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

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.