Today we are going to talk about one of the thermodynamic processes that usually occurs in nature. It's about the reverse sublimation. It occurs when an exothermic state change occurs from a gas to a solid without first being converted by its liquid phase. It has other names such as regressive sublimation or deposition.
In this article we are going to tell you all the characteristics, how it happens and how important reverse sublimation is.
Key features
It is an exothermic process since the gaseous particles must lose energy in the form of heat and give it to the environment. In this way, it is achieved that the product of this reaction has less energy than that of the reactants. In such a way that it cools down enough to that can form crystals, solidify or freeze on a surface. This reverse sublimation process can be seen where there is a sufficiently icy surface so that crystals can be deposited on it directly.
When we speak of deposition, we are not referring to the fact that the particles are deposited from the gas phase without actually wetting the surface. Normally we find reverse sublimation phenomena on icy objects such as frost that is deposited on the leaves during winter. We can detect this deposition since it is formed by a thin layer of crystals, although it can also be an apparent dust or clay.
Thanks to the control of this process New multilayer materials can be obtained where each layer consists of a specific solid that is deposited by both physical and chemical processes.
The role of reverse sublimation
It is, as its name suggests, a compound process sublimation. It does not start from a solid evaporating, but from a gas that solidifies or freezes. It can be quite surprising to think that a gas can be cooled to such an extent that it does not even need to pass through it has been liquid in the first place.
Let's see what the role of the surface has in reverse sublimation. When a gas is highly disordered and diffuse, it begins to rearrange its particulars and establish itself as a solid when temperatures drop. This rearrangement is thermodynamically difficult to carry out. And it is that it needs a type of support that is capable of receiving the gas particles so that they can be concentrated. Once the particles are concentrated, they can interact with each other to exchange heat with the colder surface.
This is how they lose energy thanks to the surface acting as a heat exchanger. As the particles exchange heat with the colder surface, they slow down and the first crystalline nuclei are formed. These nuclei serve so that other groups of particles and the rest of the surrounding gas can be deposited. Thanks to this structure, reverse sublimation can begin to form. The end result of this process is that a solid crystal layer forms on the surface.
Conditions for reverse sublimation to occur
For this process to take place, first of all there must be several conditions. The first is that the surface of contact with the particles must have a temperature below its freezing point. This means that the gas must be supercooled in such a way that as soon as it touches the surface, all its stability can be disturbed.
On the other hand, if the surface is cool enough, the high temperature of the gas can be transferred more quickly to make all the particles adapt to the structure on the surface. There are various reverse sublimation methods where the contact surface does not even have to be cold for a reaction to occur. In the technology industry, a lot of work is done with this process and it is called chemical vapor deposition by combustion.
Examples
We are going to see which are the main examples of this type of process. When we take a beer out of the refrigerator, the glass is coated in white. And it is that the bottle offers a sufficient surface so that the water vapor molecules collide and lose all the energy quickly. If the glass that covers the beer is black, the white color will be much more noticeable. We can tear with a fingernail to see that the steam has become solid.
Sometimes this process is such that the beer becomes covered in a white frost. The effect lasts for a short time since as minutes go by it condenses and becomes damp in the hand.
Another example is frost. As occurs on the walls of the beer bottle, the frost that is deposited on the internal walls in some refrigerators also has this process. These layers of ice crystals can also be seen in fin tuna at ground level. It is a freeze that does not fall from the sky like snow does. The air is simply so cold that when it hits the surface of the plants it freezes directly. They go from a gaseous state to a solid state.
Physical and chemical deposition
So far we have only talked about water. However, it can also occur with other substances or compounds. Let's suppose that we have a chamber where there are gaseous gold particles. Here we can introduce an icy and resistant object and layers of gold will be deposited on this object. The same would happen with other metals or compounds as long as an increase in pressure is not required to be able to create a vacuum.
On the other hand, we have chemical deposition. If there is a chemical reaction between the gas and the surface, it is a chemical deposition. This is commonly used for polymer coating in industry. Thanks to chemical deposition, surfaces such as diamond, tungsten, nitrides, carbides, silicon, graphene, etc. are treated.
As you can see, reverse sublimation is a natural process that humans benefit from for various uses in industry. I hope that with this information you can learn more about reverse sublimation and how it happens.