arXiv subjects

Peter Lebedev-Stepanov

Publications and source records attributed to Peter Lebedev-Stepanov.

8 recordsLinked to original sources

arXiv2026

Tangential and normal partial slip at the liquid-fluid interfaces: application to a small liquid droplet, gas bubble, and aerosol

An analytical solution is obtained for the problem of the slow movement of a small drop of a fluid in another immiscible fluid in an infinitely large reservoir with the boundary condition of the normal slip and/or tangential partial slip at the interface. That generalizes the conventional Navier and Maxwellian boundary conditions of partial slip. Normal slip is accompanied by the density gradient in the fluid and is applicable only if one of the phases in contact at the interface is a gas. Although tangential partial slip and the associated generalization of the Hadamard-Rybczynski equation (HRE) have been considered previously, they were done using the friction coefficient formalism. Here, this issue is discussed within the more general formalism of slip lengths. It is proven that each of the two fluids separated by an interface has its own slip length. New equations describing the terminal velocity of gas bubble rise and aerosol falling have been obtained. The result is compared with experiment. It has been shown that the gas density within a rising bubble and around a falling droplet in the air is not uniform. The relative magnitude of the density increment increases with the size of the bubble or aerosol. Presumably, the best applicability of the generalized HRE should be expected for the interface of hydrophobic liquid and hydrophilic one (water and hydrocarbons, water and higher alcohols, in general: aqueous emulsions, water, lipophilic organic liquids and oils, etc.). These are quite important emulsions in practical terms, for example, for the oil industry and medicine. Experimental methods for determining the slip length are considered.

physics.flu-dyn↗

arXiv2026

Stokes flows in a sessile hemispherical drop due to evaporation and surface tension gradient

Viscous hydrodynamic flow in a small, slowly evaporating, sessile hemispherical droplet with a pinned contact line is considered. Analytical solutions are obtained for the Deegan outward flow, which is responsible for the coffee ring effect, as well as the Marangoni flow excited by a surface tension gradient. It is assumed that the surface tension gradient may be caused by anisotropic cooling of droplet surface or other factors, such as nonuniform illumination of an optically active surfactant. Two main types of boundary conditions, no-slip and full-slip, are considered in describing the flow-substrate interaction. It is shown that under the no-slip condition, there is a rigid relationship between the evaporation rate and the surface tension gradient, which imposes strict requirements on the temperature regime inside the droplet. This result offers a new vision of the critical Marangoni number, which describes the threshold for the transition of an evaporating droplet from capillary flow to developed Marangoni convection. The results of this work may attract the attention of experimenters to the study of the sensitivity of viscous flow in an evaporating droplet to the liquid-substrate boundary conditions, especially if the system under consideration passes into the Marangoni regime, when the no-slip condition changes to a partial or full slip condition due to the increase in viscous shear stress near the substrate.

physics.flu-dyn↗

arXiv2025

Sliding of a liquid spherical drop in an external fluid: a generalization of the Hadamard-Rybczynski equation

An analytical solution is obtained for the problem of the slow movement of a small drop of liquid in another immiscible liquid in an infinitely large reservoir with the boundary condition of partial slip at the liquid-liquid interface. That generalizes the conventional Navier condition of partial slip that given at the liquid-solid interface, since the solid (rigid) state can be considered as a liquid with infinite viscosity. A generalized Hadamard-Rybczynski equation (HRE) is obtained. If slip length {\lambda}=0 that equation transforms into the conventional HRE. For infinite viscosity of the droplet, generalized HRE becomes a well-known relation generalizing the Stokes drag force for a solid sphere, taking into account the boundary condition of partial slip. At certain {\lambda}, we arrive at a model with continuity of the all components of the viscous stress tensor at the interface of two fluids, including diagonal tensor components. Generalized HRE is applied to the interpretation of the experiment in which the velocity of falling of a spherical droplet of silicate oil in the castor oil is investigated. Streamlines have been built corresponding to both the Hadamard-Rybczynski model and the partial slip approach. Presumably, the best applicability of the generalized HRE should be expected for the interface of hydrophobic liquid and hydrophilic one (water - hydrocarbons, water - higher alcohols, in general: aqueous emulsions, water - lipophilic organic liquids and oils, etc.). These are quite important emulsions in practical terms, for example, for the oil industry and medicine. Experimental methods for determining the slip length are discussed.

physics.flu-dyn↗

arXiv2025

Vector Laplacian in Spherical Coordinates: An Unnoticed Typo in Landau and Lifshitz's Fluid Mechanics Course

