Mechanism Of Bubble Nucleation Research Articles

Insights into heterogeneous surface induced bubble nucleation mechanisms in cellulose reinforced polylactic acid foams

As the most abundant natural homo-polymer, cellulose has the potential to enhance polymer properties reducing the cost of raw materials. In this work, the carboxylate cellulose nanofiber (CNF-C) was selected to modify polylactic acid (PLA) foams, and the density functional theory was constructed to help analyze the foaming mechanism quantitatively. The theoretical results showed that the ordered structure, the carboxyl and the hydroxyl of CNF-C were more conducive to providing much stronger CO2 adsorption for bubble nucleation, where the predicted critical bubble size decreased and the cell density increased with the addition of CNF-C. The experimental results revealed that the CNF-C promoted the rheological properties and crystallization behaviors of PLA samples, the PLA/CNF-C foams were characterized with uniform structures, the average cell size decreased from 21.39 μm to 0.19 μm, and the cell number density increased from 2.65×1010cell/cm3 to 2.30×1014cell/cm3. Those improvements resulted in an increase of 394.0 % for the compressive strength of the prepared foams. Generally, the high-performance PLA/CNF-C foams were fabricated successfully without compromising the properties of bio-based and biodegradable, the foaming mechanism was analyzed combining theoretical results with experimental data, and it was believed to provide a guide for cellulose reinforcing biodegradable polymer materials.

Effects of the wall temperature on the boiling process and the molecular dynamics behavior of the liquid hydrogen on a flat aluminum wall

Understanding the nucleation mechanism and boiling heat transfer characteristics of liquid hydrogen is a key issue in the development of liquid hydrogen storage systems. However, the current methods of experimental and mesoscopic or macroscopic scale continuous flow simulations suffer from the problems of large study scales and undetermined initial boundary conditions. The purpose of this study is to reveal the bubble nucleation mechanism and bubble development process of liquid hydrogen on the surface of heating plate at the molecular scale, and to investigate the effect of different heating plate temperature on bubble growth, during thin film boiling. In this research, molecular dynamics simulation is used to combine molecular position distribution with energy analysis and explain the molecular behavior. The effect of heating temperature on different phase transition stages of liquid hydrogen is also investigated by varying the heating surface temperature. In this study, the bubble nucleation, bubble development and merging of liquid hydrogen on the heating surface were successfully observed. It is shown that increasing the heating temperature has a small effect on the bubble growth rate and liquid layer thickness in the nucleation boiling stage, and mainly affects the gas film growth rate in the film boiling stage. The bubbles are extremely unstable at the initial stage of generation. Within 1 fs of the first appearance of bubbles, which are associated with random micro-pits caused by thermal motion of solid atoms, bubbles move on the surface of the heating plate. This study found a phenomenon that bubbles firstly generated and then disappeared. This paper provides a new idea for the nucleation mechanism and boiling heat transfer study of liquid hydrogen. Moreover, it provides a solution to the problems of liquid hydrogen phase change research.

Investigation of bubble nucleation on inhomogeneous wettability surfaces

ABSTRACT Surface wettability plays an essential role in bubble nucleation. This paper investigates the phase transition behaviour of argon films placed on inhomogeneous wettability substrates using molecular dynamics simulations. The location of bubble nucleation and initial nucleation time is investigated to reflect the effect of hydrophobic area fraction of inhomogeneous wettability surfaces on bubble nucleation. The results show that the bubbles appear first on the hydrophobic part of the inhomogeneous wettability surface. The relationship between hydrophobic area fraction and initial nucleation time is nonlinear. The initial nucleation time first decreases and then increases as the area of the hydrophobic region increases. In addition, the bubble nucleation process is restricted when the hydrophobic occupation is relatively large on inhomogeneous wettability surfaces. At last, the mechanism of bubble nucleation on inhomogeneous wettability surfaces is discussed in combination with the restrictive relationship between heat absorption and solid–liquid interaction energy. The amount of heat absorption does not directly determine the bubble nucleation. Bubble nucleation can arise in the location with a lower surface energy barrier once the heat absorption can overcome the solid–liquid interaction energy.

