Wed. Sep 28th, 2022
Two grown men blow giant bubbles on a lawn.
enlarge Physicist Justin Burton (left) experiments with giant soap bubbles on Emory University’s Quad with graduate student Stephen Frazier.

Everyone loves bubbles, regardless of age – the bigger the better. But to blow really big bubbles on a world record scale, a very precise bubble mixture is needed. Physicists have determined that a key ingredient is mixing polymers of different strand lengths, according to a new paper in Physical Review Fluids. That produces a soap film that can be stretched thin enough to make a giant bubble without breaking.

Bubbles may seem frivolous, but there is a complex physics underlying them, which is why their study has long been serious science. In the 1800s, Belgian physicist Joseph Plateau outlined four basic laws of surface tension that govern the structure of soapy films. Surface tension is why bubbles are round; that shape has the least surface area for a given volume, so it requires the least energy to maintain. Over time, that shape will look more like a soccer ball than a perfect sphere, as gravity pulls the liquid downwards (“become coarse”).

Bubbles and foams remain an active area of ​​research. In 2016, for example, French physicists worked out a theoretical model for the exact mechanism for how soap bubbles are formed when jets of air hit a soapy film. They found that bubbles only form above a certain speed, which in turn depends on the width of the air jet. If the jet is wide, there will be a lower threshold for bubbles to form, and those bubbles will be larger than those produced by narrower jets, which have higher velocity thresholds. That’s what happens, physically speaking, when we blow bubbles through a small plastic wand: the jet forms near our lips and is wider than the soapy film hanging in the wand.

In 2018, we reported how mathematicians at New York University’s Applied Math Lab had refined the method of blowing the perfect bubble even further, based on similar experiments with thin soapy films. They concluded that it is best to use a circular rod with a circumference of 1.5 inches and blow gently at a constant 6.9 cm/sec. Blow at higher speeds and the bubble will burst. Use a smaller or larger wand and the same thing will happen.

But what about blowing giant bubbles or long, thin soap films that can span two floors? Justin Burton, co-author of the latest paper and a physicist at Emory University specializing in fluid dynamics, first became intrigued by the topic at a conference in Barcelona. He saw street performers producing giant bubbles the diameter of a hula hoop and as long as a car.

He was particularly intrigued by the changing rainbow of colors on the bubbles’ surface. This effect is due to interference patterns created when light reflects off the two surfaces of the film. For Burton, this was also an indication that the thickness of the soap was only a few microns, roughly equal to the wavelength of light. He was surprised that a film of soap could remain intact when stretched so thin into a giant bubble and began doing his own experiments, both in the lab and in his own backyard.

While perusing the open access Soap Bubble Wiki, he noted that most of the favored bubble solution recipes contain a polymer, usually natural guar (a common thickening food additive) or a medical lubricant (polyethylene glycol).

Using those recipes as a guideline, “We basically started making bubbles and popping them, recording the speed and dynamics of that process,” Burton said. “Focusing on a liquid at the most violent moments can tell you a lot about the underlying physics.”

The ultimate goal was to determine the perfect proportions for a bubble mixture to produce giant bubbles: something with a little stretch, but not too much, where the liquid flows a little but not too much – in other words, the Goldilocks of bubble mixtures. .

As Lissie Connors writes at Physics Buzz:

For their experiment, the researchers created different mixtures of water, soap and long-chain polymers to make their bubbles. Unfortunately blowing a 100 m3 bubble is a poor use of the lab space and quite difficult to measure accurately, so the soap films were made with a cotton thread and the thickness was measured with infrared light. In addition to measuring thickness, they also tracked the life of each film.

Burton and his team concluded that it was the polymeric strands that were the key to producing giant bubbles, confirming the collective wisdom online. “The polymer strands get entangled, kind of like a hairball, forming longer strands that don’t want to come apart,” Burton said. “In the right combination, a polymer causes a soap film to reach a ‘sweet spot’ that is viscous but also stretchy — just not so stretchy that it tears apart.”

The team also found that varying the length of the polymer strands resulted in a firmer soap film. “Different sized polymers become even more entangled than single sized polymers, reinforcing the elasticity of the film,” Burton said. “That’s a fundamental physics discovery.”

You’ll find Burton’s giant bubble recipe in the sidebar. But be warned: there are some factors that cannot be controlled in a real world (unlike Burton’s lab environment), such as humidity levels.

DOI: Physical Review Fluids, 2020. 10.1103/PhysRevFluids.5.013304 (About DOIs).

Physical secrets of giant soap bubbles.

By akfire1

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