Alexey Rednikov

Vapor-Based Interferometric Measurement of Local Evaporation Rate and Interfacial Temperature of Evaporating Droplets

The local evaporation rate and interfacial temperature are two quintessential characteristics for the study of evaporating droplets. Here, it is shown how one can extract these quantities by measuring the vapor concentration field around the droplet with digital holographic interferometry. As a concrete example, an evaporating freely receding pending droplet of 3M Novec HFE-7000 is analyzed at ambient conditions. The measured vapor cloud is shown to deviate significantly from a pure-diffusion regime calculation, but it compares favorably to a new boundary-layer theory accounting for a buoyancy-induced convection in the gas and the influence upon it of a thermal Marangoni flow. By integration of the measured local evaporation rate over the interface, the global evaporation rate is obtained and validated by a side-view measurement of the droplet shape. Advective effects are found to boost the global evaporation rate by a factor of 4 as compared to the diffusion-limited theory.

Singularity-free description of moving contact lines for volatile liquids.

We here show that, even in the absence of “regularizing” microscopic effects (viz. slip at the wall or the disjoining pressure/precursor films), no singularities in fact arise for a moving contact line surrounded by the pure vapor of the liquid considered. There are no evaporation-related singularities either even should the substrate be superheated. We consider, within the lubrication approximation and a classical one-sided model, a contact line advancing/receding at a constant velocity, or immobile, and starting abruptly at a (formally) bare solid surface with a zero or finite contact angle.

Effect of Marangoni Flows on the Shape of Thin Sessile Droplets Evaporating into Air.

Freely receding evaporating sessile droplets of perfectly wetting liquids, for which the observed finite contact angles are attributed to evaporation, are studied with a Mach-Zehnder interferometer. The experimentally obtained droplet shapes are found to depart, under some conditions, from the classical macroscopic static profile of a sessile droplet. The observed deviations (or the absence thereof) are explained in terms of a Marangoni flow due to evaporation-induced thermal gradients along the liquid-air interface. When such a Marangoni effect is strong, the experimental profiles exhibit a maximum of the slope at a certain distance from the contact line. In this case, the axisymmetric flow is directed from the contact line to the apex (along the liquid-air interface), hence delivering more liquid to the center of the droplet and making it appear inflated. These findings are quantitatively confirmed by predictions of a lubrication model accounting for the impact of the Marangoni effect on the droplet shape.

Leidenfrost drops on a heated liquid pool

We show that a volatile liquid drop placed at the surface of a non-volatile liquid pool warmer than the boiling point of the drop can experience a Leidenfrost effect even for vanishingly small superheats. Such an observation points to the importance of the substrate roughness, negligible in the case considered here, in determining the threshold Leidenfrost temperature. A theoretical model based on the one proposed by Sobac et al. [Phys. Rev. E 90, 053011 (2014)] is developed in order to rationalize the experimental data. The shapes of the drop and of the substrate are analyzed. The model notably provides scalings for the vapor film thickness. For small drops, these scalings appear to be identical to the case of a Leidenfrost drop on a solid substrate. For large drops, in contrast, they are different and no evidence of chimney formation has been observed either experimentally or theoretically in the range of drop sizes considered in this study. Concerning the evaporation dynamics, the radius is shown to decrease linearly with time whatever the drop size, which differs from the case of a Leidenfrost drop on a solid substrate. For high superheats, the characteristic lifetime of the drops versus the superheat follows a scaling law that is derived from the model but, at low superheats, it deviates from this scaling by rather saturating.

Self-propelled Leidenfrost drops on a thermal gradient: A theoretical study

We theoretically investigate the behavior of Leidenfrost drops on a flat substrate submitted to a horizontal thermal gradient and highlight that they are able to self-propel in a preferential direction. Namely, they are found to travel towards the colder parts of the substrate, as if they were trying to maximize their lifetime. In particular, a centimetric water drop can reach velocities of the order of cm/s for thermal gradients of the order of a few K/mm. In general, the presented model, based upon the lubrication approximation in the vapor cushion as in the work of Sobac et al. [“Leidenfrost effect: Accurate drop shape modeling and new scaling laws,” Phys. Rev. E 90, 053011 (2014)] and here formulated for simplicity for a 2D drop, enables predicting the values of these velocities as a function of the thermal gradient, drop size, superheat, and fluid properties. Surprisingly, the variability of vapor properties with temperature turns out to be instrumental for the drop to move, even if the vapor film pr...

Impact of an insoluble surfactant on the thresholds of evaporative Bénard-Marangoni instability under air

Abstract.It is known that the addition of an insoluble surfactant to a Bénard-Marangoni (BM) layer heated from below or cooled from above can give rise to a supplementary, oscillatory mode of instability. Here the objective is to see how exactly this plays out in the framework of a recently studied and experimentally tested case of a non-long-wavelength BM instability driven by diffusion-limited evaporation into air in isothermal surroundings. Linear stability analysis is accomplished within a now standard reduction to a one-sided model. In the absence of surfactant, we just recover the classical Pearson problem, albeit with an evaporation-specific wavenumber-dependent Biot number potentially attaining large values for strongly volatile liquids. Adding a surfactant not only sharply stabilizes the monotonic Pearson-like mode, but also leads to a more dangerous oscillatory mode, a parametric study of which is here undertaken. Although slanted towards the evaporative case, the present study is also of interest from the general viewpoint of the Pearson problem + an insoluble surfactant, the results for which are scarce in the literature without being obscured by further effects. In particular, an asymptotic analysis based on small interfacial Lewis numbers (diffusion coefficients) is undertaken near the codimension-2 point.Graphical abstract

Complex fluis droplets in leidenfrost state

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.

Precursor Films and Contact Line Microstructures

This chapter focuses on the small-scale structure of a contact line formed by a pure liquid, a gas (in particular, the vapor of the liquid considered), and a smooth and chemically homogeneous solid substrate. On the basis of the lubrication approximation (small slopes of the liquid/gas interface), various situations are considered, including volatile or nonvolatile liquids, partial or complete wetting, and advancing or receding contact lines. Two types of microfilms ahead of the contact line are considered: extended (i.e., covering the substrate everywhere) and truncated (i.e., formally ending at some “dry” spot). On the macroscopic side, results useful for the study of many configurations, such as droplet evaporation and boiling, are provided in terms of the Cox–Voinov relationship, relating the scale-dependent apparent contact angle to the velocity of the contact line. Parameters entering this relationship—namely, the apparent contact angle of the steady microstructure and the microlength below which singularities of the contact line are regularized—are calculated in various cases. Particular attention is devoted to discussing the pertinence of disjoining pressure and the Kelvin effect (curvature dependence of saturation conditions) at small scales in relation with the resolution of contact line singularities. The selection between microstructures with extended and truncated films is also discussed.

Chapter 24 – Complex Fluids Droplets in Leidenfrost State

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.