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Bubble growth inside the die is one of the critical issues in the PVC foam extrusion process. Burning and clogging might have occurred when bubble growth exceeds the critical size, leading to non-uniform outlet velocity and pressure profile inside the die. Therefore, a design modification for die geometry is necessary. This study aims to suppress the bubble generation by making the pressure inside the die greater than the bubble nucleation pressure and to make the velocity distribution uniform at the die exit. A new concept of adding choker bars inside different dies (namely, T-die, fishtail die, and coat-hanger die) is proposed. Three choker bars consecutively placed one after another inside the dies slit sections were numerically simulated to observe and discuss the effects of the proposed geometric shape on the outlet velocity profile in the non-isothermal generalized Newtonian flow. The results show that the velocity distributions at the exit of the die were nearly uniform in all three dies. Among the dies, the fishtail die shows the best outlet velocity uniformity. The pressure values inside all dies are found to be higher than the nucleation pressure except at the last 0.5 mm near the die outlet. This confirmed that the bubble growth could only begin near the 0.5 mm die outlet region in all dies.

Understanding liquid motion in nanoenvironment is of fundamental importance in nanofluidics-based systems. While the liquid outflow from hydrophobic nanochannels can significantly affect system performance, its underlying mechanism remains unclear so far. Here, we present an experimental study of the gas-phase effect on liquid outflow behavior from hydrophobic nanochannels in a liquid nanofoam (LN) system. Four LN samples, consisting of same liquid-solid composition but different amounts of the gas phase, are characterized by cyclic quasi-static compression tests. A remarkable difference in the LN system reusability has been observed, indicating that the liquid outflow behavior is highly sensitive to the amount of the gas phase. As the gas amount increases, the degree of liquid outflow from hydrophobic nanochannels is considerably promoted. This promotive effect is because of the suppression of gas outflow and acceleration of bubble nucleation in the nanochannels. These fundamental findings open a new perspective on liquid outflow behavior and can facilitate the design of reusable nanofluidics-based energy absorbers.

Thermo-optically generated bubbles in water provide a powerful means for active matter control in microfluidic environments. These bubbles are often formed via continuous-wave illumination of an absorbing medium resulting in bubble nucleation via vaporization of water and subsequent bubble growth from the inward diffusion of gas molecules. However, to date, such bubbles tend to be several microns in diameter, resulting in slow dissipation. This limits the dynamic rate, spatial precision, and throughput of operation in any application. Here we show that isolated plasmonic structures can be utilized as highly localized heating elements to generate thermoplasmonic nanobubbles that can be modulated at frequencies up to several kilohertz in water, orders of magnitude faster than previously demonstrated for microbubbles. The nanobubbles are envisioned as advantageous localized active manipulation elements for high throughput microfluidic applications.

The bubble nucleation of liquid argon over patterned surfaces with different wettabilities is investigated by using molecular dynamics (MD) simulations. In order to study effects of different wall temperatures on bubble nucleation process, bubble nucleation positions and bubble dynamics behaviors are observed. Simulation results show that at low wall temperatures bubble nucleation tends to occur over the hydrophobic part of the wall. With increasing wall temperature, the bubble nucleus gradually changes its position from the hydrophobic to hydrophilic part. When the wall temperature increases to a certain value, bubble nuclei first appear on both the hydrophobic and hydrophilic parts. By analyzing heat flux and change trend of argon state points, differences in bubble nucleation process at different wall temperatures are explained. Moreover, the effect of area fraction of hydrophobic part on bubble nucleation temperature is investigated. It is found that there exists an optimal area fraction which makes the bubble nucleation temperature the lowest.

During the impact of a liquid droplet on a sufficiently heated surface, bubble nucleation reduces the contact area between the liquid and the solid surface. Using high-speed imaging combined with total internal reflection, we measure and report how the contact area decreases with time for a wide range of surface temperatures and impact velocities. We also reveal how formation of the observed fingering patterns contributes to a substantial increase in the total length of the contact line surrounding the contact area. Ministry of Health (MOH) Nanyang Technological University This work was supported by the Nanyang Technological University (NTU) and Ministry of Education (MOE, grant number MOE2018-T2-2-113), Singapore.

An in-depth understanding of the formation of intergranular helium bubbles and its relation to embrittlement is an important issue in the nuclear industry. In this paper, a thermodynamic model is d...

