Evaporation-Induced Microphase Separation at the Interface of a Sessile Droplet: Formation of a Regular Lattice Pattern

Evaporation alters molecular interactions and can induce phase separation in complex fluids. Whether evaporation can drive localized phase separation at the receding contact line and lead to ordered pattern formation in small liquid volumes remains an open question. Here, we investigate the evaporation-induced microphase separation of a block copolymer (Pluronic F-127) in a salt-containing buffer within an evaporating sessile droplet. We observe that salt-mediated phase separation occurs at the dynamic interface of the droplet, leading to the formation of isolated polymer clusters or highly regular lattice patterns depending on the initial polymer concentration. Our results demonstrate that localized evaporative concentration and salt–polymer interactions at the droplet interface drive salting-out effects and microphase separation into ordered mesophases. This study highlights the utility of evaporating sessile droplets for engineering self-assembled, regular polymeric patterns via interface-mediated microphase separation.