Electrohydrodynamics of particle-covered drops

We experimentally investigate the effect of surface-absorbed colloidal particles on the dynamics of a leaky dielectric drop in a uniform DC electric field. Depending on the particle polarizabilty, coverage and the electrical field intensity, particles assemble into various patterns such as an equatorial belt, pole-to-pole chains or a band of dynamic vortices. The particle structuring changes droplet electrohydrodynamics: under the same conditions where a particle-free drop would be a steady oblate spheroid, the belt can give rise to unsteady behaviours such as sustained drop wobbling or tumbling. Moreover, particle chaining can be accompanied by prolate drop deformation and tip-streaming.

Coarsening of a Quiescent Electrorheological Fluid I: Experiment

We report a real-time, two-dimensional light scattering study of the evolution of structure in a concentrated electrorheological fluid during the ‘liquid-solid’ phase transition. We find that after particle chaining along the electric field lines, strong light scattering lobes appear at a finite scattering wavevector q orthogonal to the field lines, and then brighten as they move to q=0. This indicates the existence of an unstable concentration fluctuation that signifies the segregation of chains into columns. In fact, the observed growth kinetics of the characteristic length, as well as the form of the structure factor, are qualitatively similar to two-dimensional spinodal decomposition in a system with a conserved order parameter.