Colloidal stability dictates drop breakup under electric fields*

The electric field induced breakup of a squalane drop containing a colloidal suspension of carbon black particles with polyisobutylene succinimide (OLOA 11000) surfactant is studied. The drop is suspended in silicone oil. The breakup mode of the drop depends strongly on the suspension stability. A drop of a stable suspension has the same breakup mode as a drop with surfactant alone, i.e., without added carbon black. At lower electric fields, the drop breaks by the formation of lobes at the two ends of the drop; and at higher fields the homogeneous lobes break in a non-axisymmetric manner. However, a drop of an unstable suspension shows a drastically different breakup mode. These drops elongate and form asymmetric lobes that develop fingers and eventually disintegrate in an inhomogeneous fashion. As a basis for comparison, the breakup of a pure squalane drop, and a squalane drop with equivalent surfactant concentrations but no carbon black particles is examined. Axisymmetric boundary integral computations are used to elucidate the mechanism of breakup. Our work demonstrates that colloidal instability on the time scale of drop deformation leads to rich and unexplored breakup phenomena.