HYDRODYNAMIC THICKENING OF DEPLETION LAYERS IN COLLOIDAL SOLUTIONS

We report a new surface phenomenon in flowing colloidal suspensions of anisotropic particles. It is characterized by a decrease in the probability of presence of the solute particles near the cell walls when the solution is submitted to Poiseuille flows exceeding a minimum shear rate. The effect has been detected on dilute aqueous solutions of xanthan, a wormlike polysaccharide chain, by using the optical evanescent-wave-induced fluorescence method. The results obtained at various shear rates are in quantitative agreement with a simple model based on the chain dynamics near the repulsive wall, and taking into account the increased molecular tumbling rate induced by the shearing flow.