Formation Of Microlayer Research Articles

The microlayer and force balance of bubbles growing on solid in nucleate boiling

This study uses interface-resolved computational fluid dynamics simulations to investigate the dynamics of bubble growth on a horizontal solid surface, with a focus on the characteristics of the microlayer and the forces involved in the process. The simulations exclude phase change effects to concentrate on hydrodynamics and employ an external mass source for controlled bubble inflation. This mass source follows a time-dependent bubble radius growth law, R(t)∝t1/2, which is typical for heat-transfer-controlled growth of a vapour bubble during nucleate boiling of water at atmospheric pressure. The numerical framework is validated against recent experimental measurements of bubble shape and microlayer profile. The results of this work indicate that the rate of bubble growth significantly influences microlayer formation. Faster growth rates produce a near-hemispherical bubble shape with an extended radial microlayer on the solid, while slower rates yield a taller, more spherical bubble with a shorter microlayer. All microlayer profiles exhibit an outwardly-curved shape, with a maximum microlayer thickness increasing with the growth rate. The study also examines the force balance on the bubble, revealing that the net vertical force of the bubble does not equal zero even when the bubble remains attached to the solid surface. Our analysis of the bubble motion demonstrates that previous force balance models used for determining bubble detachment lack robustness. The microlayer profile obtained in this work is important for boiling heat transfer studies as the microlayer contributes significantly to local heat transfer. The force balance analysis shows the need for a new approach to determine bubble detachment behaviour, which is vital for predicting flow boiling rates.

Predicting initial microlayer thickness in nucleate boiling using Landau–Levich theory

A phenomenological model is proposed to estimate the initial thickness of the liquid microlayer forming beneath a vapour bubble growing on a solid surface upon nucleate boiling. The model employs an analogy between the microlayer formation and the classic plate withdrawal problem. It calculates the microlayer thickness by considering it as a Landau–Levich film, where the thickness is a function of the meniscus speed and radius of curvature. Given the nearly hemispherical shape of the bubble during the early growth stage when the microlayer is first deposited, we assume that the meniscus speed can be approximated by the bubble expansion rate, and estimate the meniscus curvature using the Rayleigh equations. Unlike previous theories that assume that the bubble radius growth is proportional to the square root of time, the proposed model does not rely on any specific law of growth for vapour bubbles. The model is validated for predicting the microlayer thickness in water and ethanol, showing good agreement with experimental measurements and empirical correlations. Subsequent analyses of the microlayer interface profile address inconsistent reports – some described a wedge-like shape, whereas others reported a slight outward curvature with decreasing thickness in the outer region. This discrepancy is attributed to a reduction in the expansion rate of the microlayer's outer edge, particularly when the bubble reaches its maximum width. Our model provides insights into microlayer dynamics, essential to boiling heat transfer, as the evaporative heat flux through the microlayer is very sensitive to its initial thickness.

Acoustically controlled bubble nucleation

Fundamental studies of single bubble heat transfer are crucial to improve our understanding of boiling phenomena and develop modeling and simulation tools for the design and optimization of boiling systems. However, conducting these kinds of experiments over a wide range of conditions is challenging and may involve expensive and complicated experimental techniques (e.g., lasers) to isolate and control bubble nucleation. In this work, we introduce a new technique to control nucleation with an ultrasonic beam. A low-cost and widely available piezoelectric disc driven at megahertz frequencies directed at a heated surface can increase the time-averaged pressure and suppress evaporation until the moment the transducer is turned off, allowing us to control the incipience of boiling to within 150 µs. We demonstrate this technique by measuring the physical and thermal footprint of natural and acoustically controlled bubbles with high-resolution infrared thermometry and phase detection diagnostics. We were able to raise the bubble nucleation temperature by over 30 K compared to a reference case with no acoustic control, which significantly changes bubble growth rate, microlayer formation and evaporation and departure diameter. We highlight the potential of this new low-cost and easy-to-use technique in a variety of different boiling heat transfer studies.

