Tailoring Dynamic Reconfiguration in Assemblies Driven by Convection and Thermophoresis

Manipulating the structure and dynamics of self-assembled microscopic particles on demand remains a central challenge in materials science. We address this challenge by investigating how colloidal particles assemble in spatiotemporally modulated light under photothermal convection and thermophoresis. Prior work has shown that polystyrene microspheres can be self-assembled into crystalline monolayers using photothermal convection, thermophoresis, and hydrodynamic interactions. We find that the phase behavior of this system strongly depends on various parameters, such as particle concentration, the area of the illuminated region, and the sample thickness. We observe two separate phenomena: (i) particles crystallize at the illuminated region, and (ii) particles avoid the illuminated region and gather around it, causing the formation of a void. The interplay between convection and thermophoresis likely causes the occurrence of this rich phase behavior. By applying the fundamental understanding of the phase behavior, we demonstrate dynamic reconfiguration of a self-assembled crystal from one shape to another on demand. Our findings could pave the way for reconfigurable colloidal systems with potential applications in microscale colloidal robotics.