Figure 5: Drop deformation as a function of the square of the electric field strength.

Pure silicone drops are oblate (drop deformation D<0) and the measurements for weak deformations (−0.04<D<0) quite accurately follow the prediction of Taylor–Melcher perturbation theory. Drops with low clay concentration almost follow the same curve as drops without clay. However, with higher clay concentrations the behaviour is qualitatively different: at low field strengths the drop is oblate, but as the field strength increases the drop deformation reverses from oblate to prolate (D>0). This behaviour coincides with the nonlinear electric response of clay particles, at high field strengths clay form increasingly longer conductive dipolar chains that effectively can short-circuit the silicone drop. The three images at the bottom show how a drop with clay concentration in the medium to high regime (that is, 1–1.5 wt%) deforms reversibly as the electric field strength is increased and decreased. The drop radius is about 1 mm. The E-field direction is horizontal in the plane of the panels, as indicated by the arrows.