The Brownianmotion of 0:5 mmand 1 mmpolystyrene
spheres used as probes for a photonic force microscope is
recorded at various distances from a surface with nanometer
spatial and 25 ms temporal resolution. Analyzing timewindows
of 0:4 s of the Brownian motion, the diffusion
constant and local viscosity are determined. Using 0:5 mm
spheres the viscosity can be measured in volumes as small
as 1 mm3 with 15% precision within 0:4 s. The decrease of
the diffusion coefficient due to the partial confinement near
a surface is measured continuously over sphere’surface separations
from 3 mm down to 200 nm. The increase in viscous
drag is found to agree with Brenner’s analytical result [1].