On visualization of sub-micron particles with dark-field light microscopy

Dark-field light microscopy is widely employed to visualize colloidal particles much smaller than the light wavelength used. In the captured images, the colloidal particles appear, against a dark background, as bright "specks" much larger than the geometrical size of the particles. To verify whether the "specks" are for individual particles or clusters of particles, experiments are performed which used low bulk concentrations of five suspensions of monodispersed particles (approximately 0.3 microm in diameter) and a dark-field video microscopic system with an optical resolution of approximately 0.5 microm to count the particles after they all have deposited onto the inner surfaces of a parallel-plate glass channel. The average size and the size distribution of the particles are also determined at the end of each experiment. The results confirmed that the visualized "specks" are for individual particles. The measured and prepared particle bulk concentrations in the five experiments closely matched, to within +/-5%, and the measured average size of the particles and their size distribution at the end of the five experiments were in agreement with the known values.