Two-dimensional dynamics and microstructure of dense suspensions of ideally polarizable particles in an electric field: The nontrivial effect of confinement

The dynamics and microstructure of two-dimensional suspensions of ideally polarizable particles in an electric field are studied using large-scale numerical simulations. The particles are assumed to carry no net charge and thus known to undergo a nonlinear electrokinetic phenomenon termed dipolophoresis-the combination of dielectrophoresis and induced-charge electrophoresis. For ideally polarizable particles, the effect of induced-charge electrophoresis is dominant, which results in chaotic and diffusive motions. Up to an area fraction ϕ≈50%, the diffusive particle dynamics get hindered due to increasing excluded volume interactions. At ϕ=50%, the suspension intermittently exhibits a nearly frozen state, resembling a static equilibrium state, which arises only at this area fraction. A nonmonotonic behavior is then observed in the suspension dynamics, where the hydrodynamic diffusivity starts to increase with the area fraction and reaches a local maximum at ϕ=60% before decreasing again as approaching random close packing. We illuminate this nonmonotonic suspension dynamics in connection to the transition in particle contact mechanisms and suspension microstructure. Furthermore, the effects of confinement on suspension dynamics and microstructure are examined, where another nonmonotonic and nontrivial behavior arises in terms of the level of confinement. Upon increasing confinement, velocity fluctuations indeed increase compared to its unbounded values and reach a maximum at moderate confinement before getting diminished at extreme confinement. This nonmonotonic and nontrivial behavior is the most prominent at ϕ=50%. We explain the nonmonotonic behavior as a consequence of symmetry breaking in particle interactions due to confinement, resulting in suspension instability. The confinement-induced instability causes the emergence of coherent vortical structures that lead to the formation of layering microstructure. The confinement-induced effects are well correlated with the level of chaos in a suspension. These results can offer the potential use of confinement to effectively manipulate such suspensions.