Dry sand can flow like a liquid, a new study finds. All it takes are the right conditions. When heavy grains are placed atop lighter ones in a container, the lighter grains can burble upward like the blobs in a lava lamp.
Scientists had known that wet grains can act like liquids. Strong earthquakes, for instance, can “liquefy” solid sand that is soaked with water. Less well-known is how to mix grainy materials without water. Indeed, this research is the first to show how mixing dry sand grains can mimic fluids.
Understanding such flows could help scientists better explain the behavior of mudslides or explosive flows from volcanoes. The research could also help create uniform mixtures of particles for use in industries that make drugs, chemicals or foods. For instance, dry, grainy particles of different sizes and densities tend to move around one another. This causes them to separate out — or “demix” — says Richard Lueptow. He’s a mechanical engineer at Northwestern University in Evanston, Ill., who was not involved in the new study.
Researchers reported their new findings online April 22 in the Proceedings of the National Academy of Sciences.
Mix it up
Christopher McLaren is a mechanical engineer at ETH Zurich in Switzerland. His team placed two types of round grains in a thin glass chamber. A layer of heavy grains sat atop a layer of lighter ones. To get the grains moving, the team vibrated the chamber. They also sent gas upward through it.
The grains “flowed” around one another like liquids in a lava lamp would. Bubbles and fingerlike streams of lightweight grains rose to the top. Heavy grains flowed lower in the container.
Patterns like the ones seen in lava lamps form as a lighter fluid pushes its way into a denser fluid. This results from what physicists call the Rayleigh-Taylor instability. The new research shows for the first time that dry grains can mimic that instability.
That was unexpected. This instability is related to the fact that the fluids can’t mix together. But that’s not true for solid grains. They can mix fully — and yet didn’t.
“We realized there was a completely different realm of physical mechanisms [here],” says Christopher Boyce. An author of the new study, he works as a chemical engineer at Columbia University in New York City.
His team used a computer model to simulate how the grains would mix. It showed that the gas flow and vibrations controlled how the grains moved. Gas flowing through the chamber traveled differently between grains of different sizes and densities. That created upward drag. This force kept the lighter grains clumped together. Drag also stopped the clumps from mixing with heavy grains. It is somewhat like how oil and water might behave.
When the team changed the rate of gas flow or the vibration level, the grains formed larger fingers or bigger blobs. This showed that both the gas and the vibrations were crucial for the grains to mimic patterns of fluid instability.
A different pattern emerged when researchers placed a blob of heavy grains within a layer of lighter ones. The grains behaved more like fluids that can mix — such as red dye in water.
Heavy grains split into two blobs that fell at an angle through the layer of lighter grains. The blobs split again and again in a branching pattern. That pattern also resulted from the flow of the gas up through the grains.
Past studies that used vibrations only had shown grains of different sizes can move around each other in fluidlike ways, says Paul Umbanhowar. He is a mechanical engineer, also at Northwestern University.
Those moves resulted from small grains slipping down between larger ones during the shaking. Bigger grains ended up on top. It’s much the way Brazil nuts will end up at the top of a shaken can of mixed nuts. In physics, this phenomenon is actually called the Brazil nut effect.
By adding gas flow, Umbanhowar concludes, the new results now are much more similar to the behavior of normal fluids.
angle The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.
behavior The way something, often a person or other organism, acts towards others, or conducts itself.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemical engineer A researcher who uses chemistry to solve problems related to the production of food, fuel, medicines and many other products.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
drag A slowing force exerted by air or other fluid surrounding a moving object.
earthquake A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
fluid dynamics The study of liquids and gases in motion.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
liquefy (in geology) A term for the movement of soil particles during an earthquake that prevents them from holding firm and serving as a solid foundation for buildings, roads, bridge footings and other structures.
mechanical engineer Someone trained in a research field that uses physics to study motion and the properties of materials to design, build and/or test devices.
mechanism The steps or process by which something happens or “works.” It may be the spring that pops something from one hole into another. It could be the squeezing of the heart muscle that pumps blood throughout the body. It could be the friction (with the road and air) that slows down the speed of a coasting car. Researchers often look for the mechanism behind actions and reactions to understand how something functions.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
physicist A scientist who studies the nature and properties of matter and energy.
Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.
simulate (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.
Journal: C.P. McLaren et al. Gravitational instabilities in binary granular materials. Proceedings of the National Academy of Sciences. Published online April 22, 2019. doi:10.1073/pnas.1820820116.