Optical microscopy measurement of pair correlation functions

We studied the pair correlation function g(r) of silica particles with a fluorescent core and a nonfluorescent shell which were confined between two glass plates by optical video microscopy. To investigate the possible role of optical artifacts due to overlapping particle images, we compared experiments, where, first, the whole particle (white image) and then, only the fluorescent core (fluorescent image) was used for determining particle positions. While under white-image conditions the observed g(r) exhibits a main peak at about 1.2 times the particle's diameter; under fluorescent image conditions the obtained g(r) resembles a short-ranged repulsive system where the main peak is close to contact. This discrepancy points towards artifacts of video microscopy, leading to erroneous g(r) and in turn to erroneous effective-pair potentials.publishe