Dynamic Coupling of Optically Evolved Assembling and Swarming of Gold Nanoparticles with Photothermal Local Phase Separation of Polymer Solution

Optical trapping of gold nanoparticles (Au NPs) at a glass/solution interface initially generates a periodically aligned structure of a few NPs along the direction perpendicular to a linearly polarized laser. When the number of NPs was increased, this alignment was expanded to the outside of the irradiated focus, forming a single large assembly where the Au NPs dynamically fluctuated like a swarming of bees. The morphology was dumbbell-shape, consisting of two swarms at both sides of the focus, and its size reached about 10 μm. This optically evolved assembling and swarming was studied in poly(N-isopropylacrylamide) (PNIPAM) solution, where liquid-liquid phase separation (LLPS) was induced by photothermal heating of the trapped Au NPs forming a microdroplet of highly concentrated PNIPAM. Dynamic coupling of the NPs assembling and swarming with the droplet formation of PNIPAM leads to cooperative optical evolution, through which the assembly was embedded in the single microdroplet.