Microscopic mechanism of effects of nanostructure morphology on bubble nucleation: A molecular dynamics simulation

In this study, the effect of nanostructure morphology on bubble nucleation of liquid argon on a platinum solid surface was explored using molecular dynamics methods. We examined the differences in nucleation behavior on smooth and nanostructured surfaces, including trapezoidal, rectangular, and reentrant cavities. The simulation results demonstrated that nanostructured surfaces outperformed smooth surfaces in terms of bubble nucleation, and the corresponding morphology had a significant effect. The reentrant cavity had the shortest bubble initial nucleation time, followed by the rectangular cavity, and the trapezoidal cavity had the longest bubble initial nucleation time. The mechanism of bubble nucleation on solid surfaces was further investigated from two perspectives: heat transfer through the solid–liquid interface and the potential energy constraint. Compared with other surfaces, surfaces with reentrant cavities exhibited a significantly higher heat transfer efficiency. The liquid atoms on the nanostructured surfaces absorb a large amount of heat within a short time, resulting in rapid accumulation of heat in the cavity. The thermal motions of the liquid atoms are enhanced, and the potential energy constraints from the surrounding atoms are overcome to achieve rapid bubble nucleation. Thus, the nanostructures promote bubble nucleation in the heated liquid. Our results provide novel insights into the design of nanostructures to improve the boiling heat transfer performance.

Nanoscale investigation of surface wettability distribution on bubble nucleation with variable temperature boundary condition

Surface wettability plays an essential role in bubble nucleation. In this article, the molecular dynamics simulation method is conducted to study the bubble nucleation of liquid argon film on a copper substrate with hybrid wettability. Unlike the constant temperature boundary condition, the variable temperature boundary condition is adopted to simulate the bubble nucleation process. The temperature gradually increases from 86 K to 200 K, so the substrate temperature at the bubble nucleation time can be obtained. The potential energy, kinetic energy, total energy, and temperature during bubble nucleation are analyzed. The results show that bubble nucleation occurs when the kinetic energy exceeds the potential energy barrier. A method is used to capture the bubble nucleation process, described from the perspective of total energy. The initial nucleation time of the hybrid wettability surface gradually reduces with the rise of the proportion of the hydrophilic part. Compared with the pure hydrophilic surface, the hybrid wettability surface can achieve faster initial nucleation time and lower nucleation temperature by altering the proportion of wettability. The potential energy and temperature distribution reveal the microscopic mechanism of bubble nucleation. This study provides theoretical guidance for the design of cooling surfaces of micro/nanoelectronic devices.

A revised Lennard-Jones potential for bubble nucleation study of argon based on the molecular dynamics simulation method

Bubble nucleation in boiling heat transfer is a microscale phenomenon, which is usually investigated by the molecular dynamics simulation method with the study object of argon. The simple potential model of argon favors revealing the bubble nucleation mechanism. However, the published works indicate that the required heating temperature for achieving bubble nucleation inside liquid argon is unreasonable and usually over the critical temperature. Therefore, in this study, a revised Lennard-Jones (L-J) potential for bubble nucleation study of argon is proposed based on the “PK” norm that bubble nucleation happens when the average kinetic energy of a group of liquid atoms exceeds their potential barrier (absolute value of atomic potential energy). The parameters α and β are added to the original L-J potential to weaken the phase transition barrier and reduce atomic balance distance, corresponding to the functions of lowering onset nucleation temperature and avoiding the unreasonable change of liquid argon properties, respectively. Besides, a moving top wall based on the force balance is conducted to maintain the simulation system pressure at the expected value. Simulation results indicate that based on the revised L-J potential and pressure controlled at one atmosphere, only the evaporation phenomenon happens when the heat source temperature is fixed at 85 K within the simulation time of 10 ns, while it converts into the bubble nucleation phenomenon as the thermostat temperature is raised to 90 K, which is very close to the onset boiling temperature of liquid argon (87.2 K). In addition, the relative differences between the original and revised potentials in the density, thermal conductivity, and thermal diffusion coefficient are 1.31%, 33.50%, and 233.33%, respectively. The difference in the third property is significant, but compared with the benchmark value obtained by the popular property database RefProp, the computational accuracy of the revised potential is higher than the original potential. Hence, the revised L-J potential is more suitable for the bubble nucleation study of argon by using the molecular dynamics simulation method.

