Self-diffusion of particles in shear flow of a suspension

Abstract

Self-diffusion coefficients were determined experimentally for lateral dispersion of spherical and disk-like particles in linear shear flow of a slurry at very low Reynolds number. Using a concentric-cylinder Couette apparatus, recurrent observations were made of the lateral position of a particular radioactively labelled particle. The self-diffusion coefficientDwas calculated by means of random-walk theory, using the ergodic hypothesis. Owing to great experimental difficulties, the calculated values ofDare not of high accuracy, but are correct to within a factor of two. In the range 0 < ϕ < 0·2,D/a2ω increases from zero linearly with ϕ up toD/a2ω ≅ 0·02 (where ϕ = volumetric concentration of particles,a= particle radius, ω = mean shear rate of suspending fluid). In the range 0·2 < 0·5, the trend ofD/a2ω is not clear because of experimental scatter, but in this rangeD/a2ω ≅ 0·025 to within a factor of two. Within the experimental accuracy, spheres and disks have the same value ofD/a2ω.