Electrolubrication in flowing liquid mixtures

We describe the ``electrolubrication'' occurring in liquid mixtures confined between two charged surfaces. For a mixture of two liquids, the effective viscosity decreases markedly in the presence of a field. The origin of this reduction is field-induced phase separation, leading to the formation of two low-viscosity lubrication layers at the surfaces. These layers facilitate larger strain at a given stress. The effect is strong if the viscosities of the two liquids are sufficiently different, the volume fraction of the less viscous liquid is small, the gap between the surfaces is small, and the applied potential is large. The phase separation relies on the existence of dissociated ions in the solution. The effective viscosity is reduced by a factor $\alpha$; its maximum value is the ratio between the viscosities of the two liquids. In most liquids, $\alpha \sim1$ -- $10$, and in mixtures of water and glycerol $\alpha \sim 80$ -- $100$ under relatively small potentials.