Optical Force-Induced Dynamics of Assembling, Rearrangement, and Three-Dimensional Pistol-like Ejection of Microparticles at the Solution Surface

Optical trapping and assembling dynamics of polystyrene microparticles (MPs) of 1 μm in diameter are studied at its solution-air surface using a widefield microscope. Upon switching on the intense 1064 nm laser, the MPs are gathered, forming a single concentric circle (CC)-like assembly larger than the focus. It consists of a few tens of MPs, and the central part of the assembly shows structural color, which indicates that the assembly is also growing in the axial direction. The MPs are dynamically fluctuating in the assembly, and some of them are ejected when newly coming MPs collide with the CC-like assembly from the bulk solution. The MPs speedily leaving the assembly are aligned in a linear manner, which we refer to as "pistol-like ejection". The three-dimensional (3D) dynamics was elucidated by changing laser power, MP concentration, and surface chemical property. It is directly observed that the trapping laser was scattered radially from the CC-like assembly, and the ejection was induced along the scattered laser path. This pistol-like ejection is stochastically repeated upon the collision. After prolonged irradiation, the assembly rearranges to a hexagonal close packing (HCP)-like assembly, in which no pistol-like ejection was observed. We note that our observation is a characteristic of the solution surface and were never observed in bulk solution. We conclude that the kinetically driven assembly formation gives rise to a CC-like structure that is metastable and shows the pistol-like ejection phenomenon. Later, the assembly rearranges to a thermodynamically stable HCP-like assembly. The assembling, pistol-like ejection, and its rearrangement are all driven by optical force, which is common for optical trapping-induced molecular crystallization and optically evolved assembling and swarming of gold nanoparticles.