Direct Measurement of Suspension Structure in Pressure Driven Flow

Many non-Newtonian properties of concentrated suspensions, including shear thinning, normal stress, and shear migration, are attributed to the evolution of structural anisotropy. Investigations of this anisotropy have largely been conducted using simulations such as Stokesian Dynamics and indirect measurement via scattering. Using high speed confocal laser scanning microscopy, we directly investigate the local structure via particle tracking after structural evolution in a pressure driven flow ranging from Péclet number (the ratio of convective to diffusive forces) of 0 < Pe < 2000 over bulk volume fractions 0.2 ≤ Φ ≤ 0.4. Due to the shear gradient, a single experiment allows both sampling over a range of Péclet numbers and concentrations that vary due to shear migration. Clear structural anisotropy similar to that demonstrated by simulations of Foss and Brady, 2000 and Morris and Katyal, 2002 show strong deviations from homogeneity in radial distribution profiles at high Péclet number and high volume fraction. Moreover, in regions of low Pe located in the center of the pressure driven flow, signatures of colloidal crystallization driven by the high concentration that results from the normal forces generated this anisotropy are apparent.