Like many sharks, the great hammerhead shark is negatively buoyant, meaning that, absent other forces, it would sink in water. To compensate, sharks generate lift with their pectoral (side) fins to offset their weight. Their dorsal (top) fin is used to generate the horizontal forces needed for control and turning. However, both captive and wild great hammerhead sharks tend to swim rolled partway onto their sides. The reason for this unusual behavior is hydrodynamic – it is more efficient for the shark. Unlike other species, the great hammerhead has a dorsal fin that is longer than its pectoral fins. By tipping sideways, the shark effectively creates a larger lifting span and is able to induce less drag than when it swims upright. Models show that swimming on their sides requires ~8% less energy than swimming upright! (Image credit: N. Payne et al., source)
Category: Research
Hagfish Escape Mechanisms
The hagfish is an eel-like creature that has not changed much in the past 300 million years in part because the hagfish is very good at escaping would-be predators. When attacked, the hagfish excretes mucins that combine with seawater to form slime. This gel-like viscoelastic fluid forms quickly and has some handy properties. For example, when stretched, the slime becomes extremely viscous. Many fish feed using a suction method, in which they thrust their jaws forward and enlarge their mouths to suck water and prey inside. This strong unidirectional flow stretches the slime, which thickens it and clogs the fish’s gills. Suddenly, the fish is much more concerned with being unable to breathe, allowing the hagfish to flee.
Being surrounded by all that slime could smother the hagfish, too, if it were not for another clever feature of the slime. When sheared, hagfish slime collapses, losing its viscosity. The hagfish actually ties itself in a knot to create this shear and slide the slime right off. (Image credit: V. Zintzen et al.; L. Böni et al., source)
The Evaporation of Ouzo
Ouzo is an aperitif made up of ethanol (alcohol), water, and anise oil. This three-part, or ternary, mixture undergoes an intriguing evaporation process thanks to the characteristics of its components. An ouzo drop’s evaporation can be divided into four phases, each shown above. Initially, the drop is well-mixed and transparent (upper left).
Since ethanol is the most volatile of ouzo’s components, it evaporates the most quickly. As the ethanol evaporates, the drop becomes oversaturated with oil (upper right). Oil droplets form, giving the ouzo a milky appearance. At the same time, the ethanol evaporating causes gradients in surface tension, which drive a vigorous Marangoni flow inside the drop.
Eventually, the ethanol finishes evaporating and the oil drops collect in a ring around the outside of the drop (lower left). Slowly, the water inside the drop evaporates. Eventually, a tiny microdroplet of water is left to dissolve in the anise oil (lower right). (Image and research credit: H. Tan et al., source; via Inkfish)
The Knuckleball
For more than a century, athletes have used the zigzagging path of a knuckleball to confound their opponents. Knuckleballing is best known in baseball but appears also in volleyball, soccer, and cricket. It occurs when the ball has little to no spin. The source of the knuckleball’s confusing trajectory, according to a new study, is the unsteadiness of the lift forces around the ball. As the ball flies, tiny variations occur in the flow on either side, causing small variations to the lift as well. Using experiments and numerical models, the researchers established that this white noise in the lift forces is sufficient to cause knuckleball-like path changes.
They were also able to explain why some sports see the knuckleball effect and others don’t. The wavelength of the deviations – the distance between a zig and a zag – is relatively long, so knuckleballing can only be noticed if the distance the ball flies is long enough for the deviation to be apparent. Additionally, the side-to-side motion is largest when flow on the ball is transitioning from laminar to turbulent flow, so knuckleballing also requires a very particular (and usually low) initial speed. (Image credit: L. Kang; research credit: B. Texier et al.; submitted by @1307phaezr)
Granular Plugs
Imagine filling a narrow tube with a mixture of water and tiny glass beads. Then take a syringe and very slowly start drawing out the water. As the water gets sucked out of the tube, air will be pulled into the opposite end. The meniscus where the air and water meet sweeps up the glass beads like a liquid bulldozer. As the experiment continues, pressure builds up and air starts filtering through the beads, changing the viscous and frictional forces the system experiences. Eventually, the grains break off, leaving a chunk of glass beads – known as a plug – behind. Keep draining the tube and more plugs form. Check out the video below to see it in action! (Image/video credit: G. Dumazer et al., source; research paper; open synopsis; submitted by B. Sandnes)
The Seabird That Can’t Get Wet
Unlike most seabirds, the frigatebird does not have waterproof feathers. Landing in the water during a transoceanic flight would quickly drown the bird, so instead they stay aloft. But until recently, scientists did not realize just how adept the birds are. Studying tagged frigatebirds in flight, researchers found that the birds could reach altitudes of 4000 meters and that they could soar without flapping for up to 64 kilometers! They achieve these heights by seeking out clouds, which mark strong atmospheric updrafts. The birds ride these thermals up to the cloud tops – well into freezing conditions – and then glide slowly back down.
