MRSEC Seminars

The MRSEC holds seminars presenting research at the frontier of Bioinspired Soft Materials. The seminars are targeted towards graduate students and other researchers in the field, although everyone is invited to attend. As the topic is highly interdisciplinary, seminars are designed to be accessible to a wide range of backgrounds.

The seminars take place on Thursdays at 4PM on Zoom.

Organizer: John Berezney (Dogic/Fraden Lab Postdoc)

Fall 2020

Shear Thickening in Active Gels and Frictional Colloids

September 24, 2020

Daniel Blair, Georgetown University

Abstract:  In this talk I will discuss our recent results on the microscopic physical origins of shear thickening in two vastly different materials: colloidal suspensions and active gels. In the first part of my talk, I will introduce a method we have developed that allows us to resolve the spatial distribution of stresses in sheared soft-materials, known as Boundary Stress Microscopy. We have applied this technique suspensions undergoing continuous and discontinuous shear thickening. I will present our results on the existence of clearly defined dynamically localized regions of substantially increased stress that appear intermittently at stresses well above the applied stress. Surprisingly, we find that these spatially distinct and dynamic phases account quantitatively for the observed shear thickening seen in sheared colloidal dispersions (e.g. Oobleck). In the second part of my talk I will discuss our results on the rheology of active matter. Our system is composed of microtubules and kinesin motor proteins that self-assemble to form complexes that propel themselves through the fluid. What results is a dramatic increase in the viscosity of the material with applied shear rate. 

Recording of seminar

Nanoparticle, Segmental & Chain Dynamics in Polymer Nanocomposites

October 8, 2020

Karen Winey, University of Pennsylvania

Abstract: Polymer nanocomposites encompass a wide variety of systems comprised of thermoplastics or thermosets and nanoparticles of various shapes and sizes. While a few polymer nanocomposite systems have very well-established applications, the scope of these materials could be expanded further, perhaps even into construction and infrastructure applications that require improved creep performance. This talk will focus on the dynamic properties of polymer nanocomposites wherein there are either neutral or favorable interactions between the nanoparticles and thermoplastics. Using a combination of experimental methods (single particle tracking, ion beam methods, dielectric spectroscopy, modulated DSC, and quasielastic neutron scattering), as well as coarse-grained molecular dynamics simulations, we are developing a comprehensive description of the nanoparticle diffusion and hierarchical polymer dynamics. I will specifically discuss the importance of nanoparticle size and loading, as well as polymer molecular weight and nanoparticle dispersion. Finally, I will present our recent work of the role of nanoparticles on creep properties.

 Recording of seminar

 

Toward controlling the kinetics of colloidal self-assembly

November 12, 2020

Miranda Holmes-Cerfon, New York University

Abstract: Proteins fold reliably and quickly, much more quickly than if they explored their configuration space in equilibrium. Can we design colloids to also self-assemble into a target structure on a desired timescale? I will talk about our efforts to understand the kinetics of colloidal self-assembly and how these can be controlled by varying the interactions between the colloids. First I will derive a theory for calculating transition rates between clusters of colloids, which captures the effects of both short-ranged interactions and hydrodynamics that are critical in our experiments. Then I will illustrate the tradeoff between the thermodynamic stability and kinetic accessibility of a target cluster as one varies the interaction parameters. This tradeoff forms a "Pareto front" for self-assembly that we may compute using a genetic algorithm, and I will show how to use the algorithm to find systems where there is almost no tradeoff. Finally, I will touch upon our attempts to find time-varying, nonequilibrium protocols for the interactions that cause a target state to fold as rapidly as desired. 

RECORDING OF SEMINAR