Separation and transportation of powders are important processes in various technological applications. Although mechanical, chemical, or electrical methods can provide possible solutions, operational or environmental constraints may... more
Separation and transportation of powders are important processes in various technological applications. Although mechanical, chemical, or electrical methods can provide possible solutions, operational or environmental constraints may require alternative methods. Spreading and levitation of clusters (aggregates) of fluorinated fumed silica nanoparticles placed under atmospheric pressure on a hot plate is reported. The powder spreading in the chamber continued until the temperature-dependent saturation value of the spot radius, which grew linearly with the temperature. Open space experiments clearly demonstrated levitation of the powder clouds. Qualitative physical analysis of the observed phenomena is suggested. The effect of levitation is explained by the lifting thermo-phoretic force emerging in the Knudsen layer of air. The levitation of the powder under atmospheric pressure becomes possible due to the combination of low adhesion of the fluorinated fumed silica clusters to the substrate, low density of the particles and clusters, and their high specific surface area.
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Paraffin wax stores energy in the form of latent heat at a nearly constant temperature during melting and releases this energy during solidification. This effect is used in industrial energy storage. An unusual change in the shape of a... more
Paraffin wax stores energy in the form of latent heat at a nearly constant temperature during melting and releases this energy during solidification. This effect is used in industrial energy storage. An unusual change in the shape of a melted droplet of paraffin wax placed on a relatively cold glass plate is studied. As the droplet solidifies, its upper surface becomes nearly flat and a dimple is formed in the center of this surface, making the droplet look like a fruit (pumpkins are more commonly shaped like this, but the authors prefer apples). A series of experiments, as well as physical and numerical modeling of the droplet's thermal state, taking into account the formation of a mushy zone between liquidus and solidus, made it possible to understand the role of gravity and gradual increase in viscosity and density of paraffin wax on changing the droplet shape and, in particular, to clarify the mechanism of formation of the dimple on its upper.
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Freezing of water droplets placed on the bare and superhydrophobic surfaces of polymer wedges are studied both experimentally and computationally. Two-dimensional numerical calculations of the transient temperature field in a chilled... more
Freezing of water droplets placed on the bare and superhydrophobic surfaces of polymer wedges are studied both experimentally and computationally. Two-dimensional numerical calculations of the transient temperature field in a chilled polymer wedge show that the direction of heat flux from the droplet through the thermal contact region with the wedge differs significantly from the normal to the wedge surface. This is the physical cause of the recently observed asymmetric cooling of the droplet. A novel approximate computational model is proposed that takes into account the variable area of the water freezing front in the droplet. This model gives a quantitative estimate of the faster freezing of the droplet on the bare surface. The obtained numerical results agree with the data of laboratory experiments. The velocity of the crystallization front and the droplet deformation including the so-called freezing tip formation are monitored in the experiment. The direction of the freezing cone axis appears to be noticeably different for the cases of bare and superhydrophobic wedge surfaces. This deviation is explained by the fact that the direction of the freezing cone axis is controlled by the local direction of the heat flux. For a hydrophobic wedge surface, the deviation of the freezing tip from the vertical is smaller, because the reduced thermal contact area reduces the influence of the heat flux direction at the wedge surface.
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Research Interests: Mechanical Engineering, Chemical Engineering, Materials Science, Mechanics, Radiation, and 11 moreComputational Fluid Dynamics, Modeling, Heat Transfer, Transparency, Smoothed Particle Hydrodynamics, Thermal conduction in Nanomaterials, Absorption, Radiative Transfer, Thermal Radiation, Particulate Matter, and Interdisciplinary Engineering
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A novel physical approach based on relative mutual independence of absorption and scattering properties of weakly-absorbing materials is formulated. The resulting simplifications in both experimental characterization and theoretical... more
A novel physical approach based on relative mutual independence of absorption and scattering properties of weakly-absorbing materials is formulated. The resulting simplifications in both experimental characterization and theoretical modeling of spectral properties of dispersed materials are discussed in some details. The suggested approach is verified using the published experimental data for materials of different nature. It is shown that the methods developed enable us to simplify radically the identification procedure for material properties, determine extremely low absorption coefficients of the bulk substance from the measurements for highly-scattering porous material, and estimate the effect of non-uniform distribution of adsorbed water on scattering characteristics of nano-porous super-insulations.