Bubble Contact Angle Research Articles

Effect of extreme wetting scenarios on pool boiling conditions

This study focuses on the detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity. The wettability is changed by modifying the surface chemistry and without significant variations in the mean surface roughness. Under these conditions and for the range studied here the effect of the extreme wetting regimes was dominant, thus the influence of surface topography was not addressed. A particular trend is observed for the boiling curve obtained with the superhydrophobic surfaces, as the heat flux increases almost linearly with the superheat, although with a much lower slope than the hydrophilic surfaces. This occurs due to the formation of a large stable vapour film over the entire surface just at around 1K superheat, because of the almost immediate coalescence of the bubbles generated on the surface. This behaviour agrees with the so-called “quasi-Leidenfrost” regime recently reported in the literature and with a theoretical prediction of the heat flux that is presented in this study. Furthermore, a comprehensive analysis of bubble dynamics, useful for comparison with numerical simulations is given. Such analysis is based on the temporal evolution of the bubble diameter together with bubble contact angle and with the velocity of the contact line. The results suggest that the existing models and correlations can predict the trends of the bubble growth using a modified contact angle value, called the bubble contact angle (or its supplemental value), for the hydrophilic surfaces, even if they cannot accurately predict bubble sizes. Approximating the modified contact angle with the quasi-static contact angle, obtained during surface characterization is practical for a qualitative evaluation, but the results obtained here do not support for its use when estimating the bubble departure diameter. On superhydrophobic surfaces, the effect of the vapour film must be considered, since although this is not the starting point of the boiling process, it represents the actual working conditions when using this kind of surfaces.

REMOVED: An adaptive mesh refinement based flow simulation for free-surfaces in thermal inkjet technology

Abstract The microscopic multiphase flows of thermal inkjet (TIJ) devices usually involve a constantly changing liquid–gas interface. Often the curvature of the interface can become too high at some localized positions to be accurately resolved using a conventional static computational grid. A good solution to such challenging problem is to use the Adaptive-Mesh-Refinement (AMR) method, since AMR technique can concentrate the computational effort in the regions where it is needed most. In recent years, an open-source AMR code, Gerris , has been extensively used and benchmarked for a large variety of surface-tension dominated flows. However, the code cannot be directly used to model the TIJ process due to the lack of bubble dynamics and contact angle model for solid boundaries in the code. Therefore in the paper, we propose the following new approaches to model the TIJ flows and implement these algorithms in Gerris : (1) the vapor bubble is treated as an empty cavity so that its complex behavior of the vapor bubble after the bubble nucleation can be modeled using the adiabatic gas expansion and compression; (2) a height-function method based contact angle model is developed and implemented on the embedded solid surfaces. The new code has been verified and validated through a series of numerical examples and experimental tests. The proposed AMR method and the simulation code can be used in a broad range of applications in TIJ technology as well as multiphase flows of microfluidics.

Drainage of particle stabilized foam film

Abstract A model for drainage of aqueous foam film with a partial coverage of particles accounts for an inner clean air-liquid film surrounded by a film stabilized by close packed particles at the outer periphery. The drainage of the inner film occurs due to capillary pressure gradient. The film thickness is found to decrease fastest at the center with the inner film being thinner than the outer film eventually leading to its rupture as a result of film thickness becoming zero at the center. The velocity of film drainage is found to be higher for lower particle concentration and higher surface tension. The lifetime (time of rupture) of the film is found to be larger for larger bubble size and liquid holdups (higher initial film thickness). For a film stabilized by close packed monolayer or multilayer of particles, the disjoining pressure is larger for larger particle number concentration and particle size, increases with surface tension only for film stabilized by single or two layers of particles and is a function of bubble size only for film stabilized by multilayers of particles. The maximum disjoining pressure decreases with contact angle approaching zero for a critical contact angle of 129° and increases linearly with surface tension. At mechanical equilibrium, the particle stabilized foam gets drier for larger bubble sizes, contact angles and particle number concentrations.

Theoretical and experimental investigations of dynamics of bubble growth and triple line

AbstractThe dynamics of triple line and bubble growth were investigated inside water, gold, silver, and alumina nanofluids. The presence of nanoparticles was found to modify the behaviour of the triple line, depending on nanoparticle materials. Two analytical expressions were developed, one for the contact angle of bubbles and another for the contact angle of droplets, using force balance in the horizontal direction. It was mathematically proven that these analytical expressions are the exact analytical solutions of the Young‐Laplace equation. These analytical expressions prove that the Young equation cannot be an accurate force balance at the triple line for axisymmetric bubbles and droplets. The analytical expressions predict the contact angle of bubbles and droplets and prove theoretically that the contact angles of axisymmetric bubbles and droplets depend on volume or size, which agreed with existing experimental data. The predictions of the analytical expressions were compared with experimental data, and close agreements were observed. To reduce uncertainty and error, these two analytical expressions can be used instead of the Young equation, particularly in CFD calculations, to predict the behaviour of triple line or fluid flow in the vicinity of substrate for droplets and bubbles.