Experimental investigation of electrically enhanced boiling of FC 72 using high-resolution phase-detection diagnostics

This work features the boiling of FC72 on a transparent sapphire substrate. Tests are performed using a thin ITO film (indium tin oxide) coated on the sapphire substrate as the heat source, a fast speed video camera to capture the boiling process using a technique to track the phase (liquid or vapor) in contact with the heating surface, an InfraRed video camera to capture the average temperature on the surface, and a metallic grid to impose an electric field perpendicular to the boiling surface. The maximum average electric field tested in this work is 3.3kV/mm, which led to a 18% increase of the critical heat flux. This study analyses in detail the phase distribution data showing that (1) there is no evidence of microlayer formation, and suggesting that (2) the triple contact line evaporation accounts for approximately 20% of the total heat flux, while (3) the quenching stage accounts for approximately 80%. Finally, phase distribution images are processed to characterize the size of vapor patches, showing that a boiling crisis occurs when the distribution of the vapor patches becomes scale-free, and corroborating the hypothesis that the boiling crisis can be modelled as a bubble interaction instability using a percolation model.

Direct numerical simulations of microlayer formation during heterogeneous bubble nucleation

In this article, we present direct numerical simulation results for the expansion of spherical cap bubbles attached to a rigid wall due to a sudden drop in the ambient pressure. The critical pressure drop beyond which the bubble growth becomes unstable is found to match well with the predictions from classical theory of heterogeneous nucleation imposing a quasi-static bubble evolution. When the pressure drop is significantly higher than the critical value, a liquid microlayer appears between the bubble and the wall. In this regime, the interface outside the microlayer grows at an asymptotic velocity that can be predicted from the Rayleigh–Plesset equation, while the contact line evolves with another asymptotic velocity that scales with a visco-capillary velocity that obeys the Cox–Voinov law. In general, three distinctive regions can be distinguished: the region very close to the contact line where dynamics is governed by visco-capillary effects, an intermediate region controlled by inertio-viscous effects away from the contact line yet inside the viscous boundary layer, and the region outside the boundary layer dominated by inertial effects. The microlayer forms in a regime where the capillary effects are confined in a region much smaller than the viscous boundary layer thickness. In this regime, the global capillary number takes values much larger then the critical capillary number for bubble nucleation, and the microlayer height is controlled by viscous effects and not surface tension.

Hydrodynamic analysis of liquid microlayer formation in nucleate boiling of water

This paper presents analyses of the formation of "microlayers" i.e., very thin (∼ 1μm) liquid films often found beneath steam bubbles growing on a solid surface in low-pressure boiling. The hydrodynamics of bubble growth on a surface in pool boiling of water at atmospheric pressure have been modelled with interface-capturing Computational Fluid Dynamics (CFD) in a range of conditions that include a recent set of experiments, and transcend the remit of extant computational studies. Numerical predictions of bubble and microlayer behaviour were found in broad agreement with experimental measurements, and consistent with the current theoretical understanding of the microlayer hydrodynamic formation process. From recently reported direct measurements of microlayer thickness, a range of values of the likely thickness of the liquid film as formed on the solid surface have been inferred, and used to validate the current simulations, which clearly indicated the formation, beneath a growing steam bubble, of a liquid film of thickness comparable to values inferred from measurements. Notably, CFD simulations revealed the formation of a characteristic "dewetting ridge" i.e., an area where the film surface is markedly convex, in a microlayer region close to the triple contact line not accessible to any present-day optical measurement technique, and indicate that, in an otherwise flat microlayer, a noticeably curved region should be expected to form near the contact line. This paper provides enhanced insight into micro-scale features, such as the shape and thickness of microlayers, that typically evolve over extremely small time scales, and are therefore challenging to measure, and at present not amenable to any observation technique. A parametric study, covering a range of values of the capillary number between 7.86 × 10−4 and 7.86 × 10−2, and surface contact angles between 15˚ and 90˚, typical of wetting or partially wetting fluids on common engineering surfaces, confirmed the trends in microlayer behaviour predicted for the case of atmospheric-pressure water-boiling in laboratory conditions.

