Dynamics of low-density gels

Complex fluids exhibiting yield stress, thixotropy, and elastoplasticity have gained much attention in practical applications. Among them, low-density gels, such as Laponite and fibril cellulose (FCs) materials, due to their fantastic time-dependent rheological features and unique properties arising from their lightweight and highly porous nature are advantageous in a wide range of industries. More specifically, cellulose low-density gels, such as TEMPO-CNFs, exhibit a large surface area and an interconnected pore network, excellent mechanical properties, and biocompatibility features that make them promising materials for diverse sustainable environment-friendly applications in areas such as biomedicine, energy-harvesting pieces of equipment, filtration, and packaging. In this regard, this study highlights the settling dynamics of solid objects into low-density gels and their complex rheological behavior, emphasizing the importance of microstructure, and aging in understanding their flow characteristics. First of all, motivated by the already reported results on the semi-oscillatory settling velocity of tiny steel spheres in Laponite gel, this work started with a one-point simulation to understand such a non-linear solid-fluid interaction. The research study then proceeded with experimental observations on the rheological behavior of TEMPO-CNFs and their response flow behavior to free-falling solid spheres. Consequently, rheological measurements exhibit shear-thinning, thixotropic, yield stress, and elastic features for TEMPO-CNFs where both yield stress and storage modulus vary with the state of the microstructure. Furthermore, Stoke's experiments indicate continuously decreasing settling velocity and irregular semi-oscillatory regimes, comparable to the results in Laponite gels. A Computational Fluid Dynamics (CFD) simulation is then carried out to investigate the structural dynamics of a locally heterogeneous microstructure in response to gravity-induced stress. As a result, the sphere can move inside the fluid when its gravity-induced stress is above the local yield stress of the fluid. Then, the irregular regime is justified to occur when the settling time of the sphere is long enough to capture the associated local non-homogeneity of the microstructure. In addition, the continuously decreasing settling velocity regime is due to a continuous shear-independent restructuring away from the settling object. In contrast, a continuously increasing settling velocity regime is also observed due to further shear-dependent destructuring in front of the settling object. Ultimately, settling regimes are explained based on the rates of forming and breaking the microstructure. Interestingly, the same dynamics with different time scales are associated with linear and non-linear regions of the flow curve, along with, that related time scales vary with the degree of structuring of the fluid. Finally, the geometry of the fluid flow field around the moving sphere in terms of symmetry breaking is characterized.