Putting out fires can be a difficult, water-intensive task. In this video, scientists demonstrate how using a non-Newtonian fluid can make it easier to extinguish and suppress flames. Where water tends to splatter and scatter against an object, a yield-stress fluid can cling and coat to smother the flame. The fluid used here is water with a 0.1% polymer additive, which is enough to significantly change the fluid’s rheological properties. Pre-treating flammable objects with the fluid is also effective at suppressing combustion, raising additional possibilities for using such techniques in fighting the spread of wildfires. (Video credit: B. Blackwell et al.)
Category: Research
Pollock-Style Physics
Here on FYFD, we like to show off the artistic side of fluid dynamics. But some researchers are actively studying how artists use fluid dynamics in their art. In this video, they examine one of Jackson Pollock’s painting techniques, in which filaments of paint were applied by flinging paint off a paintbrush. Getting the technique to work requires a fine balance of forces and effects. Firstly, the paint must be viscous enough to hold together in a filament when flung. Secondly, the centripetal acceleration of the rotation must be high to both form the catenary filament and throw it off the brush. And, finally, the Reynolds number needs to be high enough to add some waviness and instability to the filament so that it looks interesting once it hits the canvas. Also be sure to check out the group’s previous work exploring Siqueiros’s painting techniques. (Video credit: B. Palacios et al.)
Flowing Water on Mars?
Scientists have known for years that Mars once had liquid water on its surface, and they have many contemporary examples of frozen water ice on the Red Planet. But this week NASA announced the strongest evidence yet that liquid water still flows on Mars. Researchers have observed from orbit dark line-like features called recurring slope lineae (RSL) that develop, darken, and grow seasonally in many locations on Mars. The appearance of these features coincides with warmer surface temperatures (above -23 degrees Celsius), and the lines fade again when temperatures cool. Although scientists suspected the dark lines might be related to flowing water, the evidence remained circumstantial until spectral observations of multiple sites indicated that the darker features contained hydrated salts. In other words, briny salt water is still flowing at or near the Martian surface. (Image credits: NASA)
Healing Soap Films
As fragile as a soap bubble seems, these films have remarkable powers of self-healing. The animation above shows a falling water droplet passing through a soap film without bursting it. An important factor here is that the water droplet is wet–passing a dry object through a soap film is a quick way to burst it, as those who have played with bubbles know. The droplet’s inertia deforms the soap film, creating a cavity. If the drop’s momentum were smaller, the film could actually bounce the droplet back like a trampoline, but here the droplet wins out. The film breaks enough to let the drop through, but its cavity quickly pinches off and the film heals thanks to the stabilizing effect of its soapy surfactants. (Image credit: H. Kim, source)
Miniature Bursting Bubbles
Fizzy drinks like soda or champagne contain dissolved carbon dioxide which forms bubbles when the pressure inside its container is released. The tiny bubbles rise to the surface where the liquid film covering them can rupture, creating a small cavity at the surface. The cavity collapses in a matter of milliseconds (bottom animation). Above the surface, the cavity reverses its curvature to create a liquid jet (top animation) which can expel multiple tiny droplets. These droplets can tickle a drinker who hovers too close, but they also carry and distribute the aroma molecules that are part of the experience of a drink like champagne. (Image credit: E. Ghabache et al., source)
(Today’s topic brought to you by my impending nuptials to my favorite physicist/spacecraft engineer.)
Printing in Glass
A group at MIT have created a new 3D printer that builds with molten glass. This allows them to manufacture items that would difficult, if not impossible, to create with traditional glassblowing or other modern techniques. One of the coolest aspects of this technique is that it can use viscous fluid instabilities like the fluid dynamical sewing machine to create different effects with the glass. You can see this around 1:56 in the video. Varying the height of the head and the speed at which it moves will cause the molten glass to fall and form into different but consistent coiling patterns. All in all, it’s a very cool application for using some nonlinear dynamics! (Video credit: MIT; via James H. and Gizmodo)
Io’s Magma Ocean
Jupiter’s moon Io is the most volcanically active world in our solar system. The energy that drives its geological activity comes from tidal forces the moon experiences from Jupiter and from other Jovian moons. These forces flex the moon and heat its interior via friction. Previous models of Io’s tidal heating assumed a solid body, but their results predicted volcanoes in locations that did not match observations of the moon. A new study suggests that the missing piece of the puzzle is a subsurface ocean of magma. Highly viscous liquids like magma also generate heat when deformed by tidal forces, and applying this model to Io allowed scientists to better match the volcano distribution actually seen on the world. For more, check out NASA’s article. (Image credit: NASA; via Gizmodo; submitted by jshoer)
How Dogs and Cats Drink
We humans do our hands-free drinking via suction, using the shape of our lips and mouths to create low pressure that draws liquids in. Dogs and cats, on the other hand, have no cheeks and, therefore, no suction. Instead, both cats (top) and dogs (bottom) drink using adhesion, or the tendency of a liquid to stick to a surface. Both species flatten part of their tongue against the water surface, then pull it up rapidly. This draws a column of water up after their tongue, which they then snap their jaws closed around. Although they use the same method, cats are daintier drinkers than dogs, which sometimes leads to the misconception that the animals drink differently. (Image credits: NYTimes, source; research credit: S. Jung et al.)
Boiling Water in Oil
Most people know that throwing water into hot oil is a bad idea. But, as dramatic as the results can be, the boiling of a water droplet submerged in oil is remarkably beautiful, as seen in the animations above. The initial water droplet expands as it shifts from liquid to vapor (top). At a critical volume, the expansion occurs explosively (middle), causing the bubble to overexpand relative to the pressure of the surrounding fluid. The higher pressure of the oil around it collapses the drop, which then re-expands, creating the cycle we see in the final two animations. This oscillation triggers a Rayleigh-Taylor type instability along the bubble’s interface, causing the surface corrugations observed. The vapor bubble will continue to rise through the oil, eventually breaking the surface and scattering hot oil droplets. (Image credits: R. Zenit, source)
Shock Waves in Flight
Schlieren optical systems have been used to visualize shock waves in labs for more than a century, but the technique did not translate well to photographing shock structures outside the lab. But now NASA’s Armstrong Research Center and Ames Research Center have developed a method that allows them to capture highly-detailed images of the shock waves around airplanes while they are flying. This is incredible stuff. Be sure to check out the high-resolution versions on this page, along with more description of the coordination necessary to pull off the photos.
The light and dark lines you see emanating from the airplane are places with strong density gradients. The dark lines are mostly shock waves, with the strongest shock waves appearing black due to the large change in air density. Many of the light streaks are expansion fans, areas where the density and pressure drop as air speeds up.
The goal of this research is to better understand shock wave structures around supersonic planes in order to reduce the noise supersonic aircraft cause when flying overhead. As you can see in the photos, the shock waves at the nose and tail of the aircraft persist far away from the aircraft; these are what cause the twin sonic boom heard when the plane flies by. (Photo credit: NASA; via J. Hertzberg)
