Transient cellular convection in electrically polarized colloidal suspensions: Letter to the editor

We present theoretical evidence from computational fluid dynamics for transient isothermal roll-cell convection induced by very strong vertical electric fields in thin horizontal layers of waterin-oil emulsions initially at sedimentation equilibrium under gravity. This convection is driven by the Kelvin body force, which depends on the concentration gradient of droplets and on differences between the isotropic dielectric permittivities of the two phases of a nonionic emulsion. Provided that the applied voltage and hence a nondimensional electric Rayleigh number N is sufficiently large compared to a gravitational Rayleigh number R, roll-cell convection sets in, grows, and then decays. If N is sufficiently large this roll-cell convection disturbs the local composition of the suspension and increases the spatial homogeneity of the emulsion. This electroconvective effect, which can accompany dielectrophoresis, is of interest as a novel form of transient pattern formation and as an example of electrically induced stirring. Similar convection patterns should be observable in related colloidal suspensions of fine particles with high dielectric permittivities dispersed in carrier fluids with lower dielectric permittivities and densities.