Direct measurement of depletion potentials in mixtures of colloids and nonionic polymers

In colloidal suspensions containing a binary mixture of hard spheres depletion forces occur which substantially contribute to the interaction of the larger spheres among themselves and a wall, respectively. We investigated the depletion force acting on a large colloidal polystyrene sphere immersed in a solution of small, noncharged polymer coils close to a flat glass surface by means of total internal reflection microscopy. When the distance between the polystyrene sphere and the wall is smaller than the diameter of the polymer coils, an attractive potential acting on the sphere is observed which depends strongly on the polymer concentration. Our results are in agreement with theoretical predictions.