S. L. Krasnolutskii

Methods of Measuring the Diffusion Coefficient and Sizes of Nanoparticles in a Rarefied Gas

In view of rapid progress in nanotechnologies, it is necessary to develop new methods of determining the diverse physical properties of nanoparticles. The sizes and diffusion coefficient of particles are basic properties. In practice, these properties are determined by socalled differential mobility (electromobility) analyzers (DMAs). 1 The feasibility of using DMAs in this range was shown in [1]. The interpretation of data obtained by this method is based on the Stokes law of resistance with the Canningham‐Millikan‐Davis corrections. For example, the diffusion coefficient is determined by the formula

Diffusion of nanoparticles in a rarefied gas

It is suggested to describe the diffusion of nanoparticles in rarefied gases in terms of the kinetic theory. For this purpose, the potential of interaction between a carrier gas molecule and a dispersed particle is constructed by summing the interactions of the given gas molecule with all atoms (molecules) of the dispersed particle. With this potential, a formula for the diffusion coefficient of the dispersed nanoparticle is derived. The dependence of the diffusion coefficient on the radius and temperature is studied. Analytical results are compared with experimental data. The well-known experimental Cunningham-Millikan correlation is shown to apply only in the range of near-room temperatures, for which the parameters of this correlation were determined.

Simulation of the nanofluid viscosity coefficient by the molecular dynamics method

The viscosity coefficient of several model nanofluids is simulated by the molecular dynamics method. As nanofluids, argon mixtures with aluminum and lithium particles are used. The size of nanoparticles is varied from 1 to 4 nm; their volume concentration, from 1% to 12%. It is shown that the viscosity of the nanofluids is considerably higher than that of the carrier fluid. The finer the particles, the higher the viscosity of the nanofluids with the volume concentration of the particles being the same. The reason for such an effect is explained qualitatively. It is also found that the viscosity of the nanofluids depends on the material of nanoparticles.

Physics and mechanics of heat exchange processes in nanofluid flows

Nanofluids present a new type of dispersed fluids consisting of a carrier fluid and solid nanoparticles. Unusual properties of nanofluids, particularly high thermal conductivity, make them eminently suitable for many thermophysical applications, e.g., for cooling of equipment, designing of new heat energy transportation and production systems and so on. This requires a systematic study of heat exchange properties of nanofluids. The present paper contains the measurement results for the heat transfer coefficient of the laminar and turbulent flow of nanofluids on the basis of distilled water with silica, alumina and copper oxide particles in a minichannel with circular cross section. The maximum volume concentration of particles did not exceed 2%. The dependence of the heat transfer coefficient on the concentration and size of nanoparticles was studied. It is shown that the use of nanofluids allows a significant increase in the heat transfer coefficient as compared to that for water. However, the obtained result strongly depends on the regime of flow. The excess of the heat transfer coefficient in the laminar flow is only due to an increase in the thermal conductivity coefficient of nanofluid, while in the turbulent flow the obtained effect is due to the ratio between the viscosity and thermal conductivity of nanofluid. The viscosity and thermal conductivity of nanofluids depend on the volume concentration of nanoparticles as well as on their size and material and are not described by classical theories. That is why the literature data are diverse and contradictory; they do not actually take into account the influence of the mentioned factors (size and material of nanoparticles). It has been shown experimentally and by a molecular dynamics method that the nanofluid viscosity increases while the thermal conductivity decreases with the decreasing dispersed particle size. It is found experimentally for the first time that the nanofluid viscosity coefficient depends on the particle material. The higher is the density of particles, the higher is the thermal conductivity coefficient of nanofluid.

On thermal diffusion of nanoparticles in gases

Thermal diffusion of nanoparticles in gases is studied with the help of the kinetic theory and using the nanoparticle-molecule interaction potential developed earlier by the authors. The dependence of the thermal diffusion coefficient of nanoparticles on their radius, volume concentration, and the temperature of carrier gas is analyzed. The results are compared with the data for gas mixtures.

Simulation of the thermal conductivity of a nanofluid with small particles by molecular dynamics methods

The thermal conductivity of nanoliquids has been simulated by molecular dynamics method. We consider nanofluids based on argon with aluminum and zinc particles with sizes of 1–4 nm. The volume concentration of nanoparticles is varied from 1 to 5%. The dependence of the thermal conductivity on the volume concentration of nanoparticles has been analyzed. It has been shown that the thermal conductivity of a nanofluid cannot be described by classical theories. In particular, it depends on the particle size and increases with it. However, it has been established that the thermal conductivity of nanofluids with small particles can even be lower than that of the carrier fluid. The behavior of the correlation functions responsible for the thermal conductivity has been studied systematically, and the reason for the increase in the thermal conductivity of nanofluid has been explained qualitatively.

Simulation of nanoparticle thermal diffusion in dense gases and fluids by the molecular dynamics method

This paper is devoted to the study of the thermal diffusion of nanoparticles in dense gases and fluids by the method of molecular dynamics with Rudyak–Krasnolutskii nanoparticle–molecule and Rudyak–Krasnolutskii–Ivanov nanoparticle–nanoparticle potentials. The thermal diffusion and binary diffusion coefficients were calculated with the help of the fluctuation-dissipation theorem. Nanofluids simulated consisted of argon as а carrier medium and aluminum nanoparticles. Dependences of the nanoparticle thermal diffusion and Soret coefficients on the particle diameter and volume concentration were derived. The thermal diffusion coefficient showed a significant dependence on the particle size for small nanoparticles (1–4 nm diameter).