# Active Colloid Phase Transitions and Living Binary Crystal Formation

## TL;DR

This paper shows how light-controlled colloids can mimic atomic phase transitions and chemical reactions, creating a new tool for studying material behavior.

## Contribution

A new platform using photoactive colloids to emulate phase transitions and chemical reactions with light.

## Key findings

- Photochemical reactions on colloids create tunable hydrodynamic interactions for phase transitions.
- Adding passive colloids mimics chemical reactions, forming compounds with defined ratios.
- Phase transitions and reactions in colloidal systems can be controlled with external illumination.

## Abstract

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.