Is directed percolation in colloid-polymer mixtures linked to dynamic arrest?

TL;DR

This study uses simulations to examine how directed percolation relates to dynamic arrest in colloid-polymer mixtures, finding that arrest occurs near the spinodal and is not directly linked to directed percolation.

Contribution

It demonstrates that dynamic arrest in colloid-polymer mixtures occurs near the spinodal, independent of directed percolation, challenging previous hypotheses linking the two phenomena.

Findings

Directed percolation shifts the percolation line to higher densities.

Dynamic arrest occurs near the spinodal, not at the directed percolation transition.

Competing repulsions do not fundamentally alter the gelation mechanism.

Abstract

Using computer simulations, we study the dynamic arrest in a schematic model of colloid-polymer mixtures combining short-ranged attractions with long-ranged repulsions. The arrested gel is a dilute rigid network of colloidal particles bonded due to the strong attractions. Without repulsions, the gel forms at the spinodal through arrested phase separation. In the ergodic suspension at sufficiently high densities, colloidal clusters form temporary networks that percolate space. Recently [Nat. Commun. 7, 11817 (2016)], it has been proposed that the transition of these networks to directed percolation coincides with the onset of the dynamic arrest, thus linking structure to dynamics. Here, we evaluate for various screening lengths the underlying gas-liquid binodal and the percolation transitions. We find that directed percolation shifts the continuous percolation line to larger densities, but even beyond this line the suspension remains ergodic. Only when approaching the spinodal does dynamic arrest occur. Competing repulsions thus do not modify the qualitative scenario for non-equilibrium gelation, although the structure of the emerging percolating network shows some differences.