Some of the experimental techniques that have been applied in recent years to the problem of measuring grain position and motion in two- and three-dimensional granular flows are briefly reviewed. The so-called "spin tagging" technique in magnetic resonance imaging provides a direct measure of a single displacement component for a slice through a three-dimensional granular material. Positron emission particle tracking allows the position vector of a single particle to be measured with better time resolution than with MRI, though with lower accuracy. High speed imaging using a video camera provides valuable data in two-dimensional flows, allowing, for example, velocity distributions, self-diffusion coefficients, static structure factors and intermediate scattering functions to be calculated from direct observations of the particle motion. The techniques are illustrated by applications including rotating drum experiments, and convection and fluidization in vertically-vibrated systems.
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