Jun 26, 2019 | Updated: 09:24 AM EDT

When Sand Behaves Like Oil

May 08, 2019 03:09 PM EDT

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The Similarity in Behavior of Sand and Oil
(Photo : ETH Zurich / Alexander Penn)

Parts of the examples of granular materials are rice, sand, and coffee. The conduct of granular substances plays a vital function in various natural processes, such as avalanches and the motion of sand dunes, but they are also essential in the industry. In the manufacture of pharmaceuticals or foods, it is crucial to process granular materials as efficiently as possible.

The physical laws that govern how granular materials behave are only partly understood, despite the variety of practical applications. In the case of liquids, the opposite is right in the case: several well-established physical laws and mathematical instruments are used to describe their behavior. This analysis is particularly true for unstable, complex mixtures such as emulsions which have structures that quickly rearrange themselves.

A new order began with the researchers from the group led by Christopher Muller, Professor of Energy Science and Engineering at ETH Zurich, in collaboration with scientists at Columbia University in New York have found out that under some conditions, mixtures made of granular materials exhibit striking similarities to mixtures of immiscible liquids and can even be described by similar physical laws.

The team placed heavy and light grains in different configurations in a tight container for them to carry out their experiments which they vibrated while simultaneously passing air through it from below - these two procedures "fluidized" the grains whereby they started to behave similarly to liquids. Then, from the outside, the researchers observed how the materials in the container rearranged over time.

The instance of placing a layer of heavy sand on top of lighter sand, fluidization will cause the lighter grains to migrate upwards because of their lower density and form globule-like structures much like viscous liquids. A doctoral student in Muller's group explained that the grains behave similarly to oil in water would. A complex interaction occurs between the two materials.

If they embed a small quantity of light sand in heavy sand, the light sand will more or less move upwards in small globules. In heavy sand, however, a more complex pattern emerges: a ball of heavy grains, surrounded by light grains, will not merely sink to the bottom intact. Instead, it will gradually disintegrate into several small globules, and the material will continue to branch out as time passes.

A postdoctoral involved in the experiments, Alexander Penn said that their discoveries are significant for several applications. In the event of a pharmaceutical manufacturer aspires to create a very homogeneous powder mixture, it has to understand the physics of these materials in detail so that it can control the process. Also, the findings are expected to be of interest to geologists, helping them to understand better the processes involved in landslides or how sandy soils behave during earthquakes.

Ultimately, the research will also be relevant to the current energy debate.

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