Stripe-like wave clouds can often form downstream of mountains. This satellite image shows such clouds in the South Pacific where rocky mountains jut 600 meters (2,000 ft) above the sea. This disrupts air flowing east by forcing it to move up and over the island topography. The air does not simply settle back down on the other side, though. It must come back into equilibrium with its surroundings in terms of density and temperature. While doing so it will travel up and down along a wavy path. As it reaches the crest of the wave, humid air cooling condenses and forms a cloud. At troughs, the air warms and the condensation disappears. This creates the stripey cloud pattern in the mountain’s wake, which fades out as the atmospheric gravity waves die out. (Image credit: NASA/J. Schmaltz; via NASA Earth Observatory)
Search results for: “waves”
Visualizing Acoustic Levitation
The schlieren photographic technique is often used to visualize shock waves and other strong but invisible flows. But a sensitive set-up can show much weaker changes in density and pressure. Here, schlieren is used to show the standing sound wave used in ultrasonic levitation. By placing the glass plate at precisely the right distance relative to a speaker, you can reflect the sound wave back on itself in a standing wave, seen here as light and dark bands. The light bands mark the high-pressure nodes, where the pressure generated by the sound waves is large enough to counteract the force of gravity on small styrofoam balls. This allows them to levitate but only in the thin bands seen in the schlieren. Move the plate and the standing wave will be disrupted, causing the bands to fade out and the balls to fall. (Video and image credit: Harvard Natural Sciences Lecture Demonstrations)
Space Shuttle Sonic Booms
The Space Shuttle had a famous double sonic boom when passing overhead during re-entry. This schlieren flow visualization of a model shuttle at Mach 3 reveals the source of the sound: the fore and aft shock waves on the vehicle. The nose of the shuttle generates the strongest shock wave since it is the first part of the vehicle the flow interacts with. This initial shock wave turns the flow outward and around the shuttle. The second boom comes from the back of the shuttle and serves to turn the flow back in to fill the wake behind the shuttle. (The actual shock wave would look a little different than this one because there’s no sting holding the shuttle like there is with the model.) The other major shock wave comes from the shuttle’s wings, but, at least for this Mach number, the wing shock wave merges with the bow shock, making the two indistinguishable. (Image credit: G. Settles, source)
Catching Particles with Sound
Acoustic levitation traps particles using specially shaped sound waves, but, thus far, it’s only been useful for small particles. One common method of trapping forms the sound waves into a vortex-like shape. Particles in one of these acoustic vortices will spin rapidly, become unstable, and get ejected from the vortex if they’re larger than about half the wavelength of sound used. Recently, though, researchers have stabilized much larger particles by trapping them between two acoustic vortices with opposite spins. The researchers alternate between the two vortices so that each can counteract the other in order to hold the particle in the center of the trap. The new technique has enabled them to trap particles up to 4 times larger than those in previous experiments. (Image and research credit: A. Marzo et al., source; via Science)
Seeing the Wake
Hot exhaust gases churn in the wake of this climbing B-1B Lancer. The high temperature of the exhaust changes the density and, thus, the refractive index of the gases relative to the atmosphere. Light traveling through the exhaust gets distorted, making the highly turbulent flow visible to the human eye. Note how the four individual engine exhaust plumes quickly combine into one indistinguishable wake. This is typical for turbulence; it’s hard to track where any given fluctuations originally came from. The airplane’s wingtip vortices are just visible as well, if you look closely. (Image credit: T. Rogoway; submitted by Mark S.)
Pilot-Wave Hydrodynamics: Resources
This is the final post in a collaborative series with FYP on pilot-wave hydrodynamics. Previous posts: 1) Introduction; 2) Chladni patterns; 3) Faraday instability; 4) Walking droplets; 5) Droplet lattices; 6) Quantum double-slit experiments; 7) Hydro single- and double-slit experiments; 8) Quantum tunneling; 9) Hydrodynamic tunneling; 10) de Broglie’s pilot-wave theory
Thanks for joining us this week as we explored nearly two centuries’ worth of scientific discoveries around vibration, fluid dynamics, and quantum mechanics. For those who’d like to learn more about these and related topics, we’ve compiled some helpful resources below.
