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  • The Capillary Channel Flow Experiment

    The Capillary Channel Flow Experiment

    Moving fluids around in microgravity can be a challenge. On Earth we experience buoyancy and other gravitational effects that dominate how fluids move. In space, on the other hand, the only options are to move fluids mechanically with pumps or fans or to use capillary action. Even on earth, adhesive forces between a liquid and its solid container can draw fluids in narrow tubes upward against the force of gravity. In microgravity, this capillary flow can be even more effective. But the best way to study and understand this flow regime is to do so in space. The Capillary Channel Flow experiment and similar studies have allowed astronauts on the space station and researchers back on Earth to explore the effects of capillary action on microgravity fluid transport. The results will be used to improve propulsion systems, heat exchangers, and life support systems used in space. (Photo credits: NASA, M. Dreyer et al., and A. Agrawala; submitted by jshoer)

  • Space Balls (of Water!)

    Space Balls (of Water!)

    The microgravity environment of space is an excellent place to investigate fluid properties. In particular, surface tension and capillary action appear more dramatic in space because gravitational effects are not around to overwhelm them. In this animation, astronaut Don Petit injects a jet of air into a large sphere of water. Some of the water’s reaction is similar to what occurs on Earth when a drop falls into a pool; the jet of air creates a cavity in the water, which quickly inverts into an outward-moving jet of water. In this case, the jet is energetic enough to eject a large droplet. Meanwhile, the momentum, or inertia, from the air jet and subsequent ejection causes a series of waves to jostle the water sphere back and forth. Surface tension is strong enough to keep the water sphere intact, and eventually surface tension and viscosity inside the sphere will damp out the oscillations. You can see the video in full here. (Image credit: Don Petit/Science off the Sphere)

  • Convective Impressionism

    Convective Impressionism

    Buoyant convection, driven by temperature-dependent changes in density, is a major force here on Earth. It’s responsible for mixing in the oceans, governs the shape of flames, and drives weather patterns. The images above show flow patterns caused by buoyant convection. The colors come from liquid crystal beads immersed in the fluid; red indicates cooler fluid and blue indicates warmer fluid. You can see plumes of warmer fluid rising in some of the photos. At the same time, though, the images are beautiful simply as art and are strongly reminiscent of works by Vincent van Gogh. (Image credit: J. Zhang et al.)

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    Holiday Fluids: What is Fire?

    Snowy holidays and long, dark nights are a great time to sit by the fire or enjoy some candlelight. We’ve talked before about how buoyancy affects a flame’s shape, how atomization mixes liquid fuel and oxidizers, how flames propagate, how internal combustion works and how instabilities can end combustion. But in all that we haven’t addressed what fire actually is! Combustion is a chemical process–a reaction between a hydrocarbon fuel and oxygen, but the flame we’re accustomed to seeing is a combination of blue light produced by the complete reaction and incandescent red/orange/yellow light from glowing soot particles produced when there is insufficient oxygen for the reaction. If you have time after the Minute Physics version, this video from Ben Ames has a wonderful explanation of flames. Of course, if you just prefer your holiday fun with more explosive high-speed videos, you’re going to want to see this Christmas tree made from detonation cord (see 2:40 for the start of the best part). This wraps up our holiday-themed fluid dynamics series. Happy holidays from FYFD! (Video credit: Minute Physics)

  • Fluids Round-up – 27 July 2013

    Fluids Round-up – 27 July 2013

    Fluids round-up time! Here are our latest fluidsy links from around the web:

    (Photo credit: T. Thai)

    Reminder: This weekend is your final chance to take the reader survey! Thank you to everyone who has taken a couple minutes to share their thoughts.

  • Drop-Tower Droplets

    Drop-Tower Droplets

    A microgravity environment can cause some nonintuitive behaviors in fluids. Many of the effects that dominate fluid dynamics in space are masked by gravity’s effects here on Earth. As a result, it can be very difficult to predict how seemingly straightforward technologies like heat exchangers, refrigeration units, and fuel tanks will behave. The photos above show two bubble jets–created by injecting a liquid-gas mixture into a liquid–colliding in microgravity. This particular experiment was conducted in a drop tower rather than on-orbit, which produced some side effects like the large bubbles seen in the images. These were created by the coalescence of smaller bubbles that congregated near the top of the tank shortly before the experiment attained free-fall. (Photo credit: F. Sunol and R. Gonzalez-Cinca)

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    Washing Your Face in Space

    What happens to a wet washcloth when wrung out in space? Astronaut Chris Hadfield answers this question from students with a demonstration. Without gravity to pull the water downward, surface tension effects dominate and the wrung cloth forms a tube of water around it. Surface tension and capillary action draw the fluid up and onto Hadfield’s hands as long as he holds the cloth. After he lets go, we see that the water remaining around the cloth soaks back in (again due to capillary action) and the wet, twisted washcloth simply floats without releasing water or relaxing its shape. While pretty much what I would have expected, this was a very cool result to see! (Video credit: C. Hadfield/CSA; submitted by Bobby E)

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    Gravity’s Effect on Bursting Bubbles

    In a gravitational field, the pressure in a fluid increases with depth. You can consider it due to the weight of the fluid above. Outside of scuba diving or hiking at altitude, this effect is not one typically given much thought. But what effect can it have at a smaller scale? This video shows the collapse and rebound of three initially spherical cavitation bubbles inside a liquid. Each bubble is created in a different gravitational field – one in microgravity, one in normal gravity, and one at 1.8x Earth gravity. The bubble in microgravity remains axisymmetric and spherical, but the two bubbles recorded in gravitational fields develop jets during rebound. Even at a scale of only a few millimeters, gravity causes an imbalance in pressure across the bubble that creates asymmetry. (Video credit: D. Obreschkow et al.)

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    Spitting Droplets

    Any phenomenon in fluid dynamics typically involves the interaction and competition of many different forces. Sometimes these forces are of very different magnitudes, and it can be difficult to determine their effects. This video focuses on capillary force, which is responsible for a liquid’s ability to climb up the walls of its container, creating a meniscus and allowing plants and trees to passively draw water up from their roots. Being intermolecular in nature, capillary forces can be quite slight in comparison to gravitational forces, and thus it’s beneficial to study them in the absence of gravity.

    In the 1950s, drop tower experiments simulating microgravity studied the capillary-driven motion of fluids up a glass tube that was partially submerged in a pool of fluid. Without gravity acting against it, capillary action would draw the fluid up to the top of the glass tube, but no droplets would be ejected. In the current research, a nozzle has been added to the tubes, which accelerates the capillary flow. In this case, both in terrestrial labs and aboard the International Space Station, the momentum of the flow is sufficient to invert the meniscus from concave to convex, allowing a jet of fluid out of the tube. At this point, surface tension instabilities take over, breaking the fluid into droplets. (Video credit: A. Wollman et al.)

  • Liquid Mushrooms

    Liquid Mushrooms

    The Rayleigh-Taylor instability can form at the interface between two liquids of different density under the influence of gravity, but a similar instability can occur in the absence of gravity. The image sequence above shows the Richtmyer-Meshkov instability, which occurs between two liquids of differing densities (regardless of their orientation) when impulsively accelerated. In this case, the experiment was conducted in a drop tower to simulate microgravity with the apparatus dropped on a spring to provide the impulse. As the instability grows, asymmetries appear.  Nonlinear dynamics will amplify these distortions, eventually leading to turbulent breakdown. (Photo credit: C. Niederhaus/NASA Glenn, J. Jacobs/University of Arizona)

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