  Diffusion in bidisperse Brownian hard-sphere suspensions is studied by
Stokesian Dynamics (SD) computer simulations and a semi-analytical theoretical
scheme for colloidal short-time dynamics, based on Beenakker and Mazur's method
[Physica 120A, 388 (1983) & 126A, 349 (1984)]. Two species of hard spheres are
suspended in an overdamped viscous solvent that mediates the salient
hydrodynamic interactions among all particles. In a comprehensive parameter
scan that covers various packing fractions and suspension compositions, we
employ numerically accurate SD simulations to compute the initial diffusive
relaxation of density modulations at the Brownian time scale, quantified by the
partial hydrodynamic functions. A revised version of Beenakker and Mazur's
$\delta\gamma$-scheme for monodisperse suspensions is found to exhibit
surprisingly good accuracy, when simple rescaling laws are invoked in its
application to mixtures. The so-modified $\delta\gamma$ scheme predicts
hydrodynamic functions in very good agreement with our SD simulation results,
for all densities from the very dilute limit up to packing fractions as high as
$40\%$.
