Numerical study of neutral and charged microgel suspensions: from single-particle to collective behavior

TL;DR

This study uses Molecular Dynamics simulations to explore how neutral and charged microgel suspensions behave from single particles to dense collective states, revealing structural reentrance and the limitations of simple models.

Contribution

It provides a detailed analysis linking single-particle deformation and shrinking to collective structural reentrance in microgel suspensions, highlighting differences between neutral and charged systems.

Findings

Structural reentrance observed in radial distribution functions

Dynamics always slow down despite non-monotonic structural changes

Charged microgels show shell fusion at high packing fractions

Abstract

We perform extensive Molecular Dynamics simulations of an ensemble of realistic microgel particles in swollen conditions in a wide range of packing fractions $\zeta$. We compare neutral and charged microgels, where we consider charges distribution adherent to experimental conditions. Through a detailed analysis of single-particle behavior, we are able to identify the different regimes occurring upon increasing concentration: from shrinking to deformation and interpenetration, always connecting our findings to available experimental observations. We then link these single-particle features to the collective behavior of the suspension, finding evidence of a structural reentrance, that has no counterpart in the dynamics. Hence, while the maximum of the radial distribution function displays a non-monotonic behavior with increasing $\zeta$, the dynamics, quantified by the microgels' mean-squared displacement, always slows down. This behavior, at odds with simple Hertzian model, can be described by a phenomenological multi-Hertzian, which takes into account the enhanced internal stiffness of the core. However, also this model fails when deformation enters into play, whereby more realistic many-body models are required. We are able to unveil the key physical mechanisms, shrinking and deformation, giving rise to the structural reentrance that holds up to very large packing fractions. We further identify key similarities and differences between neutral and charged microgels, for which we detect at high enough $\zeta$ the fusion of charged shells, previously invoked to explain key experimental findings, and responsible for the structural reentrance. Overall, our study establishes a powerful framework to uncover the physics of microgel suspensions, paving the way to tackle different regimes and internal structural architectures, where experimental evidence is still limited.