A deep dive into the relationship between cohesion and erosion

Landslides are a striking example of erosion. When the bonds that hold soil and rock particles together are overwhelmed with power – often in the form of water – sufficient to pull rock and soil apart, that same power breaks the bonds with other rocks and soils that hold them together. retain. position. Another kind of erosion is to use a small jet of air to remove dust from a surface area. When the force of turbulent air is strong enough to break the bonds that hold individual dust particles, or grains, together and adhere them to the surface, that is also erosion.

In the pharmaceutical industry, the cohesion/erosion dynamic is extremely important to successfully transform powders into drugs. They also play a key role in another, fairly remote example: the landing of a spacecraft on an area, such as the moon. As the spacecraft lowers, the exhaust from its engines causes the granular material to erode and transport to the floor. The displaced material forms a crater, which must be of the correct dimensions too narrow or too deep, and this will cause the spacecraft to tip over.

We often encounter split material composed of small particles – think beach sand, soil, snow and dust – which can be affected by more than just frictional forces, sharing additional cohesive forces with their neighbours. If cohesion only acts between a particle and its immediate neighbours, it also produces macroscopic effects, for example by causing the aggregation of divided pieces of material and adding additional resistance to the composite. Cohesion is what causes powders, like flour, to clump together and allows us to build castles on the beach by adding a small amount of water to dry sand.

Alban Sauret, Associate Professor in the Department of Mechanical Engineering at UC Santa Barbara, has a keen interest in these processes. Published in the journal Actual physical Critique Fluids, his group, including in first year of doctorate. student Ram Sharma and colleagues in France, present new research examining how cohesion between particles can influence the onset of erosion. By using a recently developed system that allows them to control the cohesion between model grains, and then performing experiments in which they used a jet of air to move the grains, they were able to better understand cohesion, which holds particles together. together erosion, which causes them to separate, and transport, which involves the distance traveled by the displaced particles.

The research suggests an approach to quantifying how the magnitude of the cohesion changes the amount of regional stress needed to trigger erosion. This understanding could be used in civil engineering, for example, to measure the strength and stability of the ground in an area where construction is planned. But the researchers also hope their model will provide empirical evidence for a physical theory of erosion that includes cohesion and is relevant to a massive range of applications, from dust removal from solar panels (dust can reduce the creation of energy up to 40%) to land rockets on other planets.

In the presence of external forces, such as wind or water, the cohesion between the particles can be overcome. The onset of erosion refers to the point at which the drag force, exerted by fluid or air, causes the particles to lose get hold of with the granular bed, both separating from each other as than neighboring ones and of the area to which they adhere. This reflects our current fairly basic understanding of erosion: if the local external forces on a particle are greater than the forces holding it together, it erodes – another way of saying it is displaced.

When fluids or air apply greater stresses, such as moving fast enough to become turbulent flows, they can cause greater erosion. An extremely huge range of turbulent flow patterns acting on an equally massive range of materials leads to the erosion that we see, at the macro level, in the form of huge canyons, worn away for eons by turbulent rivers, and gigantic sand dunes, shaped by turbulent air currents. Surprisingly, since erosion drives the sediment cycle and constantly reshapes the Earth’s surface, current understanding of the forces of erosion is not sufficient to explain the rich variety of landforms that result.

Although the erosion of non-cohesive grains can be predicted satisfactorily, the interaction between flows turbulence and erosion in the presence of inter-particle cohesion has not been well studied. But it’s worth investigating, says Sauret, motor vehicle “cohesion is everywhere! — and always have a dirty floor.”

To control the cohesion between the particles, the researchers applied a polymer coating on identical glass spheres (analogous for the particles) d a diameter of 8 millimeters. Coating thickness could be increased or decreased precisely to increase or decrease cohesion. The turbulent flow is modeled by a variable air jet directed towards the granular bed.

The experiments allowed the team to determine a scaling law for the sill at which erosion overcomes interparticle cohesion, regardless of system specifics, such as particle size. By quantifying the relationship between these two forces, the research presents a procedure that can be used to predict the erosion threshold for different grain sizes.

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

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