Phase separation and turbidity of electrorheological fluids

The turbidity of a suspension of silica particles in a silicone oil is studied in the presence of an electric field. We present experimental results which show a very strong attenuation of a laser beam when the electric field is switched on. We give a theoretical interpretation of these results with the help of a Rayleigh Gans Debye theory applied to cylinders. This model allows us to obtain the average length of the aggregates as a function of the coupling parameter lambda which is the ratio of the electrostatic energy to kT. These results agree with a theoretical prediction and also with the results of Monte-Carlo simulations. The kinetics of the beginning of field induced aggregation can be obtained from the change of the turbidity with time after the application of a voltage step.