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Two Inks, One Glass

Pink and blue drops of ink fall into a glass of water, curl into ribbons and slowly get pulled into a growing whirlpool. At the top a golden drop swells the whole time. When it finally falls into the vortex, the glass turns into a golden spiral galaxy, then clears and the first drop falls again.

Nothing here is a video of real ink. The water is a grid of 135 × 175 cells, and every frame the computer solves a simplified version of the Navier-Stokes equations that describe how fluids move: ink is carried along by the flow, and the flow is corrected so water never gets squeezed or stretched. The method is Jos Stam's "Stable Fluids" from 1999, which cannot blow up no matter how hard you stir, the reason it became a favorite for games.

Fun fact: simple grid fluids tend to lose their small swirls. A trick called vorticity confinement detects the spinning regions and gives them a tiny extra push, which is why the curls here stay crisp. On the website you can tap to drop ink and drag to stir.