We often tilt bottles or cartons to get the last milk into our coffee or drizzle every bit of olive oil on a salad. How long must we wait for those thin films of liquid to drain? Two physicists at Brown University, Thomas Dutta and Jay Tang, tackled that everyday puzzle; their results were published in Physics of Fluids.
The project began from simple, familiar annoyances: Dutta recalled his grandmother struggling to coax out the last drops, and Tang wanted to know how long to wait for water to run out of his cast-iron wok after washing without harming the seasoning. Tang, who studies bacterial biophysics and needs a solid grasp of fluid mechanics for his work, used the problem as a way to teach and explore fluid dynamics in a common setting.
Dutta and Tang used the Navier–Stokes equations to predict how liquids flow as thin films down inclined surfaces. They then tested their predictions experimentally by letting liquids run down a plate tilted at 45 degrees and weighing what had flowed off to determine when 90% of the liquid had drained.
Their experiments matched the theory. Low-viscosity water ran off in a few seconds. Milk, being a thin liquid but more viscous than water, took about 30 seconds for 90% to drain. Olive oil, thicker still, required more than nine minutes. Cold maple syrup drained far more slowly—on the order of hours.
For Tang’s wok question, Dutta built a computer simulation based on fluid dynamics to estimate how long residual water takes to coalesce at the bottom so it can be poured away. Tang found he had been underestimating the wait: instead of one or two minutes, the model indicated roughly 15 minutes are needed for about 90% of the residual water to collect.
The researchers’ playful study reflects a broader appreciation for curious, unconventional questions in science. The Mario Markus Prize for Ludic Science, awarded by the German Chemical Society since 2022 and worth €10,000, recognizes inventive curiosity in natural science. History shows playful or accidental observations can produce major advances: Alexander Fleming’s accidental discovery of penicillin, Charles Goodyear’s vulcanization of rubber after spilling a rubber–sulfur mix on a hot stove, and Kay Zufall’s adaptation of a moldable wallpaper cleaner into Play‑Doh.
This article was originally written in German.
