Recent developments in digital image processing provide revolutionary new tools for studying colloidal suspensions. Combined with video microscopy, image processing greatly facilitates time-resolved measurements of individual colloidal particles' trajectories. In the 90 years since Perrin's pioneering photographic study of diffusion and Brownian motion, quantitative analysis of colloidal images has been used to study phase transitions in three- and two-dimensional systems , to probe the effects of external fields on colloidal dynamics, and to measure directly the interaction between isolated pairs of colloidal microspheres[5, 6, 7]. While both image processing and the ultramicroscopy of colloidal suspensions are well developed fields, applying the former to the latter poses some problems whose resolution has not been discussed in a unified way. A rapidly growing community of researchers will come up against these practical hurdles as technological advances associated with multimedia computing make these techniques more widely available. In this light, we present some practical methods which we have found useful in studying the microscopic structure and dynamics of colloidal systems. While this treatment centers on suspensions of submicron spheres, many of the methods described below can be generalized to suspensions of non-spherical particles.
In section II, we describe typical instrumentation required for acquiring digital video images of colloidal particles. Section III describes in some detail the steps required to convert a digital movie of colloidal particles into an ensemble of single-particle trajectories. We stress those aspects of the analysis which allow us to track particles with spatial resolution much finer than the wavelength of light used to create the images. High-resolution trajectory data makes possible a wide range of quantitative measurements of colloidal processes at the microscopic scale. As practical examples, in Section IV we describe measurements of microspheres' self-diffusion coefficients and of the separation dependence of the pair-wise interaction potential for charge-stabilized colloid.