Rotational Diffusion and Streaming Birefringence

The longitudinal axes of nonspherical particles have a random distribution of angles with respect to each other in dilute solutions. This angular distribution of the longitudinal axes is disturbed by externally applied shear gradients or other fields. The longitudinal axes orient themselves parallel or perpendicular to the direction of the field, according to the type of field and its 

For the rod-shaped particles of tobacco mosaic virus of length 280 nm at room temperature, for example, rotational diffusion coefficients of 550 s were found. The rods thus require 0.0018 s in order to return to their equilibrium (randomly oriented) positions. 

The orientation of the molecules counteracts the rotational diffusion. The smaller the molecules, the faster will be the rotational diffusion. For a given ratio of the molecular axes, therefore, there is a lower limit for which a particle length can be measured by birefringence. It is at 20 nm. Shorter 

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