Sat. Oct 1st, 2022
Eggs fried in a pan are arranged in such a way that they look like a smiling face.
enlarge Food sometimes sticks to a heated surface, even when using oil or a non-stick frying pan. Scientists have concluded that convection may be to blame.

Home cooks around the world have relied on non-stick cookware for decades for quick and easy cleanup after meal preparation. But sometimes food still sticks in the middle of nonstick pans. A new paper published in the journal Physics of Fluids offers a likely explanation: Food lingers because of the same underlying mechanism that gives rise to the coffee ring effect and so-called “wine tears.”

The first non-stick frying pans were made possible by the invention of Teflon in 1938 by a chemist named Roy Plunkett, who was researching possible new chlorofluorocarbons as part of a joint venture with DuPont. In April of that year, Plunkett later recalled, his assistant selected one of the cylinders they used to store tetrafluoroethylene gas (TFE) at dry ice temperatures until the jerry cans were ready to be chlorinated for their experiments. When the assistant opened the valve, the gas did not flow out of the container under its own pressure, as expected.

Astonished, the researchers opened the container to find that the gas was gone. Instead, they found a white powder. The TFE was polymerized to a waxy solid called polytetrafluoroethylene (PTFE), which turned out to have some interesting properties: it was chemically inert and heat resistant, and it had very low surface friction. It may not have been useful as a coolant, but it turned out to be a great non-stick coating.

Teflon was first used as a seal for the atomic bomb during World War II, but Dupont registered the Teflon trademark in 1944 and began marketing it as a non-stick coating for commercial bread and baking pans. It eventually made its way into everyday consumer cookware, and the rest is history. Today, several other non-stick coatings are also available in consumer products, and while culinary purists prefer cast iron skillets or non-stick pans for sauces and gravies, there’s no doubt that most cooks appreciate the time saved in cleanup.

But sometimes food sticks to the center of a skillet, even with a non-stick coating. Researchers at the Czech Academy of Sciences were curious as to why this might be the case and decided to experiment. They filmed sunflower oil in a non-stick pan covered with ceramic particles that was heated, and they noted the rate at which a suspicious dry spot formed and grew in the oil. They conducted similar experiments with Teflon-coated pans.

According to co-author Alexander Fedorchenko, a physicist at the Czech Academy of Sciences, food that gets stuck in the center of the pan is “caused by the formation of a dry spot in the thin sunflower oil film due to thermocapillary convection.” It is a variant of the so-called Marangoni effect – after the Italian physicist Carlo Marangoni – which is responsible for both wine tears and the infamous ‘coffee ring effect’, which has also sparked a lot of interest among physicists.

As we have previously reported, British physicist James Thomson (Lord Kelvin’s elder brother) first noticed wine tears in 1855. The effect is most noticeable in wines (or other spirits such as rum) with an alcohol content of at least 13, 5 percent. That’s because alcohol has a lower surface tension than water. If you spread a thin layer of water on your counter and put a single drop of alcohol in the center, you’ll see the water flow out, away from the alcohol. The difference in the alcohol concentrations creates a surface tension gradient, which drives the current.

In the case of food that sticks to the center of non-stick frying pans, we are talking about a temperature gradient created in the oil film when a pan is heated from below – the aforementioned thermocapillary convection. As the temperature increases, there is a corresponding decrease in the surface tension, creating a surface tension gradient. The oil then flows out, getting thinner and thinner until it cracks, leaving a dry spot in the center of the pan.

So what should a frustrated cook do to keep food from sticking to the center of their skillet? It’s mostly common sense advice. To avoid unwanted dry spots, the following set of measures should be applied: increase the thickness of the oil film, heat moderately, completely wet the surface of the pan with oil, use a pan with a thick bottom or stir food regularly while cooking. cooking,” Fedorchenko said.

In terms of relevance, the researchers’ findings are not limited to home cookware. “The formation of dry patches and streams in thin films of fluid is ubiquitous in nature and engineering,” the authors wrote. “Everyone could see how initially a continuous film of water on a window pane suddenly tears, leaving islands of dry patches.” Thin liquid films are also used in liquid distillation columns, industrial scrubbers and other electronic component devices. “Dry spotting or film breakage plays a negative role, resulting in sharp overheating of the electronic components,” Fedorchenko said. “The results of this study may therefore have broader application.”

DOI: Physics of FluidS, 2021. 0.1063/5.0035547 (About DOIs).

By akfire1

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