Dynamic and Microstructural Response of Soft Colloidal Materials to Constant Stress Deformation
Author | : Hubert Kenneth Chan |
Publisher | : |
Total Pages | : 129 |
Release | : 2014 |
ISBN-10 | : 1303717239 |
ISBN-13 | : 9781303717239 |
Rating | : 4/5 (39 Downloads) |
Book excerpt: Under low stress, soft disordered materials such as colloidal gels and glasses are kinetically-jammed, solid-like materials; as stress is increased, a transition to a liquid-like phase capable of steady flow is expected. This ability makes these materials attractive for personal care products, paints, foods, and other applications that require a material to exhibit distinct deformation behavior at different steps in its manufacturing or consumer use. However, the origins and implications of the solid-liquid transition are still often unclear, and insight into the conditions and mechanisms that govern this intricate rheology is imperative for the improvement and extension of numerous applications. In this dissertation, viscoelastic colloidal systems arrested through different routes are experimentally probed to uncover the determinants of their complex deformation behavior near the jamming transition. Results include the discovery that dilute, depletion-induced colloidal gels can yield via a two-step process, with proposed origins associated with interparticle bond rotation followed by bond rupture. Possible implications include an apparent anisotropic strengthening of the gel in response to applications of constant stress between the two yield points. Recognizing that complex rheological phenomena such as these can never be fully understood without insight into the microstructural and dynamic events that accompany them, a custom-built shear cell coupled to a confocal microscope is introduced for the direct visualization of constant stress deformation of soft materials. Included among its features is the ability to conduct measurements with dramatically reduced "creep ringing" than typically observed with commercial rheometers, attributable to its low moment of inertia and its unique torque generation mechanism. For the primary study in this dissertation, the shear cell is utilized to investigate the creep and flow behavior of repulsive and attractive colloidal glasses. With increasing stress, topological caging effects diminish, and dynamics become more spatially and temporally homogeneous. For creep deformation, each relaxation event occurs intermittently and involves many neighboring particles. During flow induced by large stresses, rearrangements are more frequent and require fewer cooperative particles. If interparticle attraction is introduced, significant displacements are prevented while interparticle bonds remain intact, and rearrangements require more cooperativity.