Local viscosity probed by photonic force microscopy

The Brownianmotion of 0.5 μmand 1 μmpolystyrenespheres used as probes for a photonic force microscope is recorded at various distances from a surface with nanometer spatial and 25 μs temporal resolution. Analyzing timewindows of 0.4 s of the Brownian motion, the diffusion constant and local viscosity are determined. Using 0.5 μm spheres the viscosity can be measured in volumes as small as 1 μm³ 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 μm down to 200 nm. The increase in viscous drag is found to agree with Brenner's analytical result [1].