Electro-rheological fluids:
- ER fluids are colloidal suspensions of particles of size 1-100 microns in non-conducting solvents
- The viscosity of ER suspensions can increase 2-3 orders of magnitude when electric fields ~1kV/mm are applied across them. The response takes ~ 1ms. This increase in viscosity is attributed to the energy required to disassociate the electric field induced structures.
We study the structure of electrorheological (ER) fluids(ER Bibliography).
As shown in the diagram below, an external electric field induces a dipole oriented parallel to the field in a dielectric particles (a sphere here), and dipoles interact such that they align themselves along the field direction.
(below) Schematic of a ER suspension forming chains across electrodes and undergoing shear:The force required to break apart the electric-field induced structures manifests itself in an increase in viscosity.
from Klingenberg and Zukoski, Langmuir 1990, 6, 15-24
Because of the large increase in viscosity, and the rapid, reversible viscous response, technological applications have been proposed for ER fluids, in devices such as shock absorbers, clutches and valves.