Understanding depletion forces beyond entropy

The effective interaction energy of a colloidal sphere in a suspension containing small amounts of nonionic polymers and a flat glass surface has been measured and calculated using total internal reflection microscopy and a novel approach within density functional theory, respectively. Quantitative agreement bet een experiment and theory demonstrates that the resulting repulsive part of the depletion forces cannot be interpreted entirely in terms of entropic arguments but that particularly at small distances (less than or similar to 100 nm) attractive dispersion forces have to be taken into account.