  We study aging in a colloidal suspension consisting of micron-sized particles
in a liquid. This system is made glassy by increasing the particle
concentration. We observe samples composed of particles of two sizes, with a
size ratio of 1:2.1 and a volume fraction ratio 1:6, using fast laser scanning
confocal microscopy. This technique yields real-time, three-dimensional movies
deep inside the colloidal glass. Specifically, we look at how the size, motion
and structural organization of the particles relate to the overall aging of the
glass. Particles move in spatially heterogeneous cooperative groups. These
mobile regions tend to be richer in small particles, and these small particles
facilitate the motion of nearby particles of both sizes.
