  We report extensive molecular-dynamics simulation results for binary mixtures
of hard spheres for different size disparities and different mixing
percentages,for packing fractions up to 0.605 and over a characteristic time
interval spanning up to five orders in magnitude. We explore the changes in the
evolution of glassy dynamics due to mixing and discover two opposite scenarios:
for large size disparity, increasing the mixing percentage of small particles
leads to a speed-up of long-time dynamics, while small disparity leads to a
slowing down. These results agree with predictions based on the mode coupling
theory for ideal glass transitions.