Numerical simulation of hydrogen isotope bubbles evolution and their effect on fuel retention in tungsten

• A high nucleation rate and self-clustering of hydrogen isotope (HI) in the near-surface layer. • The appearance of the densest and largest bubbles in the near-surface layer enhances the hydrogen isotope retention significantly. • The behavior of the bubbles is strongly dependent on the operation regime in a tokamak. The 2D Hydrogen Isotope Inventory Process code with Monte Carlo bubble growth model (HIIPC2D-MC) has been upgraded by adding surface retention and self-clustering bubble nucleation to study the evolution of hydrogen isotope (HI) bubble and its impact on fuel retention. In this code, fuel retention is simulated by HIIPC2D, while the nucleation and growth of the bubble is modeled by 3D Monte Carlo code. Firstly, the dynamic behavior and mechanism of bubble nucleation is studied via self-clustering nucleation module. The effects of irradiation flux, material temperature ( T ) and trap concentration on bubble nucleation are systematically studied. The results show that the low T promotes the self-clustering bubble nucleation, large irradiation flux and low trap density lead to high nucleation rate. The long irradiation time facilitates the formation of bubbles via hydrogen self-clustering. Then, the time evolution of nucleation and growth of the bubbles is demonstrated. After the nucleation, both cascade-coalescence and pressure drive the bubbles to grow. The most and largest bubbles appear in the near-surface layer, and the HI inside the bubble plays important role on the total fuel retention. The simulation indicates the behaviors of the bubbles depend strongly on the operation regime in a tokamak. However, the bubble burst (blistering burst in experiment) is not included in the model, which will affect the bubble performance during long term irradiation.

Deciphering the molecular mechanism of water boiling at heterogeneous interfaces

Water boiling control evolution of natural geothermal systems is widely exploited in industrial processes due to the unique non-linear thermophysical behavior. Even though the properties of water both in the liquid and gas state have been extensively studied experimentally and by numerical simulations, there is still a fundamental knowledge gap in understanding the mechanism of the heterogeneous nucleate boiling controlling evaporation and condensation. In this study, the molecular mechanism of bubble nucleation at the hydrophilic and hydrophobic solid–water interface was determined by performing unbiased molecular dynamics simulations using the transition path sampling scheme. Analyzing the liquid to vapor transition path, the initiation of small void cavities (vapor bubbles nuclei) and their subsequent merging mechanism, leading to successively growing vacuum domains (vapor phase), has been elucidated. The molecular mechanism and the boiling nucleation sites’ location are strongly dependent on the solid surface hydrophobicity and hydrophilicity. Then simulations reveal the impact of the surface functionality on the adsorbed thin water molecules film structuring and the location of high probability nucleation sites. Our findings provide molecular-scale insights into the computational aided design of new novel materials for more efficient heat removal and rationalizing the damage mechanisms.

Correction to “Probing the Mechanism of Bubble Nucleation in and the Effect of Atmospheric Pressure on the Candy–Cola Soda Geyser”

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to “Probing the Mechanism of Bubble Nucleation in and the Effect of Atmospheric Pressure on the Candy–Cola Soda Geyser”Thomas S. Kuntzleman*Thomas S. KuntzlemanMore by Thomas S. Kuntzlemanhttps://orcid.org/0000-0002-2691-288X and Ryan JohnsonRyan JohnsonMore by Ryan Johnsonhttps://orcid.org/0000-0002-4849-8501Cite this: J. Chem. Educ. 2021, 98, 7, 2486Publication Date (Web):June 23, 2021Publication History Published online23 June 2021Published inissue 13 July 2021https://doi.org/10.1021/acs.jchemed.1c00604Copyright © 2021 American Chemical Society and Division of Chemical Education, Inc.RIGHTS & PERMISSIONSArticle Views436Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (508 KB) Get e-Alerts Get e-Alerts

