Tag: soap film

Lasers and Soap Films
Soap films are a great system for visualizing fluid flows. Researchers use them to look at flags, fish schooling and drafting, and even wind turbines. In this work, researchers explore the soap film’s reaction to lasers. When surfactant concentrations in the soap film are low, laser pulses create shock waves (above) in the film that resemble those seen in aerodynamics. The laser raises the temperature at its point of impact, lowering the local surface tension. That temperature difference triggers a Marangoni flow that draws the heated fluid outward. The low surfactant concentration gives the soap film relatively high elasticity, and that allows the shock waves to form.
In contrast, a soap film with a high concentration of surfactants has relatively little elasticity. In these films (below), the laser creates a mark that stays visible on the flowing soap film. This “engraving” technique could be used to visualize flow in the soap film without using tracer particles. (Image and research credit: Y. Zhao and H. Xu)
When surfactant concentrations are high, a laser pulse “engraves” spots onto a flowing soap film. Shown in terms of interference (left) and Schlieren (right) imaging.
April 2, 2024
Colorful Drainage
Bright colors mark this slowly draining soap film. The film sits slightly off-horizontal, so flow shifts over time from the top of the frame to the bottom. The fluid is also evaporating. All the faster shifts are caused by ambient air currents from the room. The colors of the film are directly related to the local thickness; as the film thins and evaporates, the bright colors shift to darker ones. Eventually, that black region at the top will expand and the film will break up. (Video credit: B. Sandnes/Complex Flow Lab)
August 1, 2023
A 2D Splash
We see plenty of droplets splash when they fall into a pool, but what happens when the drop and pool are two-dimensional? Here researchers captured the familiar process of a splash in an unfamiliar way by looking at a falling drop contained within a soap film. As the drop reached the thicker lower boundary of the soap film (which acts like a pool), its impact sent up ejecta that stretch and curl, much like the three-dimensional splashes we’re accustomed to. (Image credit: A. Alhareth et al.)
April 19, 2023
Soap Film Ruptures
Soap film ruptures are well understood for your typical bubble solution, but what happens when tiny particles get added to the soap film? That’s the question in this recent study. Researchers added 660-nanometer particles, in varying amounts, to their soap films to see how it affected rupture. When they broke the films just after formation (top image), they found results that were quite similar to the usual, particle-free case. But when the films sat for awhile before breaking spontaneously (bottom image), the rupture caused wrinkling and folding similar to a piece of fabric. The researchers hypothesize that aging allowed the soap film to thin until the film and the particles were similar in size. Then, when the film ruptured, the particles affected how it broke up. (Image and research credit: P. Shah et al.)
After aging and thinning, a colloidal film ruptures spontaneously, forming fabric-like wrinkles.
April 11, 2023
Magnetic Soap Films
Soap films naturally thin over time as fluid evaporates and differences in film thickness cause surface-tension-driven flows. In this video, researchers experiment with adding magnetic nanoparticles to the soap film. In the second image, the white structures near the center of the film contain nanoparticles, and they’re drawn toward the magnet that sits (out-of-frame) to the left of the film. With more nanoparticles and a stronger magnetic field (Image 3), the entire soap film takes on a distinctive profile that thins from left to right. The effect is so strong that the film quickly thins to the point of rupture. (Image and video credit: N. Lalli et al.)
March 27, 2023
Within the Bubble’s Pop
To our eyes, a soap bubble appears to pop instantly, but when observed in high-speed video, the process is far more complex. In this video, the Slow Mo Guys pop human-sized bubbles, giving us an opportunity to appreciate the rupture process at speeds up to 50,000 frames per second.
Once the rupture starts, the hole spreads very symmetrically. But as the hole grows, the remaining soap film starts distorting. As Gav and Dan observe, the far side of the bubble actually wrinkles up before the rupture front arrives and tears the remaining fluid into droplets! (Image and video credit: The Slow Mo Guys)
April 19, 2022
“Delusion”
Soap films are ephemeral and ever-changing. The shifting concentration of surfactants along the surface of the film, combined with thermally-driven convection, keeps the fluid in motion. The shifting colors reflect subtle changes in the soap film’s thickness. Over time, gravity drains fluid from the top of the film, thinning it to the point that it appears black. This photo from Bruno Militelli captures all of that detail in a striking and fascinating image that earned him 2nd place in the Manmade category of the Close-Up Photographer of the Year awards. You can find more winners of the competition here, and more of Militelli’s work on his website and Instagram. (Image credit: B. Militelli)
March 4, 2022
Flying on Soap Films?
YouTube channel Viral Video Lab has two videos showing 3D-printed gliders flying on wings formed from soap films. It’s a neat idea for a toy aircraft, though obviously not practical. But are the videos real? The channel features plenty of obviously fake concepts, like perpetual motion machines, and explicitly states in its About page that “videos shown on the channel may contain CGI effects.” They’re clearly not strangers to stretching the truth.
Sadly, I don’t have the means to properly test the concept, but it at least seems plausible (although there are some flight sequences in the videos themselves that I don’t think are totally real). There are bubble solutions out there capable of making quite giant, long-lasting bubbles, though they are more complicated than the simple soap and water solution suggested in the video. And having essentially flat wings doesn’t preclude gliding, as long as you have a positive angle of attack. I’d be interested to see if someone with a 3D-printer can recreate the effect. Let me know if you give it a try! (Video credit: Viral Video Lab; via Gizmodo)

November 29, 2021
“Flux Capacitor”
Sandro Bocci’s short film “Flux Capacitor” explores the geometry and dynamics of soap films. When you dip wire models into soapy solution, the films that cling to the model can form complicated shapes as surface tension works to minimize the overall surface area. Bocci’s macro photography highlights the intense flows going on in the narrow regions where films meet. It’s a different take on soap films and neat to see! (Image, video, and submission credit: S. Bocci et al.)
March 5, 2021
Spinning Bubbles
Fluid dynamics is largely about figuring out the relationship between forces. For a soap bubble sitting still, that’s primarily the effect of gravity, which makes the fluid in the soap film drain downward, and surface tension, which tries to maintain a spherical shape for the bubble.
Once you start spinning the bubble, though, there are new forces that come into play. One is the centrifugal force caused by the rotation, and another is the drag force between the rotating soap bubble and the air inside and outside of it. The addition of these forces drastically changes the bubble’s shape. It becomes wobbly and flattens out. Watch the contact line where the bubble meets the surface and you’ll also see it creeping outward toward the edge of the platform. (Image credit: C. Kalelkar and S. Paul, source)
November 9, 2020