I often receive questions about how fluids react to extremely hard and fast impacts. Some people wonder if there’s a regime where a fluid like water will react like a solid. In reality, nature works the opposite way. Striking a solid hard enough and fast enough makes it behave like a fluid. The video above shows a simulated impact of a 500-km asteroid in the Pacific Ocean. (Be sure to watch with captions on.) The impact rips 10 km off the crust of the Earth and sends a hypersonic shock wave of destruction around the entire Earth. There’s a strong resemblance in the asteroid impact to droplet impacts and splashes. Much of this has to do with the energy of impact. The asteroid’s kinetic (and, indeed, potential) energy prior to impact is enormous, and conservation of energy means that energy has to go somewhere. It’s that energy that vaporizes the oceans and fluidizes part of the Earth’s surface. That kinetic energy rips the orderly structure of solids apart and turns it effectively into a granular fluid. (Video credit: Discovery Channel; via J. Hertzberg)
Month: March 2015
Popcorn Popping
The familiar popping behavior of popcorn is the combination of several events. When heated, unpopped kernels act like pressure vessels, managing to contain their boiling water content until a critical temperature of 180 degrees Celsius. At this temperature, nearly all kernels fracture. Popcorn’s jump doesn’t come from the fracture, though. Most of its acrobatics occur when a leg of starch branches out of the popping kernel. The starch acts somewhat like a muscle – after being compressed against the ground, it springs back, propelling the corn upward. Finally, by synchronizing high-speed video and audio recordings of popping corn, researchers determined that the pop in popcorn is not caused by fracture or rebound but instead is the result of the release of water vapor. (Image credit: TAMU NAL, source; research credit: E. Virot and A. Ponomarenko; submitted by Chad W.)
How Eyelashes Work
New research shows that eyelashes divert airflow around the eye, serving as a passive filter that reduces dust collection and controls evaporation. Mammal hairs in places like the nose act as ram filters that trap the particles that hit them and which require regular cleaning via sneezing. Eyelashes, on the other hand, prevent dust collection by altering airflow at the surface of the eye. At the optimal length of roughly 1/3rd the width of an eye, eyelashes create a stagnation zone near the eye surface that forces air to travel above rather than through the eyelashes. This results in lower shear stress and lower flow speeds at the eye surface, both of which help reduce evaporation and shield the eye from dust. Lashes can get too long, though; the researchers found that longer lashes tended to channel higher flow speeds toward the eye surface, leading to faster evaporation rates. Thus, donning longer fake eyelashes may dry out your eyes. (Image credit: G. Diaz Fornaro; research credit: G. Amador et al.; via skunkbear)
