Microstructure, Dynamics and Rheology of Model Soft Particulate Gels

Abstract

Soft particulate gels consist of a small amount of solid material (colloidal particles or small aggregates) embedded in a fluid and are industrially relevant materials for applications in a wide range of fields including foods, personal care products and, biomedicine. Due to attractive inter-particle interactions, the solid component self-assembles into an open, porous network which controls the overall mechanical response of the material. In this dissertation, I have studied a class of numerical models for particulate gels in which the particle contacts are described by an effective interaction combining a two-body attraction and a three-body angular repulsion. Using molecular dynamics, I have shown how varying the model parameters allows us to sample, for a given gelation protocol, a variety of gel morphologies. For a specific set of the model parameters, I have identified the local elastic structures that get interlocked in the gel network. Starting from the analytical expression of their elastic energy from the microscopic interactions, I have estimated their contribution to the emergent elasticity of the gel and gained a new insight into its origin. Using large scale simulations with Optimally Windowed Chirp (OWCh) signals, I have investigated the microscopic origin of the linear viscoelastic response of soft particulate gels. My results indicate that the viscoelastic spectrum of a wide range of gels, with different microstructures, is controlled by an underlying fractal characteristic of the gel network, i.e., its initial rigid backbone, and by the associated hierarchy of relaxation time scales. Building on these methods and analysis, I have also studied how the viscoelasticity of composite gels can be tuned by changing the nature of the interactions in different mixed components in close connection with experiments. In particular, I have shown how a significant increase of the elastic modulus in nanofiller composites can be achieved at unexpectedly low volume fractions of fillers when the interactions between the fillers and the gel matrix are attractive. Moreover, mixing components of different interaction strengths in a binary network can result in a non-monotonic dependence of modulus on composition, which can then be used to fine-tune the mechanical properties in novel gel materials. The architecture of the gel networks and properties such as bending stiffness of the gel strands embedded in soft gels and the prestress also enter non-linear properties such as strain-stiffening, yielding and failure, which I have studied especially in connection with experiments on biopolymer gel networks. My work supports the idea suggested by the experiments, that the linear and nonlinear moduli are controlled by both the network architecture and the fiber mechanics while the onset the nonlinear, stretch-dominated regime and the yield strain are controlled predominantly by the network architecture. The increased bending stiffness of gel strands finally together with prestress can result in a dramatic reduction of the bond-reforming capability after yielding, possibly leading to a more brittle mechanical failure. The stiffer gel strands, together with prestress, promote a more brittle failure.