When drops of some unusual fluids hit the ground fast enough, the bottoms solidify and bounce while the tops stay liquid and spill over. Researchers have now taken the first high-speed images of this process.
A complex fluid is an unusual type of fluid whose viscosity changes when put under pressure. For example, ketchup is a complex fluid that becomes less viscous when you press on the sides of its container, while a mixture of cornstarch and water turns fully solid when compressed. However, the exact details of how such changes happen aren鈥檛 fully understood, so at Northwestern University in Illinois and her colleagues came up with an experiment to investigate further.
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The researchers prepared a complex fluid by vigorously mixing tiny silica spheres into water until the particles stayed suspended throughout. They dribbled drops of this fluid onto a slab of glass and recorded the impact at a rate of 100,000 images per second.
The recordings show that as a drop hits the glass, it experiences lots of pressure at its bottom, which then spreads upwards through the drop. This pressure causes the fluid to undergo a process called shear jamming, where the silica spheres pack together so closely that they can no longer move.
In some of the tests, the entire drop turned solid, while in others only the bottom solidified. This was related to the concentration of silica spheres in the fluid, with the more silica, the greater the amount of shear jamming.
鈥淚t is remarkable to be able to see a drop go from spreading like water to bouncing like a solid,鈥� says at the City College of New York. Previous experiments haven鈥檛 been able to record both the solid and liquid parts of complex fluids at the same time, he says.
at the University of Twente in the Netherlands says these further details could advance our understanding of how complex fluids behave on a microscopic level.
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