Electrorheological (ER) fluids are suspensions of extremely fine particles (up to 50 micrometres) in non-conducting fluids. The apparent viscosity of these fluids changes reversibly by an order of 100000 in response to an electric field.

For example, a typical ER fluid can go from the consistency of a liquid to that of a gel, and back, with response times on the order of milliseconds. The change in viscosity is proportional to the applied potential.