Shimmering soap bubbles have a dark side

Dark spots in soap film grow and merge moments before bubble pops

soap bubble film

HELLO DARKNESS  Just before a soap bubble bursts, its rainbow surface becomes peppered with dark spots in the thinnest regions of the bubble film. Once the darkness spreads over a large area, the bubble pops.

L. Shen

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Zoom in on a soap bubble just before it bursts and brilliant, complex patterns emerge. Shimmery rainbows appear in thicker portions of the soap film, while clusters of dark spots appear in the thinnest regions.

The thickness of the soap film determines the color seen. Light rays that reflect off of the top and bottom of the film combine to amplify particular wavelengths, an effect known as constructive interference. In thicker areas of the film, longer wavelength colors appear, producing a rainbow. Under gravity’s pull, the soap film flows, causing the patterns to shift over time until finally the bubble pops.

When the film becomes even thinner than the shortest wavelength of visible light, dark spots appear, due to destructive interference that cancels out light reflecting off the bubble’s surface. Over about a minute, those spots grow and merge, physicist Li Shen of Imperial College London and colleagues reported November 22 at the American Physical Society’s Division of Fluid Dynamics meeting in Portland, Ore.

COME TOGETHER A time sequence shows how black spots on a bubble’s surface merge and grow over the span of about a minute, until the bubble pops. L. Shen

This effect, known as “coarsening,” pops up in other places, too. Shake up a vinaigrette salad dressing and it will gradually separate, as oil and vinegar droplets form and coalesce. This is the first time the effect has been observed in bubbles, says Shen, who used dishwashing detergent and a digital camera to study the phenomenon.

Bad news for the bubbles: The melding black spots are precursors to their demise. These bubbles popped shortly after the images were taken. 

BUBBLE TROUBLE Dark spots on the surface of a bubble mark thin regions of the soap film. These regions merge as the bubble nears its breaking point. L. Shen/APSPhysics

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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