Diffusion of an isolated colloidal sphere confined between flat plates

Constrained diffusion of an isolated uncharged PMMA sphere (similar to 1 mu m diameter) confined between two parallel flat walls was studied using vid eo microscopy in combination with optical tweezers. This technique allows u s to measure directly the hindered diffusive motion of the sphere as an exp licit function of the distance of the sphere from a wall without interferen ce of electrostatic interaction between the sphere and the walls, nor inter ference from sedimentation. The measured diffusion coefficients are used to test the predictions of the wall drag effect predicted by several approxim ate theoretical analyses. We find quantitative agreement with the behavior predicted using a hydrodynamic analysis that independently superposes the w all drag effects arising from each wall. (C) 2000 Elsevier Science B.V. All rights reserved.