When sand behaves like oil

Sand, rice and coffee are all examples of granular materials. The behaviour of granular substances plays a key role in many natural processes, such as avalanches and the motion of sand dunes, but they are also important in industry. In the manufacture of pharmaceuticals or foods, it is important to process granular materials as efficiently as possible.

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

A new order

Researchers from the group led by Christoph Müller, Professor of Energy Science and Engineering at ETH Zurich, in collaboration with scientists at Columbia University in New York, have discovered that under certain circumstances, mixtures made of granular materials exhibit striking similarities to mixtures of immiscible liquids and can even be described by similar physical laws.

To carry out their experiments, the researchers placed heavy and light grains in different configurations in a narrow container, which they vibrated while simultaneously passing air through it from below. These two processes "fluidised" the grains, so that they began to behave similarly to liquids. From the outside, the researchers then observed how the materials in the container rearranged over time.

Contrasting structures

If, for example, a layer of heavy sand is placed on top of lighter sand, fluidisation will cause the lighter grains to migrate upwards due to their lower density and form globule-like structures much like viscous liquids. "The grains actually behave similar as oil in water would," explains Christopher McLaren, a doctoral student in Müller's group. "A complex interaction occurs between the two materials."

If a small quantity of light sand is embedded in heavy sand, the light sand will more or less move upwards in compact globules. However, in heavy sand, a more complex pattern emerges: a ball of heavy grains, surrounded by light grains, will not simply sink to the bottom intact. Rather, it will gradually disintegrate into several smaller globules, and the material will continue to branch out as time passes.

Diverse applications

"Our findings are significant for several applications," says Alexander Penn, a postdoc involved in the experiments. "If, for example, a pharmaceuticals manufacturer wants to produce a very homogeneous powder mixture, it has to understand the physics of these materials in detail, so that it can control the process." The findings are also likely to be of interest to geologists, helping them to better understand the processes involved in landslides or how sandy soils behave during earthquakes.

Moreover, the work will also be relevant to the current energy debate. "If you analyse industrial processes, you can see that a significant share of the needed energy is used to process granular materials," explains Penn. "If we know how to better control granular materials, we can develop more energy-efficient manufacturing processes."

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