微細周期構造の付与で検出特性を改善した気泡センサ

In solid oxide fuel cell, carbon dioxide is generated in the water circulator and tends to accumulate in the reformer. If the air bubble forms a fair size, it disturbs the water flow and gives damage to the water circulator. For practical applications, long-term operation with an air bubble detection capability is needed. The purpose of this paper is to enhance the sensitivity of the air-bubble sensor by increasing the physical water repellency of its detecting surface using micro- and nano-periodic structures. The air-bubble sensor comprised a cylindrical sleeve with two coil shaped-electrodes and a ground electrode. Four types of sleeves for the air-bubble sensor with different micro- and nano-periodic structures on the surface of a detection area were fabricated. As the results, it was found that i) a micro-periodic structure increased the apparent contact angle of air bubble from 50° to 89°, and ii) a V-shaped micro-periodic structure could degrease the accidental error by improving the wettability. The electrode voltage was increased toward the detection threshold level and the detected time was prolonged when the apparent contact angle of water was small, showing that which increase the probability of false detection. Additionally, the detecting sensitivity of the air-bubble sensor was increased by forming a micro-periodic structure on the surface. In summary, a micro-periodic structure has the effects improving the detecting precision of air-bubble sensor.

The Influence of Long-Range Surface Forces on the Contact Angle of Nanometric Droplets and Bubbles

For a droplet or a bubble of dimensions below 100 nm, long-range surface forces such as long-range van der Waals forces can compete with capillarity, which leads to a size dependence of the contact angle. This is discussed in this work, where we also show that the effect cannot simply be described by a normalized line tension. We calculate interfacial profiles for typical values of van der Waals forces and discuss the role of long-range surface forces on the contact angle of nanobubbles and nanodrops.

Construction of a blood-compatible interface based on surface segregation in a polymer blend

The technique of surface segregation was applied to prepare a bio-inert polymer interface. A small amount, 10 wt%, of poly(2-methoxyethyl acrylate) (PMEA), which exhibits excellent bio-inertness properties, fed into a matrix polymer was able to suppress platelet adhesion sufficiently to be of practical use. PMEA was effective because it was preferentially segregated at the outermost region of the polymer blend. Combining interfacial-sensitive analyses such as the air bubble contact angle and neutron reflectivity measurements and sum-frequency generation spectroscopy with the platelet adhesion test gives a better understanding of how the bio-inert property is expressed at the water interface.

In Situ ATR FTIR Spectroscopic Study of the Formation and Hydration of a Fucoidan/Chitosan Polyelectrolyte Multilayer

The formation of fucoidan/chitosan-based polyelectrolyte multilayers (PEMs) has been studied with in situ Fourier transform infrared (FTIR) spectroscopy. Attenuated total reflectance (ATR) FTIR spectroscopy has been used to follow the sequential build-up of the multilayer, with peaks characteristic of each polymer being seen to increase in intensity with each respective adsorption stage. In addition, spectral processing has allowed for the extraction of spectra from individual adsorbed layers, which have been used to provide unambiguous determination of the adsorbed mass of the PEM at each stage of formation. The PEM was seen to undergo a transition in growth regimes during build-up: from supra-linear to linear. In addition, the wettability of the PEM has been probed at each stage of the build-up, using the captive bubble contact angle technique. The contact angles were uniformly low, but showed variation in value depending on the nature of the outer polymer layer, and this variation correlated with the overall percentage hydration of the PEM (determined from FTIR and quartz crystal microbalance data). The nature of the hydration water within the polyelectrolyte multilayer has also been studied with FTIR spectroscopy, specifically in situ synchrotron ATR FTIR microscopy of the multilayer confined between two solid surfaces. The acquired spectra have enabled the hydrogen bonding environment of the PEM hydration water to be determined. The PEM hydration water is seen to have an environment in which it is subject to fewer hydrogen bonding interactions than in bulk electrolyte solution.

Phase-Field Modeling of Vapor Bubble Growth in a Microchannel

A numerical model based on Cahn–Hilliard phase-field method is introduced for the first time in the literature to investigate the hydrodynamics and heat transfer characteristics of a vaporized elongated bubble in a rectangular microchannel. In the simulations, the initially nucleated bubble starts growing as it comes in contact with superheated water. The effects of the water inlet velocity and the bubble contact angle on the temporal evolution of the average heat transfer coefficient are also reported. Both qualitative and quantitative comparisons indicated that the numerically obtained bubble shape and the growth rate are in very good agreement with the experimental results available in the literature.

