Ultrasmall Force Measurement

The concept of force plays a central role in our understanding of nature. Due to the ongoing trend towards miniaturization, the investigation of forces relevant at microscopic and nanoscopic length scales therefore is an important field of research. Over the past decades several methods to measure ultra-small forces with resolution in the sub-pico Newton range have been developed. Total Internal Reflection Microscopy (TIRM) is an extremely sensitive non-invasive technique to measure the interaction potentials between a micron- or submicron sized particle and a wall with femto Newton resolution. The equilibrium distribution of the particle-wall separation distance z is sampled monitoring the intensity I scattered by the Brownian particle under evanescent illumination. Central to the data analysis is the knowledge of the relation between I and the corresponding z, which typically must be known a priori. For purely dielectric particles, an exponential dependence can be assumed, however, severe deviations may occur when e.g. surfaces with metallic coatings are considered.