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Curated by RSF Research Staff

The liquidity of granular materials

It is easy to walk across the sand on a beach. But stepping into a ball pit is right away more difficult. Sand and ball pits are both granular materials. This kind of materials are made of collections of small particles or grains. Their particularity is they can behave like solids or liquids depending on their density and their other physical parameters.

The physics of granular materials is complex and many models have been proposed over the last decades. However, the industry uses a lot these physical properties to process materials using for example “fluidized bed”. These particular beds consist of a fluid-solid mixture and exhibits fluid-like properties. It is considered a heterogeneous mixture of fluid and solid that can be represented by a single bulk density.

Illustration of the different phases of a granular material under an increasing gas velocity injection.

The resulting phenomenon is called fluidization. Fluidized beds are also used for efficient bulk drying of materials. Fluidized bed technology in dryers increases efficiency by allowing for the entire surface of the object to be suspended and therefore exposed to the air. This phenomenon can be quite surprising when experienced!


In a recent work, researchers from Brandeis University studied a broad class of disordered materials including granular materials. These materials are interesting because they can form jammed states. A jammed system can resist small stresses without deforming irreversibly, whereas unjammed systems flow under any applied stresses.

The team showed that jamming of frictional, disk-shaped grains can be induced by the application of shear stress at densities lower than the critical value. These jammed states have a much richer phenomenology than the isotropic jammed states. A minimum shear stress is needed to create robust, shear-jammed states with a strong force network percolating in all directions. Shear strain is sort of like cupping sand between your hands, and then, without changing the width between them, moving one hand forward and the other hand backward. Not much sand flows between your hands with a force like this.

" It's not just the number of particles that put them in a jam, it's also the strain and the real-world forces, like friction, that cause the back-up. […] The discovery could change the design of coal and grain silos and even the bulk dispensers at Whole Foods.”

R. P. Behringer, Duke University, North Carolina

The friction and shear reveal the richness of possible states of granular matter and they give good insight for potential new discoveries. Studying these new states of granular matter may also expose deeper connections between jamming and seemingly unrelated phenomena, from earthquakes to transformations occurring in other kinds of matter, like water to ice.

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