Refreshments will be served in the College of Computing Building hall outside Room 016 at 1:30 p.m. Seminar will be held in the College of Computing Building Room 016 at 2:00 p.m.
Ahmad Omar, Department of Chemical Engineering, California Institute of Technology
“Exploring the Dynamic Landscape of Physical Gels”
Gels assembled by the reversible association of polymers or colloids represent a common and fascinating class of soft materials of wide-spread technological interest. Despite the ubiquity of these materials, bridging the microscale physics of associative networks to their appealing dynamical and mechanical properties remains an outstanding challenge. In this talk, I will describe our use of theory, computation and experiment in formulating a microscopic portrait of the equilibrium and nonequilibrium dynamic landscape of physical gels. This multiscale approach has enabled us to elucidate that quiescent diffusion in associative telechelic polymer networks is controlled by single-polymer (activated) dynamics in the form of chain hopping. Far from equilibrium, the dynamic landscape of physical gels is fundamentally altered. Computer simulations reveal that shearing a polymer gel results in a fast (non-activated) diffusive mode in the form of freely-diffusing mulitchain aggregates. The rapid emergence of this diffusive mode with applied shear destabilizes homogeneous flow for gels sufficiently close to the two-phase boundary resulting in a shear-induced phase separation. These findings motivate the tantalizing hypothesis that applied force may aid in the phase separation/assembly of kinetically frustrated materials. Indeed, doping a colloidal gel with self-propelled particles (which offer a level of precision in material assembly not afforded by shear) is found to drive the system from a state of arrested metastablity to a state of lower free energy. Our work thus lends support to the idea that by carefully tuning the magnitude of internally or externally applied fields, new diffusive modes can emerge that facilitate metastable materials circumventing thermally insurmountable kinetic barriers.
Ahmad Omar earned a B.S. in Chemical Engineering from UT Austin and subsequently moved to Caltech where he is currently a Chemical Engineering Ph.D. candidate working with Professor Zhen-Gang Wang. His research at Caltech focuses on understanding the rheology, dynamics and thermodynamics of complex fluids (including polymer and colloidal gels, topologically constrained polymers, and active materials) by leveraging both computational and theoretical tools while in close collaboration with experiment. Ahmad is a recipient of an NSF Graduate Research Fellowship and an HHMI Gilliam Fellowship for Advanced Study.