Two drops of water resting on a stretched soap film will circle one another, stretch into thin trailing arms, and finally crash together — a tabletop echo, in miniature, of galaxies merging across the cosmos.
Millimeter-sized droplets deposited on a soap film pull toward each other, orbit for several revolutions, and then collide and merge into looping, spiral-armed structures that closely resemble real interacting galaxies, the researchers report in PNAS Nexus.
Scientists have long studied small floating particles that drift together on water because of surface tension, an effect nicknamed the “Cheerios effect.” But those particles are rigid, only interact over short distances, and lose energy quickly, so they never build anything as elaborate as a spiral galaxy. “These simple systems based on a limited number of nondeformable particles cannot give rise to complex structures, such as the ones resulting from the interactions of galaxies,” researchers write in the paper.
J.-P. Martischang et al./PNAS Nexus, 2026
The researchers stretched a soap solution across a 10-centimeter circular frame, then used a syringe to inject droplets of water onto the film. Each droplet settles into a thick, lens-shaped pool that sags the surrounding film downward, much like a marble resting on a stretched sheet of rubber. When a single droplet sits at the center of the film, it always forms a lens with the same edge width, about 6 millimeters, no matter how much water is in it. Nudge that droplet off-center, and it swings back and forth like a pendulum, trapped in a bowl-shaped dip created by its own weight pressing on the film.
Things get more interesting with two droplets. Give a second droplet a sideways push near a resting one, and the pair begins to orbit, drawn together by the dip each creates in the film. The researchers found that this attraction weakens with distance in the same mathematical way Newton’s law of gravity does, just adapted to two dimensions. Eventually the droplets spiral inward, stretch into curling tidal arms and connecting bridges, and merge into a single new lens.
Those spiraling, colliding shapes turned out to closely match images of real galaxy pairs pulled from a public astronomical survey. To check the resemblance more rigorously, the team compared their footage to computer simulations of merging elliptical and spiral galaxies from an existing galaxy-merger database, matching up similar-looking stages of each collision. The comparison revealed a rough scaling between the two: one second in the lab experiment corresponds to roughly 460 million years of simulated galactic time.
“Perspective for investigating, on a human time scale, simplified representations of phenomena that occur on time scales beyond the reach of human observation,” the researchers conclude.
Journal Reference: PNAS NEXUS. DOI: 10.1093/pnasnexus/pgag079
