ChE Seminar – ““Phase Mechanics” of arrested colloidal gels: A new paradigm for non-equilibrium phase transitions in soft matter” by Dr. Roseanna Zia (Stanford University)

Host: Dr. Roger Bonnecaze


Understanding kinetically arrested phase transition in complex media, and its influence on structure-property relationships, has been identified as one of the Grand Challenges for the future of soft matter science. Colloidal gels and glasses are an important class of such materials and are the subject of an emergent field of study in which much focus is placed on predicting yield behavior. More fundamentally, 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 visco-elastic 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, specializing in the theory of colloidal hydrodynamics and suspension mechanics, where she developed a novel non-equilibrium equation of state for colloidal dispersions.  Zia subsequently conducted post-doctoral research in the study of colloidal gels via theory and large-scale dynamic simulation at Princeton University, in collaboration with Professor William B. Russel.  Between undergraduate studies (University of Missouri, BSME) and graduate studies at Caltech she worked as a mechanical engineer in the automotive industry in Detroit, specializing in the design of mechanisms and pyrotechnically actuated devices for occupant restraints; during this time she earned an M-Eng degree at the University of Michigan.

Zia serves as an Associate Editor for the Journal of Rheology, and on the Advisory Board of the journal Physics of Fluids.

Dr. Zia’s work in colloidal systems focuses on the development of predictive theory to elucidate the micro-mechanical underpinnings of macroscopic material behaviors in complex fluids and other soft matter, with a focus on non-equilibrium systems.  Among these are so-called “Grand Challenge” questions, including theory of the glass transition and kinetically arrested states of complex media, uncovering elusive transition states in biochemical reactions, and mapping the mechanical nature of the origin of life. A central hypothesis in Zia’s work is that answers to many of these questions are held in the vast separation between colloidal versus solvent-molecule relaxation time scales in complex fluids, where dynamic phenomena are set by an interplay between comparatively slow colloidal dynamics and the durable but temporary nature of physical interparticle bonds.

Zia began her faculty career at Cornell in January 2013.  That same year she received the NSF BRIGE Award for research in confined suspensions as a model for intracellular transport.  At the end of that year she received the NSF CAREER Award for research on the evolution and collapse of colloidal gels.  Her research on the relationship between fluctuation and dissipation in non-equilibrium complex fluids was selected for the 2013 Publication Award by the Society of Rheology. In April 2014 Zia was selected for the Office of Naval Research Young Investigator award.  Most recently she was selected for the National Academy of Engineering Frontiers in Engineering (NAE-FOE) and symposium, and received the Cornell Engineering Sonny Yau (’72) Teaching Award.  In November 2016 she was awarded the 2017 ONR Director of Research Early Career Award.  She moved her research group to Stanford University in September, 2017.

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