The study of nucleation site interactions on the mixed wettability rough surface

In order to achieve the best heat transfer performance on a mixed wettability surface, engineering of the wettability pattern is often used as heat transfer performance is significantly dependent on pitch distance between hydrophobic spots, which is maybe linked to the nucleation site interaction effects. In this work, the effects of nucleation site interactions between two hydrophobic cavities on a hydrophilic surface (a mixed wettability rough surface) are investigated, using a two-dimensional pseudopotential phase-change lattice Boltzmann method. The results of the mixed wettability rough surface are compared with completely hydrophilic rough surface and mixed wettability smooth surface to understand the effects of wettability and surface roughness. The focus is placed on the hydrodynamic interaction (which has promotive effect on the bubble formation activity) and thermal interaction (which has inhibitive effect). There exists an optimum pitch distance at which the heat flux and heat transfer coefficient are highest for mixed wettability surface, which are due to larger intensity of hydrodynamic interaction and nearly zero intensity of thermal interaction. Moreover, heat flux is found to be larger for the mixed wettability rough surface than the mixed wettability smooth surface. The bubble nucleation mechanism and time are found to be different for all surfaces.

Ethanol as a Probe for the Mechanism of Bubble Nucleation in the Diet Coke and Mentos Experiment.

The Diet Coke and Mentos experiment involves dropping Mentos candies into carbonated beverages to produce a fountain. This simple experiment has enjoyed popularity with science teachers and the general public. Studies of the physicochemical processes involved in the generation of the fountain have been largely informed by the physics of bubble nucleation. Herein, we probe the effect of ethanol addition on the Diet Coke and Mentos experiment to explore the impact that beverage surface tension and viscosity have on the heights of fountains achieved. Our results indicate that current descriptions of the effects of surface tension and viscosity are not completely understood. We also extend and apply a previously reported, simplified version of Brunauer–Emmett–Teller theory to investigate kinetic and mechanistic aspects of bubble nucleation on the surface of Mentos candies in carbonated beverages. A combination of this new theory and experiment allows for the estimation that the nucleation sites on the Mentos candy that catalyze degassing are 1–3 μm in size, and that between 50,000 and 300,000 of these sites actively nucleate bubbles on a single Mentos candy. While the methods employed are not highly sophisticated, they have potential to stimulate fresh investigations and insights into bubble nucleation in carbonated beverages.

Bubble nucleation in core-shell structured molten oxide-based membranes with combined diffusion-bubbling oxygen mass transfer: experiment and theory.

Oxygen-selective membranes are likely to play a leading part in the future separation processes relevant to energy engineering. A newly developed molten copper and vanadium oxide-based diffusion-bubbling membrane with core-shell structure and fast combined oxygen mass transfer is a promising candidate for efficient oxygen separation. In this work, the oxygen bubble nucleation and transport properties of the diffusion-bubbling membrane were experimentally and theoretically studied. Bubble size distribution and cumulative oxygen flux have been plotted as functions of oxygen partial pressure. The relationship between the bubble density, oxygen partial pressure, and oxygen permeation flux was established. The oxygen flux and bubble density vary in the ranges of 3.2 × 10-8-1.4 × 10-7 mol cm-2 s-1 and 1.3 × 1013-5.8 × 1013 m-3 at ΔPO2 = 0.1-0.75 atm, respectively. The mechanisms of homogeneous, heterogeneous, pseudo-classical and non-classical nucleation are reviewed within the framework of the Cahn-Hilliard model. It is shown that the homogeneous nucleation mechanism is most likely in the membrane core. The estimated values of the interfacial tension, energy barrier, and rate nucleation are 0.02 J m-2, 5 kT, and 4 × 1029 m-3 s-1, respectively.

Transport-Based Modeling of Bubble Nucleation on Gas Evolving Electrodes.

