Why Does Non-stick Cooking Stick To The Pan? Why Is The Sticking Part in The Center Again?

There are many brands of non-stick pans on the market. No matter which one you choose to buy, you will occasionally stick to the pan during your cooking process. If you observe carefully, the sticking part is often in the center of the pan. What is causing this? Why do the two situations of "sticky pan" and "non-stick pan" always alternate during the cooking process?

What is the sacred "dry spot"?

If you want to understand the reason, you must understand what is a "dry spot." If you pour a basin of water on the clean glass, the liquid will definitely flow down the glass, but you will find that the water will not completely flow down, and there will be small droplets on the surface of the glass. Have you ever wondered why it is a small drop of water, why not a whole water film? In fact, when water flows along the glass, the entire glass surface will be covered with a layer of water film. This layer of water film will be partially broken in a very short time. The part that is not broken is the small water droplets we see. The dry area left by the rupture of the water film is the "dry spot". At this point, the content we are talking about now seems to have nothing to do with the problem of "sticky pot", but it is not. These content is the key to solving doubts. You can read it first.

The phenomenon of liquid film rupture abounds in daily life, and it was Soviet physicist Y. Karetnikov who first observed this phenomenon. By repeatedly observing the rupture of liquid films in life, he found that the liquid will rupture when it descends along a vertical surface, whether it is water, gasoline, edible oil, etc., all without exception. Y. Karetnikov compiled a paper related to this and published it in 1954.

Uncover the mystery of "sticky pot"

Why does non-stick cooking stick to the pan? Because the oil film has "hot capillary convection" phenomenon. In order to better understand, let's start with an interesting experiment. Two physicists from the Czech Academy of Sciences found a ceramic pellet non-stick pan, and poured a thin layer of rapeseed oil into it. The thickness of the oil film was only 1.5 mm. Then they began to heat the bottom of the pot and set up a video camera to record.

The results of many experiments have a common phenomenon: during the heating process, the oil film suddenly ruptures from the middle, and continues to extend outward over time, finally reaching the maximum diameter, and this ruptured "hole" is the "dry spot". Later, they measured the rate of formation of "dry spots" of sunflower oil through the images recorded by the camera.

In order to make the experiment more rigorous, they applied polytetrafluoroethylene (Teflon) on the non-stick pan, which can effectively weaken the adhesion of the pan and eliminate interference factors. With the pouring of rapeseed oil, the temperature of the bottom of the pot continued to rise, the oil film still ruptured in the center, exposing the dry bottom of the pot, and "dry spots" appeared again!

The "dry spots" that appear on the bottom of the pan are not covered by oil film. If you put the vegetables in and fry, it is equivalent to cooking without adding oil. Can it not stick to the pan? The "dry spot" is a hole where the oil film ruptures from the middle and continues to extend, that is, the central part of the pan will have no oil due to the appearance of the "dry spot" during the cooking process, so it will be particularly easy to stick to the pan.

Why does the oil film break? Let's talk about the phenomenon of "hot capillary convection" objectively. All homes use natural gas stoves. When the pan is heated, the burning of the flame on the bottom of the pan will cause uneven heating of the oil film, resulting in a temperature gradient in the oil film. Someone should say, why is the pan heated unevenly? The flame is divided into outer flame and inner flame, and there is a certain difference in temperature between the two.

As the temperature rises further, the temperature gradient on the surface of the oil film will become more pronounced, which will cause the surface tension of the oil film to decrease, resulting in a tension gradient. The stronger tension on the edge will pull the oil with weaker adhesion in the center, which will cause the oil to flow from the center to the edge of the pan, and the oil film will become thinner and thinner. Eventually, "dry spots" will be formed, and the cooking will stick to the pan. .

A variant of the "hot capillary convection" phenomenon is called the "Marangoni effect", and this phenomenon is even more interesting! When you use a goblet to hold red wine, many people will gently shake the glass clockwise (counter-clockwise).

When the wine glass is placed on the table, carefully observe the surface of the red wine, you will find that the liquid film is slowly rotating, even after a long time, this phenomenon will still occur. This is not inertia, but also caused by surface tension. After shaking, red wine will remain on the side of the wine glass. Because the wine film attached to the glass is thinner and evaporates faster, the surface tension of the wine film on the glass wall is greater than the wine in the wine glass. This thin wine film" Towing" the wine in the glass to do a rotating motion.

Application value behind

Those who are experienced in cooking must speak up: pour more oil to increase the thickness of the oil film; or use a thicker pan to heat the bottom of the pan evenly. Yes, these methods are indeed a panacea to avoid non-stick pans, but "dry spots" are more harmful in the food, chemical and manufacturing industries, because a common situation will inevitably appear on the production lines of these industries: liquid in solid Surface flow.

And this time to have a deeper understanding of the formation of "dry spots", which has more important application value for science and engineers.