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When cooking, pour the oil into the pan and shake the pan to smooth a thin layer of oil, so as to prevent the pot from sticking when heated, which may be the experience of many cooks.

Let the oil smooth so that it won't stick to the pan | giphy, however, there is one thing that may lead to obsessive-compulsive disorder: why is the oil on the pan always uneven? As long as it is heated, the oil at the bottom of the pot will automatically slip to both sides, which makes it easier for the fried food to stick to the pan. Isn't the pan itself flat?

But the oil is always uneven | Wiki so whose "pot" is this problem? Facts have proved that it is not only the "pot" of the pot, but also the "pot" of oil.

First of all, some pans are not "as named" because their bottoms are not really flat. If some pots are in use, they may automatically "bend". This is because a pan that is too thin is likely to deform under rapid changes in temperature, that is, to expand when heated.

That's why you don't pour cold water into a very hot pot right away. | Giphy for this reason, some pots may maintain a curved shape with a bulge in the middle in order to adapt to this thermal expansion. This principle is the same as keeping some gaps in the tracks, otherwise the severely deformed pot may not be able to stand on the stove. The bending that comes with this design may also have two shapes, one is concentric circles and the other is longitudinal.

The bottom of the pot is curved and there are two different forms. | reference [2] if you want to know whether the bottom of your pot is flat, you can put the same amount of warm water on the cold pot. If it moves to both sides without heating, it may mean that the bottom of the pot is bent. Sometimes, however, this bending may not be visible in a thick-bottomed pan, or it may be specially designed.

However, even if the pot is really flat, the oil will still slip to the edge of the pot, because when heated, the oil itself will run out. Researchers at the Czech Academy of Sciences used a high-speed camera to record the changes in sunflower oil as it warmed up in a pan.

The researchers want to explore why the oil film in the middle is always ruptured. | reference [1] the unheated oil is laid out in the pan, depending on the surface tension. Because the oil in the middle of the pot is heated faster. As the temperature increases, the surface tension of the liquid decreases, so the surface tension of the oil in the pot shows a gradient of low in the middle and high at the edge. The oil with stronger edge tension will pull the middle oil out.

There is less and less oil in the middle, and the study suggests that when the volume flow rate of the intermediate oil reaches the critical value, the oil film will break. Scientists have even calculated that the speed at which the edge of the oil "slips away" is 5.5 centimeters per second.

The flow of this oil caused by the surface tension gradient is called the Marangoni effect and is also known as "thermo-capillary convection" at temperature.

There is also the phenomenon of "wine and tears". | COMSOL in many life scenes, we can see the Marangoni effect, such as the famous "wine and tears". After pouring the wine into the glass, a circle of water droplets on the top of the wine kept falling back into the wine. According to the Malangoni effect, the wine climbs up to the inside of the glass through capillarity, and the alcohol evaporates more quickly. at this time, the surface tension of the liquid on the wall of the glass increases, while the surface tension of the wine in the glass is lower, and more wine climbs up the wall of the glass. But because of gravity, the climbing wine fell back into the cup. Some people say that the more "wine tears", the higher the quality of wine, but in fact, the wine with higher alcohol content, the phenomenon of wine tears is more obvious.

Wine and tears is also a kind of Marangoni effect. | COMSOL you can spread a thin layer of water on the table, and then drop a drop of alcohol in the middle, and you will find that the water will stay away from alcohol, because the alcohol concentration causes the liquid to produce a surface tension gradient.

With some colored liquids such as alcohol or soapy water, you can also make the Marangoni effect at home. | KieranBreton / youtube in addition, there is a "coffee stain ring" formed by spilled coffee. When we leave the coffee liquid on the table, when it dries, it will not form a solid brown spot, but will always have a dark outer ring on the edge. This is because when droplets full of fine solid particles, such as coffee, stay on the table, the different rates of evaporation at the inner and outer edges cause the solid particles to end up on the "outer ring". Such as red wine and tea can also produce this phenomenon.

A dark outer ring left by a coffee stain. | Wiki although this phenomenon may seem innocuous, it may have a greater impact if it occurs on inkjet printers or gouache pigments, so the researchers also try to "control" the particles in these droplets to make them more obedient and avoid the phenomenon of coffee stains.

Similarly, the "uneven oil shop" may be small, but the flow of thin liquid on solids may also occur in the industrial field, so it is necessary to study it. The researchers also suggested several options to avoid sticking to the pot while cooking. For example, use "wide oil", that is, to make the oil a little thicker. Or use a pan with a thicker bottom and more uniform heating. In addition, more stirring can also avoid sticking to the pot.

Kuang you won!

ChatGPT friendly reminder: please have the right amount of oil. Reference:

[1] Fedorchenko, A. I., and J. Hruby. "On formation of dry spots in heated liquid films." Physics of Fluids 33. 2 (2021): 023601.

[2] https://www.pfannenprofis.com/en/pages/boden-nicht-plan-hohlwoelbung

[3] https://gizmodo.com/the-mystery-about-the-coffee-ring-effect-continues-1785521172

[4] https://cn.comsol.com/multiphysics/marangoni-effect?parent=modeling-conservation-mass-energy-momentum-0402-432-362

[6] https://en.wikipedia.org/wiki/Coffee_ring_effect

This article is from the official Wechat account: bring Science Home (ID:steamforkids), author: everything.

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