From diffusive motion to local aggregation: Effect of surface contamination in dipolophoresis

We investigate the effects of surface contamination, modeled as a thin dielectric coating, on the dynamics in suspensions of ideally polarizable spheres in an applied electric field using large-scale direct particle simulations. In the case of clean particles (no contamination), the suspensions are known to undergo dipolophoresis, or a combination of dielectrophoresis, which tends to cause particle chaining and aggregation, and induced-charge electrophoresis, which dominates the dynamics and drives transient pairings, chaotic motions, and hydrodynamic diffusion at long times. As surface contamination becomes significant, induced-charge electrophoresis is gradually suppressed, which results in the simulations in a transition from diffusive dynamics to local aggregation and chaining as a result of dielectrophoresis. This effect has a strong impact on the suspension microstructure, as well as on particle velocities, which are strongly reduced for contaminated particles. This transition is also visible in the particle mean-square displacements, which become sub-diffusive in the case of strong contamination. We explain this sub-diffusive regime as a consequence of the slow dynamics of the particles trapped inside clusters and chains, which result in non-integrable local waiting time distributions.