Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "capillary action"

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)

It is common in many industries to use oil as a defoamer to break up existing foams or prevent foams from forming. But with the right surfactants—additives that change the foam’s surface tension—it’s possible to make aqueous foams that are actually stabilized by the presence of oil. This video explores some of the ways that oil can interact with these kinds of foam, beginning with capillary action, which draws the oil up into the junctions between foam films. For more, see Piroird and Lorenceau. (Video credit and submission: K. Piroird)

Water bridges that seem to float on air are an electrohydrodynamic phenomenon. By filling two beakers with extremely pure deionized water and applying a large voltage across them, flow is induced from one beaker to the other, as seen in the first few seconds of the video above. This flow is stable enough that the beakers can then be separated by a few centimeters without disturbing the bridge. Gravity tends to make the water bridge sag and capillary action tries to thin the bridge, but both effects are countered by the polarization forces induced in the water by the electric field. You can learn much more about the effect and see both photos and videos of it in action at Elmer Fuchs’ webpage. The flow visualization videos are especially neat! (Video credit: E. C. Fuchs)

Anyone who has eaten a bowl of Cheerios is familiar with the way solid objects floating on a liquid surface will congregate. This is a form of capillary force driven by the wetting of the particles, surface tension, and buoyancy. Using ferromagnetic particles and a vertical magnetic field, one can balance capillary action and lock the particles into a fixed configuration relative to one another. By adding a second, oscillating magnetic field, it’s possible to make the beads dance and swim together. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: M. Hubert et al.)

Back in 1999 Len Fisher earned an Ig Nobel Prize in Physics for explaining the physics of dunking a biscuit or cookie in a liquid. The cookie is porous, with many tiny, interconnecting channels run throughout it. When dipped in a liquid, capillary action pulls the fluid up into these channels against the force of gravity. As most people discover, this wetting can soften the cookie to the point of collapse. The optimal manner of dunking then is to hold the cookie at a shallow angle; this allows the lower surface to soak in milk (or the hot beverage of your choice) while keeping the upper surface dry and structurally sound. Fisher further argued that Washburn’s equation, which describes the time necessary for capillary action to draw a liquid up a given length of a cylindrical pore gives a good estimate of the length of time for a cookie dunking. This proved so popular he even wrote a book about it. This is a part of a series on fluids-related Ig Nobel Prizes. (Photo credit: C. Lindberg; research credit: L. Fisher)

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.

At very small scales, the interaction of solids and liquids is governed by molecular forces. Here researchers demonstrate how carbon nanowires of only a few nanometers in diameter draw liquid up in a film or bead when inserted in a pool. Capillary action is the name we give this gravity-defying force generated between the liquid and solid molecules. Although this behavior was predicted theoretically, it had not been previously observed at this scale due to the need for electron microscopy. Such microscopes require a vacuum, which boils off almost any liquid instantaneously. Researchers used a special fluid that remained in a liquid state even under near-vacuum pressures in order to make these observations. (Video credit: J. Li et al/MIT News; submitted by 20percentvitaminc)

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)

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.)

This short film offers an artistic look at the phenomenon of the water bridge. When subjected to a large voltage difference, such as the 30 kV used in the film, flow can be induced between water in two separated beakers. This creates a water bridge seemingly floating on air. There are two main forces opposing the bridge: gravity, which causes it to sag, and capillary action, which tries to thin the bridge to the point where it will break into droplets. These forces are countered by polarization forces induced at the liquid interface due to the electrical field separating the water’s positive and negative charges. This separation of charges creates normal stresses along the water surface, which counteracts the gravitational and capillary forces on the bridge. The artist has done a beautiful job of capturing the unsteadiness and delicacy of the phenomenon. (Video credit: Lariontsev Nick)