We have measured the hindered mobility of microscopic spheres (7 to 15
mu m diameter) suspended in aqueous solutions very near a wall (0 - 5
00 nm) using the technique of total internal reflection microscopy. Th
e elevation of the sphere above the plate is monitored by measuring th
e 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 rad
iation force to the same Brownian particle several hundred times and m
easuring the resulting ensemble-average change in intensity as a funct
ion of time due to that force. The intensity of scattered light can be
translated into an absolute separation distance, thus enabling simult
aneous, independent measurements of the mobility and elevation of the
sphere. When the sphere comes within a few radii of the wall, its mobi
lity is expected to be significantly hindered owing to hydrodynamic in
teractions with the wall. Results show excellent agreement with Brenne
r'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 ra
dius, the measured mobilities ranged from 2 % to 10 % of the Stokes va
lue far from the wall.