Imaging Colloidal Dynamics with Digital Holographic Microscopy

Although colloidal dynamics in concentrated suspensions have been studied extensively with optical and confocal microscopy, very few real-space experiments have explored the dynamics of interacting particles in dilute suspensions.  In these systems, standard microscopy techniques often prove too slow to image the rapidly diffusing particles.  We use an interferometric technique called digital holographic microscopy to reveal the dynamics and structure of colloidal suspensions with millisecond time resolution and nanometer-scale spatial precision in all three dimensions.  These experiments have revealed unexpected aspects of seemingly simple phenomena, such as the diffusion of particles in clusters or on emulsion droplets, or the binding of a particle to a liquid interface. In the latter case, we find that the dynamics are surprisingly slow: particles may take weeks or even months to relax to equilibrium at oil-water interfaces, a consequence of the hindered motion of the contact line near the solid substrate.  These studies may yield some insights into the mechanisms and failure modes of colloidal self-assembly.