Probing Glassy and Granular Physics in a Model Soft Glassy Material

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

Non-technical Abstract: Soft Glassy Materials (SGMs) contain microscopic particles densely crowded together in a surrounding liquid and examples are found all around us-- soap suds contain crowded air bubbles, mayonnaise is filled with oil droplets, and toothpaste is filled with solid abrasives. SGMs are generally squishy solids that have in common rather unusual mechanical properties whose physical origins are poorly understood, such as the tendency to soften dramatically when they're deformed, followed by extremely slow re-stiffening afterwards. This project will perform an intensive study of an oil in water emulsion engineered to be transparent -- essentially clear mayonnaise-- using state of the art microscopy to make three dimensional movies of individual droplets rearranging during deformation, softening and re-stiffening. The 3-D movies of actual rearrangement events will be compared with corresponding simulations to test the events and their triggers relative to the predictions of currently untested theories. Because of the ubiquitous nature of these materials, this project has the potential to impact numerous practical problems ranging from food and cosmetic processing and formulation to improved strategies for enhanced petroleum recovery. The arresting visual nature of the work will facilitate outreach efforts to high-school students in the Philadelphia public schools. Technical Abstract: The term 'Soft Glassy Material' (SGM) describes an ubiquitous set of materials including concentrated emulsions, foam and pastes that lay at the intersection of two outstanding problems in soft condensed matter physics: the colloidal glass transition and the granular jamming transition. SGMs remain deeply mysterious due the difficulty of experimentally probing the microscopic dynamics in these typically opaque or turbid materials. This project will directly test models for flow and relaxation in SGMs, including the Soft Glassy Rheology model, using three-dimensional confocal imaging of an index- and density-matched emulsion that is fluidized by coarsening. By replicating the observed structures in a simulation environment similar to Bubble Dynamics, multiple deformation 'experiments' on each structure can then be performed to identify and characterize the subtle strain and energy barriers for microscopic droplet rearrangements. A deep understanding in this model system will further understanding in colloidal glasses and granular materials. The rich, visual nature of the data and its association with an everyday material-- soap foam--will facilitate outreach efforts to high-school students in the Philadelphia public schools, while providing graduate and undergraduate students hands-on experience with state of the art three-dimensional microscopy.