Category: Research

Flames in Space
The jellyfish-like light show in the animations above shows the life and death of a flame in microgravity. The work is part of the Flame Extinguishment Experiment 2 (FLEX-2) currently flying aboard the International Space Station. When ignited, the fuel droplet creates a blue spherical shell of flame about 15 mm in diameter. The spherical shape is typical of flames in microgravity; on Earth, flames are shaped like teardrops due to the effects of buoyancy, which exists only in a gravitational field. The bright yellow spots and streaks that appear after ignition are soot, which consists mainly of hot-burning carbon. The uneven distribution of soot is what causes the pulsating bursts seen in the middle animation. When soot products drift back onto the fuel droplet, it causes uneven burning and flame pulses. The final burst of flame in the last animation is the soot igniting and extinguishing the flame. Fires are a major hazard in microgravity, where oxygen supplies are limited and evacuating is not always an option. Scientists hope that experiments like FLEX-2 will shed light on how fires spread and can be fought aboard spacecraft. For more, check out NASA’s ScienceCast on microgravity flames. (Image credits: NASA, source video; submitted by jshoer)
September 12, 2014
The Archer Fish’s Arrow
Archer fish hunt by shooting jets of water at their prey to knock them into the water where the fish can eat them. Previous research showed that the archer fish’s projectile jet is pulsed such that the water released at a later time has a greater velocity. This makes the jet bunch up so that a ball of liquid hits the prey with greater force than the jet would otherwise. A recently released paper shows that the archer fish actively adjust their liquid jets in order to strike targets at different distances while maintaining this bunching effect. To control the jets, the fish adjust both how long they jet and what speed they impart to the fluid by changing how they open and close their mouths. (VIdeo credit: Nature; research credit: P. Gerullis and S. Schuster; via phys.org; submitted by @jchawner)
September 11, 2014
Antibubble Vortex Rings
Bubbles are familiar, but antibubbles are a bit more unusual. An antibubble typically has a liquid-air-liquid interface, with a thin shell of air separating a liquid droplet from the surrounding fluid. Although they look rather like bubbles, antibubbles behave differently. Antibubbles are, for example, very sensitive to pressure changes. A sinking antibubble like the one in the video above, experiences a higher pressure on its lower face. This pressure compresses the gas shell and thins it on the bottom. The air shell bursts at the thin point and the antibubble collapses, generating two vortex rings and a small, buoyantly rising bubble. (Video credit: S. Dorbolo et al.)
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September 10, 2014
The Churning of Corals
Corals may appear static, but near the surface the tiny hair-like cilia of these polyps are churning the water. Although it has been known for some time that corals have cilia, scientists had previously assumed they only moved water parallel to the coral’s surface. Instead recent flow visualizations show that the cilia’s movements generate larger-scale vortical flows near the coral that can help draw fresh nutrients in as well as flush waste away. This means that, instead of being reliant on currents and tides, corals can exert some control on their environment in order to get what they need. This insight into coral cilia may shed some light on the micro- and macroscopic flows generated by other cilia, like those in our lungs. For a similar example of seemingly-passive organisms generating their own flows, check out how mushrooms create air currents to spread their spores. (Image credits: O. Shapiro et al. and MIT News; source video; h/t to Katie B)
September 3, 2014
Breaking Drops with Vibration
Atomization is the process of breaking a liquid into a spray of fine droplets. There are many methods to accomplish this, including jet impingement, pressure-driven nozzles, and ultrasonic excitement. In the images above, a drop has been atomized through vibration of the surface on which it rests. Check out the full video. As the amplitude of the surface’s vibration increases, the droplet shifts from rippling capillary waves to ejecting tiny droplets. With the right vibrational forcing, the entire droplet bursts into a fine spray, as seen in the photo above. The process is extremely quick, taking less than 0.4 seconds to atomize a 0.1 ml drop of water. (Photo and video credit: B. Vukasinovic et al.; source video)
August 29, 2014
Seahorse Hunting
Those who have observed the languid pace of seahorses or seadragons swimming might think these fish only hunt slow prey. In fact, the tiny crustaceans on which they feed are extremely quick, capable of velocities over 500 body lengths per second. Instead of speed, the seahorse relies on stealth to capture its prey, as shown in the holographic video above. Seahorses use a pivot method to feed, simultaneously shifting their snouts up and sucking water in to catch their target. But this method of feeding only works for distances of about 1 mm. To get that close in the first place, the seahorse must approach its prey without alerting it. Researchers found that both the seahorse’s head shape and its natural posture create a hydrodynamic quiet zone just off the seahorse’s snout, directly in its strike zone. Fluid velocity and deformation rates in this region are significantly lower than those around the rest of the seahorse’s face when it moves, allowing the fish to sneak up on its prey. These adaptations are remarkably effective, too; the researchers observed that the seahorses were able to position themselves within 1mm of their prey without alerting them 84% of the time. (Video credit: B. Gemmell et al.; via Discover)
August 27, 2014
Water-Based Tractor Beam
Researchers in Australia have demonstrated a “tractor beam” capable of manipulating floating objects from a distance using surface waves on water. And, unlike some research, you can try to replicate this result right in the comfort of your own bathtub! When a wave generator oscillates up and down, it creates surface waves that move objects and particles on the water’s surface. When the wave amplitudes are small, the outgoing wave fronts tend to be planar, as in part (a) of the figure above. These planar waves push surface flow away from the wave generator in a central outward jet, and new fluid is entrained from the sides to replace it. This creates the kind of flowfield shown in the streaklines of part (b).
