Liquid-vapor Interface Research Articles

Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies.

The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch+) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch+ into the anti-conformer in relatively lower ChCl concentrations. Finally, the depletion of Ch+ from the interface (probing depth = 0.4 nm) is demonstrated by aerosol-based velocity map imaging XPS measurements of glyceline and water mixtures. The nanostructure of liquid-vapor interfaces and structural rearrangement by hydration can provide critical insight into the molecular origin of the deep eutectic behavior and gas-capturing application of DESs.

Experiment of contact angle hysteresis of a droplet on hydrophobic fibrous membrane

Fibrous membranes are widely used in liquid-gas filtration and separation, porous structures formed by randomly cross-layered fibers have a significant influence on interfacial free energies, showing complex changes in contact angle. The present paper aims to measure the pore structures of a hydrophobic ((C2F4)n, PTFE) fibrous membrane by Confocal Laser Scanning Microscope (CLSM), and analyze the droplet contact angle hysteresis along its circumference. The capillary force of the solid-liquid interface is calculated by the surface integral of Laplace pressure on half droplet surface, and the solid-liquid and liquid-vapor interface area ratios are obtained according to the Cassie-Baxter model. The liquid penetration depth in pore structures is calculated by the quasi-equilibrium of the interface free energy. The experimental result shows that for the PTFE fibrous membrane, the liquid-vapor interface area ratio (f2The experimental result shows that for the PTFE fibrous membrane, the liquid-vapor interface area ratio (f2

Liquid evaporation from nanochannel in the presence of non-condensable gas by direct simulation Monte Carlo

Liquid-vapor phase change in the form of evaporation and with the confinement of nanostructures is of vast importance in many engineering applications. The direct simulation Monte Carlo (DSMC) approach was employed to simulate the liquid evaporation from two-dimensional nanochannels in the presence of non-condensable (NC) gases. The thermodynamic properties of vapor and NC gas flow fields as well as the evaporation rate of liquid were calculated. Factors such as channel opening ratio, meniscus receding depth and the NC gas density were examined to reveal their impacts on the liquid evaporation rates. The results showed that the gas mixture flow field transforms to one-dimensional away from the channel exit. The overall vapor pressure drop can be divided into that across the liquid–vapor interface, that within the channel, outside the channel, as well as that caused by the binary diffusion. The effect of NC gas on the evaporation rate was found to be the most outstanding with pinned meniscus and with a wide channel opening, in which case the normalized evaporation rate drops to 0.43. Moreover, the Maxwell-Stefan equation could capture the evaporation data with modified diffusion lengths. For receded meniscus (250 nm deep) and small opening ratio (0.417), the effective diffusion length was as large as 1.6 times that in the one-dimensional case.

Adaptive thermodynamic consistency control via interface thickness in pseudopotential lattice Boltzmann method across wide temperature ranges

Multiphase flow phenomena are ubiquitous in real-life applications, and the pseudopotential-based lattice Boltzmann multiphase simulation method has gained extensive usage in fields such as fuel cells, energy storage materials, and boiling heat transfer. Over the past two decades, significant improvements have been made to the pseudopotential model. These advancements have greatly enhanced its accuracy in simulating various processes. In this paper, we employ a numerical stability-based unit conversion scheme to ensure an accurate representation of real-world material properties. Additionally, we introduce a more precise non-ideal two-parameter gas state equation Modified Peng-Robinson (MPR) that closely aligns with experimental data, surpassing the commonly used single-parameter Peng-Robinson state equations. Furthermore, we compare the accuracy of the two-state equations for polar and non-polar substances, finding improved accuracy for non-polar substances and a higher degree of fidelity for polar substances when using the MPR equation. We analyze the constant correction coefficients chosen for thermodynamic consistency regulation from the perspectives of vapor-liquid interface thickness and numerical stability, both when held constant and when employing a dynamic correction approach that balances vapor-liquid interface width, numerical stability, and thermodynamic consistency. Finally, we validate to ensure its practical applicability.

