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Digital Video Microscopy

Colloidal spheres larger than a couple hundred nanometers are easily observed with a good quality light microscope. Their images can be captured by a video camera, digitized, and analyzed with a computer to measure their positions precisely in each video frame. Identifying each particle's centroid with its scattering pattern's center of brightness yields resolutions exceeding 20 nm in the plane [2]. Consecutive snapshots of $ N$ particles' positions can be linked [2] into trajectories, $ \vec r_i(t)$, to yield the in-plane distribution function

$\displaystyle \rho (\vec r, t) = \sum_{i=1}^N \delta(\vec r - \vec r_i(t)).$ (1)

Our goal is to use these measured trajectories to shed new light on the interactions responsible for colloidal dynamics.

David G. Grier 2001-01-16