High‐Speed Microscopy and Optofluidic Modulation of Dynamic Oscillations in Liquid Metal Microdroplets

Abstract Autonomous interfacial oscillations of gallium‐based liquid metals hold great promise for soft robotics and adaptive photonic devices, yet their rapid transient dynamics remain insufficiently characterized due to the limitations of conventional imaging techniques. Here, a high‐speed microscopy study of self‐sustained, asymmetric oscillations in eutectic gallium–indium (EGaIn) microdroplets partially immersed in hydrochloric acid (HCl) solution is presented. Using a cost‐effective smartphone‐based imaging platform capable of 7680 frames per second, a pronounced temporal asymmetry in the oscillation cycle, consisting of a rapid 3 ms contraction driven by surface oxidation, followed by a 86 ms recovery governed by acid‐mediated oxide dissolution at the triple‐phase boundary, is uncovered. The system supports stable, high‐frequency oscillations, sustaining up to 31 Hz for over 30 min, a performance that contrasts markedly with previously reported behavior in alkaline environments. As proof of concept, a Janus EGaIn‐HCl droplet functioning as an autonomous optical modulator, producing tunable laser reflection and interference patterns without external input, is demonstrated. The oscillation frequency is readily tunable via HCl concentration, offering a strategy for environmentally regulated, redox‐driven soft matter dynamics. These findings support the development of intelligent, self‐regulating soft devices for chemical‐to‐mechanical energy conversion as well as adaptive photonics.