A numerical study on the intrinsic error involved in the use of a shear cell to measure liquid diffusion coefficients

Abstract

The shear cell technique has some merits not possessed by the long capillary method for the measurement of diffusion coefficients. However, it also possesses the possibility of an intrinsic error arising from any liquid mixing associated with its shearing operation. A dynamic numerical simulation, with a grid sliding technique, was developed and was used to explore the solute exchange between the two liquid charges in a diffusion couple, the concentration distribution along the liquid charges, and to investigate the error of the shear cell method. This was shown to be associated with the shear rate and annealing time involved. It was found that the intrinsic error was caused mainly by the convection resulting from the shear and strongly depends on the mass Peclet number and the Reynold’s number. The intrinsic error arising from the shearing action in the shear cell required to create the diffusion couple deceases with increasing time of the diffusion anneal. The variation of the maximum intrinsic error with the elapsed diffusion time was calculated for a capillary diameter of 1.5 mm similar to that which was used in diffusion coefficient measurements.