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 "soap bubbles"

Soap bubbles are ephemeral creations. The slightest prick will send them tearing apart in the blink of an eye. It may come as a surprise, therefore, that dropping a water droplet through a bubble will not break it. Instead, the bubble will heal itself using the Marangoni effect. In a soap bubble, the soap molecules act as a surfactant, lowering the surface tension of the water and allowing the fragile structure to hold together. When the water drop impacts the bubble, the local surface tension increases because of the relative lack of soap molecules. This increase in surface tension pulls at the rest of the bubble, drawing more soap molecules toward the point of contact. The effect evens out surface tension across the surface and stabilizes the bubble. You can test the effect at home, too. If you wet your finger, you can poke a soap bubble without popping it. (Video credit: G. Mitchell; via io9)

In their latest video, Gavin and Dan of The Slow Mo Guys demonstrate what giant bubbles look like in high-speed video from birth to death. Surface tension, which arises from the imbalance of intermolecular forces across the soapy-water/air interface, is the driving force for bubbles. As they move the wand, cylindrical sheets of bubble film form. These bubble tubes undulate in part because of the motion of air around them. In a cylindrical form, surface tension cannot really counteract these undulations. Instead it drives the film toward break-up into multiple spherical bubbles. You can see examples of that early in the video. The second half of the video shows the deaths of these large bubble tubes when they don’t manage to pinch off into bubbles. The soap film tears away from the wand and the destructive front propagates down the tube, tearing the film into fluid ligaments and tiny droplets (most of which are not visible in the video). Instead it looks almost as if a giant eraser is removing the outer bubble tube, which is a pretty awesome effect.  (Video credit: The Slow Mo Guys)

Though seemingly instantaneous to the naked eye, the bursting of a soap bubble is fascinating when slowed down. Here it is at about 2200 frames per second. Initially, the bubble is approximately spherical - its shape determined by a balance between surface tension, gravity, and pressure. The prick of a pinpoint disrupts the balance, and surface tension pulls the thin film away from the defect. The liquid sheet of the bubble retracts swiftly into a filament of fluid and a cloud of tiny droplets. (Video credit: soapbubble.dk)

Snowflakes aren’t the only frozen crystals to play with outside in the winter. Photographer Angela Kelly recently posted a series of frozen soap bubbles made by her and her son. In temperatures well below freezing, the thin film of the soap bubble does not survive long before it begins to freeze. The bubbles do not freeze all at once; instead the frost creeps gradually across it. For bubbles sitting on a surface, the ice front expands upward, much the same as with a freezing water drop. Once frozen, the bubbles crack or rip when touched instead of melting and popping. (Photo credit: A. Kelly; via BoredPanda; submitted by jshoer)

A bullet passes through a soap bubble in the schlieren photo above. The schlieren optical technique is sensitive to changes in the refractive index and, since a fluid’s refractive index changes with density, permits the visualization of shock waves. A strong curved bow shock is visible in front of the bullet as well as weaker lines marking additional shocks waves around the bullet. Impressively, the bullet’s passage is so fast (and the photo’s timing so perfect) that there are no imperfections or signs of bursting in the soap bubble. The photo’s caption suggests that the bubble may be filled with multiple gases. If they are unmixed and of differing densities, this may be the source of the speckling and plume-like structures inside the bubble. Incidentally, if anyone out there has high-speed schlieren video of a bullet passing through a soap bubble, I would love to see it. (Photo credit: H. Edgerton and K. Vandiver)

Originally posted: 24 Aug 2011 That soap bubbles burst in the blink of an eye is a pity considering how fascinating their disappearing act is. This photo set from photographer Richard Heeks captures the bubbles mid-burst. Once the bubble’s film is breached, surface tension rips the smooth film back like a broken balloon, causing the liquid that used to be part of the bubble to erupt into droplets. (Photo credit: Richard Heeks)

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If you find yourself some place really cold this holiday season, may I suggest stepping outside and having some fun freezing soap bubbles? The crystal growth is quite lovely, as seen in this photograph. If you live in warmer climes, fear not, you can always experiment in your freezer. It would be particularly fun, I think, to see how a half-bubble sitting on a cold plate freezes in comparison to a droplet like this one. (Video credit: Mount Washington Observatory)

Some soap films are capable of self-healing after a solid object passes through them, as shown in the video above. The behavior is primarily dependent on Weber number—a nondimensional ratio of the film’s inertia to its surface tension. Although demonstrated for positive curvature in the video, the same behavior is observed in negatively curved soap films as well. For a look at how the behavior varies with projectile velocity and size, check out this video. (Video credit: J. Bryson, BYU Splash Lab)

Accidental releases of combustible gases in unconfined spaces can be difficult to recreate in a laboratory environment.  Here researchers simulate the conditions using detonation inside a soap film bubble. Combustible gases are pumped inside the soap film and then a spark creates ignition. The resulting flame propagation is visualized using high-speed schlieren photography, making the density gradients in the flame visible. When the mixture of hydrogen fuel to air is balanced, the flame is spherically symmetric with a high flame speed.  In contrast, weaker mixtures of fuel/air produce slow flame speeds and mushroom-like flames that leave behind unreacted fuel.  This is due to buoyant effects; the time scale associated with buoyancy is smaller than that of the flame speed and chemical reactions when the fuel/air mixture is lean.  (Video credit: L. Leblanc et al.)

This high-speed video shows a soap bubble being blown via didgeridoo, a wind instrument developed by the Indigenous Australians. The oscillations of the capillary waves on the surface of the bubble vary with the frequency of note being played. High frequency notes excite small wavelengths, whereas lower notes create large wavelength oscillations. For more fun, check out what you can do with didgeridoos in space. (submitted by Christopher B)