February 4, 2023

The polymer coated spherical glass beads used in the experiments. The grains at the top have a very thin coating and no cohesion; those on the underside have a much thicker coating and are cohesive. Credit: UC Santa Barbara

Landslides are a striking example of erosion. When the bonds that hold particles of dirt and rock together are overwhelmed by a force — often in the form of water — sufficient to pull the rock and soil apart, that same force breaks the bonds with other rocks and soil that keep them in place. Another type of erosion is the use of a small jet of air to remove dust from a surface. If the force of the turbulent air is strong enough to break the bonds that hold the 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 extremely important to successfully process powders into medicines. They also play a key role in another, rather distant example: the landing of a spacecraft on a surface, such as the moon. As the spacecraft descends, the exhaust from its engines causes the granular material on the surface to erode and be transported. The displaced material forms a crater, which must be properly sized; too narrow or too deep, and it will cause the spacecraft to tip over.

We often come across divided materials composed of tiny particles — think beach sand, soil, snow and dust — that can be affected by more than just frictional forces, sharing some extra cohesion forces with their neighbors. Although cohesion only works between a particle and its immediate neighbors, it also produces macroscopic effects; for example, causing divided bits of material to aggregate and add extra strength to the composite. Cohesion causes powders, such as flour, to clump and we can make castles on the beach by adding a small amount of water to dry sand.

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Alban Sauret, an associate professor in the Department of Mechanical Engineering at UC Santa Barbara, is very interested in these processes. Published in the magazine Physical Assessment Liquids, his group, which includes freshman Ph.D. student Ram Sharma and colleagues in France present new research examining how cohesion between particles can influence the onset of erosion. Using a recently developed technique that allowed them to check the cohesion between model grains and then conduct experiments where they used a jet of air to move the grains, they gained a better understanding of cohesion, which holds particles together; erosion, causing them to separate; and transport, which means how far the displaced particles then travel.

When particles move

The nozzle (top) creates a flow of turbulent air that disrupts the cohesion between particles (bottom) and the surface, leading to erosion and transport of the particles. Credit: UC Santa Barbara

The study provides an approach to quantify how the magnitude of cohesion changes the amount of local stress required to initiate erosion. For example, this concept could be used in civil engineering to measure the strength and stability of the soil 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 encompasses cohesion and is relevant to a wide variety of applications, from removing dust from solar panels (dust can reduce energy production by as much as 40%) to land rockets on other planets.

In the presence of external forces, such as wind or water, the cohesion between particles can be overcome. The onset of erosion refers to the point at which the resistive force, exerted by liquid or air, causes particles to lose contact with the granular bed, separating them from each other as neighbors and from the surface to which they adhere. This summarizes our fairly basic, current understanding of erosion: if local external forces on a particle are greater than the forces holding it in place, it erodes — another way of saying it’s displaced.

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Because fluids or air exert greater stresses, for example by moving fast enough to become turbulent flows, they can cause greater erosion. An extraordinarily wide range of turbulent flow configurations acting on an equally wide range of materials lead to the macro-level erosion we see in the form of huge canyons, carved for centuries by turbulent rivers, and gigantic sand dunes, formed by turbulent air currents. Surprisingly, the current understanding of erosional forces is not sufficient to explain the rich variety of resulting landforms, as erosion drives the sediment cycle and constantly reshapes the Earth’s surface.

Although erosion of non-cohesive grains can be predicted satisfactorily, the interplay between turbulent flows and erosion in the presence of interparticle cohesion has not been well studied. But it deserves study, Sauret says, because “Cohesion is everywhere! If you’re modeling something as simple as cleaning a surface, for example, and your model doesn’t properly account for cohesion, you’re probably going to get a wrong one. approach – and still have a dirty surface.”

When particles move

Associate professor Alban Sauret and PhD student Ram Sharma in the lab, with the experimental setup behind them. Credit: UC Santa Barbara

To control the cohesion between particles, the researchers applied a polymer coating to identical glass spheres (analogue for particles) with a diameter of 0.8 millimeters. The thickness of the coating can be increased or decreased precisely to increase or decrease cohesion. The turbulent flow is modeled by a variable air jet directed at the grain bed.

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

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

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More information:
Ram Sudhir Sharma et al, Erosion of cohesive grains by an impacting turbulent jet, Physical Assessment Liquids (2022). DOI: 10.1103/PhysRevFluids.7.074303

Provided by University of California – Santa Barbara

Quote: When Particles Move: A Deep Dive into the Relationship Between Cohesion and Erosion (2022, Aug. 3), retrieved Aug. 3, 2022 from https://phys.org/news/2022-08-particles-deep-relationship-cohesion- erosion. html

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