Study on the size and spatial configuration of liquid metal droplets in conductive hydrogels induced by surface acoustic waves†

Abstract

Conductive hydrogels based on liquid metal microdroplets are widely used as wearable electronic devices. Droplet uniformity affects sensor sensitivity for weak signals, such as heart rate and pulse rate. Surface acoustic waves at micrometer wavelengths allow precise control of a single droplet, and have the potential to make uniformly discrete liquid metal droplets and distribute them in hydrogels. But the control law of liquid metal droplet size and its spatial configuration by acoustic surface waves is not clear. The aim of this paper is to present an analysis of the acoustic regulation mechanism in the interfacial evolution of fluids with high interfacial tension coefficients, and to investigate the influence of microdroplet generation characteristics (size and spacing) on the conductive and mechanical properties of conductive hydrogels. The results showed that the combined action of acoustic radiation force, shear force and pressure difference force helped to overcome interfacial tension and speed up the interfacial necking process during the filling and squeezing stages. The use of acoustic surface waves serves to diminish the influence of droplet size on the two-phase flow rate. This provides an effective approach for achieving decoupled control of microdroplet size and spacing, alongside the formation of a homogenous array of liquid metal droplets. The acoustic surface wave effect makes the liquid metal microdroplets more uniform in size and spacing. As the liquid metal content relative to the hydrogel substrate solution increases, the liquid metal size decreases. The hydrogel's initial conductivity and conductivity after self-healing increase by 10% and 25%, respectively, which can realize the effective monitoring of ECG and EMG signals. This study helps to reveal the evolution mechanism of liquid-metal interfaces induced by acoustic surface waves, elucidate the effects of microdroplet size and spacing on the conductive and mechanical properties of hydrogels, and provide theoretical guidance for the high-precision preparation of wearable electronic devices.