In the movie Terminator 2, when the T-1000 is frozen in liquid nitrogen, we see the power of cryogenic substances. However, the reason why it’s okay to briefly come into contact with these substances is because of the Leidenfrost effect. When a liquid comes into contact with an object at a very high temperature, it instantly forms a vapor film that impedes heat transfer. This principle explains many phenomena that we see in our daily lives.
In the movie Terminator 2, there is a scene where the T-1000, a super-strong cyborg that cannot be killed by guns, cannons, and flamethrowers, is frozen in liquid nitrogen and then blown to smithereens with a single punch from the Terminator. Not only is this scene memorable for many, but it’s also a great visual representation of the power of cryogenic materials. Liquid nitrogen has a wide range of scientific and industrial uses, and is indispensable for a variety of experiments and storage operations, especially at very low temperatures.
Nitrogen gas boils at minus 196 degrees Fahrenheit, so liquid nitrogen is a powerful cryogenic substance that is colder than minus 196 degrees Fahrenheit and can freeze most materials in an instant. In fact, liquid nitrogen plays an essential role in the laboratory when studying superconductors or storing biological samples for long periods of time. Thanks to this powerful cooling ability, liquid nitrogen is also used in cryogenic experiments, to regulate the temperature required in certain processes, and even to shrink mechanical parts.
So what happens when you put your hand in and out of liquid nitrogen? Well, if you do it for longer than two seconds… you get what you imagine. But if you put it in and out for just a second, your hand will be completely fine. Similarly, dry ice, a common refrigerant, can be held on your tongue for a short time and then removed without any damage. Why is brief contact with these cryogenic substances okay?
The short answer is that it’s because of the Leidenfrost effect. When a liquid comes into contact with an object whose temperature is significantly higher than its boiling point, only the surface of the liquid in contact with the object evaporates momentarily, forming a vapor film. When a vapor film forms, heat transfer between the two objects is impaired. This vapor film, which can be thought of as a naturally occurring thermal barrier, has a very short duration, meaning that it only impedes heat transfer for a short time and then disappears. This is why it’s okay to stick your hand in liquid nitrogen for a second and then remove it, and why it’s okay to stick your tongue in dry ice for a second and then remove it.
The Leidenfrost effect is also found in other common phenomena. For example, you may have noticed that when you drop a drop of water into a hot skillet, it dances and rolls around the surface of the pan before evaporating. This is due to the Leidenfrost effect. When the surface of a pan reaches a temperature much higher than the boiling point of water, the bottom of the droplet instantly evaporates, forming a thin, air-like film that causes the droplet to bounce off the surface. The Leidenfrost effect is a phenomenon that we see every day, but understanding how it works is a fascinating part of science.
So why does a vapor film form and prevent heat transfer? Before we get to the reason, let’s take a quick look at the three ways heat is transferred: conduction, convection, and radiation. Conduction is the transfer of heat only through a “medium,” which is a material that transfers heat; convection is the transfer of heat with a medium; and radiation is the direct transfer of heat from a higher temperature to a lower temperature through electromagnetic waves, with or without a medium. For example, when one side of a metal is heated, the other side also heats up, which is a phenomenon of conduction; when hot water and cold water are mixed, the water becomes lukewarm, which is a phenomenon of convection; and when the heat energy emitted by the sun reaches the earth, it is a phenomenon of radiation. However, actual heat transfer is mostly a mixture of conduction, convection, and radiation, so if only one factor is responsible for heat transfer, the effectiveness of the heat transfer will decrease. Also, conduction is usually the largest heat transfer, followed by convection, and then radiation.
Now, let’s answer the above question by looking at the difference between two objects without and with a vapor film between them. First of all, in both cases, the effect of radiation is not significant and can be ignored. First, when the vapor film is not formed, the heat transfer between the two objects is a mixture of conduction and convection. This is the case in most real-world situations, such as sticking your hand in boiling water or rolling a snowball with your bare hands, where the temperature difference between the two objects is not very large. However, when a vapor film is formed, heat transfer between the two objects can only occur by convection. As mentioned above, this is the case when you put your hand or tongue in liquid nitrogen or dry ice, where the temperature difference between the two objects is very large. Therefore, what was originally a mixture of conduction and convection, once a vapor film is formed, heat transfer occurs only by convection, so the amount of heat transferred is small.
To summarize, the Leidenfrost effect is a phenomenon in which when a liquid comes into contact with an object at a temperature significantly higher than its boiling point, only the surface of the liquid in contact with the object momentarily evaporates to form a vapor film, which temporarily reduces heat transfer. It is important to note that this phenomenon only occurs between a liquid and a substance whose temperature is significantly higher than the liquid’s boiling point. For example, as mentioned above, sticking your hand in liquid nitrogen or your tongue in dry ice is safe for a short period of time because the temperature of your hand is much higher than the boiling point of the liquid nitrogen or dry ice, so a vapor film forms on the surface of your hand or tongue, which momentarily stops heat transfer, but sticking your hand in boiling water is never, ever safe because the temperature of your hand is much lower than the boiling point of the water! Of course, even in the case of liquid nitrogen and dry ice, this phenomenon doesn’t last long, so don’t be so reckless as to actually do it just to test it out.
There are more phenomena around us than you might think that can be explained by simple scientific principles. We just don’t pay attention and don’t think about it. Science is everywhere: in the air we breathe, in the plants growing on our porch, in the pot of ramen noodles we boil. For example, physics can explain why your glasses fog up when you walk down the street on a cold winter day and come into a warm room. It’s caused by water vapor in the air condensing on the cold lenses of your glasses due to the temperature difference between indoors and outdoors, another example of a natural phenomenon that we experience every day. It’s fascinating and meaningful to realize that the small things we encounter in our daily lives actually contain amazing scientific principles.
