How do dynamic heterogeneities evolve in time?

We present simulations of a hard disc system and analyze the time evolution of the dynamic heterogeneities. We characterize the time evolution of slow regions and slow particles individually. The motion of slow clusters turns out to be very restricted, i.e. a cluster is generated and annihilated in a spatial region four times the size of its maximum extent. The residual motion of the cluster can be traced back to subdiffusive motion of the constituent particles and the process of absorption and loss of adjacent particles. The subdiffusive dynamics is independent of how long the particles remain slow. Clusters of fast particles show an even smaller reach, which seems to be due to their short life time.