Interactions between colloidal particles induced by polymer brushes grafted onto the substrate

TL;DR

This study uses self-consistent field theory to analyze how polymer brushes on substrates influence colloidal interactions, revealing entropic barriers that can stabilize colloidal mixtures and guide self-assembly.

Contribution

It demonstrates the existence of entropic repulsive barriers between colloids on polymer-grafted substrates, providing insights for controlling colloidal self-assembly.

Findings

Repulsive energy barrier of several kBT identified.

Interaction energy decomposed into interfacial, depletion, and elastic contributions.

Barrier can stabilize colloidal mixtures against phase separation.

Abstract

We investigate the interaction energy between two colloidal particles on or immersed in nonadsorbing polymer brushes grafted onto the substrate as a function of the separation of the particles by use of self-consistent field theory calculation. Depending on the colloidal size and the penetration depth, we demonstrate an existence of repulsive energy barrier of several $k_{B}T$, which can be interpreted by separating the interaction energy into three parts: colloids-polymer interfacial energy, entropic contribution due to ``depletion zone" overlap of colloidal particles, and entropically elastic energy of grafted chains by compression of particles. The existence of repulsive barrier which is of entirely entropic origin, can lead to kinetic stabilization of the mixture rather than depletion flocculation or phase separation. Therefore, the present result may suggest an approach to control the self-assembling behavior of colloids for the formation of target structures, by tuning the colloidal interaction on the grafting substrate under appropriate selection of colloidal size, effective gravity (influencing the penetration depth), and brush coverage density.