Summer! The sun, warm, in the air the smell of freshly cut grass. The grass is far away, and the smell is felt. This is physics, a similar phenomenon is called diffusion. And it is defined as the penetration of particles of contacting substances into each other. For example, as now, the grass is in contact with the air, and the smell from the mowed grass spreads through the air far from the place of mowing. A similar process is usually characterized by such a value as the diffusion coefficient.
We can say that everything that happens around us is largely connected with the processes of interpenetration. Thanks to this phenomenon, we even live. What's so surprising? The penetration of oxygen or nutrients into the blood - this is the real diffusion, one substance penetrates another. Widespread diffusion in nature is much wider than it seems to us. Such a phenomenon is not something exotic, but rather, it is widely represented in the surrounding world. Even the fact that the air exhaled by a person does not accumulate around, but is scattered in space, is also due to diffusion.
Interpenetration can occur in a variety of bodies - liquid, gaseous, solid. Its cause is the chaotic motion of molecules of matter. By the way, such a phenomenon as diffusion is considered in molecular-kinetic theory as one of the factors confirming it. So, returning to the state of aggregation of the body: the penetration rate of a substance depends on its state of aggregation and on such a value as the diffusion coefficient.
What is this mysterious concept? This is the name of the quantitative characteristic of the rate of transfer of molecules of one substance to another. The diffusion coefficient, the formula of which is quite specific, allows one to estimate the amount of a substance that has passed through a unit area (square meter) per unit time (second).
In practice, it has been established that in gases mutual penetration occurs at the highest rate, while in solids the penetration rate is minimal. The diffusion coefficient is influenced by body temperature and the mutual concentration of nearby substances. With increasing temperature, the rate of interpenetration increases, as does the concentration of a substance.
Thus, the interpenetration phenomenon is due to the so-called concentration gradient or temperature gradient. By type, diffusion is divided into free and forced. Forced arises under the influence of external forces. Depending on their type, forced diffusion is defined as thermo-, electro-, baro- and ascending.
Diffusion is widely used in technology. One of the most typical examples is diffusion welding. The essence of this technology is simple: they connect two different bodies (let it be two metals), and then expose them to pressure and temperature. The latter is smaller in magnitude than the melting point of substances.
The result is a combination of two dissimilar materials. This technology is widely used in instrument engineering and the electronics industry, in the manufacture of large-sized parts of complex shape, in pilot and small-scale production. Such welding can be carried out in different conditions, up to vacuum conditions, everything is determined by the specific requirements for the finished product.
Another, no less common, use of the phenomenon of diffusion is its application to obtain semiconductor structures. One of the technologies for creating pn junctions is based on the phenomenon of interpenetration. Under the influence of high temperatures approaching the melting temperature, regions with the desired concentration of impurities are obtained in the crystal structure.
Consideration of the concept of "diffusion coefficient" made it possible to verify the extremely widespread occurrence of the phenomenon of the interpenetration of substances in nature, as well as in various options for its use in technology.