Direct measurement of hydrodynamic cross correlations between two particles in an external potential

We report a direct measurement of the hydrodynamic interaction between two colloidal particles. Two micron-sized latex beads were held at Varying distances in optical tweezers while their Brownian displacements were measured. In spite of the fact that fluid systems at low Reynolds number are generally considered to have no "memory," the cross-correlation function of the bead positions shows a pronounced, time-delayed anticorrelation. We show that the anticorrelations can be understood in terms of the standard Oseen tensor hydrodynamic coupling.