Roseanna N. Zia: “Phase Mechanics” of arrested colloidal gels: A new paradigm for non-equilibrium phase transitions in soft matter
Plats: Kemicentrum, Lecture hall F
Kontakt: peter [dot] schurtenberger [at] fkem1 [dot] lu [dot] se
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A Physical Chemistry seminar by Assistant Professor Roseanna N. Zia, Chemical Engineering, Stanford University, Stanford, California, U.S.A.
Kinetically arrested phase transitions in complex media exert a remarkable influence on material behavior, yet structure-property relationships remain challenging to model. Colloidal gels and glasses are an important class of such materials. The physico-chemical nature of inter-particle colloidal attractions has permitted the construction of colloidal phase diagrams via molecular theories, where metastable and unstable phase separation closely parallels that in molecular systems. As such, colloids have long been viewed as paradigmatic model systems for molecular phase transitions, but where the vast separation of timescales between colloidal and solvent particles provides a means by which to “slow down” relaxation processes and study phase behavior. However, colloidal gels represent “arrested” states of phase separation, where the same interparticle attractions that promote phase separation also inhibit it, freezing in a non-equilibrium microstructure to form a viscoelastic network. In contrast to attempts to place them on equilibrium phase diagrams, we argue that such gels must exit the equilibrium phase diagram. We show that when interparticle bonds are O(kT), thermal fluctuations enable ongoing particle migration and a (logarithmically) slow march toward full phase separation. Our work reveals the surprising result that gel yield can occur with loss of fewer than 0.1% of particle bonds, with no network rupture; rather, localized re-entrant liquid regions permit yield and flow. Analysis of the evolving osmotic pressure and potential energy reveals the interplay between bond dynamics and external stress that underlies mechanical yield, and provides a compelling connection to stress-activated phase separation. I will show that external fields and forces open a pathway of escape from arrested phases toward equilibrium, and I will propose a ‘non-equilibrium phase diagram as the foundation for “phase mechanics”, a new view of states of arrested colloidal matter.
Roseanna N. Zia is an Assistant Professor of Chemical Engineering at Stanford University. She received her Ph.D. from the California Institute of Technology in Mechanical Engineering in 2011 with Professor John F. Brady, for development of theory in colloidal hydrodynamics and microrheology. Zia subsequently conducted post-doctoral study of colloidal gels at Princeton University, in collaboration with Professor William B. Russel. Zia began her faculty career at Cornell in January 2013, then subsequently moved her research group to Stanford University in 2017.
Dr. Zia’s body of work in colloidal hydrodynamics includes developing micro-continuum theory for structure-property relationships of flowing suspensions, elucidating the mechanistic origins of the colloidal glass transition, and microscopic modeling of reversibly bonded colloidal gels, which resulted in discovery that gel aging is actually ongoing but very slow phase separation and the finding that mechanical yield of colloidal gels is actually a non-equilibrium phase transition, triggered by changes in osmotic pressure. More recently she is developing models of biological cells, examining biological processes orchestrated by colloidal-scale forces.
Dr. Zia’s work has been recognized by several awards, including the NSF BRIGE Award, the NSF CAREER Award, the Publication Award from the Society of Rheology, the Office of Naval Research (ONR) Young Investigator award, the ONR Director of Research Early Career Award, the Engineering Sonny Yau (’72) Teaching Award, and the PECASE Award. In addition, Zia serves as an Associate Editor for the Journal of Rheology, and on the Advisory Board of the journal Physics of Fluids.