LOCAL VISCOSITY PROBED BY PHOTONIC FORCE MICROSCOPY

The Brownian motion of 0.5 mu m and 1 mu m polystyrene spheres used as probes for a photonic force microscope is recorded at various distanc es from a surface with nanometer spatial and 25 mu s temporal resoluti on. Analyzing time-windows of 0.4s of the Brownian motion, the diffusi on constant and local viscosity are determined. Using 0.5 mu m spheres the viscosity can be measured in volumes as small as 1 mu m(3) with /-15% precision within 0.4 s. The decrease of the diffusion coefficien t due to the partial confinement near a surface is measured continuous ly over sphere-surface separations from 3 mu m down to 200 nm. The inc rease in viscous drag is found to agree with Brenner's analytical resu lt [1].