A previously unaccounted fundamental typo has been discovered in Course of Theoretical Physics, vol. 6, by Landau and Lifshitz Fluid Mechanics, 1987, Pergamon, which corresponds to the same typo in the Russian original of this book. This concerns the first of the Navier-Stokes equations in spherical coordinates (15.21), which includes the radial component of the vector Laplacian, namely, an extra square in the denominator of the sine. This error migrates to secondary publications and can complicate theoretical studies related to the application of the Navier-Stokes equations, which the author of this note has encountered first-hand. This typo is not present in An Introduction to Fluid Dynamics by Batchelor. The present short article makes a detailed derivation of the radial component of the vector Laplacian from general principles to show what the corresponding Navier-Stokes equation should look like, and that the typo is indeed present. In addition, more compact equivalent forms of the Navier-Stokes equations in spherical coordinates are proposed.

physics.flu-dyn↗

arXiv2025

Sliding of a liquid spherical droplet in an external insoluble liquid at low Reynolds numbers

The experiment shows that small liquid droplets under the action of gravity and the Archimedes force move in the external viscous liquid practically according to the Stokes drag force equation, and not in accordance with the Hadamard-Rybczynski (HR) formula, which was specially developed to describe the motion of a liquid droplet in an external viscous liquid. Various mechanisms are proposed to explain this: increased viscosity at the interface between two liquids and the presence of unaccounted surfactants. However, there is another fundamental mechanism that has not been taken into account. It can be expected that the velocities of such liquids, insoluble in each other, may not equalize at the boundary of the droplet. No slip condition may be may be unnatural at the droplet interface. In this paper, the Navier condition is applied to the liquid-liquid boundary for the first time. A generalized HR equation is obtained. If slip length {\lambda}=0 that equation transforms into the usual HR equation. At certain {\lambda}, we arrive at a model with continuity of the components of the viscous stress tensor at the interface of two fluids. For infinite viscosity of the drop, it becomes a well-known relation generalizing the Stokes drag force for a solid sphere, taking into account the boundary condition of partial slip.

physics.flu-dyn↗

arXiv2024

Stokes flow of incompressible liquid through a conical diffuser with partial slip boundary condition

An alternative form of the general solution of the linearized stationary Navier-Stokes equations for an incompressible fluid in spherical coordinates is obtained by the vector potential method. A previously published solution to this problem, dating back to the paper by Sampson, is given in terms of a stream function, which leads to formulas that are difficult to apply in practice. The presented form of solution is applied to the problem of liquid flowing through a conical diffuser under a partial slip boundary condition for a certain slip length lambda. Recurrent relations are obtained that allow us to determine the velocity, pressure and stream function. The solution is analyzed in the first order of decomposition with respect to a small dimensionless parameter (lambda divided by r). It is shown that the sliding of the liquid over the surface of the cone leads to a vorticity of the flow. At zero slip length, we obtain the well-known solution to the problem of a diffuser with no-slip boundary condition corresponding to strictly radial streamlines.

physics.flu-dyn↗

arXiv2023

New exact solutions for the evaporation flux density of a small droplet on a flat horizontal substrate with a contact angle in the range of 135-180 degrees

Previously [arXiv:2103.15582v3], an expression was proposed for the evaporation flux density of a small liquid droplet having the shape of an axisymmetric spherical segment deposited on a horizontal substrate. The dependence of the flux density on the polar angle was established for arbitrary contact angles. This formula has the form of an integral and is rather complicated for use in modeling algorithms. An approximate expression was obtained for the evaporation flux density at small contact angles. However, the question of which simplified formulas should be appropriate to apply in other ranges of contact angles, for example, in the case of obtuse angles remains open. In this paper, we propose new exact solutions for the set of discrete "hydrophobic" contact angles. As an example, very simple exact expressions are obtained explicitly for the evaporation flux density for droplets with contact angles 135 and 150 degrees that do not contain integral dependencies. They can also be used as approximate solutions for a narrow range of contact angles around the specified values.

cond-mat.soft↗

arXiv2021

Sessile liquid drop evaporation: analytical solution in bipolar coordinates

The investigation of evaporating liquid drop deposited onto a flat surface is of great importance for physical, engineering and medical applications. Novel analytical expressions are proposed to calculate the evaporation rate of sessile drop (mass loss per unit surface area per unit time) and total evaporation rate (mass loss per unit time). To obtain these results, the H.M. Macdonald's solution for a flat wedge was transformed by method of inversion in a sphere originally developed by J.C. Maxwell in the Treatise on Electricity and Magnetism with further derivation the solution for a lens based on consideration given in bipolar coordinates by G.A. Grinberg. These solutions are mathematically equivalent to expressions proposed earlier by Yuri O. Popov [Phys. Rev. E 71, 036313 (2005)], but, in some cases, probably, the new solutions can be more useful from a computational point of view.

physics.flu-dyn↗