Particulate gels formed by colloidal particles dispersed in a solvent under a range of conditions and in a broad variety of formulations often undergo syneresis, a phenomenon where the gel structure remains intact but shrinks expelling a small volume fraction of the particle dilute phase. The overall mechanical strength of the gel can delay or hinder particle flocculation, sedimentation, or separation. This can promote the mid- or long-term stability of products. The structural and, especially, the mechanical heterogeneities along with changes in the nature of particle contacts are ubiquitous in these materials, with these factors certainly playing an important role in the poorly understood phenomenon of syneresis.
This project investigates how changes in particle contacts and, in the forces acting on them, may translate into stress redistribution, triggering changes in the gel structures at larger scales inducing syneresis. The research approach is based on computer simulations of particle based models and statistical physics analysis of gel properties that emerge from the evolution of microscopic structures and dynamics of the particles.
During this first year, we have developed a new computational model for particulate gels which allows us to include, in the description of the gels, specific information not only of the particle effective interactions through the solvent, but also of the particle surface contacts, characterized in terms of adhesion, and sliding and rolling friction. The idea is to use contact models, previously introduced for grains, in a general computational framework for colloidal gels. We have focused on two formulations of this model, which correspond to respectively soft and adhesive or stiff and cohesive particles. Regular meetings with IFPRI industrial partners and discussions with IFPRI academic researchers (Vermant and Hsiao) have guided choices towards an experimentally relevant parameter space.
In an extensive simulation study, we have prepared gel structures for different particle volume fractions between 10% and 30% and for different adhesion strengths. Different preparation protocols have been systematically explored to identify conditions that guarantee reproducibility and mechanical stability of the gel structures. We have characterized all gels obtained in terms of the distribution of local pressures and distributions of particle contacts, which display clear differences with varying the contact mechanics, for the same particle volume fraction and preparation protocols. Preliminary results indicate a pronounced difference, in the tendency to contract, for gels formed with different distributions of local pressures and with different contact mechanics. These differences correspond to dramatic differences in the rheology of the gels, as explored in large amplitude shear tests, performed numerically. Finally, we have developed new simulations in which the gel is in contact with surfaces that describe containers walls to analyze how confinement and wall-gel interactions can modify syneretic behavior.
In this report, we summarize the main results achieved and discuss the plan for the coming year. The report is organized in sections that cover, respectively, the numerical model and computer simulation details, the discussion of the preparation protocols, the gel characterization in terms of local pressure and contact distributions, the preliminary results of rheological tests, and a proof of concept of gel formation under confinement.