Surface nanobubbles have been presumed to lead to the experimental observation that liquid boiling often occurs at a much lower supersaturation than expected, yet no qualitative theory exists to explain how they participate in the process. Here, we report through a simple theoretical analysis on how the metastable nanobubbles nucleate the liquid-to-vapor transition by serving as an intermediate phase. The appearance of metastable nanobubbles inhibits the shrink of the bubble nucleus and changes bubble nucleation into a multistep process. We show three possible mechanisms for heterogeneous nucleation starting from metastable surface nanobubbles: nucleation from pinned nanobubbles, nucleation via nanobubble depinning, and nucleation through nanobubble coalescence, each predicting a significant reduction in a nucleation barrier. The occurrence of a specific nucleation pathway of bubble nucleation depends on the detailed geometry of local substrate roughness. These results give insight into how the appearance of surface nanobubbles changes the nucleation mechanisms of liquid boiling.

Cavitation is widespread, but there has been a large difference between experiments and theories about cavitation. In this manuscript, the effects of oxygen molecules on the bubble nucleation of di...

In-situ hydrodynamic behavior of wicked liquid comes from interfacial dynamics at triple contact line, resulting in receding motion around expanding dry spot. We here introduce a new and creative technique of wicking experiment adopting an external pressure source equivalent to bubble nucleation pressure in order to investigate the receding behavior of wicked liquid. On the various types of surface morphology including smooth, nanostructure, nanoporous, and microstructure, it was clearly observed that wicked liquid receded from expanding dry area except for a smooth surface. The receding velocity was slower at microstructure, nanoporous, and nanostructure, in order. Clearly this result provides a hydrodynamic evidence of smaller dry area size and contact line length on microscale structure than on nanoscale structure. Moreover, the diameter of dry area showed a linear relation with CHF enhancement that indicates smaller diameter of dry area is more effective to delay irreversible expansion of dry spots. This novel observation is expected to provide reliable analysis of contact line dynamics with CHF enhancement on wicking-dominant surfaces.

The pore shape affected by initial contact angle of a bubble entrapped by a solidification front is predicted. Initial contact angle affects bubble nucleation, surface area and solute transport across the bubble cap, solid gas pressure in the pore and shape of pores in solid. Porosity influences microstructure and functional properties of materials, etc. This work accounts for solute transport across the coupled shape of cap in balance of pressures and physico-chemical equilibrium. Similar to previous work, it shows that pore shape depends on directions and magnitudes of solute across the cap. In contrast to Case 2, solute transport in Case 1 is from the pore into surrounding liquid in the early stage. The results find that a decrease in initial contact angle expedites bubble entrapment in Case 1. Solute gas pressure in the pore further exhibits three stages for initial contact angle greater than 90°. Significant drops occur in the early and late stages. In the middle stage, where contact angle is near 90°, solute concentration at the cap is about initial solute concentration in liquid, and responsible for length of the pore. Except for initial contact angle less than 90°, an isolated pore cannot be formed in Case 2. The predicted pore shape agrees with experimental data.

Molecular dynamics simulation is conducted to study the effects of surface wettability on rapid boiling and bubble nucleation over smooth surface. The simple L-J liquid is heated by smooth metal su...

Nanobubble nucleation is a problem that affects efficiency in electrocatalytic reactions since those bubbles can block the surface of the catalytic sites. In this article, we focus on the nucleation rate of O2 nanobubbles resulting from the electrooxidation of H2O2 at Pt disk nanoelectrodes. Bubbles form almost instantaneously when a critical peak current, inbp, is applied, but for lower currents, bubble nucleation is a stochastic process in which the nucleation (induction) time, tind, dramatically decreases as the applied current approaches inbp, a consequence of the local supersaturation level, ζ, increasing at high currents. Here, by applying different currents below inbp, nanobubbles take some time to nucleate and block the surface of the Pt electrode at which the reaction occurs, providing a means to measure the stochastic tind. We study in detail the different conditions in which nanobubbles appear, concluding that the electrode surface needs to be preconditioned to achieve reproducible results. We also measure the activation energy for bubble nucleation, Ea, which varies in the range from (6 to 30)kT, and assuming a spherically cap-shaped nanobubble nucleus, we determine the footprint diameter L = 8–15 nm, the contact angle to the electrode surface θ = 135–155°, and the number of O2 molecules contained in the nucleus (50 to 900 molecules).

