Electrohydrodynamic patterns in charged colloidal solutions

Various solutions of charged colloidal particles self-organize under strong ac fields into continuously sheared bands tilted with regard to the direction of the field. We report here that the angle of tilt takes essentially two values, +/-45 degrees or +/-60 degrees, depending on the amplitude of the periodic electrophoretic drift of the charged aggregate as compared to its width. We propose an electrohydrodynamic model that explains the very striking features of these patterns in colloidal solutions. Numerical solutions show good agreement with the transient and quasistationary structures observed.