Direct numerical simulation of microlayer formation and evaporation underneath a growing bubble driven by the local temperature gradient in nucleate boiling

Recently, experiments carried out with high-resolution measurement techniques showed a thin liquid microlayer (∼μm) formation underneath a growing bubble in nucleate boiling. However, a deep understanding of the heat transfer enhancement induced by this microlayer is still lacking. In the present work, we investigate the microlayer dynamics and its effect on microlayer evaporation during the microlayer formation stage. Using direct numerical simulations with the PHASTA solver, a fully resolved microlayer underneath a growing bubble driven by the local temperature gradient in nucleate boiling is reproduced. The simulation results are compared and largely validated against recent experimental observations and Mikic's model. The detailed microlayer dynamics indicates that the microlayer formation can be considered a quasi-steady process. In addition, the hydrodynamic effect shows limited influences on the microlayer thickness, thus suggesting a rather constant amount of microlayer evaporation during the entire microlayer life cycle under different hydrodynamic conditions. Here, the maximum evaporative heat flux of the microlayer occurs near the contact line and exceeds 1 MW/m2. Evaporation of the microlayer can account for ∼50% bubble volume growth after the onset of nucleate boiling. This value emphasizes the significance of microlayer evaporation in nucleate boiling modeling.

Experimental investigation on microlayer behavior and bubble growth based on laser interferometric method

High-speed laser interferometry is synchronized with a high-speed camera to visualize the dynamic microlayer behavior during bubble growth in a pool boiling under pressures from 0.1 to 0.3 MPa. An Indium–Tin-Oxide (ITO) film coated on sapphire is employed as the heating unit to provide the nominal surface heat fluxes in the range from 90 to 150 kW/m2. Based on the instantaneous microlayer thickness and photographed bubble images, microlayer formation and depletion and their relationship with bubble growth are analyzed. Appreciable effects of pressure on microlayer dynamics and bubble growth have been observed. At higher pressure, the microlayer existence time decreases and consequently, the contribution of the microlayer evaporation becomes less important. At elevated pressure, the effects of liquid subcooling and surface heat flux on bubble growth become more pronounced. The dimensionless instantaneous maximum microlayer thickness, δmax/√vt, shows exponential dependence on the ratio rd/rb,1 which increases linearly with time before the microlayer depletion. A correlation is proposed to predict the instantaneous maximum microlayer thickness synthesizing the two relations. The local heat flux will be overestimated and the wall temperature profile is contrary to the experimental observation when the flow inside the microlayer is negligible. During the bubble growth period, only part of the microlayer is evaporated and the internal flow inside cannot be neglected.

Microlayer dynamics of hydrodynamically interacting vapour bubbles in flow boiling

Nucleate boiling, a ubiquitous heat transfer mode, involves multiple vapour bubble nucleations on the heater surface and offers high heat transfer coefficients. The bubble growth process on a heating substrate involves the formation of microlayer, a thin liquid film trapped between the growing bubble and the heating substrate, and contributes to the bubble growth phenomenon through evaporation. Microlayer dynamics for a single bubble have been widely investigated in the pool and flow boiling conditions. However, the literature on multiple bubbles interactions and their associated microlayer information is scarce. Notably, in the case of flow boiling, where the microlayer dynamics are not symmetric due to the bubble's movement, the bubbles’ interaction and its influence on associated microlayer dynamics have never been reported. Therefore, microlayer and bubble dynamics in multiple interactions have been investigated experimentally using simultaneous application of thin-film interferometry and high-speed videography techniques in flow boiling with water as the working fluid. Our experimental investigation revealed that the secondary nucleation could cause a reduction in lift-off time and may assist or hinder the movement of the first bubble. The experimental results also demonstrated that the secondary nucleation could deplete the microlayer of the first bubble hydrodynamically even when the bubbles are far apart. Furthermore, it has been found that the microlayer depletion rate depends on the growth rate and the location of the secondary nucleation. Hence, this experimental study emphasises the need to consider the interaction of bubbles while modelling boiling flows to avoid overestimating the contribution of microlayer evaporation.