Density functional analyses are performed to study bubble nucleation around a nanoparticle submerged in a supersaturated LJ/s fluid in this paper. Two heterogeneous nucleation modes, i.e. core-shell mode and spherical cap mode, are considered. The minimal free energy path (MFEP) of these two nucleation modes are obtained. Effects of particle wettability and supersaturation of the bulk liquid on heterogeneous nucleation are studied. It is found that the energy barrier of the spherical cap mode nucleation is lower than that of core-shell mode nucleation. However, the difference between energy barriers of the two heterogeneous nucleation modes approaches zero when the particle becomes strongly solvophobic. The disappearance of nucleation barrier is observed for strongly solvophobic particles at high supersaturation.

MAX phases have been suggested to be applicable in the next generation nuclear reactors for their advantages in thermal/mechanical properties at high temperatures and radiation damage resistance. In the present investigations, the stabilities and formation energies of helium defects in the Zr/Ti 2 AlC interface are studied by first principles calculations. The calculations are performed using four possible interface models, and the trap of helium atoms in interstitial or vacancy sites are taken into consideration. According to the predicted formation energies, a single He atom tends to reside between the Al and Ti planes. The results also imply that Al vacancies are better able to trap He atoms. The stepwise rise for the trapping energies with the increasing number of trapped He atoms are determined in an Al vacancy and the corresponding swelling effect is examined. These investigations may provide new insight into the underlying mechanisms of helium bubble nucleation and formation in the Zr/Ti 2 AlC interface.

Strongly first-order phase transitions, i.e., those with a large order parameter, are characterized by a considerable supercooling and high velocities of phase transition fronts. A very strong phase transition may have important cosmological consequences due to the departures from equilibrium caused in the plasma. In general, there is a limit to the strength, since the metastability of the old phase may prevent the transition to complete. Near this limit, the bubble nucleation rate achieves a maximum and thus departs from the widely assumed behavior in which it grows exponentially with time. We study the dynamics of this kind of phase transitions. We show that in some cases a gaussian approximation for the nucleation rate is more suitable, and in such a case we solve analytically the evolution of the phase transition. We compare the gaussian and exponential approximations with realistic cases and we determine their ranges of validity. We also discuss the implications for cosmic remnants such as gravitational waves., 40 pages, 11 figures. v2: several comments and references added

Developments in many modern applications are encountering rapid escalation in heat dissipation, coupled with a need to decrease the size of thermal management hardware. These developments have spurred unprecedented interest in replacing single-phase hardware with boiling and condensation counterparts. While computational methods have shown tremendous success in modeling single-phase systems, their effectiveness with phase change systems is limited mostly to simple configurations. But, given the complexity of phase change phenomena important to many modern applications, there is an urgent need to greatly enhance the capability of computational tools to tackle such phenomena. This article will review the large pool of published papers on computational simulation of boiling and condensation. In the first part of the article, popular two-phase computational schemes will be discussed and contrasted, which will be followed by discussion of the different methods adopted for implementation of interfacial mass, momentum and energy transfer across the liquid-vapor interface. This article will then review papers addressing computational modeling of bubble nucleation, growth and departure, film boiling, flow boiling, and flow condensation, as well as discuss validation of predictions against experimental data. This review will be concluded with identification of future research needs to improve predictive computational capabilities, as well as crucial phase change phenomena found in modern thermal devices and systems that demand extensive computational modeling.

The phenomenon of spontaneous bubble nucleation in extremely superheated binary solutions has been studied experimentally. The experiments were carried out by the pulse heating of a wire probe. Spontaneous boiling-up temperatures T* for solutions composed of components of different chemical nature have been measured in a wide range of variables, such as pressure p (up to the critical one pc) and composition (x ∼ 0 ÷ 1). For the most common types of the critical curve {Tc(x), pc(x)}, we have revealed the relationship between a change in the coordinates of the critical point and that of the attainable superheat line T*(p; x = const). This result leads to the natural choice of the program of further research including elucidation of the impact of the double metastability factor on the spontaneous boiling-up of solutions with different degree of compatibility of components.

