# Dipolophoresis in concentrated suspensions of ideally polarizable spheres

## TL;DR

This study investigates the complex dynamics of ideally polarizable spheres in concentrated suspensions under nonlinear electrokinetic effects, revealing non-trivial behaviors driven by particle contacts and microstructure changes.

## Contribution

It introduces large-scale simulations of dipolophoresis, highlighting the role of particle contacts and microstructure in concentrated regimes, a novel insight into suspension behavior.

## Key findings

- Hydrodynamic diffusivity peaks at around 35% volume fraction.
- Strong repulsive contacts dominate at high concentrations.
- Microstructure changes correlate with non-trivial suspension behaviors.

## Abstract

The dynamics of ideally polarizable spherical particles in concentrated suspensions under the effects of nonlinear electrokinetic phenomena is analysed using large-scale numerical simulations. Particles are assumed to carry no net charge and considered to undergo the combination of dielectrophoresis and induced-charge electrophoresis termed dipolophoresis. Chaotic motion and resulting hydrodynamic diffusion are known to be driven by the induced-charge electrophoresis, which dominates the dielectrophoresis. Up to a volume fraction $\phi \approx 35\%$, the particle dynamics seems to be hindered by the increase in the magnitude of excluded volume interactions with concentration. However, a non-trivial suspension behaviour is observed in concentrated regimes, where the hydrodynamic diffusivity starts to increase with a volume fraction at $\phi \approx 35\%$ before reaching a local maximum and then drastically decreases as approaching random close packing. Similar non-trivial behaviours are observed in the particle velocity and number-density fluctuations around volume fractions that the non-trivial behaviour of the hydrodynamic diffusion is observed. We explain these non-trivial behaviours as a consequence of particle contacts, which are related to the dominant mechanism of particle pairings. The particle contacts are classified into attractive and repulsive classes by the nature of contacts, and in particular, the strong repulsive contact becomes predominant at $\phi > 20\%$. Moreover, this transition is visible in the pair distribution functions, which also reveal the change in the suspension microstructure in concentrated regimes. It appears that strong and massive repulsive contacts along the direction perpendicular to an electric field promote the non-trivial suspension behaviours observed in concentrated regimes.