Dipole moments of gyricon balls

Gyricon balls consist of two hemispheres of different colors and different electrical properties. When immersed in a liquid and exposed to a uniform electrical field, they rotate so that the ball is aligned along the polar axis. The dipole moment of the ball is modeled in terms of a uniform surface charge distribution on each hemisphere, and the predicted motion is obtained from a balance of electrical and viscous torques. The motion shows a long and variable initial decay, followed by a relatively rapid turning and then a long final delay. Measurements of the rotation between parallel electrodes lead to a dipole moment and surface charge density for the balls, with typical bipolar surface charge densities in the range of 0.1–0.3 μC/m2.