Droplet coalescence kinetics: Thermodynamic non-equilibrium effects and entropy production mechanism

The thermodynamic non-equilibrium (TNE) effects and the relationships between various TNE effects and entropy production rate, morphology, kinematics, and dynamics during two initially static droplet coalescences are studied in detail via the discrete Boltzmann method. Temporal evolutions of the total TNE strength D¯* and the total entropy production rate can both provide concise, effective, and consistent physical criteria to distinguish different stages of droplet coalescence. Specifically, when the total TNE strength D¯* and the total entropy production rate reach their maxima, it corresponds to the time when the liquid–vapor interface length changes the fastest; when the total TNE strength D¯* and the total entropy production rate reach their valleys, it corresponds to the moment of the droplet being the longest elliptical shape. Throughout the merging process, the force contributed by surface tension in the coalescence direction acts as the primary driving force for droplet coalescence and reaches its maximum simultaneously with coalescent acceleration. In contrast, the force arising from non-organized momentum fluxes (NOMFs) in the coalescing direction inhibits the merging process and reaches its maximum at the same time as the total TNE strength D¯*. In the coalescence of two unequal-sized droplets, contrary to the larger droplet, the smaller droplet exhibits higher values for total TNE strength D¯*, merging velocity, driving force contributed by surface tension, and resistance contributed by the NOMFs. Moreover, these values gradually increase with the initial radius ratio of the large and small droplets due to the stronger non-equilibrium driving forces stemming from larger curvature. However, non-equilibrium components and forces related to shear velocity in the small droplet are consistently smaller than those in the larger droplet and diminish with the radius ratio. This study offers kinetic insights into the complexity of thermodynamic non-equilibrium effects during the process of droplet coalescence, advancing our comprehension of the underlying physical processes in both engineering applications and the natural world.

Nanoparticle migration in nanofluid film boiling: A numerical analysis using the continuous-species-transfer method

Investigating the boiling behavior of nanofluids remains a topic of considerable research interest. The existing theoretical framework often underscores Brownian motion and thermophoresis among the predominant mechanisms that influence nanoparticles' movement within nanofluids. However, most current numerical models addressing nanofluid boiling either neglect these mechanisms or incorporate them inadequately. A comprehensive review of the literature reveals two primary gaps. The larger set of numerical studies on nanofluids boiling does not account for any governing equation for nanoparticle concentration, neglecting the primary mechanisms of nanoparticle movement. The smaller set includes such an equation but limits its application to the vapor domain, thereby overlooking the liquid nanofluid phase. Moreover, in the context of film boiling involving nanofluids, an insulating vapor film forms between the nanofluid and the heated surface. This vapor film is dynamic, experiencing evaporation at its outer boundary, leading to an increase in nanoparticle concentration at the vapor-liquid interface—another aspect always neglected in existing numerical models. To address these research gaps, this study introduces a specialized governing equation employing the Continuous-Species-Transfer method within the framework of Computational Multi-Fluid Dynamics. Importantly, it also incorporates governing equations for thermophoresis and Brownian motion to account for the motion of nanoparticles. The method's efficacy is further corroborated by solving a two-dimensional axisymmetric film boiling problem on a vertical cylinder, with results aligning well with analytical studies that consider Brownian motion and thermophoretic behavior. Overall, the study fills a critical gap in the literature by providing a comprehensive tool for examining complex interfacial behaviors in nanofluid film boiling.

External Stefan and Internal Marangoni Thermo-Fluid Dynamics for Evaporating Capillary Bridges.

We probe the evaporation mechanisms of wettability-moderated, confined capillary bridges and bulges. For the first time, we explore the internal Marangoni hydrodynamics and external Stefan advection dynamics in the surrounding gaseous domain due to evaporative effects. A transient simulation approach based on the level set (LS) method and the Arbitrary Lagrangian-Eulerian (ALE) framework was adopted to computationally model the capillary bridge profiles and evaporation phenomenon with generic contact line dynamics (both CCR and CCA modes). The governing equations corresponding to the transport processes in both the liquid and gaseous domains are simulated in a fully coupled manner with appropriate boundary conditions to precisely trace the liquid-vapor interface and the three-phase contact point during evaporation. The effect of the bridge confinement phenomenon, i.e., the extent of confined ambient surrounding the liquid-vapor interface between the solid surfaces, is explored. Also, the role of wetting state and contact line dynamics during CCR and CCA modes of evaporation were probed, and good agreement with experimental observations was noted. Results show that the evaporation rate is primarily dictated by the confinement phenomenon, and wettability effects play a marginal role. A higher confinement curtails the evaporation rate due to an increased local vapor concentration around the liquid bridges. However, the wetting state substantially affects the internal Marangoni effect dynamics and the Stefan advection dynamics due to its explicit influence on the nonuniform evaporative flux along the liquid-vapor interface. Between superhydrophobic confinements, the contact lines are confined in the wedge-shaped region, thereby locally augmenting the vapor concentration. As a result, the large evaporative flux near the bulge region develops a higher temperature gradient, thereby inducing upscaled thermal Marangoni flow compared to hydrophilic confinements. These findings may have significant implications for the efficient designing and development of thermofluidic systems involving thermal transport, mixing, and deposition of dissolved particles in liquid bridges.