Effect of orifice size and bond number on bubble formation characteristics: A CFD study

The volume of fluid with the continuum surface force (VOF‐CSF) model has been used in the present numerical study to investigate bubble formation and shapes in a bubble column. The effects of orifice sizes ranging from 0.5–1.5 mm on the bubble formation stages (i.e., expansion, elongation, and pinch‐off), bubble contact angle, departure diameter, time, and shape of bubble are investigated under a constant inlet velocity (0.2 m/s) boundary condition (BC). It is found that the initial formation of a bubble is dependent on the orifice diameter. Consequently, the formation of the bubble's hemispherical shape at the orifice is faster for a smaller orifice diameter than for a larger orifice diameter which forms a bigger bubble. A leading bubble requires a longer time to detach itself from an orifice in comparison to the next bubble (the second bubble), but interestingly the third bubble detaches much faster than the second. This model was also used to investigate the effect of the Bond number (Boσ). An increase of the Bond number (Boσ) from 0.047 to 0.16 speeds up the bubble pinch‐off, but a further increase in the Boσ (e.g. 0.17–0.47) slows it down. Based on the simulated cases used in this study, the findings demonstrate the capacity and accuracy of the CFD method in predicting bubble formation characteristics. The findings may be useful in the design of spargers for bubble column reactors.

Influence of phase change on self-pressurization in cryogenic tanks under microgravity

Future operations of many fluid, thermal and power systems and their ability to store, transfer, and manage a variety of single or multiphase fluids in reduced gravity environment are of great importance. For many of these systems, cryogenic conditions will play an important role. Cryogenic vaporization, caused by heat leakage into the tank from the surrounding environment, is one of the main causes of mass loss and leads to self-pressurization of the storage tanks. Available publications on self-pressurization and stratification of cryogenic tanks mainly focus on the convection and surface evaporation influences. Because large superheats increase the likelihood of evaporation in the liquid, the evaporation and its effect on vapor pressure under microgravity is studied numerically in this paper. The effects of reduced gravity, contact angle of the vapor bubble, and surface tension are investigated. The computations are carried out by using the CFD software package, Ansys Fluent, and an in-house developed code to calculate the source term associated with the phase change. A coupled level set and the volume-of-fluid method (CLSVOF) are used to solve a single set of conservation equations for the whole domain and the interface between the two phases is tracked or captured. A heat and mass transfer model is implemented into the Fluent code for solving problems involving evaporation or condensation. Results show that small tiny vapor regions caused by the evaporation process change the pressure rise. Vortices are observed due to the vapor dynamics.

Amphiphilic oligopeptides grafted to PDMS‐based diblock copolymers for use in antifouling and fouling release coatings

Amphiphilic surfaces, having both hydrophobic and hydrophilic properties, have been considered for marine antifouling and fouling release applications, as they may provide an environmentally friendly non‐biocidal alternative for surface coating. Here, we report a new class of polymers modified with amphiphilic side chains constructed from non‐natural oligopeptides. These non‐natural amino acids have a diverse array of functionalities and polarities that can be arranged in any sequence to optimize antifouling and fouling release behavior. Because of advances in solid state peptide synthesis, these peptides can be prepared on a multiple gram scale in a short period of time. Different sequences of non‐natural amino acids were synthesized to determine the effect on the physical and antifouling properties of the surface. These oligopeptide sequences were attached to a PS‐b‐P(DMS‐stat‐MVS) diblock copolymer using thiol‐ene “click” chemistry. The modified polymers were spray coated onto treated glass slides and characterized using water bubble contact angle measurements and near‐edge X‐ray absorption fine structure. The coatings were tested for antifouling and fouling release performance against spores and sporelings (young plants), respectively, of the green seaweed (alga) Ulva linza and also for non‐specific protein absorption. Copyright © 2015 John Wiley & Sons, Ltd.

