Field induced structures and phase separation in electrorheological and magnetorheological colloidal suspensions

Suspensions of solid colloidal particles presenting a large mismatch of magnetic permeability or dielectric permittivity with the suspending fluid are interesting systems to correlate structure and rheology by tuning attractive dipolar forces with the help of an external field. The beginning of phase separation with formation of chainlike structure is experimentally studied in a suspension of silica particles using light scattering and in a suspension of magnetic colloidal polystyrene by means of light transmission. For a silica volume fraction of 10% we find that the mean distance between particles inside chains drops off suddenly with the increase of field. An analysis of evolution of size distribution with the field is attempted but does not show evidence for a strong increase of average sizes of chains with field. We determine the phase diagram of the magnetic suspension by measuring light transmission and show that the experimental critical volume fraction is much lower than that predicted with a theory based on transition in an isotropic medium.