Films via In‐Plane Recapillarity

Abstract

appealing because the application of voltage can turn on or off the instability at any time and therefore may represent a new way to study thin film instabilities. Recently, a similar electrochemical approach has been utilized to withdraw liquid metal from microchannels. [28,29] Electrochemical processes can pattern solid metals and other surfaces in a process called electrochemical micromachining. [30] Electrochemical micromachining locally dissolves and shapes metals through an anodic reaction (i.e., oxidation). In contrast, our approach removes only the surface metal oxide via a localized cathodic reaction (i.e., reduction) and therefore results in inconsequential loss of material while allowing the metal to flow to a new location.The patterning approach described here relates to the classical dewetting phenomenon of polymeric films, [31] which are typically trapped in an energetically unfavorable state (i.e., a film) until the temperature exceeds a sufficiently high value to enable the polymer to flow. In contrast, the method presented here induces destabilization of films using electrochemistry [6,32,33] at constant temperature. We call this technique "recapillarity" because the reduction of the oxide induces capillary action. [27,34] The studies here suggest a new way to pattern liquid metals and to study instabilities. Figure 1 depicts the in-plane patterning of liquid metal using localized electrochemistry. To demonstrate the principle, we spread a drop of EGaIn ( Figure 1a) on a glass slide to create a thin film. A copper wire contacts the edge of the film, thereby making the EGaIn the working cathode. On a separate copper wire, a small drop of electrolyte (i.e., water or salt solution) wets the tip of the wire after dipping it in a reservoir of electrolyte. Contacting this suspended drop with the metallic film completes the electrochemical circuit. Application of a reductive bias to the film removes locally the oxide skin, which causes capillary withdrawal of the metal as shown schematically in Figure 1a. We drew the letters "NCSU" (cf. Figure 1b) across the film by guiding the tip across the surface by hand. We occasionally re-dipped the tip into the electrolyte to replenish it, similar to an old fashioned ink-well pen. Although thin films of the metal are generally rough with some pinholes (cf. inset of Figure 1c), we added liquid metal to the patterned metal film to render a smooth, reflective surface (Figure 1c). Video S1 (Supporting Information) illustrates this process and Figure S1 (Supporting Information) shows an American flag composed of liquid metal produced by strategically localizing the electrochemical reduction of the oxide by the electrode "scribe."We sought to quantify and understand the primary variables that control the velocity of withdrawal of the film during reduction. To improve repeatability, we built a spreader (cf., Figure 2a(i)) to smear drops of EGaIn (≈0.1 mL) onto a glass slide cleaned with an oxygen plasma. The spreader consists of a glass tube "roller" (diameter ≈12 mm) h...

EGaIn–Metal Interfacing for Liquid Metal Circuitry and Microelectronics Integration

“…Previous methods for EGaIn and Galinstan circuit fabrication include direct‐writing, injection, ink‐jet printing, laser patterning, contact printing, imprinting, selective wetting, screen printing, spray painting, and reductive patterning . Microfluidic channels of EGaIn embedded in a soft elastomer, e.g., polydimethylsiloxane (PDMS), can function as highly stretchable wires and passive circuit elements .…”

Multiscale and Uniform Liquid Metal Thin‐Film Patterning Based on Soft Lithography for 3D Heterogeneous Integrated Soft Microsystems: Additive Stamping and Subtractive Reverse Stamping

“…Limitations of this approach are the relatively low resolution, rough EGaIn surface, and excessive EGaIn loss during the stencil lift‐off process. Subtractive direct patterning techniques using laser ablation or electrochemical reduction enable an inexpensive and facile approach to pattern fine EGaIn lines, but the serial process makes EGaIn removal slow in the case of patterning small EGaIn features on large substrates. The major technical challenge for both lithography‐enabled stencil printing and subtractive direct patterning approaches is that creating thin and uniform EGaIn films is difficult due to the high surface tension of EGaIn (γ = 624 mN m −1 ) .…”

Nanofabrication for all-soft and high-density electronic devices based on liquid metal

“…Being a liquid-phase conductor with a brittle oxide layer on the surface, the shape of EGaIn-filled microchannels can be easily changed in response to applied mechanical forces, with a new oxide layer being formed instantaneously on the EGaIn surface after deformation, thus making it shape reconfigurable 22 . The moldable characteristics of EGaIn have resulted in the development of a broad range of patterning methods based on lithography-enabled stamping and stencil printing [26][27][28][29][30][31][32][33] , microfluidic injection [34][35][36] , as well as additive [37][38][39] and subtractive [40][41][42][43][44][45] patterning processes. However, creating fine and uniform EGaIn thin-film patterns using current EGaIn patterning technologies remains a major technical challenge because of the high surface tension of EGaIn (γ = 624 mN m −1 ) 23 .…”

“…Considering the size of a single biological cell, such as platelets with a diameter of 2-3 μm, mechanotransducers should be manufactured with submicronscale features and soft, biomimetic properties 1,48,49 . Existing fabrication technologies, including the transfer printing of compliant solid metal patterns [13][14][15] , nanoprinting 17,18 , direct printing of nanomaterials [19][20][21] , and EGaIn patterning [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] , are currently not suitable to fabricate such soft and stretchable electronic devices with submicron-scale resolution.…”