theoretical and numerical simulation studies of the transport of

fluid phase tracer molecules from one wall to the opposite wall

bounding a sheared suspension of neutrally buoyant solid

particles. The experiments use a standard electrochemical method

in which the mass transfer rate is determined from the current

resulting from a dilute concentration of ions undergoing redox

reactions at the walls in a solution of excess non-reacting ions

that screen the electric field in the suspension. The simulations

use a lattice-Boltzmann method to determine the fluid velocity and

solid particle motion and a Brownian tracer algorithm to determine

the chemical tracer mass transfer. The mass transport across the

bulk of the suspension is driven by hydrodynamic diffusion, an

apparent diffusive motion of tracers caused by the chaotic fluid

velocity disturbances induced by suspended particles. As a result

the dimensionless rate of mass transfer (or Sherwood number) is a

nearly linear function of the dimensionless shear rate (Peclet

number) at moderate values of the Peclet number. At higher Peclet

numbers, the Sherwood number grows more slowly due to the mass

transport resistance caused by a molecular-diffusion boundary

layer near the solid walls. Fluid inertia enhances the rate of

mass transfer in suspensions with particle Reynolds numbers in the

range 0.5 to 7 .