Structure and dynamics of responsive colloids with dynamical polydispersity

TL;DR

This study uses Brownian Dynamics simulations to explore how responsive colloids with dynamically changing sizes behave, revealing complex many-body effects and proposing an effective potential model to describe their structure and dynamics.

Contribution

The paper introduces a simulation framework for responsive colloids with explicit size dynamics and compares their behavior to traditional models, highlighting the importance of many-body correlations.

Findings

Responsive colloids exhibit significant structural and dynamical differences from traditional models.

Many-body correlations and size dynamics influence diffusion and structure, especially at high densities.

An effective monodisperse pair potential can approximate many-body effects in RC systems.

Abstract

Dynamical polydispersity in single-particle properties, for example a fluctuating particle size, shape, charge density, etc., is intrinsic to responsive colloids (RCs), such as biomacromolecules or microgels, but is typically not resolved in coarse-grained mesoscale simulations. Here, we present Brownian Dynamics (BD) simulations of suspensions of RCs modeling soft hydrogel colloids, for which the size of the individual particles is an explicitly resolved (Gaussian) degree of freedom and dynamically responds to the local interacting environment. We calculate the liquid structure, emergent size distributions, long-time diffusion, and property (size) relaxation kinetics for a wide range of densities and intrinsic property relaxation times in the canonical ensemble. Comparison to interesting reference cases, such as conventional polydisperse suspensions with a frozen parent distribution, or conventional monodisperse systems interacting with an effective pair potential for one fixed size, shows a significant spread in the structure and dynamics. The differences, most apparent in the high density regimes, are due to many-body correlations and the dynamical coupling between property and translation in RC systems, not explicitly accounted for in the conventional treatments. In particular, the translational diffusion in the RC systems is surprisingly close to the free (single RC) diffusion, mainly due to a cancellation of crowding and size compression effects. We show that an effective monodisperse pair potential can be constructed that describes the many-body correlations reasonably well by convoluting the RC pair potential with the density-dependent emergent size distributions and using a mean effective diffusion constant.