Jeff Urbach

ABSTRACT

We usually describe the properties of materials by categorizing them as solid, liquids, or gases, but lots of stuff doesn’t fit neatly in into any one category. Toothpaste, yogurt, peanut butter, and shaving cream are familiar examples of materials that can hold their shape like a solid, but flow like a liquid when you push on them. Shear thickening fluids, such as the mixture of cornstarch and water known as Oobleck, do the opposite: flow freely when you don’t push too hard, but can thicken dramatically when subjected to large enough forces. Why does this happen, can we accurately model it, and is it something we can control to make designer materials that will thicken when we want them to, for example to absorb impacts in a shock absorber or a bullet-proof vest?

In this talk I will introduce the ways scientists and engineers measure the properties of shear thickening fluids and describe the progress that has been made in the past decade to understand the microscopic origins of the thickening transition. At Georgetown we have developed tools that allow us to visualize the dynamics of particles, fluid, and stresses with high spatial and temporal resolution, and have found that there is a surprising complexity to the shear thickening process, even under very controlled conditions. I will describe what we have learned and the challenges that remain for developing an accurate, predictive model for shear thickening fluids.

This work is part of a broader effort in soft matter research, an interdisciplinary field of materials science focused on the properties of materials that lie between solids and liquids. I will briefly describe some other soft matter research at Georgetown and discuss pathways for students to get involved in research, including the Research Experiences for Undergraduates program sponsored the National Science Foundation