We study the role of surface topology, surface chemistry, and wall superheat temperature on the onset of boiling, bubble nucleation and growth, and the possible formation of an insulating vapour film by means of large-scale MD simulations. In the numerical experiments, we control the system pressure by imposing a constant force on a moving piston. The simulations reveal that the presence of a nanostructure triggers the bubble formation, determines the nucleation site and facilitates the energy transfer from the hot substrate to the water. The surface chemistry, on the other hand, governs the shape of the formed bubble. A hydrophilic surface chemistry accelerates the bubble nucleation, however, decelerates the bubble expansion, thus postponing the formation of the film of vapour. Therefore, a hydrophilic surface provides better energy transfer from the hot wall to the water. By analysing the system energy, we show that irrespective of wall topology and chemistry, there is a wall temperature for which the amount of transferred energy is maximum., Comment: 25 pages, 8 figures, Submitted to International Journal of Heat and Mass Transfer

Shape memory materials (SMMs) recover their original shape as an external stimulus is applied. To meet the growing needs for complicated applications, it is imperative to design advanced SMMs with more functions and better performance. This paper reports a new design of liquid-based SMMs, i.e. liquid nanofoam (LN) system, by controlling the extent of liquid outflow from hydrophobic nanopores. The liquid outflow behavior is dominated by the thermoresponsive sensitive bubble nucleation process in the confined nanoenvironment. As temperature increases, the extent of liquid outflow is promoted, and the system volume is recovered. Thus, the LN system exhibits an instant volume memory behavior. As temperature decreases, the volume memory behavior is reversed. A constitutive model for the shape memory LN material has also been developed and validated. The thermoresponsive nanoconfined liquid outflow opens up new avenues for the design of advanced SMM.

In this study, a domain re-decomposition hybrid atomistic-continuum method is developed to conduct a multiscale/multiphase investigation on the bubble nucleation. In addition to the conventional co...

Nanostructures have significant impacts on incipient nucleation time and onset temperature during nucleate boiling. This study compares three types of surfaces, namely smooth, concave structured, and convex structured to investigate the mechanism of nucleate boiling via molecular dynamics simulations. The results show that nanostructured surfaces enhance bubble nucleation compared to smooth surfaces, provide fixed nucleation sites for initial embryo generation and shorten incipient nucleation time. Furthermore, nucleate boiling occurs more quickly on concave nanostructured surfaces than on convex ones regardless of the nanostructure size. However, with decreasing nanostructure size, the difference between them gradually decreases. A further analysis of the onset temperature is performed under various surface conditions. The onset temperature of nucleate boiling on the large concave nanostructured surface is much lower than on large convex nanostructured surface, owing to the stronger heat accumulation effect of the cavity. However, the onset temperature on the small concave nanostructured surface is higher, because liquid atoms are strongly absorbed into the small cavity and crystallized with the hydrophilic surface. The results improve the understanding of nanoscale nucleate boiling and provide a reference for the design of microelectronic devices.

When carbon dioxide is supersaturated in a liquid, carbon dioxide gas gets nucleated, expands, and floats on the surface of the liquid. This is a well-known phenomenon and is generally observed in carbonated drinks. This bubble generation phenomenon can be activated or suppressed by changing the properties of the solid surface in contact with the carbonated liquid. In this study, a method of exponentially increasing or suppressing the bubble generation of carbonated liquids by modifying the surface wettability is proposed. Equal amounts of carbonated liquid were poured into bare, superhydrophilic, and superhydrophobic cups to compare the amount of overflow and generated gas. In the superhydrophobic cup, bubbles were generated only at the start of pouring the carbonated liquid, after which no more bubbles were generated. When the same amount of liquid was poured into the bare cup, about 4.1% of the total mass overflowed, while in the case of superhydrophilic surfaces, about 34% overflowed. The generated gas from each cup also showed significant difference according to the surface properties. From the experimental results, it was concluded that the superhydrophobic surface can suppress bubble nucleation, thus, preventing the soda from overflowing. Furthermore, a fall in carbon dioxide concentration can be prevented.