Increasing the amplitude of the surface waves drastically changes the surface flow’s behavior. Larger wave amplitudes are more susceptible to instabilities due to the nonlinear nature of the surface waves. This means that the planar wave fronts seen in part (a) break down into a three-dimensional wavefield, like the one shown in part (c). Near the wave-maker, the surface waves now behave chaotically. This pulsating motion ejects surface flow parallel to the wave-maker, which in turn draws fluid and any floating object toward the wave-maker. The corresponding surface flowfield is shown in part (d). The researchers are refining the process, but they hope the physics will one day be useful in applications oil spill clean-up. (Video credit: Australia National University; image and research credit: H. Punzmann et al. 1, 2; via phys.org; submitted by Tracy M)
August 22, 2014
Bead-Infused Droplet
A Leidenfrost droplet impregnated with hydrophilic beads hovers on a thin film of its own vapor. The Leidenfrost effect occurs when a liquid touches a solid surface much, much hotter than its boiling point. Instead of boiling entirely away, part of the liquid vaporizes and the remaining liquid survives for extended periods while the vapor layer insulates it from the hot surface. Hydrophilic beads inserted into Leidenfrost water droplets initially sink and are completely enveloped by the liquid. But, as the drop evaporates, the beads self-organize, forming a monolayer that coats the surface of the drop. The outer surface of the beads drys out, trapping the beads and causing the evaporation rate to slow because less liquid is exposed. (Photo credit: L. Maquet et al.; research paper – pdf)
August 20, 2014
Supernova Explosion
Type 1a supernovae occur in binary star systems where a dense white dwarf star accretes matter from its companion star. As the dwarf star gains mass, it approaches the limit where electron degeneracy pressure can no longer oppose the gravitational force of its mass. Carbon fusion in the white dwarf ignites a flame front, creating isolated bubbles of burning fluid inside the star. As these bubbles burn, they rise due to buoyancy and are sheared and deformed by the neighboring matter. The animation above is a visualization of temperature from a simulation of one of these burning buoyant bubbles. After the initial ignition, instabilities form rapidly on the expanding flame front and it quickly becomes turbulent. (Image credit: A. Aspden and J. Bell; GIF credit: fruitsoftheweb, source video; via freshphotons)
August 18, 2014
Sharkskin Fluid Dynamics
Sharks have evolved some incredible fluid dynamical abilities. Instead of scales, their skin is covered in microscopic structures called denticles. To give you a sense of size, each denticle in the black and white image above is about 100 microns across. Denticles are asymmetric and overlap one another, creating a preferential flow direction along the shark. When water tries to move opposite the preferred direction, the denticles will bristle, like in the animation above. The bristled denticles form an obstacle for the reversed flow without any effort on the shark’s part. Since local flow reversal is an early sign of separation, researchers theorize that this bristling tendency prevents flow along the shark’s skin from separating. Keeping flow attached, especially along the shark’s tail, is vital not only to the shark’s agility but to keeping its drag low. Researchers have even begun 3D printing artificial shark skin to try and harness the animal’s hydrodynamic prowess. For much more shark-themed science, be sure to check out this week’s “Several Consecutive Calendar Days Dedicated to Predatory Cartilaginous Fishes” video series by SciShow, It’s Okay to be Smart, The Brain Scoop, Smarter Every Day, and Minute Physics. (Image credits: J. Oeffner and G. Lauder; A. Lang et al.; original video; jidanchaomian)
August 15, 2014