Studies of microscopic colloidal interactions with digital video microscopy, mirror image technique and blinking optical tweezers

Abstract

The microscopic interactions in a colloidal system have been intriguing to many. Such colloidal interactions include the pairwise electrostatic interaction between two colloidal particles, interaction between a particle and surrounding bounding wall, many-body interactions, entropic interaction, van der Waals force, hydrodynamic coupling and etc. It is an active field of research that provides a rich variety of phenomena that motivated many researchers. We present our effort in (a) the development of a novel technique, Mirror Image Technique, which facilitates quantitative tracking of three-dimensional motion of colloidal microspheres; (b) the observation of an anomalous attractive component in the mean interaction potential energy between an isolated charged sub-micron polystyrene sphere and the two confining walls; (c) the investigation of the anisotropic nature of the diffusion of the spheres in the confined system. The effects of varying degrees of confinement and additional self-assembly monolayer (SAM) on one of the walls were also discussed. In addition, with the use of blinking optical tweezers, we measured the localized pair-wise interaction between two microspheres and attempt to correlate the measured result with the measured rough underlying potential landscape. The observations could shed some light into the surprising reports by other researchers of long-ranged confinement-induced like-charge attractions between two sub-micron polystyrene spheres in a dilute system.