It’s a familiar scene: tipping a milk carton, angling an olive oil bottle, or waiting over a wok to coax out the last remnants. How long should you wait for that film of liquid to gather so you can pour or dump it? Two physicists at Brown University — PhD candidate Thomas Dutta and professor Jay Tang — set out to answer that everyday question. Their results were published in the journal Physics of Fluids.
Tang, whose research usually focuses on bacterial biophysics and fluid mechanics, used the kitchen problem to teach and explore how liquids behave on surfaces. He and Dutta considered practical examples: milk and oil in containers, cold maple syrup, and the thin layer of water that remains on a cast-iron wok after washing. Tang avoids thoroughly drying his wok to preserve its seasoning, but if residual water lingers too long it can cause rust — so he wondered whether his usual one-to-two minute wait was enough for most of the water to collect at the bottom.
The researchers approached the problem with the Navier–Stokes equations, which govern fluid motion, then tested their predictions experimentally. They let various liquids flow down a plate tilted at 45 degrees and periodically weighed how much had drained to determine when 90% of the initial thin film had departed.
Experiments matched the theory. Low-viscosity liquids drain quickly: water ran off in a few seconds, and milk required roughly 30 seconds for 90% to drain. Thicker fluids behaved much more slowly: olive oil took more than nine minutes, while cold maple syrup required several hours to reach the same drained fraction. For Tang’s wok, Dutta built a numerical simulation based on fluid dynamics to estimate the optimal waiting time. The result surprised Tang: about 15 minutes are needed for 90% of the residual water to collect at the bottom so it can be poured away.
The work illustrates how basic physics can explain mundane tasks and why patience—or different handling—matters with viscous fluids. It also fits a tradition of playful scientific curiosity recognized by the Mario Markus Prize for Ludic Science, a €10,000 award given by the German Chemical Society since 2022 for inventive, curiosity-driven natural science. The prize honors the late Mario Markus of the Max Planck Institute, and its name echoes the Latin ludus, meaning play.
Many major advances have roots in serendipity and playful tinkering: Alexander Fleming noticed penicillin when mold spores contaminated a bacterial culture; Charles Goodyear discovered vulcanization after rubber mixed with sulfur hit a hot surface; and kindergarten teacher Kay Zufall’s work with a wallpaper-cleaner recipe helped inspire the toy Play-Doh.
This study shows that even small, quirky questions about daily life can yield clear, quantitative answers when physics is applied. (This article was originally written in German.)