Russian scientists have conducted theoretical studies of diffusion (mixing) in a liquid in equilibrium with its saturated vapor, using the example of various simulated substances. Science has long known that if a system is heated, the rate of mixing of particles in it increases. But how exactly this happens depends on the specific substance and external conditions (for example, pressure). As a result of the study, scientists have discovered a universal pattern for a wide class of substances. This brought them closer to developing a general theory of fluids. The results of the work supported by two grants from the Russian Science Foundation (RNF) are published in the journal Scientific Reports.

The penetration of particles of one substance between particles of another is called diffusion. Thanks to this phenomenon, for example, we can feel the pleasant smell of coffee from the room, which is brewed in the kitchen, because the molecules of the drink quickly spread, mixing with air molecules. A special case of this process — self—diffusion - occurs when particles of the same substance are mixed. At the moment, scientists have a good understanding of how various processes occur in solids (crystals) and gases, but the understanding of processes in liquids, including diffusion, remains rather limited and fragmentary. However, in recent years, researchers have been able to achieve great success in the field of fluid research using molecular dynamics modeling methods.

Russian physicists from the Bauman Moscow State Technical University (Moscow) conducted theoretical studies of the behavior of liquid particles in equilibrium with their saturated vapor. This means that the number of particles evaporating from the surface of the liquid and entering it from the gas is approximately the same. The scientists carried out their calculations based on the generalized Lennard-Jones potential, a mathematical model that accurately describes noble gases (helium, neon, argon, krypton, xenon, radon), as well as any liquids in a state close to the critical point where the density of the liquid and its saturated vapor are equal. In addition, to make their results and conclusions more general, physicists also considered the ethane model.

In their work, the scientists focused on measuring the diffusion coefficient. This indicator is a measure of the speed of movement of particles and their penetration into each other's space. The diffusion rate depends on the temperature: the higher it is, the greater the velocity of the particles of the studied liquid. The researchers found out that the diffusion coefficient in a liquid that is in equilibrium with its vapor depends linearly on temperature over a sufficiently large temperature range. However, there is a certain temperature, after which the linear section is replaced by a nonlinear one: the rate of diffusion growth increases significantly with temperature. Scientists managed to discover a simple universal pattern (formula) that allowed us to accurately describe this behavior for all the liquids considered.

Also, scientists have studied in more detail the relationship of the diffusion coefficient with the excitation spectra. The motion of particles in a liquid (or in any other system) can be represented as a large number of sound waves superimposed on each other. Each liquid has its own unique set of these "waves", which is called the excitation spectrum. This is something like a fingerprint for a person. However, the researchers found that this "fingerprint" changes when the uniform increase in the diffusion rate with increasing temperature begins to accelerate. Despite the fact that the liquid itself remains the same, its spectra begin to resemble more the spectra of a gas system, where diffusion is strong, than the spectra of a crystal, where diffusion processes are weaker.

"At the moment, the theories of gases and solids (crystals) are well developed, but there is no unified theory of liquids. Our research is another step towards the development of such a theory that will allow us to better understand the physical properties of liquids, and therefore make it possible to predict and manage them. One of the approaches to the theory may be to study the connections of the excitation spectra of liquids with its various properties. We believe that the presented results will open up new prospects for further research in this direction for a wide range of liquids — from simple atomic and molecular to "liquids" formed by active or living (cellular or bacterial) particles," says Nikita Kryuchkov, Candidate of Physical and Mathematical Sciences, head of one of the projects supported by the RNF, Senior researcher of the REC "Photonics and IR Technology", Associate Professor of the Department of Physics and the Department of Biomedical Technical Systems of the Bauman Moscow State Technical University.

With all the obviously great fundamental importance of the work of Russian physicists, these studies can be useful in practice. Liquids and gases surround us everywhere. They play an important role in wildlife and various technical systems. Today it is very important to be able to model such systems, as this allows you to predict natural phenomena, such as weather, or optimize the operation of various technical systems in order to increase their efficiency — for example, the efficiency of the synthesis of useful substances in various bio- or chemical reactors. Such calculations may require very large computing power, but they can be significantly reduced if the modeling of some of the processes can be replaced with simple but fairly accurate models, one of which was obtained by Russian scientists.

Information provided by the press service of the Russian Science Foundation

Photo source (Alexey Nikolsky):

The information is taken from the portal "Scientific Russia" ( /)

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