CAREER: Shear-induced Deformation and Nonlinear Rheology of Colloidal Gels

University Of California-Irvine, Irvine CA

Investigators

Abstract

0955241 Mohraz Colloidal gels are particulate materials with constrained dynamics and remarkable viscoelasticity, and enjoy a diverse host of technological applications including ceramics, catalyst supports, membranes, food products, and composites. The quiescent microstructure and dynamics, linear viscoelasticity, and steady shear rheology of these materials have received considerable attention, both theoretically and experimentally. However, a quantitative picture of their microstructural evolution during transient large strain deformations that are typical of their processing conditions, and how it governs their nonlinear rheology, has remained elusive. The proposed CAREER program will use a combined experimental/computational scheme to address how the nature of interparticle interactions and the local structure of colloidal gels mediate their nonlinear rheology and transient deformation under well defined macroscopic flow fields. The model materials will be suspensions of fluorescent poly(methylmethacrylate) microspheres suspended in density and index matching organic solvents, in which gelation will be induced by the addition of a non adsorbing polymer. With this model system, the strength and range of attractive interparticle interactions can be controlled and systematically varied. The microstructural response of weakly aggregating colloidal gels to macroscopically imposed nonlinear deformation will be directly resolved in 3D, and with single particle resolution, by quantitative confocal laser scanning microscopy (QCLSM). Local phenomena, such as pair wise relative angular displacements, which underpin the suggested role of singly connected junctions as soft pivot points, and the microstructural origins of nonlinear rupture and stress overshoot in colloidal gels, will be directly resolved for the first time. The microstructural evolution as resolved by QCLSM will be used as input into a computational model to quantify the thermodynamic contribution from the interparticle bonds to the stress tensor, and the model predictions will be compared to experimental measurements of the transient shear stress. This innovative scheme will exploit the unique power of QCLSM in resolving the microstructure with single particle resolution, to develop a better understanding of gel nonlinear rheology, and specifically, its thermodynamic determinants. The roles of interparticle interaction geometry and local microstructure will be assessed through systematic experiments with anisometric colloids and dense systems with rigid local subunits, respectively. The techniques and findings from the proposed research activities will be directly incorporated into a graduate level course in Colloid Science and Engineering. The CAREER program will also train graduate and undergraduate students in state of the art methods of quantitative confocal microscopy, colloid synthesis, data driven modeling, and complex fluids rheology. Moreover, it will initiate a summer outreach program, aiming to instill interest in science and engineering careers in high school students from populations that are traditionally underrepresented in these disciplines, and provide the undergraduate and graduate students with a mentoring opportunity. Finally, the outreach activities and modules will be disseminated worldwide via the PI's research website.