Tag: 2016gofm

A Particle-Filled Splash
A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s – it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video. (Image credit and submission: A. Sauret et al., source)
November 16, 2016
Living Fluid Dynamics
This short film for the 2016 Gallery of Fluid Motion features Montana State University students experiencing fluid dynamics in the classroom and in their daily lives. As in her previous film (which we deconstructed), Shanon Reckinger aims to illustrate some of our everyday interactions with fluids. This time identifying individual phenomena is left as an exercise for the viewer, but there are hints hidden in the classroom scenes. How many can you catch? I’ve labeled some of the ones I noticed in the tags. (Video credit: S. Reckinger et al.)
November 7, 2016
Non-Newtonian Splashes
What happens when a stream of liquid falls through a screen? As the above video shows, water creates a beautiful flower-like burst of fluid when it hits a screen. Adding a little polymer to the water makes it non-Newtonian and more viscous. When hitting the screen, this slows it down but doesn’t prevent the fluid from flowing.
Add enough polymer, though, and the fluid becomes what’s known as a yield-stress fluid. These fluids behave much like a solid–they don’t flow–until you apply a certain amount of stress. Then they’ll flow. If you’ve ever tried to get ketchup out of a glass bottle, then you’re familiar with how these yield-stress fluids act. When dropped onto a screen, the yield-stress fluid just forms a pile–unless the impact speed is high enough to create the necessary force to get the fluid to flow! (Video credit: B. Blackwell et al.)
October 28, 2016
Clogging, In Hourglasses and Crowds
Hourglasses are pretty common, but you’ve probably never given much thought to the way they flow. An hourglass designer has to carefully select the sizing of the neck and the grains. Choosing a neck that’s too small relative to the grain size will result in frequent clogs but choosing too large a neck will make setting the timing difficult. Interestingly, it doesn’t matter whether the hourglass is filled with air or with water–the same principle holds.
Where this knowledge becomes especially useful, though, is when dealing with crowds. We’ve all experienced the frustration of being in a large crowd trying to fit through a small exit. Paradoxically, the fastest way to get a large number of particles (or sheep or people) through a narrow opening is to slow each individual down. This can either be done by instructing everyone to slow down or by forcing that same result by placing an obstacle immediately before the exit. The reduction in speed reduces clogging, which means everyone gets through faster! (Video credit: A. Marin et al.)
October 19, 2016