Direct measurements of constrained Brownian motion of an isolated sphere between two walls

We report the results of direct measurements, using video microscopy in com bination with optical tweezers, of constrained diffusion of an isolated unc harged PMMA sphere in a density-matched fluid confined between two parallel hat walls. Our experimental methodology allows us to study the hindered di ffusion of the sphere as an explicit function of its distance from the wall s, without interference from sedimentation or from electrostatic interactio n between the particle and the walls. The measured diffusion coefficients a re used to test the predictions of the wall drag effect predicted by severa l approximate theoretical analyses. We find a quantitative agreement with t he behavior predicted using a hydrodynamic analysis that independently supe rimposes the wall drag effects arising from each wall. Our results imply, i ndirectly, that neglect of multiple interactions with the colloid sphere of the perturbations of the pressure and velocity fields induced by each wall leads to an underestimate of the influence of the wall on the drag force e xperienced by the particle.