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)
Category: Research
Reducing Viscosity With Bacteria
Conventional wisdom – and the Second Law of Thermodynamics – require all fluids to have viscosity, with the noted and bizarre exception of superfluids, which can flow with zero viscosity. In essence, you cannot have work (i.e. flow) for free. Some effort has to be lost to resistance.
But scientists have discovered, bizarrely, that adding bacteria to water can result in zero or even negative viscosities – meaning that effort is required to keep the flow from accelerating. Before you ask, no, this is not a recipe for a perpetual motion machine. What happens when the bacteria-filled fluid is sheared is that the bacteria align and start collectively swimming. The local effects of each bacteria combine en masse to create a fluid that seemingly flows on its own. In the end, though, it’s the bacteria that are supplying that work. It certainly raises interesting prospects, though, for harnessing the power of bacterial superfluids. See the links below for more. (Image credit: M. Copeland, source; research credit: S. Guo et al., A. Loisy et al.; via Quanta; submitted by Kam-Yung Soh)
Grain Networks
Granular materials are complicated beasts. When packed, forces between grains create a network (above) that shifts as force is applied. And, while grains can stick and resist that force, push a little further and they may slip and avalanche. A new study of this stick-slip behavior monitors disks similar to those above by listening for changes leading up to the slip. Researchers found that vibrations inside a granular material changed measurably before the grains slipped. The scientists hope this will one day allow for monitoring of landslide and avalanche-prone areas. While the changes are not enough to definitively predict when a slide will occur, they may provide valuable estimates of when one is likely. (Research credit: T. Brzinski and K. Daniels; image credit: OIST, source; via J. Ouellette)
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)
A Burst of Microdroplets
If you hold a bubbly beverage like champagne or soda near your face, you’ll feel a light mist of tiny, nearly invisible droplets.These droplets form when bubbles reach the surface and pop, generating a tiny jet that ejects an even tinier droplet, as shown in the animation above. This process is remarkably common; its occurrence in the ocean results in billions of tons of sea salt entering our atmosphere each year. Since these tiny microdroplets stay aloft for far longer than their larger brethren, understanding how they form and just how small they can be is vital for understanding their impact on climate, pathogen spreading, and other topics. A new study suggests that the minimum size for an ejected droplet is just 1% of the size of the bubble that births it. (Image and research credit: C. F. Brasz et al., source)
A Viscous Splash
The splash of a drop may be commonplace, but it is still a mesmerizing and fertile phenomenon. When it comes to splashing, scientists are still learning how to predict the outcome. Here a drop of silicon oil impacts a film of silicon oil with an even higher viscosity. The momentum of that impact creates a crater and a splash curtain that rises and expands from the initial point of impact. Because the film viscosity is higher than the drop’s, the evolution of the corona slows down. Eventually, surface tension and gravity start pulling the splash curtain back down as the crater collapses. Meanwhile at the top of the splash, capillary forces pull fluid into the rim, which becomes unstable and grows cusps that eventually eject a cloud of smaller droplets. (Image and research credit: H. Kittel et al., source)
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.)
Breaking With a Wave
For rocket combustion and other applications, like watering your lawn with a hose, a stream of fluid may need to be broken up into droplets. While simply spraying a liquid jet will make it break up, waving that jet back and forth will break it up faster. A recent study simulated this problem numerically to determine the exact mechanisms driving that break-up. The researchers found two major culprits.
The first is a Kelvin-Helmholtz, or shear-based, instability. When a jet leaves the nozzle, there’s friction between it and the comparatively still air surrounding it. This creates tiny ripples in the surface that eventually grow into the distortions we can see, and it’s found in all jets, regardless of their side-to-side motion.
The second culprit, which is only found in the oscillating jet, is a Rayleigh-Taylor instability. By moving the jet side-to-side, you’re driving the dense liquid into less dense air, which creates a different set of disturbances that also help break up the jet. The final result: swinging the jet side-to-side breaks it into smaller droplets faster. (Image and research credit: S. Schmidt et al.)
Forming Europa’s Bands
Jupiter’s icy moons, Europa and Ganymede, are home to subsurface oceans. These moons also experience strong tidal forces from their parent planet and sibling moons that squeeze and deform them over time. A new study focuses on the bands, seen in red in the top image of Europa, that form as a result of these deformations. By simulating (bottom image) both the convective currents within the Europan ocean and the deformation of the ice over time, scientists are able to study how these geological surface features may have formed. Over the course of about a million years, material from the interior ocean works its way up into the center of a band. Because this process takes so long, the researchers point out that any attempt to collect material from the bands will yield “fossil” ocean material – essentially a glimpse of Europa’s ocean as it existed a million years ago rather than how it exists today! (Image credit: NASA; image and research credit: S. Howell and R. Pappalardo, source; submitted by Kam-Yung Soh)
Manipulating Droplets Remotely
Using acoustic levitation and an array of carefully-placed speakers, researchers can manipulate droplets without touching them. This lets scientists study the physics of droplet coalescence (top) without interference from solid surfaces, but it also provides opportunities for mixing two different substances in the final droplet.
On the bottom left, we see a droplet formed from the coalescence of a dyed droplet (visible as gray) and an undyed droplet. The swirling and mixing in the levitating droplet is fairly slow. By contrast, the droplet on the right is vibrated by manipulating the sound waves holding it aloft. This mixes the droplet quite efficiently, allowing it to reach a uniform state more than six times faster than the other droplet. (Image and research credit: A. Watanabe et al., source)
