Joanne Verweij

In this thesis, we study the structure-function relation of colloidal gels, a class of soft materials. We combine simulations and experiments to obtain insights into the microstructural response of colloidal gels in the presence or absence of external stress. Specifically, we take a closer look at processes such as fatigue, shear and gravitational collapse and study how deformation or gravity affect the microstructure and network topology of these particle gels. We employ a recently developed particle system, which can be index- and density matched using non-hazardous polar solvents, to image large volumes of colloidal gels. This confocal microscopy data allows us to determine the network topology of these particle gels using a topology mapping algorithm described in this thesis. Simultaneously, we perform simulations to obtain a better understanding of changes in the network topology of colloidal gels during deformation.

In Chapter 2 we study fatigue in colloidal gels by combining experiments and computer simulations. Repetitive loading of a soft solid leads to microstructural damage that ultimately results in catastrophic material failure. Even though fatigue poses a treat to the stability of virtually all materials, the microscopic origins of fatigue, especially for soft solids, remain elusive. Our results reveal how mechanical loading leads to irreversible strand stretching, which builds slack into the network that softens the solid at small strains and causes strain hardening at larger deformations. We thus find that microscopic plasticity governs fatigue at much larger scales. This gives rise to a new picture of fatigue in soft thermal solids and calls for new theoretical descriptions of soft gel mechanics in which local plasticity is taken into account.

To further investigate the role of plastic deformation prior to gel strand failure, in Chapter 3, we analyse whether plasticity is generally present in gel strands of different thickness and length. Our simulations show that rearrangements of particles within the strands leads to plastic lengthening and softening, which may ultimately lead to strand necking and ductile failure. This failure mechanism occurs irrespective of the thickness and length of the strands and the range and strength of the interaction potential. Here, rupture is observed to be more likely for long and thin strands and when the well width of the interaction increases.

In the following chapter, Chapter 4, we shifted our focus from single gel strands to the full network structure of colloidal gels, as mechanics of soft colloidal gels is determined by the network topology of the underlying rigid network. However, it is very challenging to quantifying this heterogeneous structure. In this Chapter, we describe an algorithm that reduces a colloidal gel to a network consisting of nodes and strands, which allows us to map the complete topology of the gel – both in experiments and simulations. Colloidal gels are quantified based on the number and coordination of nodes and the number, length and thickness of the segments. The described method allows for the mapping of the network topology of gels with different morphologies. For experimental and simulated gels, remarkable topological resemblance is shown. The developed topological mapping algorithm opens up a wide range of possibilities to study colloidal network physics in more detail.

In Chapter 5 we strived to apply the algorithm presented in Chapter 4 to colloidal gels under shear. Understanding the microstructural rearrangements in the yielding transition of colloidal gels is highly relevant to understand the initiation of flow and deformation in food products, paints and coatings. Yet, it is still unexplored how the network topology of gels affects or is affected by yielding. Here, we investigate the yielding transition in colloidal gels both in simulations and experiments. We examine different gel topologies, formed at varying interaction energies and volume fractions. Upon increasing strain, the number of segments in the gel rapidly decreases, whereas strands in the network become thicker. Simulations reveal that close to percolation applying shear increases the connectivity of the network. This is due to dangling ends in the network which can form new connections. Networks at higher volume fraction, on the other hand, decrease in connectivity due to the applied deformation. These results show how the gel topology evolves during shear deformation.

In Chapter 6, we study another type of mechanical instability in colloidal gels, namely, the collapse of a colloidal gel due to gravity. Using confocal microscopy, the gravitational collapse of colloidal gels is followed at the single-particle level. After a short delay time, the colloidal gels are observed to detach from the top of the sample chamber and a macro-scopic interface appears. The sedimentation dynamics of this interface are governed by poroelastic compression of the gel that lies beneath. Large-scale collective motions in the direction perpendicular to the gravitational field, induced by an imbalance of internal stresses in the gel, lead to shear deformation in the network that accelerate gel failure. These motions are present both during the induction period and subsequent rapid-settling stage of the gel, and subside as the gel reaches its final height. These results highlight that not only external stresses, but also internal stresses play a crucial role in the failure of these heterogeneous networks.