Light-induced Nonequilibrium Dynamics of Heterogeneous Polymer Droplets

We investigate photothermally driven shapeshifting of polymer droplets that form via Liquid-liquid phase separation (LLPS). Liquid droplets with a diameter of tens of micrometers form in a suspension of Poly(N-isopropylacrylamide) microgels mixed with gold nanoparticles and salt. When irradiated with a high-intensity LED, light is absorbed by the gold nanoparticles, triggering a photothermal response. We noted a variety of responses depending on the overall size of the phase-separated droplets. At a critical light intensity, the droplets adopt a core-shell structure comprised of a polymer-rich phase in the outer shell and a polymer-poor phase in the inner core. The smaller droplets reach an equilibrium state in the core-shell structure and remain stable. In contrast, the larger droplets experience dynamic expansion and shrinkage of the polymer-rich and the polymer-poor regions, demonstrating intriguing nonequilibrium dynamics under continuous light illumination. Since the droplet dynamics can be externally controlled by light, understanding the nonequilibrium physics can help us design smart drug delivery and microfluidic systems.