Bubble nucleation is ubiquitous in gas evolving reactions that are instrumental for a variety of electrochemical systems. Fundamental understanding of the nucleation process, which is critical to system optimization, remains limited as prior works generally focused on the thermodynamics and have not considered the coupling between surface geometries and different forms of transport in the electrolytes. Here, we establish a comprehensive transport-based model framework to identify the underlying mechanism for bubble nucleation on gas evolving electrodes. We account for the complex effects on the electrical field, ion migration, ion diffusion, and gas diffusion arising from surface heterogeneities and gas pockets initiated from surface crevices. As a result, we show that neglecting these effects leads to significant underprediction of the energy needed for nucleation. Our model provides a non-monotonic relationship between the surface cavity size and the overpotential required for nucleation, which is physically more consistent than the monotonic relationship suggested by a traditional thermodynamics-based model. We also identify the significance of the gas diffuse layer thickness, a parameter controlled by external flow fields and overall electrode geometries, which has been largely overlooked in previous models. Our model framework offers guidelines for practical electrochemical systems whereby, without changing the surface chemistry, nucleation on electrodes can be tuned by engineering the cavity size and the gas diffuse layer thickness.

Cavitation Inception from Transverse Waves in a Thin Liquid Gap

It is well known that dielectric breakdown in a liquid generates cavitation bubbles and shock waves. Here we demonstrate that when the liquid is bounded by two solid glass boundaries (10--20-$\ensuremath{\mu}\mathrm{m}$ separation), rings of microscopic bubbles can be nucleated around the laser-induced cavitation bubble. While generally acoustic nucleation is achieved with longitudinal waves of sufficient tension, this work demonstrates that acoustic cavitation can also be generated from transverse waves. Our experiments identify three waves originating at the boundaries: the fastest is the bulk wave in the solid, followed by a leaky Rayleigh wave at the liquid-solid contact, which is trailed by a Lamb-type wave. For the latter, the two solid boundaries act as a wave guide and generate intense and short-lived cavitation activity within the gap. Streak photography and high-speed photography reveal the microsecond-duration cavitation-bubble dynamics, and subpicosecond strobe photography visualizes the mechanism of bubble nucleation from the accelerated surface. Simulations coupling the solid mechanics with the acoustics support the experimentally observed mechanisms of transverse-wave-induced cavitation inception.

Probing the Mechanism of Bubble Nucleation in and the Effect of Atmospheric Pressure on the Candy–Cola Soda Geyser

The so-called Diet Coke and Mentos experiment is initiated by dropping Mentos candies into a bottle of Diet Coke or other carbonated beverage. This causes the beverage to rapidly degas, causing foam to stream out of the bottle. Simple application of the gas laws leads to the straightforward prediction that ejection of greater foam volume is expected at lower atmospheric pressure. This hypothesis is bolstered when principles of bubble physics are taken into account. This hypothesis was tested and confirmed by monitoring the foam produced during the Diet Coke and Mentos experiment at various altitudes above sea level. Upon further application of the aforementioned principles, a relationship between degassing kinetics, beverage CO2 concentration, and the size of pores on the candy surface that serve as nucleation sites can be derived. Using this relationship and experimental measurements of degassing kinetics, students estimated that the nucleation sites on Mentos candies are on the order of 2–7 μm in dimension. Students in Physical Chemistry, General Chemistry, and nonmajors’ courses have found these experiments to be of great interest.

Regulating the helium bubble nucleation in the titanium tritides by environment temperature during the early aging period

The nucleation of helium bubbles in the early aging stage of tritium storage materials will have an important influence on the bubble growth behavior in the whole of aging. Herein, the nucleation of helium bubbles was adjusted by environment temperature at the first 15 days immediately after preparation of polycrystalline titanium tritide films. The influence of the environment temperature on the nucleation of helium bubbles were studied by X-ray diffraction (XRD), positron annihilation spectroscopy (PAS), thermal helium desorption spectroscopy (THDS) and scanning transmission electron microscopy (STEM). Results demonstrate that the helium bubble nucleation is significantly affected by the environment temperature of −60 °C, 20 °C and 120 °C. The higher the environment temperature, the lower number density of helium bubble in the titanium tritide films. The helium bubble nucleation mechanism is strongly correlated to the helium atom diffusion coefficient that is controlled by the environment temperature. During the following aging, the rate of helium atoms captured by a single helium bubble increases in titanium tritides with low number density bubbles, which accelerates the growth of helium bubbles in the matrix. However, the low number density bubbles also decrease the capture probability of free helium atoms by helium bubbles, resulting in the increase of helium atoms diffusing to infinite size defects such as grain boundaries macroscopically, and accelerating the evolution of helium in titanium tritides.

