Abstract: Shaping Microscopic Particle Assemblies using Light

Controlling self-organization at the micro- and nanoscale is crucial for engineering next-generation autonomous machines that can operate in fluid environments. In this talk, I will discuss self-organization driven by photothermal effects and its capabilities of designing programmable, reconfigurable, and nonequilibrium dynamics. I will discuss a simple experimental setup that uses gold nanoparticles and an LED to drive swarming and assembly of colloidal particles toward light because of the combined effects of photothermal convection, thermophoresis, and hydrodynamic interactions. We studied how the interplay between electrostatic and hydrodynamic interactions gives rise to ordered and disordered structures.¹ In our recent work, we investigated how crystal nucleation and growth dynamics depend on particle size and light intensity. While larger particles form highly crystalline structures at a slower rate, smaller particles undergo a two-step nucleation and growth mechanism to form crystals only at higher light intensities. The size-dependent crystallization dynamics allow us to control co-crystallization in a binary mixture of small and large particles only by adjusting the light intensity. Since light projection can be dynamically controlled in both space and time, we demonstrate that the colloidal crystal and swarm assemblies can be programmed into desired shapes and reconfigured into another shape on demand. We aim to understand how to accurately program the assembly shapes for particles of different materials, sizes, and thermophoretic properties. Finally, I will briefly introduce a recent finding of oscillating light-responsive polymer droplets under constant light illumination that mimic the dynamics of embryos. Photothermal self-assembly, although simple to implement, has potential for designing smart autonomous machines whose structure and dynamics can be controlled on demand. Feedback control and reinforcement learning mechanisms could potentially be incorporated into such systems in the future to realize applications in sensing, navigation, and transport.