Droplet evaporation on two-tier hierarchical micro-pillar array surface

As surface modification becomes a promising approach to thermal management, quantifying effects of surface structure geometrical dimension is of great significance to optimize heat transfer performance. In this paper, we emphatically report droplet evaporation on three types of two-tier hierarchical micro-pillar array (THM) surfaces. Utilizing the lattice Boltzmann model, evaporation characteristics including droplet morphology, triple-phase contact line (TPCL) behavior, liquid–vapor interface evolution and heat flux distribution are elucidated. In comparison to single-tier micro-pillar array (SM) surfaces, THM surfaces enable remarkable enhancement in heat transfer when secondary pillars are configured on the side of the primary pillar or configured parallelly to the primary pillar on the substrate, which is attributed to the synergistic effect of TPCL evaporation and liquid–vapor interface evaporation. The varying trend of droplet lifetime with the primary pillar geometrical dimension on all THM surfaces are also summarized, giving a guideline for optimization of heat dissipation by tailoring structures.

Pool boiling inside micro-nano composite pores: Thermofluids behaviors and heat transfer enhancement

Pool boiling has been considered as an effective method for heat transfer, which is extensively used in semiconductor microprocessors and aerospace. The prevention of critical heat flux (CHF) and improvement of heat transfer coefficient (HTC) are the keys to strengthen the pool boiling heat transfer. It has been reported that porous pillars can delay CHF by reducing vapor–liquid counter flow and nano-porous structure can significantly improve the liquid replenishment capacity and HTC. Herein, we modulate micro-nano porous structures by etching nanostructures on the surface of microporous pillars in order to utilize the advantages of above two structures. The boiling phenomenon inside the micro-nano composite pores is clearly demonstrated based on our visualization experiment. It is observed that the main vapor motion is regular periodic growth and recession. Through the quantitative analysis of the vapor–liquid interface area and vapor movement frequency, we find that they are both positively correlated with the heat transfer performance of porous structures. Hence, it is proved that the vapor–liquid phase distribution is the decisive factor affecting the heat transfer performance of micro-nano porous structures. Following the principle of maintaining larger vapor–liquid interface area and accelerating vapor movement frequency, the micro-nano porous structures with better performance can be obtained. Our approach establishes the relationship between the internal boiling phenomenon and the performance of micro-nano porous structures. In addition, it also provides a feasible direction for improving the performance of engineered boiling micro-nano structures.

Surface Crystallization Modulation toward Highly-Oriented and Phase-Pure 2D Perovskite Solar Cells.

2D perovskites have shown great potential toward stable and efficient photovoltaic devices. However, the crystal orientation and phase impurity issues of 2D perovskite films originating from the anisotropic crystal structure and specific growth mechanism have demoted their optoelectronic performances. Here, the surface crystallization modulation technique is introduced to fabricate the high-quality 2D perovskite films with both vertical crystal orientation and high phase purity by regulating the crystallization dynamics. The solvent atmosphere condition is instituted during film processing, which promotes the formation of an oriented 2D perovskite layer in stoichiometric composition at the vapor-liquid interface and templates the subsequent film growth. The solar cells based on the optimized 2D perovskite films exhibit a power conversion efficiency of 15.04%, the record for 2D perovskites (with the perovskite slab thickness n ≤ 3 and high phase purity). The solar cells based on the highly-oriented and phase-pure 2D perovskite films also demonstrate excellent thermal and humidity stabilities.