Experiments to understand bubble base growth mechanism(s) on hydrophobic surfaces under the influence of bulk flow inertia during nucleate boiling regime

Wetting characteristics of substrate surface is one of the parameters that strongly influence the phenomena of boiling heat transfer. In this direction, importance of hydrophobic surfaces has been emphasized in the literature due to their potential to offer high boiling heat transfer rates at low heat fluxes. For such surfaces, it has been shown that microlayer ceases to exist with contact line evaporation being the dominant bubble base growth mechanism during nucleate pool boiling regime. The present work investigates the plausible mechanism(s) of bubble growth on hydrophobic surfaces under nucleate flow boiling conditions. Possibility of formation of microlayer even on low wettability surfaces under the influence of bulk flow inertia has been examined. Thin-film interferometry, in conjunction with high-speed videography, has been configured to record the microlayer dynamics and the bubble growth process in tandem. For a direct comparison, experiments under identical flow and bulk subcooled conditions have also been performed on a hydrophilic substrate, which is known to promote microlayer formation during the bubble growth process. Observations made through thin-film interferograms confirmed the formation of microlayer on hydrophobic surfaces. Experiments further revealed that the rate of growth of dry patch is higher on hydrophobic surfaces than that in the case of hydrophilic substrates. Vapor bubbles have been observed to be pinned on the hydrophobic surface whereas the phenomenon of bubble lift-off has been distinctly observed on hydrophilic substrate surface.

On the Coupled Thermal and Hydrodynamic Interaction of Adjacently Located Vapor Bubbles on Highly Wetting Surfaces.

Spatio-temporally resolved IR measurements coupled with high-speed videography have been made to propose the regime map of the interaction between two adjacently located vapor bubbles on high-wettability surfaces. The modes of interaction (thermal and/or hydrodynamic) have been studied as a function of distance between the two nucleation sites under saturated nucleate pool boiling conditions with water as the working fluid, and their effects on the wall heat-transfer rates have been quantified. Based on the comprehensive observations and analyses of the simultaneously mapped vapor bubble dynamics, substrate surface temperature, and the associated wall heat flux distributions, the possible regimes of bubble interactions have been identified. Experiments revealed three dominant mechanisms of bubble interactions: hydrodynamic interaction (HI), thermal interaction (TI), and horizontal coalescence (C). Depending on the relative spacing of the two nucleation sites, a regime map has been prepared wherein various possible bubble interactions (and/or their combinations) have been classified as HI + TI + C, HI + C, and HI. The IR thermography-based measurements revealed a strong dependence of bubble base evaporation-driven heat transfer on the possible bubble interaction mechanism(s) encountered in each regime. The dependence of microlayer formation beneath the growing vapor bubble(s) and its evaporation on HI is further corroborated through thin-film interferometric measurements. Plausible explanation(s) for mechanisms through which bubble interactions influence heat transfer have been discussed. A trend of variation of wall heat-transfer performance with bubble interaction regimes has been deduced and discussed.

Heat flux partitioning and macrolayer observation in pool boiling of water on a surface with artificial nucleation sites

In this study, the heat transfer mechanisms in pool saturated boiling of water on a sapphire heat transfer surface with a controlled nucleation site density (NSD) were observed using a high-speed infrared camera. Artificial nucleation sites were arranged on the wall surface by ink-jet printing of a superhydrophobic agent, and NSD was varied between 24 and 318 sites/cm2. The heat transfer characteristics of each fundamental heat transfer process and its contribution to the total wall heat transfer were evaluated through the heat flux partitioning analysis. There was an upper limit to the heat transfer enhancement by increasing the NSD. The boiling heat transfer coefficient (HTC) did not increase monotonically with increasing NSD but had a maximum value at an NSD between 89 and 130 sites/cm2. When NSD was increased excessively, the microlayer formation was inhibited by bubble–bubble interaction. The contribution of the microlayer evaporation to the wall heat transfer also decreased, resulting in a decrease in the boiling HTC. At all tested NSDs, liquid-phase heat transfer dominated the wall heat transfer, as in the case of the bare surface. The contribution of the microlayer evaporation that occupies a small area to the total wall heat transfer was less than 35%. In the high heat flux region, the macrolayer, a liquid layer with suppressed liquid motion, was successfully observed at the bottom of the coalesced bubble.

