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Here's one you can try at home: Fill a dish with water and drop a small amount of flour onto the surface. The flour particles disperse rapidly, like a tiny starburst, and spread out over the liquid surface.
This simple procedure, described in a paper set to be published online by the Proceedings of the National Academy of Sciences, reveals a physical phenomenon that is common to many small particles and liquids but that had remained a mystery to the study's authors.
So Pushpendra Singh and his colleagues set out to unravel the underlying mechanism. Singh, a mechanical engineer at the New Jersey Institute of Technology, explains that the explosive dispersion arises from vertical capillary forces pulling the particles into a floating equilibrium on the liquid.
"When a particle comes in contact with a liquid surface, it reduces the amount of liquid surface area, because some portion is now occupied by the particle," Singh says. "Basically what that means is that the total energy of the system is reduced. That energy has to go somewhere, and it actually is acquired by the particle."
In low-viscosity liquids such as water, particles tend to overshoot their equilibrium and bob up and down like a weighted spring, reaching surprisingly rapid vertical velocities in the process. The interfacial forces, Singh and his co-authors determined, can accelerate a nanoscale particle to roughly 160 kilometers per hour. (The smaller the particle, the greater the velocity it can reach under the surface forces.)
Those oscillations drive repulsive hydrodynamic forces that push the particles apart at a rapid clip, albeit not with the extreme velocities seen in their smaller vertical motions. "A particle is sitting on the surface, it's vibrating up and down, and it moves everything away from it," Singh explains. "If you drop more than one particle, then each one is doing that same thing." He estimates that the horizontal velocity of a particle dispersing by this mechanism is about an order of magnitude lower than the particle's vertical velocity.
Surprisingly, the height from which the particle is dropped has little influence on its energetic dispersion once it reaches the air–liquid interface. "This is not an issue, how high you drop them," Singh says, noting that in experiments where the particles settled slowly through a layer of oil floating on water the same rapid dispersion was observed when the particles reached water. "You can do some simple analysis to show that the kinetic energy the particle has because you're dropping it from some height is actually negligible compared to the interfacial energy it acquires when it's captured."
