Light-Triggered Reversible Control of Viscoelasticity and Emulsion Stability via Photoswitchable Aqueous Systems

Stimulus-responsive emulsions and gels are powerful platforms for intelligent soft materials, yet the reversible light-controlled modulation of their macroscopic properties has remained elusive. Here we report a multicomponent aqueous formulation─comprising cetyltrimethylammonium bromide (CTAB), cosurfactants, oil, water, and para-aminoazobenzene (AAB)─that, depending on water content, enables robust and reversible control of both viscoelasticity and emulsion stability with light. Within a defined composition window, the system forms entangled wormlike micelles that impart gel-like viscoelasticity. Ultraviolet irradiation disrupts this network, reducing viscosity by orders of magnitude, while visible light restores the original structure. At higher water content, the same formulation produces stable oil-in-water emulsions whose integrity can be reversibly switched by photoisomerization-driven interfacial rearrangements. This dual-level responsiveness demonstrates a simple and general strategy to couple molecular photoisomerization with macroscopic flow and phase behavior, opening new opportunities for adaptive formulations, reconfigurable soft matter, and light-controlled delivery technologies.