SEM investigation of microlayer formation and study of wear mechanism in friction composites

SEM investigation of microlayer formation and study of wear mechanism in friction composites

Direct Numerical Simulation of Microlayer Formation and Evaporation Underneath a Growing Bubble Driven by the Local Temperature Gradient in Nucleate Boiling

Direct Numerical Simulation of Microlayer Formation and Evaporation Underneath a Growing Bubble Driven by the Local Temperature Gradient in Nucleate Boiling

Microlayer dynamics during the growth process of a single vapour bubble under subcooled flow boiling conditions

The phenomena of microlayer formation and its dynamic characteristics during the nucleate pool boiling regime have been widely investigated in the past. However, experimental works on real-time microlayer dynamics during nucleate flow boiling conditions are highly scarce. The present work is an attempt to address this lacuna and is concerned with developing a fundamental understanding of microlayer dynamics during the growth process of a single vapour bubble under nucleate flow boiling conditions. Boiling experiments have been conducted under subcooled conditions in a vertical rectangular channel with water as the working fluid. Thin-film interferometry combined with high-speed cinematography have been adopted to simultaneously capture the dynamic behaviour of the microlayer along with the bubble growth process. Transients associated with the microlayer have been recorded in the form of interferometric fringe patterns, which clearly reveal the evolution of the microlayer beneath the growing vapour bubble, the movement of the triple contact line and the growth of the dryspot region during the bubble growth process. While symmetric growth of the microlayer was confirmed in the early growth phase, the bulk flow-induced bubble deformation rendered asymmetry to its profile during the later stages of the bubble growth process. The recorded fringe patterns have been quantitatively analysed to obtain microlayer thickness profiles at different stages of the bubble growth process. ForRe = 3600, the maximum thickness of the almost wedge-shaped microlayer was obtained asδ ~ 3.5 μm for a vapour bubble of diameter 1.6 mm. Similarly, forRe = 6000, a maximum microlayer thickness ofδ ~ 2.5 μm was obtained for a bubble of diameter 1.1 mm.

EVALUATION OF THE RELEASE RATE OF A CHEMOTHERAPEUTIC AGENT INCORPORATED AS A COATING ON A BIODEGRADABLE URETERAL STENT. IN VITRO STUDY

Abstract INTRODUCTION To assess the release rate of Mitomycin C (MMC) after its adherence to a biodegradable ureteral stent (BraidStent™-MMC) by dip coating in order to evaluate its potential application as an adjuvant treatment for upper tract urothelial carcinoma. MATERIAL AND METHODS The dip coating technique is applied to a total of 10 fragments of the BraidStent™ catheter which has a polymeric matrix as a coating. Each 3 cm fragment is immersed 10 times in pure MMC crystallising in methanol and finally obtaining the formation of microlayers on its surface. After drying, each fraction of the stent is immersed in 5 ml of artificial urine to study its interaction with this medium. The samples remain in an orbital shaker at 36ºC and the medium is exchanged under sterile conditions after 12, 24, 48, 48, 72, 96 and 120h. At each replacement, the remaining urine is analysed by HPLC-DAD to quantify the presence of the cytotoxic agent. RESULTS During the first 12h, MMC is completely released reaching a mean concentration of 52.22 mg/L. Comparing this result with those previously obtained with the first BraidStent ™ -MMC formulation (10.82mg/L), it is determined that this technique allows to increase the release rate up to 5 times. CONCLUSIONS The release rate of Mitomycin C from a biodegradable ureteral stent is increased by integrating a polymeric matrix with microlayers in its coating. Further trials are needed to achieve the therapeutic dose for clinical application in oncology patients.

Modelling Microlayer Formation in Boiling Sodium

During boiling at a solid surface, it is often the case that a liquid layer of a few microns of thickness (’microlayer’) is formed beneath a bubble growing on the heated surface. Microlayers have been observed forming beneath bubbles in various transparent fluids, such as water and refrigerants, subsequently depleting due to evaporation, thus contributing significantly to bubble growth and possibly generating the majority of vapor in a bubble. On the other hand, boiling of opaque fluids, such as liquid metals, is not amenable to optical observations, and microlayers have not yet been observed in liquid metals. Among that class of fluids is sodium, suitable as a coolant for nuclear reactors and as the working fluid in phase-change solar power receivers. In order to support these applications, it is necessary to understand the boiling behavior of sodium and identify the parameters that might influence microlayer formation during boiling of this important fluid. This paper presents simulations of the hydrodynamics of sodium vapor bubble growth at a surface. An interface capturing flow solver has been implemented in the OpenFOAM code and used to predict the behavior of a sodium vapor bubble near a solid surface in typical boiling conditions. The methodology has been validated using recently reported direct experimental observations of microlayer formation in water and then applied to sodium boiling cases. Simulations indicate that microlayers are formed in sodium in a similar fashion to water. Comparison of simulation results with an extant algebraic model of microlayer formation showed good agreement, which increases confidence in the current predictions of microlayer formation. Typical values of microlayer thickness thus computed indicate that the microlayer is likely to play an important role during bubble growth in sodium.

