A fundamental investigation of the aging, dynamics and rheology of thermo-sensitive colloidal dispersions near to the glass and jamming transitions is proposed. The investigation brings the PI's expertise and experience in glassy physics, aging and rheology to bear in a novel and transformative approach to the understanding of colloidal glasses. While the relevant behaviors have been extensively studied in experiments in which the suspension has been subjected to shearing deformations at a fixed concentration, there are very few works in which the response is followed after rapid, constant volume concentration changes. The advent of thermosensitive particles, such as the poly(n isopropyl acrylamide) (PNIPAAM) systems that swell and deswell in water with temperature variation have changed this situation and offer the promise of an experimental route to investigate the glass-like behavior, and specifically, the structural recovery based behavior of colloidal dispersions near to the glass or jamming transition. In addition to the thermo-sensitivity of these particles they are also barosensitive, i.e., application of pressure can change the particle swelling, hence isotropic stresses (pressure changes) impose concentration changes. The proposed research offers a transformational outcome as it promises to validate or invalidate a frequent underlying hypothesis in colloid physics that the colloidal glass is a model system for molecular glasses. Furthermore, independent of the specific outcome of the hypothesis validation, highly novel experimental results will be obtained that are relevant to the behavior of colloidal systems in the range of concentrations that are related to glass-like or jamming behaviors. It is specifically proposed to perform the temperature- and pressure-jump experiments using responsive PNIPAAM and PS/PNIPAAM core/shell particles to cross the concentration phase boundary (liquid-to- glass) along a two different paths (T and P) using diffusing wave light scattering to probe the aging dynamics of the non-equilibrium colloids. These experiments will provide the first extensive cataloguing of the structural recovery behavior of colloidal glasses in the Kovacs catalogued signatures of structural recovery, viz., intrinsic isotherm, asymmetry of approach, and memory effect. The results will provide insights to effects of particle "hardness" or "softness?"and colloidal glass fragility on the aging dynamics. Classical physical aging experiments using torsional rheometry will also be performed in the thermosensitive response regime following the methods originally proposed by Struik for molecular glasses. While there is much work in the literature that addresses the dynamics of concentrated colloidal systems in the context of the equilibrium dynamics and their divergence with concentration, the present work is transformative in that it applies concepts related to the non equilibrium behavior of molecular glasses explicitly in concentration-jumps induced by the thermo- and baro response properties of the PNIPAAM-based colloids. Thus, new understanding of the dynamics of colloidal systems will be created.
The project will provide research training and experience to students at all levels in the methods of characterizing and synthesis of colloids and the physics of glassy colloids. The PI and his graduate students will also be involved in outreach activities of middle-school girls.
