Collapse of colloidal gels

: structures, dynamics, and stresses

Abstract

The stability of formulated products is complex to predict, especially regarding gravitational stability of suspensions. Weak aggregation of colloids induced by non-adsorbing polymers form a space filling network (gel) that provides transient mechanical stability against gravitational separation. These non-equilibrium gels can either collapse immediately at a smooth rate, referred as creeping, or after a delay time undergo catastrophic structural rearrangements much faster than creeping, known as sudden collapse. A key challenge here is investigating the stresses and structures that are governing the stability and how they evolve with time. We address this by developing a model refractive-index-matched and density-difference-controlled system using PDMS emulsion with non-adsorbing polymers. This model system represents the key features of commercial suspension products, we investigate its aging using confocal microscopy at multiple lengthscales.
First, our Particle Image Velocimetry (PIV) results reveal single tracer dynamics followed by intricate collective movements even before the rapid collapse, this is reminiscent of channel formations. To reveal the mask of these channels and the exchanges between gel materials and solvents, we utilize a rotating confocal microscope to scan centimeter-height gel samples where sudden collapse occurs. Observations including channel nucleation and propagation, gel arm coarsening, and interface formation are provided with details. Remarkably, we uncover strikingly heterogeneous aging attributed to solvent flow, coexisting with still homogeneous regions. We quantify this heterogeneity by a stand deviation map, and find its correlation with the trigger from creeping to rapid collapse.
Furthermore, we delve into the distribution and transmission of stress within the colloidal gel network, which is a strong candidate contributing to structural changes. At higher resolution up to single particle level, combined with a solvatochromic dye, particle contact information is linked to compressive forces within the gel network. Elevated large contact volumes indicate high stresses, while we successfully identify force chains with lengths exceeding 10 droplets. In addition to our model emulsion system, we explore the phase behavior of an eGFP protein- PEG polymer system, showcasing rare crystallization events. By considering protein dimers as spherocylinders, we align the onset of gelation between our experiments and theoretical predictions, taking a simple step toward understanding the complexity beyond ideal spheres.

Date of Award	19 Mar 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Wuge H Briscoe (Supervisor)