A Helmholtz free energy description of the four nucleation mechanisms used to explain the bubble nucleation in electrochemical systems is presented. The mechanisms are compared based on the nucleation energy barrier and critical nuclei radius. The theoretical analysis sheds light on the effect of parameters like contact angle on the electrode surface and pre-existing gas bubbles on nucleation energy barrier. A free energy based description of surface tension (planar interface) is also obtained from the thermodynamic framework. © The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

The impact of droplets on a heated powder bed involves a wide range of phenomena with increasing complexity from spreading of liquid, to bubble nucleation, to more complex ones such as splashing, crater formation, and fluidization. In this study, we focus on the impact behaviors caused by elevation in temperature of the powder bed. We experimentally characterize the impact behaviors for wide ranges of the impact velocity and surface temperature of the powder bed. We reveal several phenomena specific to the impact on a heated powder bed such as rebound, ruptures of expanding liquid films, bubble nucleation, and fluidization. We also show that the maximum spreading diameter increases with the surface temperature of the powder bed and propose an empirical scaling law to describe such a relation.

The method of high temperature monitoring and image analysis was applied to determine the temperature at which the bubbles were nucleated on a platinum wire immersed in the melt of a float type of glass. The bubbles that rose during a slow increase of temperature were identified, monitored and subsequently, their diameter was measured and the obtained dependence between the bubble diameter and time was extrapolated to the zero size of the relevant bubbles. The bubble nucleation temperature has been determined at the temperature of bubble zero size. The obtained average value of the bubble nucleation temperature in glass with 0.185 wt.% of chemically dissolved SO3 was 1502.6 °C whereas the same quantity showed the value in glass with 0.250 wt.% of chemically dissolved SO3 to be 1501.4 °C. The intensity of bubble nucleation in the form of the number of nucleated bubbles and the volume of released gas versus time were also measured in both glass samples and described by a theoretical equation. The relation between the diffusion and thermodynamic barriers of the nucleation was determined and discussed. The results were also discussed with respect to the literature data relating to the bubble nucleation and the fining process.

Effects of nanostructured defects of a copper solid surface on bubble growth in liquid argon have been investigated through a hybrid atomistic-continuum (HAC) method. The same solid surfaces with five different nanostructures, namely, wedge defect, deep rectangular defect (R-I), shallow rectangular defect (R-II), small rectangular defect (R-III), and no defect were modeled at the molecular level. Liquid argon was placed on top of hot solid copper with a superheat of 30 K after equilibration was achieved with computational fluid dynamics–molecular dynamic (CFD–MD) coupled simulation. Phase change of argon on five nanostructures has been observed and analyzed accordingly. The results showed that the solid surface with wedge defect tends to induce a nanobubble more easily than the others, and the larger the size of the defect, the easier it is for the bubble to generate.

The bubble nucleation, growth, and departure cycle is a fundamental aspect of nucleate pool boiling. While much research on this subject has been performed in the previous century, new correlations and models have not been developed in light of important results in simulations and experiments in recent decades. In this work, we provide an updated understanding of nucleation, growth, and departure with analytical models that are validated by recent work. We found that nucleation at incipience is correlated to departure size, suggesting that the momentum-induced bubble snap-off at departure may be responsible for the size of trapped vapors in cavities. We also developed a bubble growth model that takes into account a thermal boundary layer whereas previous works typically considered a uniform superheating. In addition, we developed a bubble departure description, wherein the velocity boundary layer is responsible for closing the base of the bubble. Finally, we provide a methodology to calculate bubble growth and departure for contact angle changing with time, which is relevant in studying surfactant-enhanced boiling.

This paper introduces a passive degassing mechanism using textured surfaces to trap and transport bubbles, and then using hydrophobic porous membranes to vent out bubbles in a microfluidic system. The bubble trapping ability is achieved by creating nanostructures to promote bubble nucleation and coalescence on the sidewalls of KOH-etched concave pits in a silicon substrate. The substrate, which is bonded with a porous membrane, is placed in a liquid system with chemically generated CO2 bubbles to examine the degassing ability. The results validate that the bubbles can be easily trapped on the surfaces with nanostructures, and then vented through the porous membrane. Our proposed approach possesses the advantage of simple fabrication, great structure robustness, and effective bubble trapping and removing abilities, which show their great potential as economic, passive means of preventing the gas byproducts from blocking surfaces and improving the efficiency of microfluidic systems during operations.

