Active Colloid Phase Transitions and Living Binary Crystal Formation.

Colloids can be utilized as model "meta-atoms" to emulate phase behaviors at the atomic scale for easy observation and slower dynamics. Photoactive colloids have recently been demonstrated with on-demand directional interactions as well as tunable dynamics, which are particularly suitable to emulate the phase transition of atomic lattices due to their excellent tunability. In this study, we demonstrate that the photochemical reaction on active colloids can induce an optically tunable hydrodynamic interaction field. By spontaneously controlling the directional interaction and omnidirectional repulsion with two sets of illumination, the phase transition between the zigzag band, chains, and dispersed phase, distinguished by their 2-fold bond orientational order, can be realized. Furthermore, the addition of passive colloids, analogous to reactant atoms with different chemical natures and sizes, causes a "chemical reaction" between the colloid species, forming colloid compounds with well-defined stoichiometric ratios, while the phase transition of the colloid compound can also be emulated with external illumination. By bridging active matter physics and solid-state chemistry, our platform provides a versatile tool for studying phase diagrams and optically encoding "reaction pathways" in colloidal alloys.