Other Videos, Articles, and Resources by Topic
Chladni Patterns
- ANSYS, “Chladni Plates”
- Brusspup, “Amazing Resonance Experiment!”
- Kenichi Kanazawa, “Color Sound”
- Microfluidic Chladni patterns
- Nigel Stanford, “Cymatics”
- Peter Remco, “Chladni patterns in a violin plate”
- Steve Mould, “Random couscous snaps into beautiful patterns”
Faraday Instability
- FYFD, Alligators and water dancing
- FYFD, Liquid crystals vibrating on a tuning fork
- Gallery of Fluid Motion, “The Tibetan singing bowl”
- Nigel Stanford, “Cymatics”
- Slow Mo Guys, “Chinese spouting bowl in slow motion.”
Quantum Mechanics
Pilot-wave Hydrodynamics
- Dual Walkers, learn about the physics from the researchers themselves
- Gallery of Fluid Motion, “The pilot-wave dynamics of walking droplets.”
- Gallery of Fluid Motion, “Shedding light on pilot-wave phenomena.”
- The Lutetium Project, “Never-ending bouncing droplets.”
- The Lutetium Project, “Dual walkers: drops and waves.”
- Through the Wormhole, Interview with Y. Couder
- Wired, “Have we been interpreting quantum mechanics wrong this whole time?”
- Veritasium, “Is this what quantum mechanics looks like?”
Selected (Academic) Bibliography by Topic
Articles marked with an asterisk (*) are recommended for their approachability and/or broad overview of the subject.
Chladni Patterns
- (*) M. Faraday, “On a peculiar class of acoustical figures; and on certain forms assumed by groups of particles upon vibrating elastic surfaces,” 1831.
- Lord Rayleigh, “On the circulation of air observed in Kundt’s tubes, and on some allied acoustical problems,” 1884.
- H. van Gerner et al., “Air-induced inverse Chladni pattern,” 2011.
Faraday Instability
- (*) M. Faraday, “On a peculiar class of acoustical figures; and on certain forms assumed by groups of particles upon vibrating elastic surfaces,” 1831.
Pilot-wave Hydrodynamics
- Y. Couder and E. Fort, “Single-particle diffraction and interference at a macroscopic scale,” 2006.
- A. Eddi et al., “Unpredictable tunnel of a classical wave-particle association,” 2009.
- (*) Y. Couder et al., “Walking droplets: A form of wave-particle duality at macroscopic scale?”, 2010.
- J. Molacek and J. Bush, “Droplets bouncing on a vibrating bath,” 2013.
- J. Molacek and J. Bush, “Droplets walking on a vibrating bath: toward a hydrodynamic pilot-wave theory,” 2013.
- D. Harris et al., “Wave-like statistics from pilot-wave dynamics in a circular corral,” 2013.
- O. Wind-Willassen et al., “Exotic states of bouncing and walking droplets,” 2013.
- (*) J. Bush, “Pilot-wave hydrodynamics,” 2015.
- D. Harris et al., “Visualization of hydrodynamic pilot-wave phenomena,” 2016.
(Image credit: A. Labuda and J. Belina)
Pilot-Wave Hydrodynamics: Slit Experiments
This post is part of a collaborative series with FYP on pilot-wave hydrodynamics. Previous entries: 1) Introduction; 2) Chladni patterns; 3) Faraday instability; 4) Walking droplets; 5) Droplet lattices; 6) Quantum double-slit experiments
In quantum mechanics, the single and double-slit experiments are foundational. They demonstrate that light and elementary particles like electrons have wave-like and particle-like properties, both of which are necessary to explain the behaviors observed. Similarly, a hydrodynamic walker consists of both a particle and a wave, so, perhaps unsurprisingly, researchers tested them in both single-slit and double-slit experiments.
When a walker passes through a single-slit (top row), it’s deflected in a seemingly random direction due to its waves interacting with the slit. But if you watch enough walkers traverse the slit, you can put together a statistical representation of where the walker will get deflected. Compare that with the results for a series of photons passing through a slit one-at-a-time, and you’ll see a remarkable match-up.
