Levich Institute Seminar Announcement, 09/06/2011
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Professor Michael Solomon
University of Michigan
Department of Chemical Engineering
"Rupture and Microstructural Evolution of Colloidal Gels in Shear Flow"
Colloidal gelation results in solid-like rheology due to the effect of short-range attractive interactions on the suspension's collective dynamics and microstructure. Gelation is important for materials applications because it results in open, space filling structures that have useful mechanical properties such as a finite yield stress. Colloidal gels are also one of the few ways to induce weak (~ 1 Pa) elasticity into complex fluid formulations. In all these cases, the local Brownian dynamics of the gel network is the principle determinant of the suspension's mechanics and rheology. Non-linear mechanical properties such as the yield stress are likewise a consequence of gel microstructural deformation and restructuring. Gelation at intermediate particle loadings - between dilute, fractal cluster gels and attractive glasses - is an important range to connect microstructure, colloid dynamics and rheology both because of its implications for industry and because of the regime's structural diversity and its proximity to phase boundaries. We use the model system of charged, sterically stabilized poly(methyl methacrylate) spheres (diameter ~ 1 um) to study the microstructure and shear-induced dynamics of such gels. Short-range attractions are induced by addition of a non-adsorbing polymer. 3D direct visualization by confocal microscopy is used to characterize the microstructure of the gels. We impose simple shear flows of various strains on the gel and observe the 3D structural change after deformation. We accomplish this by using a UV light-triggered photopolymer, which allows particle configurations to be locked in place rapidly (<0.6s) after yielding. We characterize the transition from a dense network to free clusters of particles as a function of the applied strain, and we quantify this local and global structural change using the measures of contact number distribution, cluster size distribution, and number density fluctuations. We evaluate hypotheses for gel yielding from the literature in light these microstructural measurements of shear-induced gel structure.
BRIEF ACADEMIC/EMPLOYMENT HISTORY
Mike Solomon is Professor of Chemical Engineering at the University of Michigan. He received his Ph.D. at the University of California at Berkeley in 1996. After a post-doctoral appointment at the University of Melbourne, Australia, he joined the faculty at the University of Michigan.
RECENT RESEARCH INTERESTS:
Research interests are in colloidal assembly, suspension microdynamics, stability, and rheology, as well as the biomechanics of bacterial biofilms.