Brownian motion of a hydrosol particle in a colloidal force field

Abstract

Total-internal-reflection microscopy (TIRM) was used to monitor Brownian fluctuations in separation between a 10 µm polystyrene sphere and a glass plate when the two bodies are separated by a fraction of a micrometre of aqueous solution. Preliminary results clearly show evidence of double-layer repulsion which is weakened by increasing the ionic strength. Potential-energy profiles were obtained with a resolution of 2.5 nm. Although this falls short of the atomic resolution obtained with the crossed-mica cylinder technique for measuring colloidal forces, TIRM allows one of the interacting bodies to have colloidal dimensions. Besides measurement of colloidal and hydrodynamic forces, other applications include the study of hindered diffusion of the particle near a flat boundary and reversible adsorption of a single particle onto a plane surface.