We present a computational investigation on the slow dynamics of a mixture of
large and small soft spheres. By varying the size disparity at a moderate fixed
composition different relaxation scenarios are observed for the small
particles. For small disparity density-density correlators exhibit moderate
stretching. Only small quantitative differences are observed between dynamic
features for large and small particles. On the contrary, large disparity
induces a clear time scale separation between the large and the small
particles. Density-density correlators for the small particles become extremely
stretched, and display logarithmic relaxation by properly tuning the
temperature or the wavevector. Self-correlators decay much faster than
density-density correlators. For very large size disparity, a complete
separation between self- and collective dynamics is observed for the small
particles. Self-correlators decay to zero at temperatures where density-density
correlations are frozen. The dynamic picture obtained by varying the size
disparity resembles features associated to Mode Coupling transition lines of
the types B and A at, respectively, small and very large size disparity. Both
lines might merge, at some intermediate disparity, at a higher-order point, to
which logarithmic relaxation would be associated. This picture resembles
predictions of a recent Mode Coupling Theory for fluids confined in matrixes
with interconnected voids [V. Krakoviack, Phys. Rev. Lett. {\bf 94}, 065703
(2005)].Comment: Journal of Chemical Physics 125, 164507 (2006