Everyone knows the little ritual: tip a milk carton, shake an oil bottle, or hold a wok to coax the last film of liquid toward the spout. How long should you wait for that thin layer 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 study appears in the journal Physics of Fluids.
Tang, who usually studies bacterial biophysics and fluid mechanics, turned a kitchen nuisance into a teaching problem about how liquids flow on surfaces. He and Dutta examined common cases: milk and olive oil left in containers, chilled maple syrup, and the thin sheet of water that can remain on a cast-iron wok after rinsing. Tang tries not to over-dry his wok to protect its seasoning, but lingering water can cause rust. He wondered whether his routine one-to-two minute wait before putting the wok away was actually enough.
The team started from first principles, using the Navier–Stokes equations that describe fluid motion, and then ran experiments to check the theory. In the lab they let different liquids flow down a plate tilted at 45 degrees and periodically weighed how much had drained off. They defined the practical target as the time until 90% of the initial thin film had flowed away.
Theory and experiment agreed. Low-viscosity fluids evacuate quickly: water ran off in a few seconds, and milk needed roughly 30 seconds to reach the 90% threshold. More viscous substances took far longer: olive oil required more than nine minutes, and cold maple syrup took several hours to drain to the same fraction. To estimate the wok wait time, Dutta built a numerical simulation based on the same fluid-dynamics principles; the result surprised Tang. For a typical post-wash water film on a cast-iron wok, about 15 minutes are needed for 90% of the water to collect at the bottom so it can be poured away.
The findings underscore how basic physics explains mundane tasks and why patience or different handling matters when viscosity is high. The work also exemplifies playful scientific curiosity: it sits comfortably alongside questions honored by the Mario Markus Prize for Ludic Science, a €10,000 award from the German Chemical Society established in 2022 that celebrates inventive, curiosity-driven natural science. The prize commemorates the late Mario Markus of the Max Planck Institute; its name recalls the Latin ludus, meaning “play.”
History shows many major discoveries began as small, unexpected observations: Alexander Fleming noticed penicillin when mold contaminated a bacterial culture; Charles Goodyear stumbled on vulcanization when rubber mixed with sulfur encountered heat; and kindergarten teacher Kay Zufall’s experiment with a wallpaper-cleaner recipe helped inspire Play-Doh.
This study is a reminder that even quirky household questions can yield clear, quantitative answers when you apply physics. (This article was originally written in German.)