Electro-optic observations of electrodynamic band formation in colloidal suspensions

In static and low-frequency electric fields, colloidal particles in suspension tend to associate into ‘strings’ or ‘pearl chains’ along the field lines. A phenomenon has been observed in which, under long duration alternating electric fields, colloidal particles in aqueous or conducting media exhibit an electrodynamic instability in which they gather into high concentration ‘bands’ which run essentially perpendicular to the applied field vector. A detailed study is catalogued herein for aqueous suspensions of the discotic mineral kaolinite. A theory has been developed, which embraces the ‘pearl chain’ and ‘band’ formations, demonstrating that one can be formed from the other with increasing frequency and field strength and illustrating the dependence of band formation on electrophoretic mobility as observed in related electro-optical experiments. The value of the phenomenon as a mechanism for concentrating dispersed colloidal particles into regions of very high local density is apparent.