Dry spots form in the middle of frying pans and cause food to stick, even "non-stick" ones pans, because of something called thermocapillary convection -- but researchers say this situation can be avoided. Photo by Alex Fedorchenko
Feb. 2 (UPI) -- Nine out of ten times, the eggs, perfectly cooked, slide seamlessly from pan to plate.
That kind of performance is good enough for most home cooks, but researchers at the Czech Academy of Sciences found themselves perplexed by that tenth breakfast.
Even the best nonstick pans don't work 100 percent of the time -- occasionally, food sticks.
To find out why, scientists coated a variety of pans with a thin sheen of oil, turned on the burner and filmed the action from above.
Using the camera footage, researchers measured where dry spots formed and how quickly they grew inside the oiled pan.
Researchers watched and analyzed how dry spots formed and expanded inside both ceramic and teflon coated pans.
The team of physicists detailed their observations in a new paper, published Tuesday in the journal Physics of Fluids.
"We experimentally explained why food sticks to the center of the frying pan," study author Alexander Fedorchenko said in a news release.
"This is caused by the formation of a dry spot in the thin sunflower oil film as a result of thermocapillary convection," said Fedorchenko, a researcher with the Institute of Thermomechanics at the Czech Academy of Sciences.
When any pan is heated from below, a temperature gradients forms within the layer of oil -- including sunflower oil, which the researchers used for their experiments.
Because heat lowers surface tension, a surface tension gradient followed the formation of the temperature gradient in the pan. Sunflower oil slipped away from the center of the pan, where it was hottest, and concentrated around the pan's outer edges.
This gradient creates what's called thermocapillary convection, researchers said. As heat flows through the middle and toward the outside of the pan, the oil film in the middle becomes stretched thin, destabilized and eventually ruptures.
Thermocapillary convection occurs within both flowing and stationary films. When the film becomes thinner than a critical value known as capillary length, the film breaks.
This deformity inhibits the performance of the nonstick film, whether flowing oil or the nonstick coating itself, and allows food to stick.
"To avoid unwanted dry spots, the following set of measures should be applied: increasing the oil film thickness, moderate heating, completely wetting the surface of the pan with oil, using a pan with a thick bottom or stirring food regularly during cooking," Fedorchenko said.
The phenomenon can help explain the behavior of films, not just in pans, but in electronic devices and other kinds of technologies.
"Dry spot formation or film rupture plays a negative role, resulting in sharp overheating of the electronic components," said Fedorchenko. "The results of this study may, therefore, have wider application."