FYFD

Tag: soap film

  • Psychedelic Soap Film

    Psychedelic Soap Film

    Macro images of a soap film burst with color. Because the color comes from interference between light waves bouncing off the inner and outer surfaces of the soap film, the colors we see correspond directly to the thickness of the soap film. So the patterns we see reflect actual flows and variations inside the soap film. It’s not unusual for the patterning on a soap film to become increasingly complicated as the film drains and ages. Eventually black spots — areas too thin for interference to show visible colors — will appear and grow, and the film will pop.

    If you’re interested in trying out some soap film photography for yourself, Professor Andrew Davidhazy has a nice description on his website of the set-up he used for this photo. (Image credit: A. Davidhazy; via Flow Vis)

  • Flexible Filament Reduces Drag

    Flexible Filament Reduces Drag

    Most shapes aren’t streamlined for fluid flow. We call these bulky, often boxy shapes, bluff bodies. Above, we see two examples of a bluff body, a flat plate, in a soap film. On the left, the plate sits perpendicular to the soap film’s top-to-bottom flow. Two large, counter-rotating vortices form behind the plate and a wide wake stretches behind it.

    On the right, we see the same flat plate but now a long, flexible filament is attached to either end. As the flow moves past, it deforms the filament, creating a rounded shape. Researchers found that, under the right conditions, this flexible afterbody could reduce drag on the object by up to 10%. (Image and research credit: S. Gao et al.)

  • Collapsing Inside a Soap Film

    Collapsing Inside a Soap Film

    There’s a common demonstration of surface tension where a loop of string is placed in a soap film and then the film inside the loop is popped, making it suddenly form a perfect circle when the outer soap film’s surface tension pulls the string equally from every direction. In this video, researchers study a similar situation but with a few wrinkles.

    Here the loop of string is replaced with an elastic ring, which has more internal stiffness and starts out entirely round within the soap film. Then the researchers pop the outer film. That burst instantly creates a stronger surface tension inside the ring, which causes it collapse inward. As the researchers note, this is the equivalent situation to applying an external pressure on the outside of the ring. The form of the buckling ring and film depends on just how large this “pressurization” is.

    When the elastic ring is thickened to a band, popping the outer soap film makes the band wrinkle out of the plane.

    Thickening the elastic from a ring to a band alters the collapse, too. The thicker the elastic band, the harder it is to buckle in the plane of the soap film. So instead it wrinkles as the film collapses, which creates wrinkles in the soap film, too! (Image, video, and research credit: F. Box et al.; see also F. Box et al. on arXiv)

  • Soap Film Evolution

    Soap Film Evolution

    The beautiful colors of a soap film reflect its variations in thickness. As a film drains and evaporates, it turns to shades of gray and black as it gets thinner. More than fifty years ago, one scientist proposed a free-energy-based explanation for how such ultrathin films might evolve. But it’s taken another half a century for experimental techniques to reach a point where the thickness of these ultrathin films could be measured well enough to test that theory. The new mechanism, known as spinodal stratification, has been observed in both vertical films (top) and foam (bottom) but has so far not been observed in any horizontal configuration, suggesting that buoyant effects are likely important, too. (Image and research credit: S. Yilixiati et al.; submitted by James S.)

  • Foam Collapse

    Foam Collapse

    Introduce the right additive and the bubble arrays in foam will collapse catastrophically. What you see above is high-speed video of a quasi-two-dimensional soap bubble foam collapsing. There are two main mechanisms in the collapse. The first is a propagating mode. When one section of the film breaks, a stream of liquid from the broken film can impact an adjacent section, causing it to break as well. This accounts for much of the breakage you see above.

