Propagation of hydrodynamic interactions in colloidal suspensions

We describe direct measurements of the dynamics of two colloidal spheres before hydrodynamic interactions have had time to fully develop. We find that the dynamics of the two spheres are coupled at times significantly shorter than tau(nu), the time required for vorticity to diffuse between the two spheres. From the distance dependence of the measured coupling, we infer that hydrodynamic interactions develop in a sonic time scale.