Do you cook often? If so, have you noticed the problem that sometimes food always sticks to the center of the pot, even if the pot you are using is a non-stick pan wrapped in a non-stick coating.
In fact, this problem is related to a phenomenon that is common in nature and engineering, that is, the "dry spot" formed on liquid films. We have all seen that a continuous layer of water film on the glass window will suddenly break, leaving dry areas.
The originally continuous water film on the glass window will rupture, leaving behind dry areas. | Image source: pxhere
The first to notice this phenomenon was former Soviet physicist Y. Karetnikov, who described the phenomenon that liquids would rupture on the liquid film when it flows down a vertical surface. This phenomenon is almost everywhere, but for some engineering equipment, it will have some negative effects. For example, in distillation tower , industrial scrubbers, and other devices with electronic components, when dry spots appear in the liquid film formed on the solid surface, it will cause rapid overheating of the electronic components, or heat transfer and mass transfer to decrease.
If we want to avoid this phenomenon well, it is necessary to better understand how dry spots form in liquid films. Now, a new paper published in the journal Fluid Physics reveals the cracking process of liquid films on solid surfaces.
In this study, two physicists from the Czech Academy of Sciences used non-stick pans to test the problem. They heated a thin layer of sunflower seed oil in a nonstick pan coated with ceramic particles, with an oil film thickness of only 1.5 mm. Then, they recorded the process of heating sunflower oil with maximum firepower with cameras and measured the rate of dry spots in sunflower oil.
After adding cooking oil to a ceramic non-stick pan, a "dry spot" appeared in the middle. | Image source: Alex Fedorchenko
They observed that the initial continuous sunflower oil film in the pot suddenly began to break when heated, and this dry area extends outward from the center of the pot and then quickly reaches its maximum diameter.
sunflower oil is removed to the edge of the pan after being heated. | Image source: Fedorchenko et. al. /Physics of Fluids
Then, they conducted experiments with a pan coated with polytetrafluoroethylene (Teflon) and observed the same phenomenon.
The same phenomenon occurred after adding cooking oil to a non-stick pan coated with a Teflon layer. | Image source: Alex Fedorchenko
They realized that it was because of a phenomenon called "hot capillary convection", which caused dry spots to appear on the oil film. This hot capillary convection process will "suck" the oil to the edge of the pot. This phenomenon is like a variant of the "Marangoni effect", a liquid flow caused by evaporation. For example, when the wine glass rotates, a thin layer of wine will be left on the wall. Since the alcohol in the film evaporates faster than other liquids in the wine (mainly water), the surface tension of the film is higher than that of the wine at the bottom of the wine glass. The resulting surface tension gradient causes the Marangori effect, causing the wine to flow along one side of the wine glass.
, which is related to the hot capillary convection encountered by the oil in the non-stick pan, is that when the pan is heated, a temperature gradient of is generated in the oil film. As the temperature increases, the surface tension of the liquid will decrease accordingly, resulting in a surface tension gradient. The stronger tension at the edges will pull the oil with weaker viscosity in the center, causing the oil to flow out from the center, causing the oil film to become thinner and deform until it breaks, leaving a dry area in the center of the pan.
Researchers observed that when it exceeds a certain size, the holes in the oil film cannot heal anymore. For each liquid, this critical point varies by temperature. Any dry area exceeding the critical size will expand rapidly.
Researchers say that now we have a good idea why food sticks to the middle of the pot.We can take some simple measures to avoid this: for example, you can increase the thickness of the oil film to prevent it from being too thin; or you can use a pan with a thicker bottom to transfer heat more evenly and prevent the appearance of temperature gradients; without affecting the cooking effect, you can also try to use moderate firepower...
For you who may be very skilled in cooking, these suggestions may sound very obvious. But for some scientists and engineers who are deeply troubled by rupture of liquid films, the findings of this study may have important applications for them, because the flow of liquids on solid surfaces is key to many processes in the food, chemical and pharmaceutical industries.
# Creative team:
Text: Xiaoyu
Picture: Wenwenzi
# Reference source:
https://aip.scitation.org/doi/10.1063/5.0035547
https://www.newscientist.com/article/2266479-physicists-finally-figured-out-why-food-sticks-to-a- frying-pan/?utm_campaign=RSS%7CNSNSutm_source=NSNSutm_medium=RSSutm_content=physics
https://arstechnica.com/science/2021/02/the-physics-of-why-food-sometimes-sticks-to-the-center-of-a-nonstick-frying-pan/
Source: Institute of High Energy, Chinese Academy of Sciences
