Granular materials like sand, coffee grounds, and rice don’t act like water or oil, but will suddenly exhibit astonishing similarities under certain conditions, according to new research.
The behavior of granular substances plays a key role in many natural processes, such as avalanches and the motion of sand dunes. This behavior is also important in the manufacturing of pharmaceuticals and foods, when it’s 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 describe their behavior. This is particularly true for unstable, complex mixtures, such as emulsions, which have structures that quickly rearrange themselves.
Researchers have discovered that under certain circumstances, mixtures made of granular materials exhibit striking similarities to mixtures of immiscible liquids and researchers can even describe them with 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 “fluidized” 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.
If researchers, for example, place a layer of heavy sand on top of lighter sand, fluidization 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 the group of Christoph Müller, professor of energy science and engineering at ETH Zurich. “A complex interaction occurs between the two materials.”
If researchers embed a small quantity of light sand 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.
“Our findings are significant for several applications,” says Alexander Penn, a postdoctoral researcher 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 interest geologists, and potentially help 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 analyze 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.”
Additional researchers from Columbia University contributed to the work.
Source: Felix Würsten for ETH Zurich