Repulsive depletion interactions in colloid-polymer mixtures

Depletion forces in colloidal systems are known to be entirely attractive, as long as the background of macromolecules is small enough that an ideal gas approach is valid. At higher densities, however, structural correlation effects of the macromolecules which lead to additional repulsive parts in the depletion interaction, have to be taken into account. We have measured the depletion interaction between a single polystyrene sphere and a wall in the presence of non-ionic polymer coils. Although the polymer volume fraction is very small, our results clearly show the existence of repulsive parts, leading to a large over-estimation of correlation effects in a hard sphere liquid. The possible explanation of this discrepancy by additionally considering the role of van der Waals forces is discussed.