Adding just a few polymers to a liquid can substantially change its behavior. The presence of polymers turns otherwise Newtonian fluids like water into viscoelastic fluids. When deformed, viscoelastic fluids have a response that is part viscous–like other fluids–and part elastic–like a rubber band that regains its initial shape. The collage above shows what happens to a thinning column of a viscoelastic fluid. Instead of breaking into a stream of droplets, the liquid forms drop connected with a thin filament, like beads on a string. In a Newtonian fluid, surface tension would tend to break off the drops at their narrowest point, but stretching the polymers in the viscoelastic fluid provides just enough normal stress to keep the filament intact. If the effect looks familiar, it may be because you’ve seen it in the mirror. Human saliva is a viscoelastic liquid! (Image credit: A. Wagner et al.)
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Flames in Space
The jellyfish-like light show in the animations above shows the life and death of a flame in microgravity. The work is part of the Flame Extinguishment Experiment 2 (FLEX-2) currently flying aboard the International Space Station. When ignited, the fuel droplet creates a blue spherical shell of flame about 15 mm in diameter. The spherical shape is typical of flames in microgravity; on Earth, flames are shaped like teardrops due to the effects of buoyancy, which exists only in a gravitational field. The bright yellow spots and streaks that appear after ignition are soot, which consists mainly of hot-burning carbon. The uneven distribution of soot is what causes the pulsating bursts seen in the middle animation. When soot products drift back onto the fuel droplet, it causes uneven burning and flame pulses. The final burst of flame in the last animation is the soot igniting and extinguishing the flame. Fires are a major hazard in microgravity, where oxygen supplies are limited and evacuating is not always an option. Scientists hope that experiments like FLEX-2 will shed light on how fires spread and can be fought aboard spacecraft. For more, check out NASA’s ScienceCast on microgravity flames. (Image credits: NASA, source video; submitted by jshoer)
Antibubble Vortex Rings
Bubbles are familiar, but antibubbles are a bit more unusual. An antibubble typically has a liquid-air-liquid interface, with a thin shell of air separating a liquid droplet from the surrounding fluid. Although they look rather like bubbles, antibubbles behave differently. Antibubbles are, for example, very sensitive to pressure changes. A sinking antibubble like the one in the video above, experiences a higher pressure on its lower face. This pressure compresses the gas shell and thins it on the bottom. The air shell bursts at the thin point and the antibubble collapses, generating two vortex rings and a small, buoyantly rising bubble. (Video credit: S. Dorbolo et al.)
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The Physics of Sneezing
Sneezing can be a major factor in the spread of some illnesses. Not only does sneezing spew out a cloud of tiny pathogen-bearing droplets, but it also releases a warm, moist jet of air. Flows like this that combine both liquid and gas phases are called multiphase flows, and they can be a challenge to study because of the interactions between the phases. For example, the buoyancy of the air jet helps keep smaller droplets aloft, allowing them to travel further or even get picked up and spread by environmental systems. Researchers hope that studying the fluid dynamics and mathematics of these turbulent multiphase clouds will help predict and control the spread of pathogens. Check out the Bourouiba research group for more. (Video credit: Science Friday)
Soap Film Physics
Soap films consist predominantly of water, yet their thin, virtually two-dimensional nature is impossible for water alone to achieve. The small amount of added soap acts as a surfactant, lowering the surface tension of the fluid and preventing it from bursting into droplets. When forming a film, the soap molecules align themselves along the outer surfaces of the film, with their hydrophilic heads among the water molecules and their hydrophobic tails oriented outward. For the most part, the water molecules stay sandwiched between the surfactant layers, forming a film only about as thick as the wavelength of visible light. In fact, the psychedelic colors of a soap film are directly related to the film’s thickness with the black regions being the thinnest. The video above shows a horizontal soap film at the microscopic scale and some of the dynamics exist therein. (Video credit: J. Hart)
Breaking Drops with Vibration
Atomization is the process of breaking a liquid into a spray of fine droplets. There are many methods to accomplish this, including jet impingement, pressure-driven nozzles, and ultrasonic excitement. In the images above, a drop has been atomized through vibration of the surface on which it rests. Check out the full video. As the amplitude of the surface’s vibration increases, the droplet shifts from rippling capillary waves to ejecting tiny droplets. With the right vibrational forcing, the entire droplet bursts into a fine spray, as seen in the photo above. The process is extremely quick, taking less than 0.4 seconds to atomize a 0.1 ml drop of water. (Photo and video credit: B. Vukasinovic et al.; source video)
Hydrophobicity and Viscous Flow
Hydrophobic surfaces are great for creating some wild behaviors with water droplets, but they make neat effects with other liquids, too. The viscous honey in the first segment of this Chemical Bouillon video is a great example. Because the honey doesn’t adhere to the hydrophobic surface, the viscoelastic fluid does not maintain the form it had when drizzled on the surface. Instead, the honey contracts, with surface tension driving Plateau-Rayleigh-like instabilities that break the contracting ligaments apart to form nearly spherical droplets of honey on the surface. (Video credit: Chemical Bouillon)
What’s in a Splash?
A droplet falling onto a solid, dry surface seems like a simple situation, one that would be easy to understand. But splashes can be unpredictable. Velocity, viscosity, and surface tension all play clear roles, but the surrounding air also has an impact – drop the air pressure low enough and a droplet won’t splash. A new paper has tackled the problem, producing a mathematical model in agreement with experimental results. So why do some drops splash and others don’t? When a drop falls, its momentum flattens it into a pancake shape while surface tension struggles to hold it together. The spreading edge, called the lamella, can pull away from the surface. When it does, a pocket of high pressure forms beneath it due to lubrication effects, and the faster airflow over the top of the lamella creates a suction effect. This is analogous to a wing producing lift. Like the momentum that spread the droplet, the lift force pulls the lamella and ejecta sheet further up and outward, overcoming the restoring force of surface tension and tearing the droplet apart. For more on the effect, check out the research paper or this Inside Science article. (Video credit: G. Riboux and J. Gordillo; via Inside Science)
Champagne Bubble Physics
Champagne is well-known for its effervescence, but its tiny bubbles do more than affect your sensation when sipping. Champagne bubbles form when carbon dioxide dissolved in the wine nucleates along imperfections in the glass. Buoyancy causes them to flow upwards, growing as they pull more carbon dioxide from the surrounding champagne. When the bubbles reach the surface, they pop, sending an almost imperceptible fountain of tiny droplets into the air, as seen in the photo above. You can sometimes feel the droplets if you hold a glass near your face. The droplets released from the bursting champagne bubbles spread the aroma of the wine, imparting additional flavor through our olfactory sense. (Photo credit: F. Beaumont et al.)
Healing Bubbles
Soap bubbles are ephemeral creations. The slightest prick will send them tearing apart in the blink of an eye. It may come as a surprise, therefore, that dropping a water droplet through a bubble will not break it. Instead, the bubble will heal itself using the Marangoni effect. In a soap bubble, the soap molecules act as a surfactant, lowering the surface tension of the water and allowing the fragile structure to hold together. When the water drop impacts the bubble, the local surface tension increases because of the relative lack of soap molecules. This increase in surface tension pulls at the rest of the bubble, drawing more soap molecules toward the point of contact. The effect evens out surface tension across the surface and stabilizes the bubble. You can test the effect at home, too. If you wet your finger, you can poke a soap bubble without popping it. (Video credit: G. Mitchell; via io9)
