Microscopy movies used for Electric field driven dynamic assembly of active colloidal aggregates

Abstract

Active colloids self-propel and dynamically assemble in response to external fields and chemical reactions. Previous work has focused on single, monomeric active colloids. Here we show that electric fields drive binary mixtures of passive spherical colloids to aggregate into active clusters that self-propel, reshape, merge, and split. Self-propulsion arises from imbalanced electrohydrodynamic (EHD) flows, with a few large (4 μm) dielectric particles leading groups of smaller (2 μm) ones. Propulsion velocity decreases as large particles occupy more aggregate boundary, forming more symmetric, less active aggregates. Sustained self-propulsion and dynamic assembly occurs at sufficiently large particle concentration and when small particles outnumber large ones by 20–40 times. Splitting follows elongation and formation of hydrodynamically coupled clusters of large particles at aggregate poles. We present evidence that phoretic attraction (EHD flow) between aggregates drives mergers, while splitting occurs when tensile forces created by divergent self-propulsion of surface laden large particles overcomes cohesive EHD flows. Scaling analysis demonstrates the aggregate area dependence of merger and splitting rates to be consistent with these mechanisms. These results reveal how passive colloidal mixtures can be activated by electric fields to form self-organizing, reconfigurable microscale assemblies.