How to relax: Dynamics in colloid polymer composites during syneresis

Soft colloidal solids are ubiquitous to life. Their stability is crucial to consumer products and the rich mechanical properties they exhibit make them valuable subjects to investigate to meet the growing demand for sustainable, high-quality materials. Upon stressing soft colloidal solids, they experience a variety of relaxation mechanisms, one being syneresis, the compaction of the material, leading to the expulsion of fluid. Syneresis displays a wealth of complex phenomena that makes its study fascinating yet challenging. In this thesis, we looked into particle dynamics during syneresis at multiple length scales in various soft colloidal materials to reveal underlying physics governing mechanics in such soft disordered solids.

 

In Chapter 2, we unravel the internal individual particle level dynamics of three colloid polymer mixtures where the polymer architecture is altered. We compare particle dynamics between three common ‘thickeners’, linear polymer, microgel and a sub-granular gel, by creating a model colloidal system which enables fast 3D confocal microscopy. From recorded image stacks, we disentangle ensemble and individual particle dynamics based on local neighborhood and find stark differences. We utilize a common methodology of the van Hove distribution to characterize these contrasting particle dynamics between a majority of bound particles and a minority of highly mobile particles. Finally, by parameterizing these distributions, we provide an approach to distinguish separate phases which these composites resemble, namely, a colloidal gel or a colloidal glass.

In Chapter 3, we study how dynamics during syneresis in a colloidal gel is defined by stress relaxation at the strand level. We form in situ colloidal gels composed of three different types of particles with contrasting nature: liquid droplets, solid polymer, and crosslinked rubber, all with an identical averaged size. Crucially, the interparticle attraction is constant as it is triggered by a thermoresponsive surfactant, which leads to attraction upon heating to a fixed temperature. We find that syneresis occurs rapidly upon gelation only if the container walls are rendered non-adhesive. The magnitude of this syneresis is greatest for particles with high interfacial mobility, i.e. droplets and rubber, and hindered for solid particles, while being still appreciable. We perform microscopic and mechanical measurements to deduce that this magnitude is related to the modes of stress relaxation within the network by either the stretching of interparticle bonds or the bending of entire gel strands. While connectivity has recently received enormous attention in such heterogeneous networks, our results highlight that it is only one part that defines the network dynamics, with both boundary conditions and interfacial mobility playing equally important roles.

In Chapter 4, we move to the dynamics during syneresis at macroscopic level. We directly quantify syneresis in colloid polymer mixtures like low-fat mayonnaise by non-accelerated observation, avoiding disproportionate acceleration of the participating forces in the force balance. We devise a new method of mimicking syneresis triggered by scooping in a controlled and reproducible manner. This allows us to investigate quantitatively the effect of the hydrostatic driving force. Furthermore, we look into the resisting osmotic pressure which is thought to play a key role in limiting syneresis. Additionally, we characterize the composition of the expelled fluid. We find that the flow rate of expelled fluid is proportional to the difference in hydrostatic pressure over the system. We conclude that syneresis is controlled by the permeability of the network and that its kinetics can accurately be described with a 1D model based on Darcy's law by accounting for the complex geometry and microstructure.

To date, the studies on dynamics during syneresis have left out possible heterogeneities in the material. In Chapter 5, we take the first steps to investigate spatial dependence of dynamics during syneresis experimentally. We apply the technique Laser Speckle Imaging which allows us to elucidate internal dynamics within turbid materials with high spatiotemporal resolution. Syneresing colloidal gels comprising of particles with different nature, as also studied in Chapter 3, are used here. We find that both colloidal gels exhibit heterogeneous dynamics during syneresis. Furthermore, we identify the distinct spatial dependence of dynamics in gels composing of different particles; the solid particle gel shows a smooth decay in mobility from the syneresing interface into the bulk, while the droplet based gel develops a band with high mobility a few hundred micrometers behind the interface of the sample. We hypothesize that this difference is a manifestation of the different ways in which the networks bear stress, depending on the composing particle nature. Additionally, we present a proof of concept measurement with LSI on syneresing commercial food products, which opens new opportunities in investigating food stabilization in a non-invasive fashion and, ultimately contributing to predict and control the product stability.

Finally, in Chapter 6, the general discussion, we place our findings in an expanded scientific context and give an outlook for future research into and beyond syneresis in mechanics of soft disordered solids.