Measurement of the hydrodynamic forces between two polymer-coated   spheres
  The hydrodynamic forces between Brownian spheres are determined from a measurement of the correlated thermal fluctuations in particle position using a new method, two-particle cross-correlation spectroscopy (TCS). A pair of 1.3 um diameter polymer-coated poly(methyl methacrylate) were held at separations of between 2 um and 20 um using optical traps. The mobility tensor is determined directly from the statistically-averaged Brownian fluctuations of the two spheres. The observed distance dependence of the mobility tensor is in quantitative agreement with low-Reynolds number calculations. 
P. Bartlett, S. I. Henderson and S. J. Mitchell