Fig. 1: Group formation of active colloids mediated by environmental memory.

a–d Time sequence showing (a) a light-activated Janus particle (cyan) forming a path in a crowded environment of SiO₂ passive particles (densities: ρ_a = 1.1% for active particles and ρ_p = 37.5% for passive particles), b, c which is then reused by nearby Janus particles (yellow and magenta) leading to (d) the formation of a group (here, a three-particle cluster). In each image, 20-s-long trajectories are shown for both active (red colour scale) and passive (blue colour scale) particles; t represents the time of each frame and τ the time along each trajectory. Scale bar: 10 μm. e Counts C_a (represented as a heatmap) of individual Janus particles that have transited on a pixel during a 16-min acquisition in a similar crowded environment as in a–d. The heatmap is overlaid to the final frame showing how path generation and reuse (bright lines) correlate to group formation and cohesion of active units in time (white dashed circles). The heatmap was obtained from a sample with ρ_a = 1.1% and ρ_p = 37.5% and an occupied pixel was only accounted for once for each particle. Scale bar: 25 μm.