Seminar - SCM

Ideal solids obey Hooke's law, while ideal fluids follow Newton's law of viscosity. Non-Newtonian fluids, such as colloidal clay suspensions and cornstarch suspensions, exhibit complex viscoelastic behavior. This thesis investigates various non-equilibrium phenomena in complex fluids, for example, drying-induced crack formation, interfacial instabilities during forced displacement in confined geometries, colloidal adsorption on curved surfaces and evaporative self-assembly at the air-water interface. We first examine the onset of desiccation cracks in drying Laponite clay suspensions using digital imaging and microindentation using an atomic force microscope. Our findings reveal that physical aging of the clay network accelerates sample consolidation and crack formation. We propose a model incorporating poroelasticity and the Griffith’s fracture criterion, which relates the crack onset time to sample elasticity and fracture energy, to explain our experimental data. Next, we study the adsorption of Laponite nanoplatelets on like-charged latex microspheres using optical tweezer-based single-colloid electrophoresis. In addition to non-electrostatic dispersion forces, we identify an electrostatically driven adsorption mechanism when the microsphere is trapped in an aging Laponite gel. These results are validated by cryogenic field emission scanning electron microscopy. We also explore the evaporative self-assembly of amphiphilic PNIPAM microgels at the air-water interface and observe a transition from ordered clusters to loosely packed hexagonal assemblies with increasing microgel concentration. Increasing particle stiffness leads to the formation of chain-like clusters, which evolve into two-dimensional gel-like networks. We rationalize these observations by considering the interplay between short-range capillary attraction and long-range repulsions. We also investigate interfacial instabilities during the displacement of aqueous Laponite suspensions by water in a Hele-Shaw geometry. By incorporating dissociative and non-dissociative additives, we isolate three distinct instability regimes governed by the elasticity and shear-thinning behavior of the suspension. We note skewering, dense branching and snake-like propagation of the fingers in some of our experiments. In a separate study on the displacement of discontinuous shear-thickening cornstarch suspensions in a Hele-Shaw cell, we report the emergence of ‘reverse fingering’ instabilities at the outer air-suspension interface. We attribute this phenomenon to dilation-induced stress transmission, which leads to the unexpected penetration of fingers of air into the suspension.