The beautiful geometry of arrested soap bubbles

The unassuming soap bubble is one of the most important objects in physics. Bubbles fascinated Isaac Newton in the 17th century; he was intrigued by the way they refracted light. Today they’re foundational in all manner of commercial products, from eye drops to shaving cream to firefighting foams — which means we end up with better products when we figure out how to create better bubbles.

A pair of engineers at Stanford University took an important step in that direction with a paper published in September in Physical Review Fluids. There, postdoctoral fellow Saad Bhamla and professor Gerald Fuller described a method for “pausing” the chaotic motion on the surface of soap bubbles. The approach could help scientists better understand the complex physics of bubbles. It’s also already yielded a series of beautiful images that provide an intuitive glimpse of the molecular action on the surface of these familiar objects.

When a wavelength of light strikes a bubble, it slows down, inducing a wavelength-shift that transforms white light into a range of visible hues. The same process repeats when, after traversing the inside of the bubble, the wavelength exits out the other side.

“As you zoom through this liquid film, you hit the next water-air interface, and, as you exit, you have the same transformation all over again,” says Bhamla.

If that was all there was to it, the surface of a bubble would appear as though it were painted, with colors in fixed positions. Instead, the colors swirl in constant motion because the bubble’s surface is continually changing.

Bubbles are held together by surface tension (the attraction of liquid molecules to one another along a water-air boundary). The surface tension won’t be even across the bubble, though. It will be higher in some places than others, with areas of higher surface tension pulling on areas of lower surface tension. The progress of this pulling is visible because the surface of a soap bubble is covered with molecules called surfactants (for “surface active agent”). These surfactant molecules are tugged along by the changes in surface tension, creating molecular flows on the bubble’s surface — a phenomenon known as the Marangoni effect.

These molecular flows mean the thickness on a soap bubble’s surface at any given point is also always changing, which affects the light striking the bubble. Light striking the bubble in thicker places will appear more blue-green, while light striking the bubble in thinner places will appear more yellowish, with the colors changing as the thickness changes.

For scientists, this connection between the thickness of a bubble and the colors that appear on the surface provides a powerfully direct way of understanding a bubble’s properties.

“The system is telling you in real-time what’s going on inside by visually changing colors,” says Bhamla.

In their latest research, Bhamla and Fuller began by creating underwater soap bubbles. Then, using the tip of a submerged syringe, they pushed the bubbles up towards the surface. As the bubbles crowned out of the water and into the air, Bhamla and Fuller held them in position so that they only partially protruded out of the water. By fixing the bubbles in these positions — part underwater, part out of the water — they were able to pause the surface tension effects and “arrest” the flow of surfactant molecules. This pause was evident in photographs and videos taken of the bubbles as they protruded — instead of flowing, colors on the surface froze in fixed, symmetric patterns.

Bhamla and Fuller hope their technique will aid the development of products that depend on getting the surface of a bubble just right. This is important in many areas of medicine in particular. Doctors inject a thin, bubble-like film into the lungs of premature babies to help them begin breathing; researchers are also looking at using microscopic bubbles as drug-delivery systems. In all these areas, if the surface tension effects are not calibrated quite right, the bubbles pop too soon and the liquid that made them becomes a useless puddle.