Analysis of Bubble Nucleation Mechanisms in Supersaturated Solutions: A Macroscopic Perspective

In gas evolving systems, such as water electrolysis, the surface reactions on the electrode produces gas which is dissolved in the liquid surrounding the electrode. The continuous surface reaction leads to supersaturation of the liquid. In order to recover thermodynamic equilibrium, a fraction of the dissolved gas phase separates to form a bubble, a phenomena denoted as nucleation. Apart from electrochemical systems, this process is also observed during opening of a carbonated beverage, boiling and cavitation. In a seminal paper by Jones et al., four types of nucleation mechanisms in supersaturated systems are described [1]. Type 1 or homogeneous nucleation occurs in the liquid bulk at high levels of supersaturation, while type 2, or heterogeneous nucleation, happens at surface imperfections like pits and cavities at slightly lower levels of supersaturation compared to Type 1. Type 3, or pseudo-classical nucleation, utilizes pre-existing gas cavities that have radius smaller than the critical radius predicted by the classical theory ( rc =−2 γ /ΔGv , where γ is surface tension between the liquid and gas phase and ΔGv is the Gibbs free energy required to form a gas bubble). This mechanism still requires to overcome an energy barrier to form the interface, albeit lower than types 1 and 2. Type 4, or non-classical nucleation, occurs at pre-existing gas cavities whose radius of curvature is larger than the critical radius, effectively reducing the energy barrier to zero. All of the modes of nucleation have been used to explain experimental observations in saturated systems [1-3]. Considering modeling of electrochemical systems, the main proposed approach appears to be based on Type 4 nucleation as the energy barrier for nucleation is seldom considered [4,5]. In the current work, we describe the thermodynamics of the different types of nucleation, aiming to provide a theoretical framework to describe gas evolution on electrodes. The framework to explain nucleation is developed using a Helmholtz energy description of the gas-liquid solution, the interface and bubble. The interface is assumed to be diffused across a thickness and the associated free energy is calculated based on gradient theory [6]. The scenarios corresponding to Type 1, 2 ,3 and 4 nucleation are analyzed such that the change in free energy of the system is analyzed based on the difference in free energies before and after nucleation. The main assumptions used in the analysis are that the spatial distribution of molecules in both solution and gas bubble is always uniform (no gradient in concentration and other properties). Secondly, the number of molecules that gets converted into gaseous state is the same in all scenarios. Finally, the addition of a gaseous molecules into the pre-existing bubble does not alter its volume or morphology (for Type 4 nucleation). The bubble’s behavior (nucleation, growth and detachment) on the electrode effects its current distribution and exposure of the surface to electrolyte which are critical in electrochemical processes like electrowinning, chlor-alkali process, etc. The bubble behavior has also been shown to adversely effect the electrode by removing the catalyst coatings [7]. Understanding the nucleation process provides insight into the plausible sites on electrode where the bubbles subsequently growand detach, which is crucial in designing durable and efficient electrodes. This paper provides a thermodynamic and physical reasoning to explain the different nucleation energy requirement in each mechanism, corresponding to those predicted by Jones et al. [1] under certain reasonable approximations.

