Differential dynamic microscopy: probing wave vector dependent dynamics with a microscope.

We demonstrate the use of an ordinary white-light microscope for the study of the q-dependent dynamics of colloidal dispersions. Time series of digital video images are acquired in bright field with a fast camera, and image differences are Fourier analyzed as a function of the time delay between them. This allows for the characterization of the particle dynamics independent of whether or not they can be resolved individually. The characteristic times are measured in a wide range of wave vectors and the results are found to be in good agreement with the theoretically expected values for Brownian motion in a viscous medium.