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• Vibration-induced phase change dynamics of R1336mzz(Z) nanofilm are studied. • Phase change mode depends on vibrational amplitude and frequency parameters. • Film boiling and cavitation are identified through the predominant mechanism of thermal momentum and mechanism tensioning. • Interfacial wettability affects the heat transfer rate and potential barrier for bubble nucleation. Vibration-induced evaporation and boiling has attracted great attention due to its prominent efficiency. However, the understanding of low-boiling-point refrigerant phase change on various vibration and wettability conditions remains to be investigated. In this study, molecular dynamics simulations are performed to explore the vibration-induced phase change performance of R1336mzz(Z) nanofilm over a copper substrate. Results found that the phase change mode of R1336mzz(Z) nanofilm can be divided into diffusive evaporation, nucleate boiling, and film boiling/cavitation. For one thing, the residual ratio of R1336mzz(Z) molecules lies on the vibrational amplitude (A) and frequency (f), with a linear relation to Af 3/2 when Af 3/2 is less than 150. For another, the molecular motion is originated from joint action of thermal momentum and mechanical tensioning, leading to film boiling or cavitation by tunning the predominant mechanism. In addition, the impacts of interfacial wettability on vibration-induced phase change are discussed in terms of the heat transfer ability and potential barrier for bubble nucleation. Results reveal that hydrophilic surface causes strong attraction at near-wall region to increase heat transfer, while hydrophobic surface shows weak potential barrier, which promotes the occurrence of bubble nucleation. These findings deliver insights to broad the use of high-frequency vibration in industrial applications. [ABSTRACT FROM AUTHOR]

• Two techniques were employed to provide visual insights into the boiling phenomenon in an artificial cavity. • Optical sensor was effectively validated, showcasing its proficiency in analyzing vapor bubble departure frequencies. • Optical flow analysis revealed microconvection as the prevailing heat transfer mode. • Asymmetry in velocity profiles suggested a tendency for horizontal displacement of the bubble interface during growth. • Vorticity fields indicated rotational motion near the bubble's surface and in deformation movement regions. The ongoing quest for methods that enhance the efficiency of heat transfer processes, particularly those involving phase change, underscores the importance of comprehending the dynamics of vapor bubbles during boiling. This study investigates heat transfer mechanisms and the dynamics of vapor bubbles within the nucleate boiling regime. Experimental tests were conducted on a flat copper surface featuring a single cavity, focusing on analyzing vapor bubble growth and departure stages. The working fluid examined was HFE-7100 under saturated conditions. The formation and growth aspects of vapor bubbles, including their diameter (D d) and departure frequency (f), were investigated through experimental data obtained by two different techniques: by an optical sensor of variable resistance capable of generating an analog signal from a voltage change and by a high-speed camera that captures images immediately after the instant that the bubble detached from the surface. Both techniques provide visual insights into the boiling phenomenon. An escalation in heat flux and, consequently, wall superheat resulted in an increased bubble departure frequency. Furthermore, the optical flow analysis successfully identified velocity and vorticity fields induced by micro convection, the prevailing heat transfer mode in nucleated boiling. A slight horizontal displacement trend over time was observed, which may be caused by the asymmetry in the velocity profile, confirmed by velocity peaks tending toward one direction. Vortex analysis reveals rotational motion concentrated in specific regions, with noticeable deformation near the bottom of the bubble and asymmetric movement contributing to vorticity. These findings provide insights into the vapor bubble dynamics, which is important for understanding the cavity rewetting phenomena. [ABSTRACT FROM AUTHOR]