Investigation of the effect of curvature on the local mass flux of evaporating droplets using a phase field method

Droplet evaporation, a fundamental process with wide-ranging applications, involves complex physical mechanisms that significantly influence mass transfer rates and droplet lifetimes. Despite its significance, the influence of droplet deformation induced by capillary forces on local saturation conditions and its subsequent impact on phase change have received limited attention. This study delves into the impact of capillary-induced droplet deformation on local saturation conditions and its subsequent effect on phase change within a quiescent vapor environment. Contrary to most analytical and numerical studies that impose temperature and heat flux conditions at the vapor–liquid interface, our investigation employs direct numerical simulations based on the thermal Navier–Stokes–Korteweg model, capturing the dynamics of the droplet and its interface without relying on ad-hoc interface conditions. Our findings demonstrate that droplet deformation induces free oscillatory motion, which, although not significantly altering droplet lifetime due to viscous damping, leads to non-uniform mass fluxes along the droplet interface. We observe that areas of higher curvature are associated with increased local pressure and reduced temperatures, a discrepancy primarily attributed to evaporative cooling. This effect is further elucidated through an analysis of pressure and energy fluxes, revealing that regions of high curvature experience significant heat consumption, driving evaporative cooling and creating temperature gradients along the interface. Conversely, areas with negative curvature exhibit a reverse trend, indicating the complex interplay of capillarity, deformation, and phase change in droplet evaporation. Our results highlight the intricate relationship between droplet deformation, local pressure, mass flux, and temperature, underscoring the necessity for improved modeling of these factors in future studies on droplet evaporation dynamics.

Experiment of droplet contact angle hysteresis on PTFE fibrous membrane in pore-scale

The present paper aims to study the influence of pore-scale fiber structure on the Cassie-Baxter contact angle of droplets on PTFE fibrous hydrophobic membranes. The fiber structure is obtained by the Confocal Laser Scanning Microscope (CLSM), and an optical test bench is built to measure the Cassie-Baxter contact angles of a droplet on a PTFE membrane. The free energy of the solid-liquid interface is measured by the surface tension integral on the droplet profile. An iterative algorithm for liquid-vapor interface shrinking is brought forward to analyze the liquid-vapor interface area between the droplet and the membrane, and the interface parameters of the Cassie-Baxter model are recognized. The experiment shows that the Cassie-Baxter contact angles and the contact line radii along the droplet circumference are in a normal distribution due to random fiber structures, and the variance of the normal distribution increases with the membrane pore diameter. The normal distribution of Cassie-Baxter’s contact angle indicates the constraint of the solid-liquid interface to the liquid-vapor interface. On the same normal contact pressure condition, the liquid-vapor area ratio of the droplet and membrane interface changes little, and Cassie-Baxter’s contact angle increases with the solid-liquid interface area ratio.

Resistivities across the vapor–liquid interface of a simple fluid: An assessment of methods

Heat and mass transfer across the interface between liquid and vapor is studied by means of molecular dynamics simulation. Two scenarios are considered to access the interface resistivities, specifying either the evaporation rate or the temperature gradient. Spatially resolved profiles of density, temperature, chemical potential, pressure tensor elements, and hydrodynamic velocity are sampled with large-scale molecular dynamics simulations to elucidate the structural and dynamic properties across the interface under non-equilibrium conditions. The employed interaction model is appropriate for simple fluids, like argon, while its thermodynamic properties in bulk phases are fully known. Most of the temperature range from the triple point to the critical point is investigated, varying the heat flux and the particle flux over one to two orders of magnitude. Different approaches are followed to determine the interface resistivities, and their results are compared to literature data and kinetic gas theory. It is found that the interface resistivities are a sole function of the interface temperature and are independent of the chemical potential gradient or the temperature gradient. This also holds for its thickness and surface tension up to the very large gradients that are typically imposed in molecular dynamics simulations. It stands to reason that this is also the case under the presence of gradients with a magnitude that is technically relevant and thus much smaller.

Film flow boiling of a magnetic fluid over a vertical plate exposed to a magnetic field under reduced gravity conditions

In this article, the film flow boiling over a vertical flat plate was studied numerically. The saturated liquid upward flow over a vertical flat plate with a uniform free stream velocity was considered. The Volume of Fluid (VOF) model was used for capturing the liquid-vapor interface. The effects of the wall superheat, the free stream velocity of hydrodynamics, and the heat transfer of the flow film boiling were studied in the absence and presence of a magnetic induction (MI). The results showed increased free stream velocity enhances the heat transfer coefficient (HTC). In addition, applying uniform MI results in a normal force at the liquid-vapor interface. This normal force enhances the local heat transfer coefficient (HTC) during film flow boiling on the heated vertical plate. Also, the percentage of HTC enhancement is higher when the magnetic field strength increases. Moreover, the heat transfer enhancement diminishes as the free stream velocity increases. Furthermore, an increase in the magnetic permeability ratios of the phases contributes to improving the HTC and increases the wall shear stress. The effect of the variation of the acceleration of gravity is also studied, revealing that a decrease in gravity’s acceleration actually slightly enhances HTC.

