Reverse electrorheological effect: A suspension of colloidal motors.

We present an experimental evidence of a "colloidal motor" behavior of a su spension, previous attempts to observe such a phenomenon with ferrofluids u nder alternating magnetic fields have failed. Here, negative viscosity is o btained by making use of Quincke rotation: the spontaneous rotation of insu lating particles suspended in a weakly conducting liquid when the system is submitted to a DC electric field. In such a case, particles rotate around any axis perpendicular to the applied field, nevertheless, when a velocity gradient (simple shear rate) is applied along the E field direction, the pa rticles rotation axes will be favored in the vorticity direction (the direc tion perpendicular to the suspension velocity and the velocity gradient). T he collective movement of particles drives the surrounding liquid and then leads to a reduction of the apparent viscosity of the suspension. The decre ase in viscosity is sufficiently important for the liquid to flow while no submitted to any mechanical stress.