Watch Sand Defy Gravity and Flow Uphill Thanks to “Negative Friction”

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We intuitively understand that the sand pouring through an hourglass, for example, forms a neat roughly pyramid-shaped pile at the bottom, in which the grains near the surface flow over an underlying base of stationary particles. Avalanches and sand dunes exhibit similar dynamics. But scientists at Lehigh University in Pennsylvania have discovered that applying a magnetic torque can actually cause sand-like particles to collectively flow uphill in seeming defiance of gravity, according to a September paper published in the journal Nature Communications.

Sand is pretty fascinating stuff from a physics standpoint. It's an example of a granular material, since it acts both like a liquid and a solid. Dry sand collected in a bucket pours like a fluid, yet it can support the weight of a rock placed on top of it, like a solid, even though the rock is technically denser than the sand. So sand defies all those tidy equations describing various phases of matter, and the transition from flowing "liquid" to a rigid "solid" happens quite rapidly. It's as if the grains act as individuals in the fluid form, but are capable of suddenly banding together when solidarity is needed, achieving a weird kind of "strength in numbers" effect.

Nor can physicists precisely predict an avalanche. That's partly because of the sheer number of grains of sand in even a small pile, each of which will interact with several of its immediate neighboring grains simultaneously-and those neighbors shift from one moment to the next. Not even a supercomputer can track the movements of individual grains over time, so the physics of flow in granular media remains a vital area of research.

But grains of sand that collectively flow uphill? That is simply bizarre behavior. Lehigh University engineer James Gilchrist manages the Laboratory for Particle Mixing and Self-Organization and stumbled upon this odd phenomenon while experimenting with "micro-rollers": polymer particles coated in iron oxide (a process called micro-encapsulation). He was rotating a magnet under a vial of micro-rollers one day and noticed they started to pile uphill. Naturally he and his colleagues had to investigate further.

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