Name: Douglas J. JerolmackTitle: The Fragile Earth

It is all too clear that Earth is fragile, as human disturbances disrupt the natural cycles that sustain a habitable planet. Earth is also fragile matter: apparently solid soil can suddenly lose rigidity when stressed by rainfall or earthquake shaking; and earth materials exhibit creep, aging and hysteresis. In this talk I consider some existential challenges of living sustainably on a fragile (matter) planet, and advocate for the need of Soft Matter Physics to address these challenges.

To first order, we may consider Soft Earth materials -- and their associated geophysical flows -- to be composed of three ingredients: frictional particles (sand/silt), cohesive particles (clay, fine silt, organic material), and water. In this talk I demonstrate how the origins of complex behaviors of Earth materials arise from these ingredients, with surprises for both geologists and physicists. Experiments show that creep hardens hillslope soils and river beds, that landscapes `breathe' in response to thermal forcing, and that the structure and dynamics of granular creep are similar to glass. Current granular frameworks, such as jamming, cannot account for these behaviors. I then examine the surprising explanatory power that idealized models of yielding in amorphous solids have for predicting failure and flow behavior of complex Soft Earth materials. Finally, I show how nature assembles robust, hierarchical aggregates from evaporation of polydisperse suspensions, and speculate on how we may harness this process to mitigate soil erosion and create sustainable geomaterials for construction. When possible, I illustrate how concepts and experimental techniques can be taken out of the lab and into the field.