If you test the walker instead with two slits, the droplet can only pass through one slit, but its accompanying wave passes through both (bottom row). Let enough walkers through the system one-by-one, and they, like their photonic cousins, build up interference fringes that match the quantum experiment. Diffraction and interference are only a couple of the walkers’ tricks, however. In the next posts, we’ll take a look at another analog to quantum behavior: tunneling.
(Image and research credits: Couder et al., source, selected papers 1, 2; images courtesy of E. Fort)
Pilot-Wave Hydrodynamics: Walking Drops
This post is a collaborative series with FYP on pilot-wave hydrodynamics. Previous entries: 1) Introduction; 2) Chladni patterns; 3) Faraday instability
If you place a small droplet atop a vibrating pool, it will happily bounce like a kid on a trampoline. On the surface, this seems quite counterintuitive: why doesn’t the droplet coalesce with the pool? The answer: there’s a thin layer of air trapped between the droplet and the pool. If that air were squeezed out, the droplet would coalesce. But it takes a finite amount of time to drain that air layer away, even with the weight of the droplet bearing down on it. Before that drainage can happen, the vibration of the pool sends the droplet aloft again, refreshing the air layer beneath it. The droplet falls, gets caught on its air cushion, and then sent bouncing again before the air can squeeze out. If nothing disturbs the droplet, it can bounce almost indefinitely.
Droplets don’t always bounce in place, though. When forced with the right frequency and acceleration, a bouncing droplet can transition to walking. In this state, the droplet falls and strikes the pool such that it interacts with the ripple from its previous bounce. That sends the droplet aloft again but with a horizontal velocity component in addition to its vertical one. In this state, the droplet can wander about its container in a way that depends on its history or “memory” in the form of waves from its previous bounces. And this is where things start to get a bit weird – as in quantum weirdness – because now our walker consists of both a particle (droplet) and wave (ripples). The similarities between quantum behaviors and the walking droplets, the collective behavior of which is commonly referred to as “pilot-wave hydrodynamics,” are rather remarkable. In the next couple posts, we’ll take a look at some important quantum mechanical experiments and their hydrodynamic counterparts.
(Image credit: D. Harris et al., source)
Pilot-Wave Hydrodynamics: Faraday Instability
This post is part of a collaborative series with FYP on pilot-wave hydrodynamics. Previous entries: 1) Introduction; 2) Chladni patterns
In 1831, in an appendix to a paper on Chladni plate patterns, physicist Michael Faraday wrote:
“When the upper surface of a plate vibrating so as to produce sound is covered with a layer of water, the water usually presents a beautifully crispated appearance in the neighborhood of the centres of vibration.” #
Faraday was not the first to notice this, as he himself acknowledged, but it was his many clever observations and tests of the phenomenon that led to its naming as the Faraday instability. Like Chladni patterns, Faraday waves can take many forms, depending on the geometry of the vibrator and the frequency and amplitude of its vibration.
Beneath the “crispations” at the air interface, the liquid inside the pool is also moving, driven by the vibrations into streaming patterns. Sprinkling particles into this flow reveals discrete recirculation zones that depend on the vibrations’ characteristics, as seen above. This behavior can even be used to assemble particles into distinct formations.
When the vibrations are large enough at resonant frequencies, the rippling waves at the surface become violent enough to start ejecting droplets. You can experience this for yourself using a Chinese spouting bowl or a Tibetan singing bowl with some water. It’s also, bizarrely enough, a factor in alligator mating behaviors!
Next time, we’ll explore what happens to a droplet atop a Faraday wave.
(Image credits: N. Stanford, source; L. Gledhill, source; The Slow Mo Guys, source)
DIY Acoustic Levitation
Acoustic levitation is a technique where multiple speakers are positioned to create standing waves that can levitate small objects using sound. It’s even possible to manipulate the levitating objects in three-dimensions with the right set-up, but until now, the technology has been confined to the laboratory. Now a group from the University of Bristol has created kits and instructions allowing the curious to build their own acoustic levitators at home. In the video, Dianna shares some of her own adventures in building and playing with these DIY levitators and travels to the U.K. to see more from the creators.
I know what I’m adding to my list of electronics projects to try out! (Video credit: Physics Girl)