Justin Burton’s fascination with soap bubbles began in Barcelona, Spain, six summers ago. “It was hot and humid, and there were guys on the street making these huge bubbles,” he says. “I thought it was the coolest thing.” When he returned home, his sister gave him a giant bubble wand. It included a recipe for a bubble-making solution. Burton began to blow. Now he’s improved the recipe and is ready to share it.
By the way, Burton is hardly your typical bubble-blower. As a physicist at Emory University in Atlanta, Ga., he studies forces in nature. Those forces include effects on fluids that flow. The more he thought about bubbles, the more he wanted to know just how they were made.
“What is it about bubble solutions that makes them [stretch] without ripping — and for such a long time?” he asks. Now, he’s found out. And he and his team reported the results of their scientific investigation on August 1 at arXiv.org.
Big bubbles exist at just the right balance of natural forces, they found. The bubble has to be stretchy, but not so much that it rips. And the wet soapy mixture has to flow evenly to form the bubble, resisting a tendency to pop. It also has to stay together, even as gravity pulls it down and some of the water evaporates.
In their new study, Burton and his coworkers identified the secret ingredient behind these traits: a polymer. It’s what holds the bubble together but lets it flow, too.
Polymers are long chains of identical molecules. In Burton’s case, he recommends one called guar. It’s made from natural fibers. Those fibers help form long-chain polymers when dissolved in water. That’s why guar is used as a thickener in many foods, including ice cream and puddings.
Adding polymers like guar to bubble recipes can produce colossal results. The official largest free-floating bubble was blown in 2015 in Cleveland, Ohio. It had a volume of 96.27 cubic meters (almost 3,400 cubic feet). That’s about the same volume as about 13,545 basketballs. It’s also about the size as a large classroom (although not in the same shape, of course.)
Perhaps most surprisingly, that record-setting bubble probably included only about 300 milliliters, or just over a cup, of bubble solution. Burton says that in such big bubbles, the soap film thins across the ballooning structure to just about one micrometer (also called a micron) thick. That’s a millionth of a meter (0.0004 inch).
“It’s just amazing that you can have a micron-thick film that extends to these dimensions,” Burton says.
The polymer advantage
“Polymer molecules added in soapy water not only increase the viscosity [thickness] of the liquid, but also change the specific way it flows,” says Laurent Courbin. He’s a physicist at the University of Rennes, in France. He didn’t work on this study with Burton, but he does study bubbles. A few years ago, he reported on how air flow can affect the formation of bubbles.
The Emory scientists used a number of different experiments to test bubbles. To measure the thickness of soap films, they built a device that could produce a soap film and hold it in place. Then, they shone a light through it. They couldn’t use visible light, which would pass through without changing. (That’s why you can see right through a bubble.) Instead, they used infrared (In-fruh-RED) light, which has longer wavelengths than ordinary light.
Water absorbs infrared light. (This is one reason why the air gets hot: Water vapor absorbs energy from the sun, warming the air molecules.) So when the infrared light passed through the soap film, some wavelengths were absorbed by water. By measuring the infrared light that did pass through, they could see how much had been absorbed. They used that information to calculate the thickness of the film.
They also rigged up devices to let the soap film form droplets on the end of small nozzles. As the droplets grew larger, gravity pulled them down. That created a thin thread of soap between the droplet and nozzle. Measuring that thread told them how far the soap could be stretched without snapping.
Big bubble-makers often share their recipes with others online. Many have reported on different concentrations of water, detergent and polymers. But the study by Burton’s team is likely the first to seriously analyze the chemistry of bubbles.
“I’ve never been aware of a study where people really tried to make a quantitive analysis of the components of the solution,” says Pascal Panizza. He didn’t work on the study, but he has worked with Courbin on bubble studies at the University of Rennes in France.
What’s funny, says Courbin, is that scientists are just now asking questions about the physics and chemistry of bubbles. “When you think about it, it should have been in the scientific literature 200 years ago,” he says. “Everybody blows bubbles.”
arXiv A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
component Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).
concentration (in chemistry) A measurement of how much of one substance has been dissolved into another.
detergent A compound derived from petroleum products, often used for cleaning. Detergents work by breaking up and surrounding dirt particles or oily substances, so that they can be washed away with water.
dimension Descriptive features of something that can be measured, such as length, width or time.
evaporate To turn from liquid into vapor.
fiber Something whose shape resembles a thread or filament. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fibers tend to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
information (as opposed to data) Facts provided or trends learned about something or someone, often as a result of studying data.
infrared A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X-rays, radio waves and microwaves. Infrared light tends to record the heat signature of an object or environment.
literature The books, studies and other writings published on a particular subject. Scientific literature usually refers to published papers or meeting abstracts describing new research findings or the reviews of multiple papers on a topic within some field.
micrometer (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2); water is made of two hydrogen atoms and one oxygen atom (H2O).
nozzle A round spout or slot at the end of a pipe, hose or tube. Nozzles are typically used to control the flow of a jet of some high-pressure liquid or gas.
online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.
pascal A unit of pressure in the metric system. It is named for Blaise Pascal, the 17th century French scientist and mathematician. He developed what became known as Pascal’s law of pressure. It holds that when a confined liquid is pressed, that pressure will be transmitted throughout the liquid in all directions, without any losses.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. A scientist who works in such areas is known as a physicist.
polymer A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).
solution A liquid in which one chemical has been dissolved into another.
trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.
viscosity The measure of a fluid’s resistance to stress. Viscosity corresponds to the idea of how “thick” a liquid is. Honey is very viscous, for instance, while water has relatively low viscosity.
water vapor Water in its gaseous state, capable of being suspended in the air.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.