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Clustering Dynamics of SiO2-Pt Active Janus Colloids

Active colloid clustering is central to understanding non-equilibrium self-organization, with implications for programmable active materials and synthetic or biological assemblies. While most prior studies have focused on dimers or small aggregates, the dynamics of larger clusters remain relatively unexplored. Here, we experimentally investigate chemically active, monodisperse SiO2-Pt Janus colloid (JC) clusters as large as n=9 in a dynamic clustering regime, where clusters continuously form, dissolve, and merge as swimmer density increases. We show that clusters move in circular trajectories, and that both their translational and rotational dynamics can be predicted directly from the orientations of constituent JCs. Furthermore, we identify that their formation undergoes a mechanistic transition: while small clusters are mediated by chemical interactions, larger clusters are predominantly formed by steric effects. This transition arises from a mismatch of motilities between incoming JCs and clusters, combined with increased Pt-surface exposure. Our results extend prior dimer-focused studies to larger aggregates and establish a predictive description that bridges individual swimmer behavior with collective dynamics.