Reversible Phase Transitions of Anionic and Cationic Surfactant Mixtures Drive Shape Morphing Droplets

Converting chemical signals into mechanical responses is fundamental to biological systems, driving processes such as cellular motility and tissue morphogenesis. Yet, harnessing chemo-mechanical signal conversions in synthetic systems remains a key challenge in energy-dissipative materials design. While droplets can move and interact with their environment reminiscent of active biological matter, chemo-mechanical interactions are limited by the translation of chemical changes into extensive force variations required on small timescales. Droplets naturally adopt spherical shapes to minimize surface-energy and restructuring liquids into non-equilibrium geometries requires mechanisms beyond current stimuli-responsive surfactant systems, which lack the force-amplifying mechanisms needed for transient liquid structuring. Here, a spring-like charging and latch-controlled release mechanism is introduced for actuating droplets. This is based on reversible, light-induced crystal-to-coacervate phase transitions of photo-responsive surfactant assemblies, namely between anionic sodium dodecylsulfate and cationic azobenzene-based surfactants. During phase-transition, reversible partitioning of the surfactants into the oil or aqueous phases of the emulsion transiently induce rapid changes in interfacial tensions, which are up to 900 times greater than those observed for conventional stimuli-responsive surfactant systems. The insights into this novel chemo-mechanical transduction mechanism provide new control over purely liquid systems, paving the way for programmable, hierarchically structured, all-liquid matter acting with physicality.