Acoustic levitation uses the radiation forces generated by sound waves to trap small, lightweight particles at the nodes of standing waves. We’ve seen this a number of times previously, both with solid objects and liquid droplets. What makes this example particularly impressive, though, is that these researchers use an array of speakers to manipulate multiple objects at once. Check out the video above for a whole series of clips from the research. (Video credit: Science; research credit: A. Marzo and B. Drinkwater)
Search results for: “waves”
A Golden Swirl
As much as I love exploring flashy examples of fluid dynamics, like shock waves around aircraft or what happens when non-Newtonain fluids get crushed by a hydraulic press, my favorite moments are the simple, everyday ones. Getting to see fluid dynamics in my daily life, whether I’m standing in the kitchen cooking or trying to wash my hands, is what excites me the most. The photo above is an example of this kind of simple, satisfying fluid experience. The image shows wax being melted in a crockpot. As it melts and its optical characteristics change, the wax reveals the mixing pattern inside the container. There’s nothing earth-shattering or scientifically important about something like this. But it’s still a moment where the otherwise unseen and unnoticed becomes visible and beautiful. It’s the fluid dynamical equivalent of stopping to smell the roses. When did you last pause to appreciate the flows around you? (Image credit: A. Unger et al.)
What Makes Turbulence So Hard
Turbulence – that pestersome, unpredictable, and chaotic state of flow – has been a thorn in the sides of mathematicians, physicists, and engineers for centuries. It is certainly one of – if not the – oldest unsolved problem in physics. Over at Ars Technica, Lee Phillips has a nice overview of the situation, including what makes the problem so difficult:
The Navier-Stokes equation is difficult to solve because it is nonlinear. This word is thrown around quite a bit, but here it means something specific. You can build up a complicated solution to a linear equation by adding up many simple solutions. An example you may be aware of is sound: the equation for sound waves is linear, so you can build up a complex sound by adding together many simple sounds of different frequencies (“harmonics”). Elementary quantum mechanics is also linear; the Schrödinger equation allows you to add together solutions to find a new solution.
But fluid dynamics doesn’t work this way: the nonlinearity of the Navier-Stokes equation means that you can’t build solutions by adding together simpler solutions. This is part of the reason that Heisenberg’s mathematical genius, which served him so well in helping to invent quantum mechanics, was put to such a severe test when it came to turbulence.
Phillips goes on to describe some of the many methods researchers use to unravel the mysteries of turbulence computationally, experimentally, and theoretically. This is a great introduction for those curious to get a sense of how turbulence, stability theory, and computational fluid dynamics all fit together. (Image credits: L. Da Vinci; NASA; see also: Ars Technica; submitted by Kam Yung-Soh)
Kelly Slater’s Surf Ranch
Many of us who grew up visiting water parks instead of ocean beaches have spent time bobbing in a wave pool. They’ve been around for decades. But a new generation of wave pools are aiming for a different goal: the perfect surf wave. One of the foremost current facilities is Kelly Slater’s Surf Ranch, shown above. Here a hydrofoil (draped in blue tarps on the left) is pulled along an artificial lagoon to create dozens of wave profiles, all engineered to give surfers a long ride on the perfect solitary wave.
Other facilities, like the surf ranch used by USA Surfing in Waco, Texas, design their waves with different goals in mind. The Waco wave pool uses air pressure to drive their waves, and aims for a larger quantity of shorter waves. They’re designed to help young surfers practice skills they’re working on, and to give them a place where they can experience waves like those they’ll face in the upcoming 2020 Olympics in Tokyo. (Image credit: R. Young/WIRED; CNet, source; submitted by Lionel V.)
Levitating with Sound
Sound can manipulate fluids in fascinating ways, from levitation to vibration. Here researchers use sound to levitate and manipulate droplets and turn them into bubbles. Increasing the acoustic pressure on the levitating droplet flattens it, then slowly causes the drop to buckle. When the buckled film encloses a critical volume, the sound waves resonate inside it. That causes a big jump in acoustic pressure, which makes the drop snap closed into a bubble. (Image and research credit: D. Zang et al.; via Science News; submitted by Kam-Yung Soh)
Using Sound to Print
Inkjet printing and other methods for directing and depositing tiny droplets rely on the force of gravity to overcome the internal forces that hold a liquid together. But that requires using a liquid with finely tuned surface tension and viscosity properties. If your fluid is too viscous, gravity simply cannot provide consistent, small droplets. So researchers are turning instead to sound waves.
