How soft repulsion enhances the depletion mechanism

TL;DR

This paper explores how soft repulsive interactions in colloid-depletant mixtures significantly alter effective potentials and phase behavior, especially at small size ratios, challenging traditional hard-sphere models.

Contribution

It provides an exact relation for effective pair potentials in soft colloid-depletant systems and demonstrates the limitations of mapping soft systems onto hard models.

Findings

Soft repulsion enhances colloid aggregation.

Soft potentials cause dramatic changes in effective interactions at small size ratios.

Hard-sphere models are insufficient for soft colloid-depletant mixtures.

Abstract

We investigate binary mixtures of large colloids interacting through soft potentials with small, ideal depletants. We show that softness has a dramatic effect on the resulting colloid-colloid effective potential when the depletant-to-colloid size ratio $q$ is small, with significant consequences on the colloidal phase behaviour. We also provide an exact relation that allows us to obtain the effective pair potential for {\it any} type of colloid-depletant interactions in the case of ideal depletants, without having to rely on complicated and expensive full-mixture simulations. We also show that soft repulsion among depletants further enhances the tendency of colloids to aggregate. Our theoretical and numerical results demonstrate that --- in the limit of small $q$ --- soft mixtures cannot be mapped onto hard systems and hence soft depletion is not a mere extension of the widely used Asakura-Oosawa potential.