Abstract:  Understanding yielding - the transition between solid-like and fluid-like behavior in soft, gel-like materials - is of significant importance to both our daily lives (foods, pharmaceuticals, consumer products, etc.) as well as emerging applications such as 3D printing and soft robotics. This is particularly true of colloidal gels, which are often used to build viscoelasticity in the paints, inks, and creams used for these purposes. Although “simple” gels exhibit idealized yielding characterized by a single, time-independent yield stress, many concentrated colloidal gels exhibit a broader, rate-dependent transition that reveals yielding in these materials to rely on structural dynamics that are not as of yet understood.

In this talk, I will present our efforts to determine the microstructural processes underlying yielding in concentrated colloidal gels. Using a recently developed material system involving thermoresponsive nanoemulsions, we are able to study pristine gels that lack the history-dependent artifacts that can be present in other model systems. Large-amplitude oscillatory shear (LAOS) is a particularly useful flow for making time-stationary, rate-dependent rheological measurements, and reveals a wealth of information about the nonlinear mechanical response of the material, including elastic, plastic and viscous contributions to the yielding process. Coupling these measurements with unique in situ microstructural characterization by small and ultra-small angle neutron scattering (rheo-SANS/USANS) gives direct access to the structure of the fluid ranging from 10 nm to many microns under shear. I will show how these measurements reveal that structural heterogeneity and processes at the mesoscale (corresponding to thousands of primary particles) play a critical role in yielding, providing a foundation for understanding the complex nonlinear mechanical response during yielding in many colloidal gels.

Bio:  Matt Helgeson is an Assistant Professor of Chemical Engineering at UC Santa Barbara. His research involves the design and processing of complex fluids involving colloids in self-assembling fluids for soft material design. Prof. Helgeson earned a B.S. in Chemical Engineering from Carnegie Mellon University in 2004 with a minor in colloids, polymers and surfaces. He received his Ph.D. in Chemical Engineering from the University of Delaware in 2009 under the supervision of Norman Wagner and Eric Kaler. After receiving his doctorate, he performed postdoctoral research in the Novartis-MIT Center for Continuous Manufacturing under the supervision of Patrick Doyle. His honors include the American Chemical Society Victor K. LaMer Award, the Neutron Scattering Society of America Prize for Outstanding Student Research, and the inaugural Distinguished Young Rheologist Award from TA Instruments. Most recently, Prof. Helgeson received an NSF Early Career Award for his work involving the development of self-assembling nanoemulsions for material design.