Understanding depletion forces beyond entropy

TL;DR

This paper investigates the depletion forces acting on colloidal spheres near surfaces, revealing that attractive dispersion forces, not just entropy, significantly influence interactions at small distances.

Contribution

It combines experimental measurements with a novel DFT approach to show the importance of dispersion forces in depletion interactions.

Findings

Quantitative agreement between TIRM measurements and DFT calculations.

Depletion forces at small distances involve significant attractive dispersion contributions.

Entropy alone cannot fully explain the effective interaction energy.

Abstract

The effective interaction energy of a colloidal sphere in a suspension containing small amounts of non-ionic polymers and a flat glass surface has been measured and calculated using total internal reflection microscopy (TIRM) and a novel approach within density functional theory (DFT), respectively. Quantitative agreement between 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 ($\lesssim$ 100 nm) attractive dispersion forces have to be taken into account.