Science Focus
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The flow of fluids is one of the most complex, beautiful, and amazing things in physics. Slow motion pictures of drops landing on water or of two fluids mixing can be simply gorgeous. Even more amazing, the basic physics of fluid flow was worked out way back in the 19th century. Those equations, though, hold riches that are still being uncovered today.
Some of the most spectacular work in recent years has involved uncovering what happens as a drop of fluid hits a surface. And one particularly stubborn aspect—why do you get lift-off (a precursor to a splash) near the end of the impact?—has revealed itself after a barrage of high-speed camera images.
To splash or not to splash?
At first, the impact of a slow-speed droplet on a surface seemed very difficult to understand. Eventually, it was decided that the momentum of the droplet competes with its surface tension. Essentially, the momentum tries to force the drop to spread out at a speed governed by the mass of fluid and the speed at which the drop impacts. But surface tension tries to pull the droplet back together, resisting the spreading motion. Hence, a droplet rapidly expands to some radius where the forces balance. Note that viscosity—how resistant a fluid is to flow—is seemingly unimportant.
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original post: http://feeds.arstechnica.com/~r/arstechnica/science/~3/AjdSMSKJGyc/
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