Molecular dynamics study on the anomalous behavior of phase transition heats of alcohols at vapor–liquid interface

The surface adsorptions of methanol, ethanol and 1-propanol liquids at vapor–liquid interface are investigated with the surface adsorption theory and molecular dynamics simulations (MD). The surface layer thicknesses of these three liquids calculated with MD increase with temperature. It is found that the thickness of the surface layer becomes smaller as the intermolecular interaction becomes stronger. The heat of phase transition from the bulk to surface determined with the experimental surface tension data decreases from methanol to ethanol and increases from ethanol to propanol. This anomalous behavior is different from the usual one of the phase transition heats which changes monotonously with the alkyl chain length in literature. Since the reversible phase transition heat is equal to T(Ss-Sα), the entropy changes of these three liquids are calculated with MD simulations and are in agreement with the variation trend of phase transition heats determined from the experimental data. This is a qualitative confirmation of our surface adsorption theory from the viewpoint of the molecular dynamic simulations.

Interfacial structure and transport properties of concentrated lithium chloride solutions under an electrostatic field

Efficient moisture exchange between liquid desiccants and air is essential for optimizing dehumidification and air-conditioning systems, with the interfacial layer playing a critical role. This work investigates the influence of electrostatic fields (E-fields) on the structural and transport properties of the liquid-vapor interface between aqueous lithium chloride solutions and air at a microscopic scale using molecular dynamics (MD) simulations. Our research reveals that negative normal E-fields deplete ions within the interfacial layer, weakening their interactions with water molecules. Conversely, positive normal E-fields enrich surface ions, attracting a higher concentration of water molecules to the interface. The E-field enhances interactions between water molecules, increasing hydrogen bonds (H-bonds) at the interface. This weakens cohesion between water molecules and the liquid phase, ultimately reducing surface tension and the associated interfacial free energy barrier, facilitating the departure of water molecules from the liquid phase. Negative normal E-fields demonstrate greater advantages due to their pronounced alteration of the interface structure and the polarization of water molecules.

Numerical study on thermodynamic characteristics of large-scale liquid hydrogen tank with baffles under sloshing conditions

The control of the evaporation rate is essential to the long-distance transportation of liquid hydrogen (LH2). This paper conducts a numerical investigation of the thermodynamic characteristics in tanks with baffles under LH2 sloshing conditions by establishing a computational fluid dynamics (CFD) model. The pressurization rate and vapor-liquid interface sloshing behavior predicted by the numerical model is validated against experimental data. The effects of baffles, amplitude, frequency, and storage pressure on thermodynamic characteristics such as temperature, pressurization rate, and evaporation rate of the LH2 tank under sinusoidal sloshing are analyzed in detail. The simulation results suggest that adding baffles can effectively suppress the pressure fluctuation of LH2 under sloshing conditions, but baffles as a "thermal bridge" accelerate the pressurization rate in the tank. Adding baffles during smooth transportation accelerates the pressurization rate. The baffles gradually demonstrate the effect of suppressing the pressurization rate as the acceleration amplitude increases. Assuming that the LH2 evaporation rate is stable for a certain value, the daily evaporation rates in the LH2 tank account for 2.59 % and 5.13 % when the maximum acceleration is 5.92 m/s2 and 9.87 m/s2, respectively, which contradicts the long-term storage goal of LH2. The slower the sloshing frequency, the higher the maximum liquid phase level in the tank, and the faster the evaporation rate. The sloshing frequency has little effect on the LH2 pressurization and evaporation rates compared to the amplitude. The growth of pressure decreases from 139.9 Pa to 85.62 Pa for a period of 15 s when the storage pressure in the tank increases from 0.1 MPa to 0.6 MPa. This study provides guidlines for the in-depth investigation of the thermodynamic behavior in LH2 tanks during transportation.

