Phase separation at different frequencies of switching light on/off (the “trigger frequency”). (A) The change in the interparticle interaction force during one cycle at different trigger frequencies. ${\langle F\rangle }_t$ is the time-averaged interparticle forces of the intermittent attraction-repulsion during one cycle. Positive forces are attractive, and the half-cycles of the darkness and illumination are colored in green and red, respectively. See Supplementary information for measurement of these interparticle forces. (B) Optical micrograph (top) and Brownian dynamics simulations (bottom) of the evolution of the phase separation of Ag colloids of φ = 17% at different trigger frequencies. (C), (D) Characteristic lengths of the patterns in (B). Experiments were carried out in 0.25 wt% H₂O₂ and 200 μmol L⁻¹ KCl aqueous solutions, using 405 nm light of ~257 mW cm⁻². See Supplementary information for simulation details.