The mechanics of moving small particles are part of many natural phenomena, from landslides to volcanoes, and many industrial processes, from pharmaceuticals to carbon capture. In many ways, a bulk substance resembles a liquid (for example, dunes, avalanches and quicksand), but the physics underlying its characteristics is not so transparent.
A recent discovery by chemical engineers at Columbia Engineering explains the new properties of granular particles of various densities not seen in liquids. As it turned out, when the lighter grains rise to the top, the heavier ones remain at the bottom. Particles behave, for example, like oil and water: a lighter liquid repels a heavier one and rushes upward.
The study demonstrates that "bubbles" of lighter sand form and rise through heavier sand when vibrated vertically - similar to bubbles that form and rise upward in lava lamps. Just as air bubbles rise in water, because they are lighter than water and cannot mix with it, light sand bubbles rise through heavier sand, even if they are highly mixed.
The results of the study can not only explain the geological formations and processes underlying mineral deposits, but can also be used in powder processing technologies in the energy, construction and pharmaceutical industries.