Vacuum filling of complex microchannels with liquid metal

Abstract

This paper describes the utilization of vacuum to fill complex microchannels with liquid metal. Microchannels filled with liquid metal are useful as conductors for soft and stretchable electronics, as well as for microfluidic components such as electrodes, antennas, pumps, or heaters. Liquid metals are often injected manually into the inlet of a microchannel using a syringe. Injection can only occur if displaced air in the channels has a pathway to escape, which is usually accomplished using outlets. The positive pressure (relative to atmosphere) needed to inject fluids can also cause leaks or delamination of the channels during injection. Here we show a simple and hands-free method to fill microchannels with liquid metal that addresses these issues. The process begins by covering a single inlet with liquid metal. Placing the entire structure in a vacuum chamber removes the air from the channels and the surrounding elastomer. Restoring atmospheric pressure in the chamber creates a positive pressure differential that pushes the metal into the channels. Experiments and a simple model of the filling process both suggest that the elastomeric channel walls absorb residual air displaced by the metal as it fills the channels. Thus, the metal can fill dead-ends with features as small as several microns and branched structures within seconds without the need for any outlets. The method can also fill completely serpentine microchannels up to a few meters in length. The ability to fill dense and complex geometries with liquid metal in this manner may enable broader application of liquid metals in electronic and microfluidic applications.

Liquid Metal–Based Soft Microfluidics

“…The microfluidic chip is treated with vacuum. After reversal to atmospheric pressure the liquid metal is sucked into the channels, rendering an exhaust port gratuitous, as shown in Figure a . With this method, liquid metal leakage due to injection pressure is reduced while complete filling of the microchannels can be achieved in a hands‐free manner.…”

“…a) Scheme of vacuum filling of liquid metal into a microfluidic channel. Reproduced with permission . Copyright 2017, Royal Society of Chemistry.…”

Untethered soft robotics

“…The fabrication and filling of these liquid-metal microchannels has been a persistent challenge, with numerous techniques proposed in the literature, including injection 101 , freeze placement 102 and direct writing 103 . Recent work has demonstrated fast and accurate filling of complex microchannel geometries via a vacuum 104 ( Fig. 4g), as well as the fabrication of biphasic thin films by sputtering gold and thermally evaporating liquid gallium 105 (Fig.…”

Shear‐Driven Direct‐Write Printing of Room‐Temperature Gallium‐Based Liquid Metal Alloys

“…Gallium-based liquid metal alloys (GaLMAs), a class of low-melting point alloys, have been used routinely in recent years as an exceptional nontoxic, room-temperature alternative. [34] Direct-write extrusion printing has been utilized to deposit roomtemperature liquid gallium alloy structures [25][26][27][28][29][30] for applications such as stretchable conductors, [25] 3D self-supported pillars, [26] and bendable layer-by-layer conductors. [22] GaLMAs also provide lower electrical resistivity at lower density than mercury.…”