HINDERED MOBILITY OF A RIGID SPHERE NEAR A WALL

Abstract

We have measured the hindered mobility of microscopic spheres {7 to 15 μm diameter) suspended in aqueous solutions very near a wall (0-500 nm) using the technique of total internal reflection microscopy. The elevation of the sphere above the plate is monitored by measuring the intensity of light that is scattered by the sphere when illuminated by an evanescent wave which penetrates about 500 nm into the solution. The mobility is determined by using a laser beam to apply a known radiation force to the same Brownian particle several hundred times and measuring the resulting ensemble-average change in intensity as a function of time due to that force. The intensity of scattered light can be translated into an absolute separation distance, thus enabling simultaneous, independent measurements of the mobility and elevation of the sphere. When the sphere comes within a few radii of the wall, its mobility is expected to be significantly hindered owing to hydrodynamic interactions with the wall. Results show excellent agreement with Brenner's analysis for the hindered mobility of a rigid sphere near a planar surface: for separation distances from 2% to 10% of the sphere's radius, the measured mobilities ranged from 2% to 10% of the Stokes value far from the wall.