# Water drops on soap bubble films act like merging galaxies

- Link: https://www.tehranianonline.com/water-drops-on-soap-bubble-films-act-like-merging-galaxies/
- Published: 2026-03-26T16:48:56+03:30
- Author: Farid Ghoreyshi

The physics of merging galaxies has popped up in an unexpected place: the stuff 
of soap bubbles.

Water droplets placed on a flat soap film act like galaxies that orbit one another
before coalescing. As they merge, the water droplets take on shapes [reminiscent of those that appear in astronomical images of colliding galaxies](https://doi.org/10.1093/pnasnexus/pgag079),
physicist Jean-Paul Martischang and colleagues report in the April _PNAS Nexus_.
The water droplets might eventually be useful for studying gravitational attraction
in a laboratory, to better understand [how galaxies collide](https://www.sciencenews.org/article/earliest-known-galaxy-merger-occurred-shortly-after-big-bang)
and coalesce.

Like colliding galaxies, water droplets (dark circles) on a soap film orbit before
coalescing. J.-P. Martischang _et al_./_PNAS Nexus_, 2026

When plopped on a horizontal soap film — in the laboratory equivalent of a bubble
wand — a water droplet takes on a hammocklike shape, about a centimeter wide. That
disrupts the shape of the soap film, pulling it downward. That sagging causes drops
to attract one another, orbit and coalesce.

To visualize the translucent water droplets, the researchers took advantage of the
fact that each droplet acts like a lens that causes blurring. They placed a randomized
pattern of dots below the film. Mapping out where that pattern was blurred revealed
the drops’ locations and shapes.

![](https://www.tehranianonline.com/wp-content/uploads/2026/05/2539-2.webp)

Similar structures appear in water droplets coalescing on a soap film (top; dark
shapes show where blurring caused by the droplets occurs) and in merging galaxies(
bottom). That’s despite vast differences in scale: tens of millimeters for the soap
bubbles versus tens of kiloparsecs for the galaxies. (A kiloparsec is about 3,260
light-years.)J.-P. Martischang et al./PNAS Nexus, 2026

The unexpected effect appeared while the researchers were studying other features
of soap films, says Martischang, who performed the research at the University of
Lille in France. “Once we tried putting water on it and we saw those lenses, we 
just thought ‘Let’s go with that.’”

Mathematical study of the effect revealed that the attraction of the two orbiting
water drops was analogous to the attraction two objects experience due to gravity,
but in two dimensions instead of three. The researchers don’t yet know if the merging
of water droplets also follows gravity’s rules, but they did see structures in the
merging drops that are commonly seen in colliding galaxies, such as bridges and 
spiral arms.

Once the math of merging droplets is fully understood, it could allow researchers
to study phenomena that take eons in nature, but just moments in the lab. One second
for the water droplets is equivalent to 460 million years in the lives of merging
galaxies.