Marangoni instabilities of drops of different viscosities in stratified liquids

For an immiscible oil drop immersed in a stably stratified ethanol–water mixture, a downwards solutal Marangoni flow is generated on the surface of the drop, owing to the concentration gradient, and the resulting propulsion competes against the downwards gravitational acceleration of the heavy drop. In prior work of Li et al. (Phys. Rev. Lett., vol. 126, issue 12, 2021, 124502), we found that for drops of low viscosity, an oscillatory instability of the Marangoni flow is triggered once the Marangoni advection is too strong for diffusion to restore the stratified concentration field around the drop. Here we experimentally explore the parameter space of the concentration gradient and drop radius for high oil viscosities and find a different and new mechanism for triggering the oscillatory instability in which diffusion is no longer the limiting factor. For such drops of higher viscosities, the instability is triggered when the gravitational effect is too strong so that the viscous stress cannot maintain a stable Marangoni flow. This leads to a critical drop radius above which the equilibrium is always unstable. Subsequently, a unifying scaling theory that includes both the mechanisms for low and for high viscosities of the oil drops is developed. The transition between the two mechanisms is found to be controlled by two length scales: the drop radius $R$ and the boundary layer thickness $\delta$ of the Marangoni flow around the drop. The instability is dominated by diffusion for $\delta < R$ and by viscosity for $R<\delta$ . The experimental results for various drops of different viscosities can well be described with this unifying scaling theory. Our theoretical description thus provides a unifying view of physicochemical hydrodynamic problems in which the Marangoni stress is competing with a stable stratification.

Determination of kinematic parameters of falling drops of standard solution of pesticides during spraying, taking into account geometric dimensions variability

Expansion of range of applied pesticides and liquid mineral fertilizers necessitates continuous improvement of spray nozzle design, allowing to create a monodisperse spray and ensure high-quality application of chemical agents at low doses and minimal losses. The issue of studying the process of falling drops with varying geometric dimensions remains sore. Studies of drop movement in air environment make it possible to determine the falling speed and coordinates on the treated surface, to substantiate the design, dimensions, optimal operating modes and parameters of sprayers and devices for protecting the spray cone from direct exposure to wind, which is especially important at the design stage of sprayer for field spraying machines. The paper presents simulation of process of falling drops of pesticide standard solution in resisting environment, considering geometric dimensions variability. An equation for drop radius variability depending on the unit motion horizontal transverse variability, formula for variability of intensity of drop decrease depending on the initial conditions and state of environment are obtained. Dependence between coefficients of drop displacement along the horizontal transverse to the unit movement axis and time is obtained, expressions for variability of drop radius depending on the horizontal displacement and the equation for variability of velocity and vertical coordinate of drop movement on time are presented. The coefficient of mass transfer from the drop surface is determined depending on the resistance coefficient, initial velocity, medium density at the border of drop and plant medium subjected to treatment. The results obtained can be used in mechanical engineering for design and testing of sprayers and nozzles, design of wind protection devices for spray cones of standard solutions of pesticides in field sprayers, in simulation of process of drop movement with varying mass.

To the Question of Analytical Estimate of Evaporation Time of the Drop, Crossing Through the Heat Media

Due to the kinetic approach the modelling description of the drop evaporation is offer. The main equation of the theory received due to the conservation law of dissipative functions of the vapor – liquid system. The diapason of drop size it’s finding when its stability. It’s comparison of the results with the famous classical is given. The numerical estimate of the linear size of small disperse phase when take place usually evaporation (i.e. the Knudsen’s number is a small Kn = l R ≪ 1, where l is a free length path of the molecule and R is an drop radius) are given

Capillarity, wetting, and surface (interfacial) tension

Capillarity reflects the action of interfacial tension and has been central to understanding intermolecular forces. When a liquid meets a solid surface (with contact angle θ‎) it forms a meniscus which is associated with the rise/depression of liquid in a capillary tube, hence the term capillarity. Interfacial tensions also determine how a liquid wets and adheres to a solid or another liquid. Liquid menisci are curved, and Young, Laplace, and Kelvin have all thrown light upon the properties of curved liquid surfaces. The Young–Laplace equation relates the pressure difference across a curved liquid interface to both the interfacial tension and curvature of the interface. Interfacial tension also gives rise to a dependence of the vapour pressure (and solubility) of a liquid on the curvature of its surface (e.g. drop radius), as expressed in the Kelvin equation. Common methods for measurement of interfacial tensions are described in an Appendix.

Scalable single-step microfluidic production of single-core double emulsions with ultra-thin shells

We report a scalable single-step microfluidic technique for the production of monodisperse double emulsions with very thin shell thicknesses, of about 5% of the drop radius.

