When Particles Move

Landslides are a striking example of erosion. When the bonds holding soil and rock particles together are overcome by a force — often in the form of water — sufficient to pull the rock and soil apart, that same force breaks the bonds with the other rock and soil holding them in place. Another type of erosion involves the use of small air jets to remove dust from the surface. When the turbulent forces of air are strong enough to break the bonds that hold individual dust particles, or grains, together and cause them to stick to the surface, that’s also erosion.

In the pharmaceutical industry, cohesion/erosion dynamics are critical for successfully processing powders to make pharmaceuticals. They also play a key role in another, somewhat distant example: landing spacecraft on the surface, such as the moon. As the spacecraft descends, its engine exhaust causes granular material on the surface to be eroded and transported. The displaced material forms a crater, which must be the correct size; too narrow or too deep, and it will cause the spacecraft to capsize.

We often find divided materials made up of tiny particles — think sand on beaches, soil, snow, and dust — that can be affected by more than just frictional forces, sharing some additional cohesive forces with their neighbors. While cohesion only works between a particle and its nearest neighbour, it also produces macroscopic effects; for example, causes pieces of material to split into aggregates and add additional strength to the composite. It is this cohesion that causes powders, such as flour, to clump together and allows us to build castles on the beach by adding a little water to dry sand.

Alban Sauret, a professor in the UC Santa Barbara Department of Mechanical Engineering, is particularly interested in this process. Published in the journal Physical Review Fluids, his group, including the first year Ph.D. student Ram Sharma and colleagues in France, present new research examining how cohesion between particles can affect the onset of erosion. Using a recently developed technique that allowed them to control the cohesion between the model grains and then running an experiment in which they used a jet of air to replace the grain, they were able to gain a better understanding of cohesion, which holds particles together; erosion, which causes them to separate; and transport, which involves how far the displaced particles then travel.

This study offers an approach to measuring how the magnitude of cohesion changes the amount of local stress required to initiate erosion. This understanding can be used in civil engineering, say, to measure the strength and stability of the soil in the area where construction is planned. But the researchers also hope that their model will provide empirical evidence for a physical theory of erosion that is cohesive and relevant to a wide range of applications, from removing dust from solar panels (dust can reduce energy production by as much as 40%) to landing rockets on other planets.

In the presence of external forces, such as from wind or water, cohesion between particles can be overcome. The onset of erosion refers to the point at which drag, exerted by liquid or air, causes the particles to lose contact with the granular layer, becoming separated from each other as neighbors and from the surfaces to which they adhere. This captures our fairly basic and current understanding of erosion: if the local external force on a particle is greater than the force holding it back, it will erode — another way of saying that it is displaced.

Because liquids or air apply greater pressure, such as by moving fast enough to become a turbulent flow, they can cause greater erosion. A very wide range of configurations of turbulent flows acting on an equally wide range of material causes the erosion we see, at the macro level, in the form of enormous canyons, eroded over thousands of years by turbulent rivers, and giant dunes, shaped by air currents. turbulent one. Surprisingly, given that erosion drives sedimentary cycles and is constantly reshaping the Earth’s surface, current understanding of erosional forces is insufficient to explain the resulting diversity of landforms.

While non-cohesive grain erosion can be predicted satisfactorily, the interaction between turbulent flow and erosion in the presence of cohesion between particles has not been well studied. But it’s worth learning, says Sauret, because “Cohesion is everywhere! If you’re modeling something as simple as how to clean a surface, for example, and your model doesn’t properly account for cohesion, you’re more likely to take the wrong approach — and still have a dirty surface.”

To control the cohesion between the particles, the researchers applied a polymer coating to identical glass spheres (analogous to particles) with a diameter of 0.8 millimeters. The layer thickness can be increased or decreased appropriately to increase or decrease cohesion. Turbulent flow is modeled by a variable jet of air aimed at the granular bottom.

The experiments allowed the team to determine the law of scale for the threshold at which erosion overcomes interparticle cohesion, regardless of system specifications, such as particle size. By measuring the relationship between these two forces, this study presents a technique that can be used to predict erosion thresholds for different grain sizes.

The results of this study, Sauret said, could be applied directly to the process of removing cohesive sediments, such as dust and snow, from surfaces such as solar panels.

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