Rapid heating or sudden depressurization can create highly superheated liquid conditions in stationary or flowing liquid in nanochannels. In these situations, the phase stability of the liquid and the conditions for the onset of bubble nucleation are important factors in the fluid behavior and heat transfer in applications. In the investigation summarized here, a statistical thermodynamics analysis is used to derive a modified version of the Redlich-Kwong fluid property model that accounts for attractive forces between the solid wall surface atoms and liquid molecules in the fluid within a high aspect ratio (parallel plate) nanochannel. In this model, the wall–fluid attractive forces are quantified in terms of Hamaker constants, which makes it possible to assess the effect of wall–fluid force interactions on the spinodal conditions for a variety of fluid and surface material combinations. At large channel widths the fluid properties exhibit wall effects very near each wall, with bulk fluid property values...

The laser interferometric method and high-speed camera techniques are used to study dynamic characteristics of the microlayer beneath an ethanol vapor bubble during nucleation process. Clear fringe patterns of the microlayer are captured showing the movement of the gas–liquid–solid triple contact line, the dynamic changes of the micro-contact angle, bubble root radius, and volume of the microlayer during the bubble nucleation process before and after dried out have occurred.

Applications of acoustic cavitation [ frequently suffer from certain random aspects (e.g., stochastic bubble nucleation events) as well as from its sensitivity to external parameters (like gas content in the liquid). This renders for example a prediction of bubble distributions in size and space still a difficult task. To improve this situation by a better understanding of the fundamentals, a "bottom-up" approach has recently been followed which tried to model collective bubble phenomena and bubble structures on the basis of single bubbles and their interaction [. If the behavior of individual bubbles can be well captured by the models, it is hoped to gain significant insight into a larger system of acoustically driven bubbles. Indeed, several aspects of multi-bubble systems and structures could be explained by single bubble dynamics, for instance by the inversion of the primary Bjerknes force in strong ultrasonic fields. Nevertheless, many details of bubble dynamics stay partly unclear, and considerable efforts are undertaken to improve our understanding and to optimize applications of acoustic bubbles.

The mechanisms and processes of bubble nucleation and growth are still not completely solved and research in this field is ongoing. This is an important issue for materials used in fission and fusion reactors. In such environments, one of the main gaseous by-products is helium, whose presence and further production is known to decrease ductility, fatigue life and weldability, induce creep and stress rupture properties of materials, as well as promote swelling. These effects lead to the drastic modification of the material's mechanical properties. In the past, experiments have been used to offer clues into the structure of the materials; now, modelling offers the possibility to understand the structure of the material and from this information, to elucidate a fundamental understanding of material properties. This review discusses the modelling paradigms used to investigate and obtain an understanding of the mechanisms at play in helium bubble nucleation and growth in ferritic steels.

This paper presented the characteristics of puffing of carbonated emulsified fuel. Emulsified fuel saturated with CO2 under P CO 2 = 92 kPa and Te = 288 K and the degassed emulsified fuel were used in the experiment. An emulsified fuel droplet suspended from a thermocouple was inserted into an electric furnace. Puffing behavior was observed by using a high-speed video camera. The waiting time and the superheat temperature for the occurrence of puffing were measured. The mean waiting time of the emulsified fuel saturated with CO2 decreased by about 25% compared with that of the degassed emulsified fuel. When the degassed emulsified fuel was used, there was a peak of puffing occurrence at the superheat temperature of 60 K. When the emulsified fuel saturated with CO2 was used, there was a peak of puffing occurrence at the superheat temperature of 45 K. The superheat temperature at the peak decreased by 15 K. Puffing behavior of the carbonated emulsified fuel was more violent than that of the degassed emulsified fuel. These were because the dissolved CO2 caused a reduction in the bubble nucleation energy. Moreover, a new equation which described the waiting time for puffing was proposed. This equation was based on the assumption that puffing occurred when the inner droplet temperature increased linearly. The calculated results of the waiting time were in good agreement with the experimental ones when puffing occurred near the initial linear region.

Polymer foams created using supercritical carbon dioxide as a processing solvent have garnered much interest in recent years. The properties of supercritical CO 2 make it ideally suited to replace organic solvents such as chlorofluorocarbons that are being phased out for environmental reasons. A number of processes and products are already being used in industry, and we provide several examples of these developments. Yet, despite all that is now known about polymer/CO 2 systems, a complete understanding of the foaming process, particularly nucleation, is lacking. In this article, we give an overview of the state of the art in polymer foaming with CO 2 and highlight the challenges that remain to be overcome.

