Back scattering of diffusing wave spectroscopy (DWS) method is employed to measure the mean square displacements of the colloidal particles in electrorheological fluid systems right after an electric field is applied and removed. An electrorheological fluid consisting of colloidal particles 3040 nm in size shows a tailing phenomenon, namely the clusters of the colloidal ER particles do not return to their original state of Brownian motion as soon as the electric filed is removed. The DWS results of above ER fluid shows that the mean square displacement remains small and changes slowly with time right after the electric field is removed. The Van der Waals attractive force between colloidal particles is used to explain this tailing phenomenon.
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