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MIT Technology Review

The chemistry behind how you make a record-breaking giant soap bubble

The art of creating giant bubbles is more mysterious than it seems, but researchers are at last teasing apart the chemistry of thin soapy films.

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On July 20, 2015, Gary Pearlman and a small group of enthusiasts gathered in a park in Cleveland, Ohio, for a world record attempt. The goal: to create the largest free-floating soap bubble in history. 

Pearlman’s equipment consisted of a pair of fishing rods with some string tied between them. He dipped the string into a special mixture of water, soap, and polymer additives and raised it into the air. This lifted the solution, creating a thin sheet of soapy film. As Pearlman waved the rods, the movement of air stretched and extended the soapy film, forming a giant bubble.

At the same time, independent observers photographed the bubble from various angles so they could calculate its volume. It turned out to be 96.27 cubic meters (3,399.74 cubic feet)—the largest ever measured. That day, Pearlman rightfully took his place in the Guinness Book of Records

In the pantheon of scientific achievement, the creation of giant soap bubbles is sadly underappreciated. And yet it poses a set of puzzles that have fascinated Stephen Frazier and colleagues at Emory University in Atlanta.

They point out that a bubble is made of a fragile film just a few micrometers thick, and yet Pearlman’s record-breaking giant must have had a surface area of more than 100 square meters. A single hole can cause the bubble to burst. “How are such large films created, and how do they remain stable?” ask Frazier and co.

Today, the team provide some answers. These guys have studied the properties of soap films and how they change when polymers of different kinds are added. The results provide a unique insight into the science of bubble formation and the atmospheric conditions most favorable for world-record attempts.

First some background. Bubble enthusiasts have long discussed the best mixtures for their art. “For those interested in making giant bubbles, the Soap Bubble Wiki contains a wealth of empirical information and recipes for optimal bubble solutions,” say Frazier and co.

The consensus is that the best bubble mixtures contain water, a detergent in the form of dishwashing liquid (Dawn Pro seems to be the favorite), and a mix of polymers, long chain-like molecules that increase the viscosity of the fluid. The favored polymers are polyethylene oxide (also called polyethylene glycol), often used in skin creams, and guar gum, a common food thickener extracted from guar beans.

Polymers are important. The Soap Bubble Wiki states that it is almost impossible to make giant bubbles without them. But exactly what they do is poorly understood. “The precise role that polymers play is a bit of a mystery,” says the wiki.

Enter Frazier and co, who study some of the properties that polymers give to bubble mixtures. “We identify some of the underlying physical mechanisms that give rise to giant bubbles,” they say.

Their method is straightforward. They create a range of mixtures made of water, Dawn Pro, and various concentrations of either guar gum or polyethylene oxide. And they study the properties of these fluids in two different ways.

First, they perform a drip test in which a droplet forms and then falls from a pipette. They film this process using a high-speed camera. In particular, they study how, at the moment a drop falls, a thread forms between the drop and the pipette.

They also create a sheet of film using a string dipped in the fluid. They use an infrared sensor to measure the thickness of this film, how it changes before the sheet bursts, and how polymers can extend this lifetime.

Thin film research

The results make for interesting reading. The most robust bubble-making solutions are those that allow the connecting thread to be continuously drawn without breaking. it’s easy to imagine that adding polymers with longer chains in ever increasing concentrations is the best way to do this.

But Frazier and co say the results do not support this. “The most robust solutions for making bubbles have intermediate concentrations and a mixture of polymers of various molecular weights, allowing a large volume of liquid to be continuously drawn into a film without breaking,” they say.

They show this by adding polyethylene oxide that had been intentionally degraded in sunlight over six months. In the process, the sunlight would have broken down the molecules into shorter chains, creating a mixture of various lengths. This turns out to create the most robust solution.

Exactly why this works is a puzzle. Frazier and co hypothesize that the shorter chains act as links between the longest and largest polymers in solution. “To our knowledge, evidence for this behaviour has not been reported in the literature, and is left for future studies focused on extensional rheology,” they say.

The research also throws some light on the atmospheric conditions best suited to making giant bubbles. A key factor is the longevity of the soapy film: longer-lived films allow bigger bubbles.

The polymer additives increase the longevity, but nobody is sure why. One possibility is that they make the films thicker. Another is that the polymers prevent water from draining from the film and so extend its life. 

Frazier and co have gained some insight into this conundrum. They say that a key factor in strengthening the film is the concentration of polymer within it, and this increases as water is removed. However, if too much water is removed the film becomes too thin and breaks. So there is a careful balance at work.

Two factors can remove water. The first is gravity, which drains water from the soapy film. The second is evaporation, which is significant because of the huge surface area of the film.

Frazier and co say that increasing the humidity in their experiments also increases the bubble lifetime. This suggests that evaporation needs to be minimized to prevent the film from becoming too thin. “It is no wonder that many bubble enthusiasts prefer warm, humid summer days for making the largest bubbles,” they say.

That’s interesting work that throws new light onto the physics of thin films. It also reveals a previously unknown effect in the way that polymers of different lengths interact to increase the strength of a film. That’s an effect that needs to be studied in more detail.

It’s also work that Gary Pearlman might find useful, should he try to beat his world record in the future.

Ref:  arxiv.org/abs/1908.00537 : How to Make a Giant Bubble