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)
Tag: interference
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)
Draining Soap Film
The brilliant colors of a soap film are directly related to the film’s thickness. Black regions, like the one in the upper right of this image, are the thinnest regions and may be less than 100 nanometers thick. (That’s smaller than the shortest wavelength of visible light!) The colors of the peacock-feather-like blooms along the bottom of the image demonstrate significant variations in film thickness. This is caused by uneven concentrations of surfactants in the film. The variations in concentration causes differences in local surface tension, which in turn moves fluid around within the film. This is known as a Marangoni effect. (Image credit: S. Berg and S. Troian)
Bouncing in Lockstep
Droplets of silicone oil bounce on a pool of the same thanks to the vibration provided by a loudspeaker. Each droplet’s bounce causes ripples in the pool and the interference between these ripples fixes the droplets in lockstep with one another. As long as the vibration continues to feed the thin layer of air that separates the droplets from the pool during each bounce and no impurities break the surface tension at the interface, the droplets will bounce indefinitely on their liquid trampoline. Such systems can be used to observe quantum-mechanical behavior like wave-particle duality on a macro-scale. (Photo credit: A. Labuda and J. Belina)
Soap Film Butterfly
Originally posted: 14 Jan 2011 This gorgeous butterfly-like double spiral roll takes place on a horizontal soap film. The foil (seen top center) inserted in the film flaps back and forth. Each time the foil changes direction a vortex forms at the tip and gets advected away. The vortices stretch and distort in the roll, but if you look at the photograph closely, you’ll see the tiny shed vortices persisting throughout the roll structure. The bright colors that make this flow visible are due to interference patterns related to the local thickness of the film. (Photo credit: T. Schnipper et al.)
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The Colors of Soap
The brilliant and beautiful colors of a bubble are directly related the the thickness of the soap film surrounding it. When light shines on the soap film, some rays are reflected from the upper surface of the film, while others are refracted through the film and reflect off its lower surface. These reflected rays have different phase shifts and their interference is what causes the colors we observe. The color patterns themselves reveal the interior flow of the soap film, in which gravity tries to thin the film and surface tension tries to distribute the film evenly. (Photo credit: R. Kelly, A. Fish, D. Schwichtenberg, N. Travers, G. Seese)
Ripples
Capillary waves–ripples–interfere with one another after the photographer throws objects into a narrow point in a small lake. The reflections of these waves off the lake’s boundaries and against one another creates a mosaic-like geometric effect on the liquid surface. (Photo credit: Jorgen Tharaldsen/National Geographic Photo Contest)
Visualizing Fish Wakes
This novel flow visualization technique uses dilute solutions of the tobacco mosaic virus (TMV). These rod-shaped particles align with shear and produce a birefringent interference pattern visible when viewed between crossed polarizing filters. The intensity of the light is related to the magnitude of shear. The technique is benign to the fish but enables researchers to see fluid motion around fish that other techniques cannot capture. #
Soap Film Flow Viz
Flowing soap films provide an educational and beautiful method for visualizing the wakes of objects in two-dimensional flows. High-speed photography highlights the interference patterns on the soap film, providing detail without the necessity for the particulate tracking of other flow visualization methods. Highlights here include wakes behind bluff bodies, interacting cylinders, and flapping flags. (pdf) #
Tsunami Simulation
This simulation shows how tsunami waves are expected to spread from the epicenter of the Japanese magnitude-8.9 earthquake. Note the complicated interference and reflection patterns. The main wavefront moved at a speed of about 230 m/s (830 km/h) between Japan and Hawaii.