3D Simulations of Nucleate Boiling with Sharp Interface Vof and Localized Adaptive Mesh Refinement in Ansys-Fluent

Abstract The present work demonstrates the use of customized Ansys-Fluent in performing 3D numerical simulations of nucleate boiling with a sharp interface and adaptive mesh refinement. The developed simulation approach is a reliable and effective tool to investigate 3D boiling phenomena by accurately capturing the thermal and fluid dynamic interfacial vapor-liquid interaction and reducing computational time. These methods account for 3D sharp interface and thermal conditions of saturation temperature refining the mesh around the bubble edge. User-Defined-Functions (UDFs) were developed to customize the software Ansys-Fluent to preserve the interface sharpness, maintain saturation temperature conditions, and perform effective adaptive mesh refinement in a localized region around the interface. Adaptive mesh refinement is accomplished by a UDF that identifies the cells near the contact line and the liquid-vapor interface and applies the adaptive mesh refinement algorithms only at the identified cells. Validating the approach considered spherical bubble growth with an observed acceptable difference between theoretical and simulation bubble growth rates of 10%. Bubble growth simulations with water reveal an influence region of 2.7 times the departure bubble diameter, and average heat transfer coefficient of 15000 W/m2-K. In addition, the results indicate a reduced computational time of 75 hours using adaptive mesh compared to uniform mesh.

Effects of Organic Surface Contamination on the Mass Accommodation Coefficient of Water: A Molecular Dynamics Study.

The mass accommodation coefficient (MAC), a parameter that quantifies the possibility of a phase change to occur at a liquid-vapor interface, can strongly affect the evaporation and condensation rates at a liquid surface. Due to the various challenges in experimental determination of the MAC, molecular dynamics (MD) simulations have been widely used to study the MAC on liquid surfaces with no impurities or contaminations. However, experimental studies show that airborne hydrocarbons from various sources can adsorb on liquid surfaces and alter the liquid surface properties. In this work, therefore, we study the effects of organic surface contamination, which is immiscible with water, on the MAC of water by equilibrium and nonequilibrium MD simulations. The equilibrium MD simulation results show that the MAC decreases almost linearly with increasing surface coverage of the organic contaminants. With the MAC determined from EMD simulations, the nonequilibrium MD simulation results show that the Schrage equation, which has been proven to be accurate in predicting the evaporation/condensation rates on clean liquid surfaces, is also accurate in predicting the condensation rate at contaminated water surfaces. The key assumption about the molecular velocity distribution in the Schrage analysis is still valid for condensing vapor molecules near contaminated water surfaces. We also find that under nonequilibrium conditions the adsorption of the water vapor molecules on the organic surface results in an adsorption vapor flux near the contaminated water surface. When the water surface is almost fully covered by the model organic contaminants, the adsorption flux dominates over the water condensation flux and leads to a false prediction of the MAC from the Schrage equation.

A Detailed Study of Boiling Phenomena in Forced Convective Quenching Experiments

A detailed study at the microscopic level of the transient behavior of boiling phenomena during quenching of an AISI 304 stainless steel, conical-end, cylindrical probe in flowing water at 60 °C was conducted using high-speed video recordings and cooling curve data acquisition. Two free-stream velocities (0.2 and 0.6 m/s) and two initial probe temperatures (850 and 950 °C) were investigated. It was complemented by video recordings at 60 fps to calculate the wetting front velocity. Surface heat flux histories at the thermocouple positions were estimated by solving the corresponding Inverse Heat Conduction Problem. As the water velocity increases the duration of the vapor film at each thermocouple position shortens and the difference among cooling curves at the thermocouple positions decreases. Increasing the initial temperature increases the duration of the vapor film stage. The wetting front velocity increased from 4 to 6 m/s. The time to reach the maximum surface heat flux at the position of the thermocouple closest to the probe tip ranged from 9.7 to 18 s. Undulations at the vapor-liquid interface that appear periodically and propagate in the direction of quenchant flow were observed early during the vapor film stage. Later, before the collapse of the vapor film, a kind of a shock wave was generated at the probe tip which modified the appearance of the film. Once the Leidenfrost temperature is reached, a wetting front (which consists of many small bubbles that coalesce rapidly in a very small area) is formed and travels in the direction of quenchant flow while fewer and larger bubbles nucleate and grow upstream. The nucleation, growth and detachment of the larger bubbles was studied; as the free-stream velocity decreases larger values of bubble maximum diameter and half-life time were observed, while the initial temperature has a marginal effect on these quantities.