We’ve seen the effects of vibration on shear-thickening non-Newtonian fluids here on Earth before in the form of “oobleck fingers” and “cornstarch monsters”, but, to my knowledge, this is the first such video looking at the behavior in space. The vibrations of the speaker cause shear forces on the cornstarch mixture, which causes the viscosity of the fluid to increase. This is what makes it react like a solid to sudden impacts while still flowing like a liquid when left unperturbed. In microgravity there is one less force working against the rise of the cornstarch fingers, so the formations we see in this video are subtly different from those on Earth.
Tag: science
Jet Collisions
When two jets of liquid collide, they form a sheet of fluid. As the speeds of the jets change, the sheet can become unstable, forming a set of liquid ligaments and droplets that look like a fish’s bones. This is shown in the video above. For purposes of orienting yourself, flow in the video is moving right to left and the video has been rotated 90-degrees clockwise (i.e. the two out-of-frame jets forming the flow seen are falling due to gravity). (Video credit: Sungjune Jung, University of Cambridge)
Soap Bubbles Bursting
To the human eye, the burst of a soap bubble appears complete and instantaneous, but high-speed video reveals the directionality of the process. Surface tension is responsible for the spherical shape of the bubble, and, when the bubble is pierced, surface tension is broken, causing the soap film that was the bubble to contract like elastic that’s been stretched and released. Droplets of liquid fly out from the edges of the sheet until it atomizes completely.
Stalling a Wing
At small angles of attack, air flows smoothly around an airfoil, providing lifting force through the difference in pressure across the top and bottom of the airfoil. As the angle of attack increases, the lift produced by the airfoil increases as well but only to a point. Increasing the angle of attack also increases the adverse pressure gradient on the latter half of the top surface, visible here as an increasingly thick bright area. Over this part of the surface, the pressure is increasing from low to high–the opposite of the direction a fluid prefers to flow. Eventually, this pressure gradient grows strong enough that the flow separates from the airfoil, creating a recirculating bubble of air along much of the top surface. When this happens, the lift produced by the airfoil drops dramatically; this is known as stall.
Reader Question: Drafting in Cycling
jonesmartinez asks:
As a cyclist, I’m curious about drafting. How fast do I need to be going for there to be a measurable benefit? Additionally, often in a time trial a single rider is often followed by the team car and I’ve heard the rider can be pushed by the air around the team car. Any truth to this rumor? Thanks, I love the blog.
Drafting plays a major role in cycling and its tactics (check out our previous series on cycling). In general, drag increases with the square of velocity and data show this holds for cyclists. The rule of thumb I’ve heard given is that aerodynamic drag doesn’t play a large role below 15 mph, but I have not seen the numbers that inform that claim. Moreover, you have to consider the resultant airspeed around the cyclist. For example, a cyclist moving 13 mph into a 15 mph headwind (28 mph effective) will be experiencing more drag than a cyclist moving 20 mph with a 10 mph tailwind (10 mph effective). With drag being reduced 25-40% by drafting a leading rider, it is almost always beneficial to get behind someone.
That said, I have seen no measurable benefit for a leading rider with a paceline behind him, even though this should, in theory, reduce the drag on the lead rider by closing out his wake. With a large object like a car behind a solo rider, there might theoretically be some benefit. However, the car would have to be driving extremely close to the rider–far closer than they do in reality.
That said, with the prevalence of power meters in the amateur market these days, I think it would be a neat project to go out and try a few of these things firsthand and see whether such tactics actually result in a measurable difference in a cyclist’s performance–though I don’t recommend riding a foot off the front or back of a car!
Martian Lava Coils
NASA’s HiRISE spacecraft has sent back images of lava coils left on the surface of Mars. These features form when lava flows of different speeds move past one another; they’re essentially Kelvin-Helmholtz waves–like the ones often seen in clouds–in the lava flow that have solidified into solid rock! On Earth these coils appear about a foot wide; the Martian versions are 100 feet across. (Photo credit: NASA/JPL/University of Arizona; via Wired; submitted by Brian L)
Why Walking with Coffee is Tough
Almost everyone is familiar with the problem of coffee or tea sloshing over the sides of a mug as one walks, but this may be the first time researchers have systematically studied the problem. The results show that the typical frequency of the human stride closely matches the natural frequency for back-and-forth sloshing of a low-viscosity liquid in a cylindrical container the size of a typical coffee mug. Even though our natural side-to-side motion plays a role in coffee sloshing, its effect is small in comparison. A person’s maximum acceleration, which usually happens early on when walking, sets the initial sloshing amplitude, which is subsequently amplified by the stepping frequency. The researchers did find that the time to spill increased substantially if the subject was focused on not spilling the coffee, though it was unclear if this was due to the subject decreasing their acceleration and step frequency, or whether they were actively damping the oscillations with adjustments in the wrist. If you’re a perpetual coffee spiller, there’s still hope: the authors suggest that flexible cups and/or cups with a series of concentric rings–baffles–could help reduce sloshing in spite of our natural tendency to induce it. (Photo credit: dongga/Flickr; Paper: Mayer and Krechetnikov; submitted by @__pj)
Egg Spinning
Spin a hard boiled egg in a puddle of milk and you get a sprinkler. But how? The science starts at the surface. When the egg spins, the fluid touching its surface is dragged along due to friction, and, because of the fluid’s viscosity, other parts of the fluid will also be spun. Dynamics tells us that the velocity at the surface of the object varies with radius; the velocity at the bottom of a spinning sphere is much smaller than that at its equator because a particle at the equator traverses a larger distance in a single rotation. Likewise, the fluid touching the bottom of the egg is spun slower than the fluid just above it. Bernoulli’s principle tells us that, for an incompressible fluid, the pressure decreases as velocity increases, meaning that a favorable pressure gradient exists along the spinning convex surface. It is this pressure gradient that draws the fluid up the sides of the object. Near the equator, the pressure gradient is weakest and centrifugal force flings the the fluid outward. Surface tension, angular velocity, and viscosity all play a role in the jets and sheets created by the sprinklers. (Video credit: NPR Science Friday with Tadd Truscott et al)
“Compressed” Outtakes
Bubbles, viscosity, diffusion, capillary action, and ferrofluids all feature in the artistic experiments of Kim Pimmel. Be sure to check out his previous film featured here. (Video credit: Kim Pimmel)
Moving Droplets with Electric Fields
Many microfluidic devices employ techniques that manipulate droplet motion for applications like sorting, manufacturing, or precisely controlling chemical reactions at a small scale. The video above shows the oscillations of a droplet on an inclined surface as it is perturbed with an electric field. (Video credit and submission: K. Nichols)