Their bodies are impressively built for slow glides. Frigatebirds boast a low body weight for their large wing area. This ratio is known as wing loading, and it’s a fundamental characteristic of any flier, avian or otherwise. Low wing loading is key to gliding longer because it reduces the speed at which a glider loses altitude. (Image credit: D. Brossard; research credit: H. Weimarskirch et al.; via @skunkbear)
Reversing Time
Waves contain lots of information. They are also time invariant, which means that they will behave the same regardless of whether time moves forward or backward. This isn’t a property we observe often in life since time just moves forward for us. But a new experiment has demonstrated a method of wave control that can, in a sense, roll back the clock.
To do this, the scientists created a instantaneous time mirror, or ITM. When they create a disturbance on the surface of a pool of water, it sends out capillary waves in the form of ripples. A short time later, they accelerate the pool sharply downward. This universal disturbance is their instantaneous time mirror, which generates backward-propagating ripples. Those new backward-propagating waves travel back toward the source and refocus into the shape of the initial disturbance. This works for both a simple point disturbance (top image) and for a more complicated geometry like a smiley face (bottom image). (Image credit: V. Bacot et al., source; submitted by @g_durey)
ETA: To be clear, this experiment does not refute causality. It’s more like saying that the information for the initial conditions is still carried on in the later state and that you can do something to extract that information.
Amphibious Adaptation
Every year newts move to the water in the springtime to mate before returning to land for the rest of the year. This annual aquatic relocation is accompanied by changes in the newt’s body. Flaps of skin grow from their upper jaw to their lower jaw, partially closing their mouths at the corners. This can be seen in the left column of the animation compared to the center and right.
Numerical simulation shows that this mouth change has a significant impact on the newt’s ability to hunt underwater. Newts are suction feeders, who open their jaws and expand their mouth cavity to suck in water and their prey. By closing off the corners of their mouths during their aquatic phase, the newts generate more suction, reaching peak flow velocities 10% to 50% higher than in their terrestrial form and enabling them to pull prey from 15% further away. When they leave the water, the newts lose the extra flaps so that their mouths can open wider for catching prey on land. (Image credit: S. Van Wassenbergh and E. Heiss, source)
Flying in Cramped Quarters
A new study has found that budgerigars (also commonly known as parakeets or budgies) fly at only two distinct speeds. The researchers flew the birds in a tapered tunnel to see how they navigated in response to widening or narrowing paths. What they found, regardless of the flight direction in the tunnel, is that the birds fly at approximately 9.5 m/s in areas wider than 2.5 times their wingspan and drop suddenly to a speed about half that when in narrower areas. The higher speed falls within the bird’s most energy-efficient range, suggesting that the birds may prefer flying at this condition. Insects like bumblebees also change speeds when entering cluttered environments, but the insects do so gradually, not suddenly like the budgerigars. The reason for this difference is not yet known, but it could relate to how the animals sense their environment or to differences in their flight efficiency when varying speed. (Image credit: J. Bendon; research credit: I. Schiffner and M. Srinivasan; submitted by Marc A.; h/t to Irmgard B.)
Easy Squeezing
Nearly everyone has struggled with the frustration of trying to get ketchup, toothpaste, or peanut butter out of a container. These fluids and fluid-like substances are notoriously difficult to budge because they prefer to wet and adhere to solid surfaces. One way to limit this adhesion is to use a superhydrophobic surface, like the one shown in the middle image. These surfaces use micro- and nanoscale roughness to trap air pockets underneath a liquid and reduce the amount of contact between the liquid and solid. But such surfaces are delicate and prone to failure. The slippery alternative offered by LiquiGlide is a liquid-impregnated surface, shown in the bottom image. Like a superhydrophobic surface, it consists of a textured solid but one that’s filled with a liquid lubricant that preferentially wets the solid. As a result, the liquid to be shed has little to no contact with the actual solid surface and therefore slides easily off! (Image credit: LiquiGlide, source; research credit: K. Varanasi et al.; suggested by cnsidero)
