The Formation and Transformation of Colloidal Clusters under Alternating-Current Electric Fields.

Colloidal particles have long served as both model systems for atoms and molecules as well as fundamental building blocks for advanced functional materials. While anisotropic particles are often considered necessary for creating complex structures, we have previously demonstrated that simple microspheres can also form nontrivial colloidal clusters under alternating-current electric fields. However, the mechanisms behind the formation of different clusters remain unclear. Here, we explore the impacts of particle concentration, salt concentration, and electric field frequency on the formation and transformation of colloidal clusters. By experimentally measuring the strengths of dipolar and electrohydrodynamic interactions, we show that the balance of these forces under different conditions determines the morphology of the clusters. Notably, we observe the transformation of colloidal tetramers into pentamers as the frequency increases at high particle concentrations. These square-shaped pentamers can further pack into square or sigma-phase arrays, complex structures that are challenging to make. Our findings advance the understanding of colloidal assembly under electric fields, enabling the formation of new structures under optimized conditions.