Soap bubbles and other thin films are colorful thanks to wave interference across their tiny thickness, but you may have noticed that only some colors appear. Others, like red, seem to be missing. In this video, Dianna digs into the details of wave interference and color theory to explain why we don’t see pure colors in a bubble.
As she points out near the end of the video, the way to make a red bubble is to shine purely red light on the bubble, but even then, you’ll see stripes on it related to the light’s wavelength. Scientists actually use this property to measure the thickness of tiny air gaps between a droplet and a surface. (Image and video credit: Physics Girl)
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)
By shining laser light through soap bubbles, researchers have demonstrated branching flow in light for the first time. This branching occurs when waves travel through a disordered medium where the typical size of the disordered regions is larger than the wave’s length. Previously, scientists had seen evidence of this phenomenon in electrons, sound waves, and even ocean waves.
Soap bubbles serve as an excellent platform for branching in light because their exceptionally thin film varies in thickness thanks to the interplay of buoyancy, Marangoni effects, and evaporation. It’s also comparable to — but still slightly larger than — the wavelength of light. The experiment is far from simple, though. Lining the laser up with the soap bubble is tough, especially when your bubble is likely to pop! (Video credit: Nature; research credit: A. Patsyk et al.; submitted by Kam-Yung Soh)
When a hole opens in a soap bubble, it throws the surface-tension-driven capillary forces of the bubble into disarray. The rim around the hole retracts, pushing fluid away from the expanding hole. At the same time, air is pushed out of the collapsing bubble. Using microphone arrays, the researchers found they could measure and distinguish sound from both sources — the escaping air and the expanding hole.
From the sound, they developed a model that predicts the rupture location, bubble thickness profile, and other properties of the bubble. They confirmed the model’s results by comparing with high-speed photography. The authors hope their new acoustic technique will shed light on bubble bursting events that are hard to observe visually, like the bubbling of magma. (Image and research credit: A. Bussonnière et al.; via Science News; submitted by Kam-Yung Soh)
Whether young or old, everyone enjoys blowing soap bubbles, and the bigger the bubble, the more impressive it is. Researchers have been on a quest to discover how bubbles can survive with volumes measured in the tens of meters and thicknesses of mere microns.
The key to these behemoth bubbles are the polymer chains inside them. The long molecules of polymers get entangled with one another and resist further stretching, which strengthens the soap film. The researchers found that a mixture of polymer lengths are even better for long-lasting bubbles because they entangle more fully than polymers that are all the same size.
But if what you really want are practical results, I have good news for you: the researchers have released their recommended recipe for making the best giant soap bubbles. It’s included in the video below, but I’ve also reproduced it in text for easier recreation (with thanks to Ars Technica):
Giant Soap Bubble Solution From the Burton Lab, via Ars Technica
Ingredients 1 liter of water (about 2 pints) 50 milliliters of Dawn Professional Detergent (a little over 3 TBSP) 2-3 grams of guar powder, a food thickener (about 1/2 heaping TSP) 50 milliliters of rubbing alcohol (a little more than 3 TBSP) 2 grams of baking powder (about 1/2 TSP)
Directions Mix the guar powder with the alcohol and stir until there are no clumps.
Combine the alcohol/guar slurry with the water and mix gently for 10 minutes. Let it sit for a bit so the guar hydrates. Then mix again. The water should thicken slightly, like thin soup or unset gelatin.
Add the baking powder and stir.
Add the Dawn Professional Detergent and stir gently to avoid causing the mixture to foam.
Dip a giant bubble wand with a fibrous string into the mixture until it isf fully immersed and slowly pull the string out. Wave the wand slowly or blow on it to create giant soap bubbles.
Scientists have observed distinctive differences in the way soap bubbles freeze depending on their environment. If a bubble is surrounded by room temperature air but placed on a cold surface (top), it freezes from the bottom up, with a clear freeze front that slowly creeps upward.
In contrast, bubbles in an isothermal environment – one where it’s equally cold everywhere – freeze with a snow-globe-like effect of ice crystals (bottom). This freezing mode is actually triggered by a Marangoni flow. As the thin bottom layer of the soap bubble begins to freeze, it releases latent heat. That local heating changes the surface tension enough to generate an upward flow. You can see the plumes form right as the bubble touches the surface. Those plumes lift up tiny ice crystals, which continue to grow, ultimately forming the snowy crystals we see take over the surface. (Image and research credit: S. Ahmadi et al.; submitted by Kam-Yung Soh)
Though they look like jellyfish or space creatures, these images from photographer Linden Gledhill are actually explosions. What you’re seeing is the detonation of hydrogen gas with oxygen. The teal sphere with its wavy surface marks the flame front, and the crisp, stringy edges seen here and there in the foreground are the remains of a soap bubble that held the hydrogen before it sparked. You can see a similar set-up (using methane rather than hydrogen) in action here, and you can see other artistic takes on combustion in previous posts like this one. (Image credit: L. Gledhill, Flickr)
Making soap bubbles is fun, but there’s something about gigantic soap bubbles that brings out the child in everyone. The world’s largest freestanding soap bubble had more than 100 square meters of surface area, which begs an important question: how can such a thin film stay stable at that size?
The solutions used for giant bubbles have a few main ingredients: water, naturally; detergent, used for its surfactants; and polymers like polyethylene glycol that help stabilize the soap film. Exactly why polymers helped was a bit of mystery, but a new pre-print study aims to answer that.
Researchers studied how polymer concentrations affected 1) how much solution could be drawn in as bubbles formed, and 2) how long a film of solution lasted before gravity and evaporation thinned it to breaking. They found that intermediate polymer concentrations actually worked best. This gave the solution the viscoelasticity needed to draw in more solution as bubbles grew without having so much polymer that it negatively affected film lifetime. (Image credit: Pixabay; research credit: S. Frazier et al.; via MIT Tech Review; submitted by Kam-Yung Soh)
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)
File this one under awesome tricks you shouldn’t try at home. Here bubble artist Dustin Skye demonstrates his handheld inverted fire tornado. First, he blows a large encapsulating bubble, then blows butane and smoke into a smaller secondary bubble. When he breaks the wall between the two, the mixture swirls into the larger bubble. Then, by breaking a narrow hole into the remaining bubble, Skye forms a swirling tornado. He’s using conservation of angular momentum here to concentrate the vorticity he created by blowing into the original butane bubble. As the big bubble shrinks, the vorticity inside gets pulled inward and speeds up – like when a spinning ice skater pulls his arms in. That’s how you get the tornado. And from there, it’s just a matter of lighting the exiting butane and air mixture. (Video credit: D. Skye; via Gizmodo)
