WASHINGTON — It’s a mystery that’s rattled the nerves of many amateur chefs for years. Now, finally, scientists have got to the bottom of why food still sticks to pans that are supposedly “non-stick.”
The problem can easily ruin that favorite egg or french toast breakfast. It can happen even if oil or butter is used. Convection, the movement of heat by a fluid, is to blame, the new study shows.
“The results can be “very messy and unappetizing,” explains study author Alexander Fedorchenko in a statement. “We experimentally explained why food sticks to the centre of the frying pan. This is caused by the formation of a dry spot in the thin sunflower oil film as a result of thermocapillary convection.”
His team at the Czech Academy of Sciences tested ceramic and non-stick Teflon pans. A video camera was placed above and measured the speed at which a dry spot formed and grew as they were heated. When they were heated from below, a “temperature gradient” was established in the film of sunflower oil.
For common liquids like these, the surface tension decreased as it warmed up. It was directed away from the hottest area — on the center — and towards the rim. This creates the process of thermocapillary convection, which moves oil outward. When the film in the middle becomes thinner than a critical value, it “ruptures.”
The researchers determined the conditions that lead to dry spots for both stationary and flowing films. They include reductions in the local film thickness and the size of the deformed region, falling below a number known as the “capillary length.”
“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,” says Fedorchenko.
The phenomenon also occurs in other situations, such as the thin liquid films used in fluid distillation columns or other devices that may have electronic components.
“Dry spot formation or film rupture plays a negative role, resulting in sharp overheating of the electronic components,” adds Fedorchenko. “The results of may, therefore, have wider application.”
The findings were published in the journal Physics of Fluids.
SWNS writer Mark Waghorn contributed to this report.