Diffusion of an isolated colloidal sphere confined between flat plates

Constrained diffusion of an isolated uncharged PMMA sphere (∼1 μm diameter) confined between two parallel flat walls was studied using video microscopy in combination with optical tweezers. This technique allows us to measure directly the hindered diffusive motion of the sphere as an explicit function of the distance of the sphere from a wall without interference of electrostatic interaction between the sphere and the walls, nor interference from sedimentation. The measured diffusion coefficients are used to test the predictions of the wall drag effect predicted by several approximate theoretical analyses. We find quantitative agreement with the behavior predicted using a hydrodynamic analysis that independently superposes the wall drag effects arising from each wall.