Ultrasonic vibrations can boil nanoscale liquid layers, according to a new simulation-based study. Above you see a layer of water initially about 2 nm thick. When the surface it’s on vibrates at frequencies in the 100 GHz range – about a billion times faster than a hummingbird flaps – it superheats the thin layer of water. In this case, the film undergoes nucleate boiling, forming the same kinds of bubbles you see when boiling a pot of water. When the water layer gets too thin to support nucleate boiling, it stops boiling but evaporation continues. The transition occurs when van der Waals forces become significant. The technique only works with ultrathin layers of a liquid, but the authors envision broad application possibilities in industry as well as in micro- and nano-scale fluid systems. (Image and research credit, and submission: R. Pillai et al.)
Tag: acoustics
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)
Flying Beetles, Stinging Nettles, and Jellyfish
In the latest JFM/FYFD video, we tackle some of the less pleasant aspects of summer weather: stopping invasive insects, understanding how plants dispense poison, and looking at the physics behind jellyfish stings. And if you’ve missed any of our previous videos, we’ve got you covered. (Image and video credit: T. Crawford and N. Sharp)
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)
The Mystery of Carnegie Hall’s Sound
For nearly a century, the acoustics of Carnegie Hall were touted as among the very best in the world. But after a much-needed renovation in 1986, musicians and critics felt the magic of the old sound had been lost. In this video, Gizmodo explores the mystery of what changed. Was it a hole in the ceiling? The curtains that had been removed?
Eventually, a second renovation – this time for warping of the stage floor – revealed the likely culprit. Concrete had been installed to reinforce the stage in the first renovation, and this changed the stage’s resonance. Previously, instruments like the bass had caused the wooden floor to vibrate, which amplified their sound. The concrete damped that vibration, cutting out a key ingredient in Carnegie’s acoustics. When the second renovation restored the all-wooden stage, suddenly the venerable concert hall had its sound back. (Video credit: Gizmodo)
The Sound of Bubbles
When you enjoy the sound of a babbling stream on a hike, what you’re actually hearing is bubbling. Air bubbles caught in the water resonate at a frequency that depends on their size. In fact, you can use a hydrophone – basically an underwater microphone – to listen to these bubbles and learn about them. Researchers recently did exactly that with glasses of sparkling wine. By listening to the bubbles and applying a simple physical model, the researchers could characterize differences in two brands of sparkling wine, including just how bubbly they were and what size their typical bubbles are. They hope eventually to develop acoustic techniques that can monitor quality control for sparkling wines and other carbonated beverages. (Image credit: J. Kääriäinen; research credit: K. Spratt et al.; submitted by Kam-Yung Soh)
When Sound Makes You Vertiginous
For some people, a musical tone is enough to induce vertigo and feelings of being drunk. These individuals often have a small hole or defect in the bone that surrounds the canals of the inner ear. Normally, the fluid inside these canals reacts when we rotate our heads, triggering a counterrotation of our eyes that helps stabilize the image on our retinas. But when there’s a defect in the bone surrounding the canal, certain acoustic tones may pump that fluid directly. The patient’s eyes then try to correct for a rotation that’s not occurring, thereby inducing dizziness and vertigo. (Image credit: M. Moiner; research credit: M. Iversen et al.; submitted by Marc A.)
The Dangerous Clatter of Dishes
Have you ever noticed how loud dishes are when you’re handling them? Under the right (or, perhaps more accurately, wrong) circumstances, the clatter of ceramics like porcelain can be dangerously loud, as engineer Phil Metzger discovered when repairing his toilet. At one point the lid to his tank slipped from his hands and fell about 20 centimeters to strike the edge of the toilet. The lid did not break, but Metzger stumbled away stunned from the loud noise. He immediately noticed that his hearing was distorted – he described his own voice as sounding “like talking through a kazoo”. Upon further experiment, he found that the distortion occurred at specific, regularly-spaced frequencies. Like any engineer, therefore, he turned to physics to analyze the accident.
Since the lid didn’t break, he knew that the energy from the lid’s fall went into two places: the sound he heard and a small amount of dissipated heat. Using the speed of sound in a ceramic and the dimensions of the lid, he was able to calculate the frequency of sound produced by the impact, and with a little more work, he could estimate that the sound, as transmitted to his nearby ear, had been about 138 dB. Permanent damage from brief sounds can occur at 140 dB, so this was well inside the danger zone. The pressure from sounds this loud is enough to severely bend the tiny hairs in your cochlea that are responsible for sensing these vibrations. Luckily for Metzger, his hearing did recover after a few days, but it’s a good reminder to be careful. Sometimes everyday physics can be surprisingly dangerous! (”Research” credit: P. Metzger; image credit: comedynose/Flickr; via Motherboard via J. Ouellette)
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)
Pigeon Flutter
Birds are well-known for their vocalizations, but this isn’t their only way to produce noise. A new study on crested pigeons finds that the birds’ wings produce distinctive high and low notes during take-off. A low note takes place during each upstroke, and a high note is heard during the downstroke. A major source of the noise is the highly modified P8 feather. When airflow over the feather is fast enough, it sets off twisting and torsion in the feather through aeroelastic flutter. It’s this vibration that causes the noise. By playing back the notes at different speeds, researchers found that the crested pigeons use the notes’ timing as an alarm. When the cycle of high and low repeats in quick succession, they respond by taking off to escape the perceived danger.
Other bird species are also known to use aeroelastic flutter to make noise. Check out these hummingbirds, which use flutter in their mating displays. (Video credit: Science; research credit: T. Murray et al.)