The effects of irradiation temperature on the formation of He–vacancy clusters by displacement cascades in α-Fe are investigated by molecular dynamics (MD) methods. The irradiation temperatures of 100 and 600 K are considered for primary knock-on atom (PKA) energy, E p , from 500 eV to 20 keV. The concentration of He in Fe varies from 1 to 5 at.%. We find that the number of Frenkel pairs ( N F ) at 600 K is slightly lower than that at 100 K for the same He concentration and E p , but the number of He–vacancy clusters increases with increasing temperature for the same He concentration and energy recoils. However, the mean size of He–vacancy clusters is independent on temperature. The mechanisms of He bubble nucleation in displacement cascades at different temperatures are discussed in detail.

The fundamental stabilization mechanism for foaming metal powder compacts (PM foams) is investigated. It is shown that PM foams belong to the class of stable foams where the stability is controlled by the balance of interfacial forces which equilibrate after drainage has been completed. The origin of these interfacial forces is traced back to the oxide content of the underlying metal powder. The oxides are not present as compact particles but form crumpled, voluminous aggregates, the oxide network particles, which form a tenuous global network. The effective volume fraction of these network particles is about 30–35%. Analogous to a gel–sol transition, bubble nucleation leads to a fragmentation of the global oxide network structure. The isolated oxide network fragments have particle character and act as mechanical barriers within the foam structure where they induce a strong stabilizing disjoining pressure.

A unique method previously proposed by the authors was applied to the heat transfer augmentation in the flow boiling field. In this method a screen sheet was placed on the horizontal heated surface where bubble nucleation occurred. Generated vapor bubbles were trapped between the screen and the wall, became flat, and moved along the surface. This restricted bubble behavior caused the heat transfer enhancement. Three types of screen sheet were tested in the present experiment and the effect of the screen on the heat transfer and two-phase flow characteristics was investigated. In two of these cases, the screen was displaced upward by the bubble nucleation. Compared with the ordinary flow boiling case, heat transfer was enhanced by a factor of 1.2 to 6 within the present experimental range. Using a simple flow model, it was made clear that the effect of the height of the displaced screen was important in evaluating the increase in pressure drop. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 319–329, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10094

Bubble nucleation from supersaturated gas solutions generally takes place at much lower supersaturations than are expected from the theory. Furthermore, the same theory predicts that the threshold concentration of gas needed to cause nucleation should be essentially independent of the gas species used, a finding contradicted by experiment. There are two general explanations: first, that the theory is wrong, or second, that there is a previously unidentified factor which is influencing the results of the experiments. Given the success of the fundamental theory in other areas, the second explanation is preferred. The previously unrecognized factor is identified here as being the surface activity of the gases which form the bubbles.

Microscale boiling displays some quite unusual characteristics, like the occurrence of the so-called “fictitious boiling”. This paper conducted the thermodynamic and dynamic analyses regarding the phase change and bubble nucleation in microchannels. The role of perturbations on the dynamics of bubble embryos was investigated, whence the possible causes corresponding to the depression of bubble nucleation in microscale boiling were proposed through stochastic analysis. The external perturbations produced by pressure impulses accompanied with embryo growth and collapse could markedly alter the dynamic characteristics of bubble growth.

The microexplosion occurrence in droplet combustion of miscible binary-fuel droplets has been studied experimentally and theoretically. In experiments, the burning droplet was injected upward, and the induction time for microexplosion occurrence was obtained. Results show that microexplosion induction time is distributed over the quasi-steady vaporization period. The maximum frequency of microexplosion induction time and the occurrence probability of microexplosion increase with the initial droplet diameter. The microexplosion occurrence was modeled considering homogeneous bubble nucleation rate. The present stochastic model well demonstrated the experimental results. The theory shows that these stochastic characteristics of microexplosion occurrence depend on the fifth power of the initial droplet diameter. The bubble nucleation leading to microexplosion depends on the time scale and the superheated liquid volume. The time scale in droplet combustion is proportional to the second power of the initial droplet diameter, and the volume of superheated liquid is proportional to the third power of the initial droplet diameter.

