Electrohydrodynamic flow about a colloidal particle suspended in a non-polar fluid

TL;DR

This paper investigates nonlinear electrohydrodynamic flows around charge-neutral colloidal particles in non-polar fluids, revealing how particle conductivity influences flow patterns and forces, with implications for colloidal manipulation.

Contribution

It provides a detailed analysis of flow patterns and forces on particles based on conductivity, highlighting new electrohydrodynamic effects in non-polar fluids.

Findings

Flow pattern varies with particle conductivity.

Force on particles can be attractive or repulsive depending on conductivity.

Electrohydrodynamic forces can dominate dielectrophoretic effects.

Abstract

Nonlinear electrokinetic phenomena, where electrically driven fluid flows depend nonlinearly on the applied voltage, are commonly encountered in aqueous suspensions of colloidal particles. A prime example is the induced-charge electro-osmosis, driven by an electric field acting on diffuse charge induced near a polarizable surface. Nonlinear electrohydrodynamic flows also occur in non-polar fluids, driven by the electric field acting on space charge induced by conductivity gradients. Here, we analyze the flows about a charge-neutral spherical solid particle in an applied uniform electric field that arise from conductivity dependence on local field intensity. The flow pattern varies with particle conductivity: while the flow about a conducting particle has a quadrupolar pattern similar to induced-charge electro-osmosis albeit with opposite direction, the flow about an insulating particle has a more complex structure. We find that this flow induces a force on a particle near an electrode that varies non-trivially with particle conductivity: while it is repulsive for perfectly insulating particle and particles more conductive than the suspending medium, there exists a range of particle conductivities where the force is attractive. The force decays as inverse square of the distance to the electrode and thus can dominate the dielectrophoretic attraction due to the image dipole, which falls off with the fourth power with the distance. This electrohydrodynamic lift opens new possibilities for colloidal manipulation and driven assembly by electric fields.