Confocal microscopy of a dense particle system.

We report experiments utilizing confocal microscopy to determine the position of cornflour starch granules in a paste, as a function of distance from a wall. The granules have an average diameter of 15 mum. If a solvent is chosen such as to approximately match the refractive index of the granules then images of them can be obtained to a depth of 120 mum, which is the limit imposed by the working distance of the x 40 oil immersion lens used in the experiment. An algorithm is presented which successfully identifies the particle centers in 3D. In addition to experiments on static systems we show how measurements of particle number density and velocity can be obtained from images of a system in pipe flow. We find that particles are depleted from the wall over a range of around 100 mum.