Decompression induced bubble dynamics on ex vivo fat and muscle tissue surfaces with a new experimental set up

Vascular gas bubbles are routinely observed after scuba dives using ultrasound imaging, however the precise formation mechanism and site of these bubbles are still debated and growth from decompression in vivo has not been extensively studied, due in part to imaging difficulties. An experimental set-up was developed for optical recording of bubble growth and density on tissue surface area during hyperbaric decompression. Muscle and fat tissues (rabbits, ex vivo) were covered with nitrogen saturated distilled water and decompression experiments performed, from 3 to 0bar, at a rate of 1bar/min. Pictures were automatically acquired every 5s from the start of the decompression for 1h with a resolution of 1.75μm. A custom MatLab analysis code implementing a circular Hough transform was written and shown to be able to track bubble growth sequences including bubble center, radius, contact line and contact angles over time. Bubble density, nucleation threshold and detachment size, as well as coalescence behavior, were shown significantly different for muscle and fat tissues surfaces, whereas growth rates after a critical size were governed by diffusion as expected. Heterogeneous nucleation was observed from preferential sites on the tissue substrate, where the bubbles grow, detach and new bubbles form in turn. No new nucleation sites were observed after the first 10min post decompression start so bubble density did not vary after this point in the experiment. In addition, a competition for dissolved gas between adjacent multiple bubbles was demonstrated in increased delay times as well as slower growth rates for non-isolated bubbles.

Under‐Water Superaerophobic Pine‐Shaped Pt Nanoarray Electrode for Ultrahigh‐Performance Hydrogen Evolution

A pine‐shaped Pt nanostructured electrode with under‐water superaerophobicity for ultrahigh and steady hydrogen evolution reaction (HER) performance is successfully fabricated by a facile and easily scalable electrodeposition technique. Due to the lower bubble adhesive force (11.5 ± 1.2 μN), the higher bubble contact angle (161.3° ± 3.4°) in aqueous solution, and the smaller size of bubbles release for pine‐shaped Pt nanostructured electrode, the incomparable under‐water superaerophobicity for final repellence of bubbles from submerged surface with ease, is successfully achieved, compared to that for nanosphere electrode and for Pt flat electrode. With the merits of superior under‐water superaerophobicity and excellent nanoarray morphology, pine‐shaped Pt nanostructured electrode with the ultrahigh electrocatalytic HER performance, excellent durability, no obvious current fluctuation, and dramatically fast current density increase at overpotential range (3.85 mA mV−1, 2.55 and 13.75 times higher than that for nanosphere electrode and for Pt flat electrode, respectively), is obtained, much superior to Pt nanosphere and flat electrodes. The successful introduction of under‐water superaerophobicity to in‐time repel as‐formed H2 bubbles may open up a new pathway for designing more efficient electrocatalysts with potentially practical utilization in the near future.

Protein Adsorption Alters Hydrophobic Surfaces Used for Suspension Culture of Pluripotent Stem Cells.

This Letter examines the physical and chemical changes that occur at the interface of methyl-terminated alkanethiol self-assembled monolayers (SAMs) after exposure to cell culture media used to derive embryoid bodies (EBs) from pluripotent stem cells. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy analysis of the SAMs indicates that protein components within the EB cell culture medium preferentially adsorb at the hydrophobic interface. In addition, we examined the adsorption process using surface plasmon resonance and atomic force microscopy. These studies identify the formation of a porous, mat-like adsorbed protein film with an approximate thickness of 2.5 nm. Captive bubble contact angle analysis reveals a shift toward superhydrophilic wetting behavior at the cell culture interface due to adsorption of these proteins. These results show how EBs are able to remain in suspension when derived on hydrophobic materials, which carries implications for the rational design of suspension culture interfaces for lineage specific stem-cell differentiation.

Numerical study to invistigate the effect of inlet gas velocity and Reynolds number on bubble formation in a viscous liquid

Bubble formation dynamics has great value in mineral recovery and the oil industry. In this paper, a single bubble formation process through an orifice in a rectangle domain is modelled to study the bubble formation characteristics using the volume of fluid (VOF) with the continuum surface force (CSF) method. The effect of gas inlet velocities, Ug ~ 0.1 - 0.3 m/s on bubble formation stages (i.e., expansion, elongation and pinch off), bubble contact angle, dynamics and static pressure, bubble departure diameter etc. was investigated through an orifice diameter of 1 mm. The method was also used to study the effect of Reynolds number, Re? ~ 1.32 - 120 on bubble formation when all other parameters were kept constant. It is found that a high inlet gas velocity accelerated the reducing of the bubble contact angle from an obtuse angle to an acute angle and the faster development of hemispherical shape of the bubble. It is also found that an increasing of Reynolds number caused speeding up of the bubble pinch-off and formed a smaller bubble neck height due to stronger vortex ring around the bubble neck.

