Near-wall particle depletion in a flowing colloidal suspension

In this paper a method based on attenuated total reflection-Fourier-transform infrared spectroscopy is described to measure the concentration of colloidal particles dispersed in a liquid near a solid boundary in pressure driven flows. The method has been used to obtain measurements of wall depletion in an aqueous suspension of ethylcellulose particles. The method is capable of measuring the particle concentration in a thin layer near the boundary, whose thickness is between 0.3 and 1.4 Am. A considerable decrease in the concentration of particles near the wall is found for a suspension with a particle volume fraction of 0.24 over a wide range of apparent shear rates. The results from the spectroscopic experiments are compared to rheological experiments on parallel disk torsional flow at different gap heights. With this method, the order of magnitude of the thickness of an equivalent fully depleted layer (a layer at the wall assumed to contain only pure solvent and no particles) can be obtained. The results found with the two techniques are in reasonable agreement. The described method is a convenient, rapid, and relatively inexpensive way to measure the particle depletion near the wall in flowing suspensions, compared to existing magnetic resonance imaging and laser-Doppler techniques. (C) 2002 The Society of Rheology.