Observation of Brownian motion on the time scale of hydrodynamic interactions

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Abstract
We resolve the mean square displacement of particles in concentrated suspensions during the first 20 ns of their motion, before hydrodynamic interactions between particles have had time to fully develop. Nevertheless, we find that the concentrated systems exhibit clear deviations from the isolated-particle theory. These deviations are well described by several scaled versions of the isolated particle theory. The measurements also demonstrate a new multiple-scattering photon correlation technique with ultrashort time resolution.