Brownian motion of a single microscopic sphere in a colloidal force field

Total internal reflection microscopy has been used to monitor the submicroscopic distance separating a microscopic sphere and a transparent plate. Light scattered from an evanescent wave by a single sphere is exponentially sensitive to the elevation of the sphere above the plate. Changes in separation as small as 1 nm are detectable. The technique is virtually instantaneous, non-intrusive, and easily automated. Two applications are discussed: from an equilibrium distribution of separations, a direct measure of the mean potential of surface forces acting on the sphere can be obtained; from an autocorrelation of the separation as a function of time, the sphere's diffusion coefficient, D, can be obtained. Forces as small as 10–13 N can be accurately measured. Owing to its close proximity to the wall, a 10 µm polystyrene sphere in 0.5 mmol dm–3 NaCl has a value of D of ca. 2% of the bulk value. The fractional reduction in D suggests that the sphere resides ca. 100 nm above the glass plate.