Over the past decade, fluid dynamicists have been investigating tiny droplets bouncing on a vibrating fluid. This seemingly simple experiment has remarkable depth, including the ability to recreate quantum behaviors in a classical system. In this video, some of the researchers demonstrate their experimental techniques, including how they vary the frame rate relative to the bouncing of the drops. At the right frame rate, this sampling makes the droplets appear to glide along with their ripples, giving us a look at a system that is simultaneously a particle (drop) and wave (ripple). (Video credit: D. Harris et al.)
Category: Research
How Many Licks Does It Take to Get to the Center of a Lollipop?
Many a child has wondered how many licks it takes to get to the center of a lollipop. Physically, this is a problem of a solid body dissolving in a flow, and it’s one scientists are interested in for its geological, industrial, and pharmacological applications.
The animation above shows flow around a dissolving (candy!) body that was originally spherical. With both spheres and cylinders, the final shape the body takes is consistent – it has a front boundary with a curvature of nearly constant radius and a back face that is approximately flat. This creates a boundary layer of uniform thickness across the front face, and that uniform flow makes the surface dissolve steadily and evenly so that it maintains the same overall shape.
With their model and experiments, researchers have even been able to tackle the classic question of how many licks it takes:
“For candy of size 1 cm licked at a speed of 1 cm/s, we estimate a total of 1000 licks, a prediction that is notoriously difficult to test experimentally.”
(Image credit: J. Huang et al., source, pdf)
Soap Film Wakes
Soap films can create remarkable flow visualizations when illuminated with monochromatic (single color) light. Each of the photos above shows a flow moving from left to right with a small object near the left creating an obstruction. In the top two images, the objects are cylinders; in the lower one it’s a flat plate tilted at 45 degrees. All of the objects shed vortices as the flow moves past. These vortices alternate in direction – the first spins clockwise, the next counter-clockwise, then clockwise again and so on. This pattern is known as a von Karman vortex street and can even show up in the atmosphere! (Image credit: D. Araya et al.)
Sharks Swimming Sideways
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)
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.