Molecular dynamics study of bubble nucleation on a nanoscale

With the rapid development of nanotechnology, nucleate boiling has been widely applied to the thermal management of nanoelectronics, owing to its highly-efficient heat transfer characteristics. Considering the scale effects, such as temperature jump at solid-liquid interface, a further study of nucleation boiling mechanism at a microscopic level is needed. At present, extensive studies have been carried out for providing a significant insight into the formation of nano-bubbles in a nanoscale thermal system, but the effect of heat transfer efficiency affected by the surface wettability on bubble nucleation over solid substrate is rarely available in the literature. Therefore, in this paper, the effect of surface wettability on the initial nucleation process and growth rate of bubbles are investigated and the mechanism of bubble nucleation on a nanoscale is analyzed, by the molecular dynamics simulation. The modified Lennard-Jones potential is used for investigating the solid-liquid interaction. Changing the potential parameters <i>α</i> and <i>β</i> can obtain different surface wettability. The atomic sites, liquid density profiles and bubble nucleus volumes are computed to compare the processes of bubble nucleation on different surfaces. The variation of liquid temperature, potential and absorbed heat flux with heating time are evaluated to explore the mechanism of bubble nucleation. The simulation results show that the surface wettability influences the bubble nucleation and heat transfer at liquid-solid interface significantly. On the one hand, the bubble nucleation is promoted by properly increasing the liquid-solid interaction, which is distinctly different from the existing classical theory related to nano-bubble preferably formed on a hydrophobic surface. This is because the thermal resistance of the solid-liquid interface on a nanoscale cannot be neglected. The interface thermal resistance will decrease with the increase of wettability. Therefore, the heat transfer efficiency is higher for a stronger liquid-solid interaction so that the liquid over the hot wall obtains more energy to make bubble nucleus generated earlier. On the other hand, the surface wettability also influences the bubble growth rate. The stronger the liquid-solid interaction, the faster the bubble grows. When the volume of bubble reaches a certain value, a vapor film is formed on the substrate, leading to film boiling. Furthermore, it also illustrates that initial heat flux increases with time. In this stage, the heat flux curve shows two kinds of slopes, corresponding to the occurrence of evaporation and bubble nucleation, respectively. Then, after a certain time, the heat flux profile presents a declining trend, indicating a change into film boiling.

Study of the bubble nucleation and growth mechanisms in high-pressure foam injection molding through in-situ visualization

Although major efforts have been made to uncover the complex mechanisms of bubble nucleation and dynamics in foam injection molding, most theories and interpretations still require additional experimental verification. An innovative visualization mold was employed with which the mechanisms of bubble nucleation and growth in high-pressure foam injection molding were investigated. For the first time, the development of the cell structure in high-pressure foam injection molding was comprehensively explored and experimentally verified via in-situ visualization using polystyrene and supercritical carbon dioxide system. Two bubble-nucleation mechanisms were observed, namely, the nucleation due to a pressure drop at the gate during filling, and the nucleation due to melt shrinkage in the cavity after filling. It was the cavity pressure that determined which bubble-nucleation mechanism was dominated between these two in high-pressure foam injection molding. The final cellular structure and morphology of the foamed parts were, in turn, determined by the governing bubble-nucleation mechanism. It was concluded that a melt packing pressure is required to re-dissolve the bubbles nucleated at the gate back into the melt, and that a larger packing pressure was needed with a higher cell density by increasing the injection speed, the gate resistance, or the blowing-agent content. Because of the high resistance in the narrow cavity, the bubbles within the cavity were not pressurized uniformly, when the packing pressure was applied. A higher cell density increased the compressibility of the two-phase gas–melt mixture, aggravating the non-uniformity of the pressure in the cavity. Consequently, the bubbles far from the gate could not be pressurized immediately, and thereby could not dissolve into the polymer melt quickly. Additionally, the lower temperature of the melt far from the gate further delayed the permeation of gas into the polymer.

Mechanism of bubble nucleation in poly(ε-caprolactone) foaming at low temperature

CO2 bubble nucleation on the microrough poly(ε-caprolactone) surface has been investigated using a density functional theory approach. For the considered system, the effect of chemical and morphologic characteristics of poly(ε-caprolactone) on the solubility, surface tension, interfacial density distribution, nucleation free-energy barrier, and critical nucleus radius have been calculated to interpret the nucleation mechanism. All calculations are performed with the universal force field parameters. Some results have been examined by the available experimental data. According to the free-energy barrier and critical radius, the nucleation number with respect to pressure has been estimated under the instant depressurization. As a result, this work provides a theoretical explanation of bubble formation mechanism of polymer foaming at low temperature.