Investigation of nanofluid bubble characteristics under non-equilibrium conditions

We report experimental and theoretical investigations of the bubble characteristics during the oscillatory growth period for several nanofluids. The nanoparticles were found to affect liquid–gas and solid surface tensions, which modulated the bubble contact angle, radius of triple line, bubble volume and the dynamics of bubble growth. To increase the accuracy of the Young–Laplace equation predictions during the bubble growth in the oscillatory period, a new method multi-section bubble (MSB) approach was developed. In this method, the bubble was divided into n sections (i.e., n=1:N) and the Young–Laplace equation was solved for each section individually. As N increases, within each section the effects of inertia force and viscosity become reduced comparing to that of the liquid-gas surface tension. Unlike the conventional Young–Laplace approach (i.e., N=1), the new approach is able to predict the bubble characteristics reliably in the following cases: (a) the oscillatory period when bubble is fluctuating; (b) the departure period when bubble is stretched upward, right before departure; and (c) the high shear stress condition when gas velocity is relatively high.

Dimensions and the Profile of Surface Nanobubbles: Tip–Nanobubble Interactions and Nanobubble Deformation in Atomic Force Microscopy

The interactions between argon surface nanobubbles and AFM tips on HOPG (highly oriented pyrolitic graphite) in water and the concomitant nanobubble deformation were analyzed as a function of position on the nanobubbles in a combined tapping mode and force-volume mode AFM study with hydrophilic and hydrophobic AFM tips. On the basis of the detailed analysis of force-distance curves acquired on the bubbles, we found that for hydrophobic tips the bubble interface may jump toward the tip and that the tip-bubble interaction strength and the magnitude of the bubble deformation were functions of vertical and horizontal position of the tip on the bubble and depended on the bubble size and tip size and functionality. The spatial variation is attributed to long-range attractive forces originating from the substrate under the bubbles, which dominate the interaction at the bubble rim. The nonuniform bubble deformation leads to a nonuniform underestimation of the bubble height, width, and contact angle in conventional AFM height data. In particular, scanning with a hydrophobic tip resulted in severe bubble deformation and distorted information in the AFM height image. For a typical nanobubble, the upward deformation may extend up to tens of nanometers above the unperturbed bubble height, and the lateral deformation may constitute 20% of the bubble width. Therefore, only scanning with a hydrophilic tip and no direct contact between the tip and the bubble may reduce nanobubble deformation and provide reliable AFM images that can be used to estimate adequately the unperturbed nanobubble dimensions. The deformation of the bubble shape and underestimation of the bubble size lead to the conclusion that the profile of surface nanobubbles is much closer than previously thought to a nearly flat bubble profile and hence that the Laplace pressure is much closer to the atmospheric pressure. Together with line pinning, this may explain the long nanobubble lifetimes observed previously. The findings presented in this report hold independently of the material that constitutes the interrogated nanoscale surface features.

Experimental investigation and CFD modeling of the dynamics of bubbles in nanofluid pool boiling

This work investigated the dynamics of bubbles in pool boiling of nanofluid with coated and sodium dodecyl sulfate (SDS) solution with different nanoparticles. Also, computational fluid dynamics (CFD) module was used for mathematical modeling of bubbles in pure water boiling.Different macroscale parameters such as: shapes, numbers and contact angle of bubbles also were investigated experimentally and verified by CFD modeling results. Porous layers of nanoparticles on stainless steel substrate in conjunction with SDS additions were shown to modify formation of bubbles in comparison to reference condition. Improvement in surface hydrophilic conditions and boiling performance was observed by Multi-wall Carbon Nanotube (MWCNT) porous layers, in spite of coated surface by CuO and Al2O3 (γ) water based nanofluid. Growth time of bubbles also changes by the presence of porous layers and surfactant solution which resulted from change in surface tension force. Number of bubbles increased by MWCNT and SDS solution and decreased by CuO and Al2O3 nanofluid boiling. Results showed the comprehensive change in bubble dynamics and surface wettability by porous layer of nanoparticles and SDS solution.

Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions

Understanding the generation and behaviour of air bubbles in the presence of different reagents is important in the solid/liquid separation processes used to treat mineral and water and wastewater by flocculation–flotation. The presence of large air bubbles in aerated flocs, which have higher up-rising rates, can lead to the development of higher capacity units. The present work evaluated the generation and behaviour of air bubbles on hydrophilic particles of quartz in water and other aqueous solutions (10 mg L−1 920SH non-ionic polymer, 30 mg L−1 DF250 surfactant and 50 mg L−1 Flotigan EDA amine). The operating parameters were the air bubble growth time, contact angle, adhesion, attachment and detachment. The main results showed that the air bubbles grew more readily in the presence of non-ionic polymers and after adhesion in non-ionic polymer and water. The formation of large bubbles in aerated flocs in an effort to develop industrial flocculation–flotation operations with higher throughputs units is di...