Bubble nucleation could hardly be noted in microscale boiling systems even under high heat flux, denoting as the fictitious boiling phenomenon. We investigated in this work the role of perturbations on the dynamics of clusters, whence developing the physical interpretation of the fictitious boiling commonly observed in microscale boiling. Both the effects of internal and external perturbations were examined. The external perturbations could markedly depress the development of clusters, thereby correlating with the bubble-extinction characteristic noted in fictitious boiling. A pressure fluctuation model was proposed to provide a criterion to the occurrence of fictitious boiling.

This article reports the first systematic study on cavitation and bubble nucleation using the molecular dynamics simulation method. It successfully simulates the hysteretic process of bubble collapse and nucleation as a numerical counterpart of the Berthelot tube cavitation experiment. For a unary molecule system, a stable bubble regime and minimum equimolar dividing radii of bubbles are obtained with respect to computational domain sizes. For a binary molecule system, the addition of foreign molecules to the solvent molecules stimulates the nucleation more effectively in comparison to that in the unary system. The affinity between the solute and the solvent molecules controls the inception of nucleation and results in different nucleation characteristics according to its value. For an attraction coefficient greater than unity, the solute molecules spread uniformly and attract the solvent molecules, which induces bubble nucleation readily. For the coefficient less than unity, the solvent molecules segrega...

We have been requested to prepare a rather unusual article for this special publication. The article should be not only autobiographical in nature but should also have historical perspectives and critique and should contain some element of visionary forecast. Undoubtedly, the idea is unique and challenging though may prove to be less than easy to realise! In response to this challenge, I will at least attempt to present a kind of cohesive description of the evolution of my research activities in the field of defect production, diffusion, interactions and accumulation. These activities have involved widely different irradiation conditions, extending from 1 to 225 MeV electrons, fission to fusion neutrons and 3–800 MeV protons. Over the years, I have also been involved in a variety of theoretical activities dealing with different aspects of defect production, diffusion, void/bubble nucleation, kinetics of damage accumulation and irradiation hardening and embrittlement. An overview of these activiti...

This paper studies the phenomenon of spontaneous boiling-up of polymeric liquids on heating at rates \((\dot T)\) up to 107 K·s−1. A model of the thermal equation of state for (polymer + monomer) systems and a procedure for determination of the spontaneous boiling-up temperature T* for polymeric liquids, taking decomposition into account, are proposed. The experimental data on T*\((\dot T)\) for a number of polymer melts are compared with results calculated from the model.

The flow and bubbles motion in BaO-P2O5 or BaO-P2O5-Sb2O3 glass melts in a high magnetic field up to 2.0T was observed. No apparent change of flow characteristics in BaO-P2O5 glass melts was found. However, the frequency of bubble nucleation evidently decreased with the field strength. At a field strength of 2.0T, almost no nucleation of bubbles was observed. The majority of the bubbles were annihilated at the surface of the melt. This effect may apply to many optical quality glasses to be obtained bubble-free.

Nishioka and Kusaka [J. Chem. Phys. 96 (1992) 5370 (1992)] showed that the commonly used formula, W = n({\mu}^{\alpha}-{\mu}^{\beta}) + {\gamma}A, for work of formation of critical nucleus is derived by integrating the isothermal Gibbs-Duhem relation for the incompressible nucleating phase, such as an incompressible liquid phase nucleation in a vapor phase. In their paper as wel as in a subsequent paper [Li, Nishioka, and Holcomb, J. Cryst. Growth 171 (1997) 259] it was stated that the commonly used formula was valid for an incompressible nucleating phase and no longer held for such as a bubble nucleation. In this paper, we will amend this statement; that is, the commonly used formula is shown to hold for incompressible parent phase, such as a bubble nucleation in an incompressible parent phase., Comment: 5 pages, 0 figure; Journal of Crystal Growth, accepted

We report preliminary results on cavitation experiments in4He in the presence of a small number of free electrons in the liquid. At negative pressures insufficient to produce homogeneous nucleation we observed cavitation events which we ascribe to nucleation at electron bubbles. We present measurements of the threshold for cavitation at electrons over the temperature range 1.8 – 3.2 K.

Bubble nucleation has been studied on lattices using phenomenological Langevin equations. Recently there have been theoretical motivations for using these equations. These studies also conclude that the simple Langevin description requires some modification. We study bubble nucleation on a lattice and determine effects of the modified Langevin equations., Talk given at DPF2000, Columbus, Ohio; 4 pages, 4 figures