During his Penn Grad Talk—a TED Talk-style presentation by Penn Arts & Sciences graduate students—Nakul Deshpande, doctoral student in the Department of Earth and Environmental Science, presents a video showing grains of sand raining down like an avalanche to create a pyramid-like sand pile on a flat surface. Once poured, the sandpile seems ordinary enough, but there’s much more at work here.
“This is more than a sandpile—it's a tool for understanding the physics of how soil moves on hillslopes in nature,” says Deshpande. “It turns out that the earth beneath our feet is not as solid as we think.”
To look at the sandpile after all the grains are added, one would think it’s immobile; nothing happens no matter how hard you squint. It sits on a special vibration-isolating table in a basement, which is about as quiet an environment as you can get, notes Deshpande. However, using lasers and special optical techniques, the experiment reveals that much is happening beneath the seemingly calm surface. Shining a laser light on the sandpile generates an interference pattern that changes as the grains move, allowing measurement and visualization of very tiny, slow grain motions that would otherwise remain undetected. The motions are astoundingly minuscule and are estimated to be as small as a billionth of a meter—a fraction of the width of a human hair.
Pointing to the video, Deshpande says of the sandpile, “You can see little ‘popcorns’ of deformation that blip in and out of existence amidst a sea of motion.” What the audience is witnessing is a type of flow called “creep”, a slow and insidious way that materials deform when weakly stressed beneath their “yield”— a point at which all rigidity is lost. The slow motion of soils over years adds up, as the observation of cracked building foundations or leaning fence posts demonstrates. Under dynamic stresses, such as earthquakes and strong rainstorms, soils in nature can violently transition to landslides, a manifestation of yielding with devastating consequences to infrastructure and communities.
In the U.S. alone, landslide hazards cause more than $1 billion in damage and 20-50 deaths a year. The ability to see creep in the near absence of disturbance is exciting and promising, says Deshpande, and has implications for better predicting these devastating natural disasters: “Rather than thinking of landslides as an ‘on-off’ process where soil spontaneously becomes mobile, these experiments point us to a view where there is a continuous transition between creep and flow."
Creeping motion in an undisturbed sandpile is a surprise, and sharply departs from centuries-old frameworks of soil creep. To contextualize the discovery, Deshpande looked to glass, a material quite different from soil. He explains, “If you cool molten glass very quickly, and look at the molecules as they slow down, their motions become ever more sporadic, disconnected, and localized—even though their positions get locked in place. The glass flows less and less like a liquid and becomes ever more solid-like. Basically, this is what happens with the sand pile, it kind of relaxes as it creeps slower and slower as time passes.”
The experimental results not only make the previously invisible visible, but they also beg a new question: Will the sandpile—or soils in nature for that matter—creep forever, like glass? Deshpande admits to not knowing the answer, but thinks it’s a moot point. Why? Because of the ubiquitous presence of disturbances in nature, which intervene to frustrate intrinsic creep and relaxation of soils. In fact, “environmental noise” may play a role analogous to temperature in conventional glasses.
To explore this idea, Deshpande took the experiment a step further by tickling the sandpile with two types of weak disturbances: heat and gentle vibrations. Discussing results of an experiment inspired by the desolate surface of a distant asteroid—an environment even more isolated than a laboratory basement—he reports, “The sand pile responds by breathing in lock-step with the temperature cycles and creep speeds ever so slightly. Even in the deep recesses of space, grains will feel some heat from the sun and undergo slight thermal expansion, which tells us that creep will continue. The second thing that it tells us is that the sand pile is exquisitely sensitive to mechanical disturbances, and in this way, is incredibly fragile.”
Thinking about the patterns of the sand pile’s motion and connecting it to existing and emerging work on glasses will help researchers develop new diagnostics to better predict landslides, and to understand how landscapes evolve over geologic time.
The novel findings of this experiment are featured in the 2021 Nature Communications journal article, “The Perpetual Fragility of Creeping Hillslopes.” The article is co-authored with Penn professors Doug Jerolmack of Earth and Environmental Science, and Mechanical Engineering and Applied Mechanics; and Paulo Arratia of Chemical and Biomolecular Engineering, and Mechanical Engineering and Applied Mechanics, as well as David Furbish of Vanderbilt University.
Deshpande will continue his research identifying the forces that give rise to creep as a post-doc at North Carolina State University. Reflecting on what’s been unearthed thus far, he says, “This research shows that there’s a richness in the behavior of the seemingly mundane in nature, things lurking beneath our ability to readily perceive. When considering the ground beneath our feet, we must take great care and sensitivity. We are, after all, walking on glass.”
Click here to watch Deshpande's 2022 Penn Grad Talk video, "The Perpetual Fragility of Creeping Hillslopes."