Using an acoustic resonator, scientists are able to generate forces up to 100 times stronger than gravity, allowing them to precisely and repeatably form and deposit micro- and nano-sized droplets of a variety of liquids. In the images above, they’re printing tiny drops of honey, some of which they’ve placed on an Oreo cookie for scale. The researchers hope the technique will be especially useful in pharmaceutical manufacturing, where it could precisely dispense even highly viscous and non-Newtonian fluids. (Image and research credit: D. Foresti et al.; via Smithsonian Mag; submitted by Kam-Yung Soh)
Inside a Bubble Wall
Schlieren photography has an almost magical feeling to it because it enables us to see the invisible – like shock waves and the tiny currents of heat that rise from our skin. But it can also reveal new perspectives on things that aren’t invisible. Here we see soap bubbles viewed through the lens of a schlieren set-up. Schlieren is sensitive to small changes in density, so instead of appearing in their usual rainbow iridescence, the bubbles look glass-like and filled with tiny currents and bubbles. What we’re seeing are some of the many tiny flow variations across the surface of a soap bubble. They’re driven by a combination of forces – gravity, temperature, and surface tension variations, to name a few. Seen in video, you can really appreciate just how dynamic a thin soap film is! (Image credit and submission: L. Gledhill, video version, more stills)
Heating from Cavitation
When cavitation bubbles collapse, they can produce temperatures well over 2,000 Kelvin. Since cavitation near a surface can be so destructive, researchers have long wondered whether the high temperatures inside the bubble can be transmitted to nearby surfaces. A new set of numerical simulations provides some insight into that process. The researchers found that collapsing cavitation bubbles raised nearby wall temperatures in two ways: bubbles that were further away sent shock waves that heated the material, and nearby bubbles could contact the surface itself as they collapsed.
Heat transfer requires time, however; this is part of why quickly dunking your hand in liquid nitrogen and pulling it out likely won’t damage you. (Still, we don’t recommend it.) The cavitation bubbles could only transmit these high temperatures for less than 1 microsecond, which means that most materials won’t actually heat up to their melting temperature. The researchers did conclude, however, that softer materials exposed to frequent bubble collapses could show localized melting under the barrage. (Image credit: L. Krum; research credit: S. Beig et al.)
“Volumes”
“Volumes” is an experimental art film by Maxim Zhestkov using physics-based particle animation. Waves and unseen forces send billions of color-changing particles aloft in the film. The motions – especially the way the particles seem to tear themselves – are reminiscent of a complex fluid, like yogurt. These substances have both liquid-like (viscous) and solid-like (elastic) properties depending on the forces they experience. Zhestkov’s particles are similar; they move like a fluid but tear more like a solid.
I particularly like the sequence beginning at 1:30. The upwelling of particles leaves behind a lower layer that looks like a snapshot of convection in a planetary mantle while the upper layer resembles the clash of ocean waves. The whole film is quite mesmerizing. Check it out! (Video and image credit: M. Zhestkov; GIFs via Colossal)
A Star Drop
There are many ways to make a droplet oscillate in a star-shape – like vibrating its surface or using acoustic waves to excite it – but these methods involve externally forcing the droplet’s oscillation. Leidenfrost drops – liquids levitating on a film of their own vapor caused by the extremely hot surface below – turn themselves into stars. It all starts with the constant evaporation driven by the heat below. This creates a thin, fast-moving layer of vapor flowing beneath the drop. That vapor shears the drop, causing capillary waves – essentially ripples – that travel through the drop in a characteristic way. Those ripples in turn cause pressure oscillations in the vapor layer, alternately squeezing and releasing it. Feedback from the vapor layer then drives the droplet into star-shaped oscillations. Under the right conditions, water drops can form stars with as many as 13 points! (Image and research credit: X. Ma and J. Burton, source)