Airborne bacterial community diversity, source and function along the Antarctic Coast

Antarctica is an isolated and relatively simple ecosystem dominated by microorganisms, providing a rare opportunity to study the spread of airborne microbes and to predict future global climate change. However, little is known about on the diversity and potential sources of microorganisms in the marine atmosphere along the Antarctica coast. Here we explored the airborne bacterial community (i.e., bacteriome) diversity, sources and functional potential along the Antarctic coast based on 16S rRNA gene amplicon sequencing of 25 bioaerosol samples collected during the 33rd Xuelong Antarctic scientific expedition. The results showed that bacterial communities in the Antarctic bioaerosols i) were predominated by Proteobacteria (91.3%) including Sphingomonas, ii) showed relative low alpha-diversity but high spatiotemporal variabilities; and iii) were potentially immigrated with terrestrial, marine and Antarctic polar bacteria through long-range transport and sea-air exchange pathways. Moreover, canonical correspondence analysis of bacteriome composition showed that wind speed, temperature, and organic carbon had a significant effect on the bacterial community (P < 0.05), although bacterial richness (Richness index) and diversity (Simpson index and Shannon index) showed no statistically significant differences between rainy, cloudy and snowy weather conditions (Adjust P > 0.05, ANOVA, Tukey HSD test). iv) The functional profiles predicted by Tax4fun2 suggest high representation of function genes related to fatty acid biosynthesis and metabolism, amino acid metabolism, nucleotide metabolism, and carbohydrate metabolism, which is conducive to the formation of microlayers on the surface of the ocean and the survival and growth of bacteria.

The role of microlayer for bubble sliding in nucleate boiling: A new viewpoint for heat transfer enhancement via surface engineering

In an experimental study with a stainless steel heater (surface with maximum roughness Rt = 0.82 µm and contact angle hysteresis θhys = 53°), we investigated the bubble growth and motion during nucleation and departure. Complementary to that we analysed the formation of microlayer during the bubble growth and motion with computational fluid dynamics (CFD) simulation. From the simulations we found that the bubble motion leads to an expansion of the microlayer. From the experiments we obtained the drag coefficient on the bubble during bubble growth with an assumption of the absence of the wall surface tension force. From the comparison of this drag coefficient and the proposed values from the literature, we conclude that the vapor bubble does not directly contact the solid wall during the sliding. Using well-known mechanistic bubble growth models for further analysis of available microlayer area with the experimental data we conclude that a microlayer exists and the bubble must slide completely on this microlayer after leaving its originating cavity. From the change of microlayer size we can also explain the bubble regrowth after departure.

On the transition between contact line evaporation and microlayer evaporation during the dewetting of a superheated wall

We present experimental results on the transition between contact line evaporation and microlayer evaporation during the dewetting of a superheated wall. The length and thickness of the liquid microlayer is measured, both showing a dependency on the wall superheat, dewetting velocity, and heating power.This work falls within the continuing efforts of understanding the formation of the so called microlayer: a thin liquid film, that is observed for example during boiling underneath a growing vapor bubble. The short lived and small scale nature of the phenomenon make accurate measurements difficult, therefore a novel experimental set-up is proposed that allows high resolution measurements in a steady-state manner. The time resolved formation of the microlayer is described in detail and the influence of the dewetting velocity, wall superheat, and heating power is discussed. Microlayer formation is observed only if a critical velocity is exceeded for a given combination of wall superheat and heating power. Below this threshold velocity, contact line evaporation dominates the heat transfer. The dependency of the microlayer length and thickness on the dewetting velocity is described using power laws. Instability effects near the contact line are observed at high dewetting velocities and high wall superheat. It is concluded that microlayer evaporation and contact line evaporation exist in separate regimes, obviously separated by a parameter combination of dewetting velocity, wall superheat, and heating power.