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 g as approach is valid. At higher densities, however, structural correlation effects of the macromolecules which lead to additional repulsive parts in t he depletion interaction, have to be taken into account. We have measured t he depletion interaction between a single polystyrene sphere and a wall in the presence of non-ionic polymer coils. Although the polymer volume fracti on is very small, our results clearly show the existence of repulsive parts , leading to a large over-estimation of correlation effects in a hard spher e liquid. The possible explanation of this discrepancy by additionally cons idering the role of van der Waals forces is discussed.