Alexei Rednikov

Porous Media and the Bénard–Marangoni Problem

The Rayleigh–Bénard instability for a clear fluid has its equivalent for a liquid saturated porous matrix. The phenomenological Darcy momentum law cannot give rise by itself to an instability analogous to that of Bénard–Marangoni, but the Brinkman model at least allows it. A critical Marangoni number exists leading to cellullar patterns and, for realistic values of the permeability, it is proportional to the inverse of this last parameter.

A comprehensive analysis of the evaporation of a liquid spherical drop.

In this paper, a new comprehensive analysis of a suspended drop of a pure liquid evaporating into air is presented. Based on mass and energy conservation equations, a quasi-steady model is developed including diffusive and convective transports, and considering the non-isothermia of the gas phase. The main original feature of this simple analytical model lies in the consideration of the local dependence of the physico-chemical properties of the gas on the gas temperature, which has a significant influence on the evaporation process at high temperatures. The influence of the atmospheric conditions on the interfacial evaporation flux, molar fraction and temperature is investigated. Simplified versions of the model are developed to highlight the key mechanisms governing the evaporation process. For the conditions considered in this work, the convective transport appears to be opposed to the evaporation process leading to a decrease of the evaporation flux. However, this effect is relatively limited, the Péclet numbers happening to be small. In addition, the gas isothermia assumption never appears to be valid here, even at room temperature, due to the large temperature gradient that develops in the gas phase. These two conclusions are explained by the fact that heat transfer from the gas to the liquid appears to be the step limiting the evaporation process. Regardless of the complexity of the developed model, yet excluding extremely small droplets, the square of the drop radius decreases linearly over time (R(2) law). The assumptions of the model are rigorously discussed and general criteria are established, independently of the liquid-gas couple considered.

Active Drops and Drop Motions due to Nonequilibrium Phenomena

The influence of nonequilibrium phenomena such as heat and mass transfer and chemical reactions on the behavior of a drop with the surface tension sensitive to the temperature and composition is considered in linear and weakly nonlinear approximations. Particular attention is paid to the case of a drop in a homogeneous surrounding when due to the thermo(soluto)-hydrodynamical instability spontaneous breakdown of the radial symmetry of temperature and/or concentration distributions and thus translatory drop motion occur.

Leidenfrost drops: Chapter 7

This chapter is concerned with drop levitation on a vapor layer when a volatile liquid is brought into contact with a very hot solid. This phenomenon is known as the Leidenfrost effect. A simple theory of a Leidenfrost drop covering the full range of stable shapes—that is, from small quasi-spherical droplets to larger puddles floating on a pocket-like vapor film—is presented. This modeling permits an accurate description of the geometry of the drop and of the underlying vapor film. It also provides deeper insights into heat transfer, evaporation, and flow that develop inside the vapor cushion. Finally, the model presented here can serve as a basis for studying other aspects or recent discoveries related to Leidenfrost drops. For instance, using a complement to the basic model, we show how to predict the lifetime of Leidenfrost drops.

Leidenfrost droplets of complex fluids: Chapter 24

A droplet may levitate on its own vapor when dropped on a hot surface. This non-wetting situation is a great opportunity to study the evaporation of complex fluids. In this chapter, we present two case studies in order to evidence general statements about the behavior of complex fluids in Leidenfrost droplets. The first case concerns water and a surfactant mixture. While the droplet is evaporating, it is covered by a thin shell of sodium dodecyl sulfate. Eventually, the droplet explodes. The second case concerns microparticles (glass or basalt). During evaporation, the droplet is encapsulated in a monolayer of grains.A droplet may levitate on its own vapor when dropped on a hot surface. This nonwetting situation is a great opportunity to study the evaporation of complex fluids. In this chapter, we present two case studies in order to evidence general statements about the behavior of complex fluids in Leidenfrost droplets. The first case concerns water and a surfactant mixture. While the droplet is evaporating, it is covered by a thin shell of sodium dodecyl sulfate. Eventually, the droplet explodes. The second case concerns microparticles (glass or basalt). During evaporation, the droplet is encapsulated in a monolayer of grains.