    The second mode is through penetration by droplets. Watch carefully, and you’ll see that some of the breaking films generate tiny droplets which can fly through the wall of the next cell and impact against the far side. With the right conditions, that impact can trigger a new break along a non-adjacent film. Together, these two mechanisms can destroy foam in the blink of an eye. (Image and research credit: N. Yanagisawa and R. Kurita)

  • Soapy Rainbows

    Soapy Rainbows

    The swirling psychedelic colors of a soap bubble come from the interference of light rays bouncing off the inner and outer surfaces of the film. As a result, the colors we see are directly related to the thickness of the soap film. Over time, as a film drains, black spots will appear in it. This happens where the bubble’s wall becomes thinner than the wavelength of visible light. Black spots will grow and merge as the film continues to thin. Then, when it’s too thin to hold together any longer, the bubble will pop and disappear. (Image credit: L. Shen et al., source)

  • Soap Film Catenoid

    Soap Film Catenoid

    Even very simple fluid systems can have surprising complexity. What you see here is a catenoid – the hourglass-like soap film that forms between two rings. In this case, the space in the center of the catenoid has a secondary film separating the top and bottom halves of the catenoid. When the rings are pulled apart, the waist of the catenoid and the secondary film inside it collapse. The secondary film gets thicker as its diameter decreases. (The fluid has to go somewhere, after all.) As the film thickens, the pressure inside it rises, eventually pushing some of the fluid out through the catenoid. This is what causes the fingers flowing down the lower half of the catenoid in the bottom two images. (Image and research credit: R. Goldstein et al.)

  • Bouncing Off a Film

    Bouncing Off a Film

    Surface tension is the result of an imbalance between intermolecular forces near an interface. Imagine a water molecule far from the surface; it is surrounded on all sides by other water molecules and feels each of those pulling on it. Since all the nearby molecules are water, the tugs from every direction balance and there is no net force. Now imagine that water molecule near the air interface. Instead of being influenced on all sides by water, our molecule now feels water in some directions and air molecules in another. The water molecules tug harder on it than air, leaving a net force that pulls along the interface. This is surface tension, and, for a liquid-gas interface, it behaves somewhat like an elastic sheet. Surface tension is even strong enough to let a jet of soap solution bounce repeatedly off a soap film. Each bounce deforms the interface, like a trampoline dimpling when someone jumps on it, but surface tension keeps the interface taut enough for the jet to skip off without breaking it. (Image credit: C. Kalelkar and S. Phansalkar, source)

  • Self-Healing Bubbles

    Self-Healing Bubbles

    Soap films have the remarkable property of self-healing. A water drop, like the one shown above, can pass through a bubble (repeatedly!) without popping it. This happens thanks to surfactants and the Marangoni effect. Surfactants are molecules that lower the surface tension of a liquid and congregate along the outermost layer of a soap film. When water breaks through the soap film, its lack of surfactants causes a higher surface tension locally. This triggers the Marangoni effect, in which flow moves from areas of low surface tension toward ones of high surface tension. That carries surfactants to the region where the drop broke through and helps stabilize and heal the soap film. Incidentally, the same process lets you stick your finger into a bubble without popping it as long as your hand is wet! (Image credit: G. Mitchell and P. Taylor, source)

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    Songs in Soap

    There are many beautiful ways to visualize sound and music – Chris Stanford’s fantastic “Cymatics” music video comes to mind – but this is one I haven’t seen. This visualization uses a soap film on the end of an open tube with music playing from the other end. You can see the set-up here. The result is a fascinating interplay of acoustics, fluid dynamics, and optics. As sound travels through the tube, certain frequencies resonant, vibrating the soap film with a standing wave pattern (3:20). At the same time, interference between light waves reflecting off the front and back of the soap film create vibrant colors that show the film’s thickness and flow.

    When the frequency and amplitude are just right, the sound excites counter-rotating vortex pairs in the film (0:05), mixing areas of different thicknesses. With just a single note, the vortex pairs appear and disappear, but with the music, their disappearance comes from the changing tones. Watching the patterns shift as the film drains and the black areas grow is pretty fascinating, but one of the coolest behaviors is how the acoustic interactions are actually able to replenish the draining film (2:15). Because the tube was dipped in soap solution, some fluid is still inside the tube, lining the walls. With the right acoustic forcing, that fresh fluid actually gets driven into the soap film, thickening it.

    There are several more videos with different songs here – “Carmen Bizet” is particularly neat – as well as a short article summarizing the relevant physics for those who are interested. (Video and research credit: C. Gaulon et al.; more videos here)