Turbulence vs. Cancer

A new method of Russian scientists will help diagnose
oncological and infectious diseases

Grigory Kolpakov, Vladimir Koryagin, "Gazeta.Ru"Many problems associated with the mixing of liquid in microchannels can be solved by properly "organizing" non-uniform sliding on the walls of these channels.

This conclusion was reached by a joint group of Russian and German researchers, headed by Olga Vinogradova, Professor of the Faculty of Physics at Moscow State University. The theory developed by the group was published in the latest issue of the journal Physical Review E, the impact factor of which is 2.3 (Nizkaya et al., Flows and mixing in channels with misaligned superhydrophobic walls).

This work belongs to the field of microfluidics, a promising and rapidly developing interdisciplinary field of research that studies fluid flow in microchannels. Microfluidics is especially in demand in chemistry and biomedical research, where there is a need to carry out chemical synthesis of small doses of a substance or perform separation of biomaterial particles.

"Microfluidics is the basis of the so–called laboratories on a chip – miniature devices that allow multi–stage chemical processes, including chemical reactions, mixing, concentration and separation on a single chip the size of a small coin," Olga Vinogradova said, "Such systems are promising not only as microreactors in synthetic chemistry, but also in as portable analytical devices, for example, for the diagnosis of oncological and infectious diseases."

One of the problems that researchers face when working with microchannels is difficult mixing of liquids. The fact is that the flow in such channels is laminar, that is, layered. With laminar flow, there is no convection, so liquids mix very slowly, only due to diffusion.

Physicists managed to find a simple solution to the problem based on the use of superhydrophobic surfaces. Such surfaces are made of hydrophobic (water-repellent) material and at the same time are micro-rough. As a result, micro-bubbles of air are retained in the recesses of the texture of the superhydrophobic surface. The presence of such an "air cushion" makes the superhydrophobic surface very slippery.

In this work, the scientists proposed using a superhydrophobic texture in the form of parallel grooves rotated at some angle to the axis of the channel, and the grooves on the upper wall were turned to the right, and on the bottom – to the left. Such grooves gave the channel walls anisotropic properties: liquid flows faster along them than across. In addition, it turned out that in addition to the main flow along the axis of the channel, there is a secondary shear flow of fluid in the transverse direction. As a result, near the walls, the liquid begins to twist slightly, just as a bullet twists, moving along the rifled barrel of a rifle. Investigating the formed vortex, scientists have discovered a very interesting effect:

"If the liquid moves very slowly, then a single, very elongated transverse vortex forms in the channel," said Tatiana Nizkaya, co–author of the article from the IFHE RAS, "However, already with an increase in the flow velocity, the liquid begins to "drift" on turns."

"This vortex is overlaid with many small, limited by neighboring grooves, that is, an "artificial turbulence" is created in the flow," said Evgeny Asmolov, co–author of an article from the IFHE RAS and TsAGI, "Such flows can be useful for mixing liquids or for separating particles of different sizes."

Together with co-authors from the University of Mainz (Germany), a computer simulation of the predicted effect was carried out by the method of dissipative particle dynamics. The scientists analyzed the trajectories of motion of model liquid particles in the microchannel and studied the dependence of the shape and number of vortices on the flow velocity. Based on the simulation results, the authors concluded that there is a critical value of the velocity at which one large vortex is divided into many small ones, which eventually leads to a new effective mechanism for mixing the liquid.

"There are already systems for efficient mixing in microchannels based on the use of a special "pattern" of the channel surface. For example, in order to twist the liquid, special obstacles at the bottom of the channel are placed "in a herringbone pattern". At the same time, the vortex arises due to the side walls," Tatiana Low summed up, "Our method is much simpler: it is enough just to take two superhydrophobic planes with strips of gas and rotate them at an angle to each other. In addition, the splitting of the vortex into many small ones allows mixing simultaneously across the entire width of the channel."

Portal "Eternal youth" http://vechnayamolodost.ru20.04.2015