Influence of Surface Wettability on Dropwise Condensation Heat Transfer

For finding influence of the surface wettability on dropwise condensation heat transfer, a model for dropwise condensation heat transfer has been established based on the drop size distributions and the heat transfer rate through a single drop with considering influence of contact angle to heat transfer. It has been shown based on the proposed model that up to a drop radius of 5μm, the rate of decrease in the drop population density is not as steep as the rate for a drop radius greater than 10μm, because coalescence between drops starts taking place. Varying the contact angle changes the drop distribution; higher the contact angle, lower the departing droplet size and large number density of small droplets. Heat flux first increases and then decreases with increasing contact angle under the temperature difference condition.

Modulation equations for strongly nonlinear oscillations of an incompressible viscous drop

Large-amplitude oscillations of incompressible viscous drops are studied at small capillary number. On the long viscous time scale, a formal perturbation scheme is developed to determine original modulation equations. These two ordinary differential equations comprise the averaged condition for conservation of energy and the averaged projection of the Navier–Stokes equations onto the vorticity vector. The modulation equations are applied to the free decay of axisymmetric oblate–prolate spheroid oscillations. On the long time scale, only the modulation equation for energy is required. In this example, the results compare well with linear viscous theory, weakly nonlinear inviscid theory and experimental observations. The new results show that previous experimental observations and numerical simulations are all manifestations of a single-valued relationship between dimensionless decay rate and amplitude. Moreover, if the amplitude of the oscillations does not exceed 30% of the drop radius, this decay rate may be approximated by a quadratic. The new results also show that, when the amplitude of the oscillations exceeds 20% of the drop radius, fluid in the inviscid bulk of the drop is undergoing abrupt changes in its acceleration in comparison to the acceleration during small-amplitude deformations.

Redistribution and Removal of Particles From Drop Surfaces

It was recently shown by us that particles distributed on the surface of a drop can be concentrated at the poles or equator of the drop by subjecting the latter to a uniform electric field. In this paper, we present experimental results for the dependence of the dielectrophoretic force on the parameters of the system such as the particles’ and drop’s radii and the dielectric properties of the fluids and particles, and define a dimensionless parameter regime for which the technique can work. Specifically, we show that if the drop radius is larger than a critical value, that depends on the physical properties of the drop and ambient fluids and the particles, it is not possible to concentrate particles and thus clean the drop of the particles it carries at its surface because the drop breaks or tip-streams at an electric field intensity smaller than that needed for concentrating particles. However, since the dielectrophoretic force varies inversely with the drop radius, the effectiveness of the concentration mechanism increases with decreasing drop size, and therefore the technique is guaranteed to work provided the drop radius is sufficiently small.

MBE GROWTH OF GaAs NANOWHISKERS STIMULATED BY THE ADATOM DIFFUSION

The growth mechanisms of GaAs nanowhiskers (NWs) during molecular beam epitaxy (MBE) are studied theoretically and experimentally. A kinetic model of the diffusion-induced NW growth is presented that allows one to predict the dependence of NW length on the drop radius and on the technologically controlled MBE growth conditions. The results of scanning electron microscopy studies of GaAs NWs grown at different conditions on the GaAs (111) B surface activated by Au are presented and analyzed. It is shown that the length of NWs increases with decreasing the drop radius and with decreasing the deposition rate of GaAs , while its temperature dependence has a certain maximum. The aspect ratio of MBE-grown GaAs NWs is higher than 100. The maximum length of NWs is several times larger than the effective thickness of the deposited GaAs . The obtained results demonstrate that the NW growth is controlled by the adatom diffusion toward their tip rather than by the adsorption-induced vapor–liquid–solid mechanism. The growth conditions' influence on the NW morphology may be used for the controlled fabrication of NWs by MBE for different applications.

Important Parameters in Drop Coalescence at Planar Surfaces

An experimental study has been performed to establish the principal elements that govern drop coalescence. The study consisted of placing drops of various sizes and physical properties on a planar interface. The coalescence process was recorded with the aid of a high speed digital camera. The experimental portion of the project was aimed at capturing the time of coalescence and the size of the secondary drop that formed after coalescence had finished. Results of the experiments, when scaled properly, showed clear patterns with respect to inertial and viscous terms. Dimensional analysis indicated that Ohnesorge number, Oh, had a strong influence on the behavior of drop coalescence. The ratio of secondary drop radius to primary drop radius, ri, was calculated to be approximately constant when Oh was much smaller than unity. However, as Oh approached unity from the lower bound, the value of ri decayed. No secondary drop was observed when Oh was greater than unity. Normalized coalescence times confirmed this trend by being properly scaled with inertial time scales for small Ohnesorge number and preferring viscous time scales when Ohnesorge number was greater than unity.