• Water nucleation on silicon surfaces for the Cassie and Wenzel states is studied. • The role played by surface topography in controlling water nucleation is focused. • Nucleation for the two wetting states shows different dependence on cavity size. • Transition in the wetting state significantly affects nucleation temperature. • Microscopic mechanisms are elucidated in depth by molecular dynamics simulations. The etching process of the silicon die often produces nanoscale roughness, wherein nanostructures alter the solid-liquid heat transfer process and thus influence the bubble nucleation. The difference in the initial wetting state further complicates the effect of nanostructures on the nucleation process. To fully understand the nucleation process at a rough water/silicon interface for different initial wetting states, we employ molecular dynamics simulations to study and characterize this phenomenon. The nucleation processes for the nanostructured surface with different cavity widths, depths and numbers are systematically analyzed in terms of the position, time and temperature of nucleation. The relevant effect mechanisms of surface topography for different initial wetting states are elucidated in depth. The results manifest that regardless of the initial wetting state, the cavity always controls the nucleation site by the original vapor nucleus for the Cassie state or a local heat accumulation effect for the Wenzel state. Moreover, in general, the cavity delays the onset of bubble nucleation for the Cassie state yet promotes the nucleation event for the Wenzel state compared with the smooth surface because of the change of the local solid–liquid contact area. Importantly, the nucleation time for the two wetting states shows a complex and different dependence on the cavity size. Furthermore, the nucleation temperature (529.8–550.3 K) for the Cassie state is significantly lower than that (576.7–592.1 K) for the Wenzel state because the original vapor nucleus can significantly lower the energy barrier of nucleation for the Cassie state. This study is of importance to promote further insights into the onset of water film boiling on rough silicon surfaces and provide guidance for the nanostructure design of the silicon die for enhancing nucleate boiling. [ABSTRACT FROM AUTHOR]

Highlights • The bubble nucleation position changes with increasing wall temperature. • Differences in nucleation process at different wall temperatures were explained. • An optimal area fraction exists for the lowest nucleation temperature. Abstract 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. [ABSTRACT FROM AUTHOR]

Highlights • We made simultaneous observations of the dynamics and thermal behavior of dry patches. • The critical heat flux phenomenon occurred during the rewetting of a large dry patch. • The bubbles nucleating at the advancing liquid front played a key role in the failure of the rewetting. • We determined an empirical value for the critical line density of the bubbles at the triple contact line. Abstract We observed the dynamics of dry patches underneath massive bubbles during the pool boiling of saturated water under atmospheric pressure, and measured the associated temperature distribution. We synchronized the observations both spatially and temporally using high-speed total reflection and infrared thermometry techniques. The observations presented in this paper provide evidence that the critical heat flux phenomenon is triggered during the rewetting of large dry patches with periphery temperatures that are much lower than minimum film boiling temperature, so-called Leidenfrost point. As the liquid meniscus advanced toward the dry patch, numerous secondary bubbles nucleated and impeded the flow of liquid toward the dry patch. This prevented the liquid from rewetting the dry patch. The key physical mechanism for triggering CHF is initiated when the line density of the secondary bubbles nucleating at the periphery of a shrinking dry patch underneath a departing mushroom bubble exceeds a critical value. These bubbles completely block the liquid inflow into the dry patch and prevent rewetting, eventually causing the dry patch to expand irreversibly. We used our experimental data to determine an empirical value of the critical line nucleation site density. [ABSTRACT FROM AUTHOR]

The startup characteristics of micro oscillating heat pipe (micro-OHP) have been experimentally investigated using a high-speed CCD camera in conjunction with the temperature measurement. The micro-OHP was fabricated on a silicon wafer by the MEMS technology, having trapezoidal channels with a hydraulic diameter of 357 μm. HFE-7100 was used as the working fluid with volumetric filling ratios ranging from 31% to 72%. The effects of filling ratio and heating power input on the start-up process were presented. Two different start-up behaviors, so-called start-up with and without bubble nucleation (STWBN/STWOBN), subject to different heat input levels were observed, and the underlying mechanisms were elaborately analyzed. The occurrence of STWBN or STWOBN depends mostly upon the spatial distribution of slugs/plugs in the micro-OHP. For the STWOBN mode, the fluid movement could be triggered by intense liquid film evaporation without nucleation with evaporator temperature less than 80 °C. However, the STWBN mode is sometimes indispensable to excite the two-phase oscillations in the micro-OHP due to the non-desirable slug/plug distribution, leading to much higher evaporator temperatures even greater than 140 °C to initiate the bubble generation. To predict the startup temperature at the STWBN mode, a theoretical model to describe the wall superheat required for incipient bubble nucleation in microchannels was propounded and largely agreed with the present experimental data. Micro-OHPs worked at the STWBN mode may greatly hinder their applications in electronics cooling, and artificial cavities fabricated or embedded on silicon channel wall surfaces are proposed to facilitate bubble nucleation and then the startup at low temperatures. [ABSTRACT FROM AUTHOR]

• The bubble nucleation and growth in flat microchannels at localized ultra-high heat fluxes were studied. • The time duration of bubble growth was less than 200 μs. • A linear growth rate of about 20 m/s was evidenced at about the first half of bubble growth stage. • The short micro-pin fin arrays lowered the threshold heat flux and onset temperature of bubble nucleation. The two-phase flow boiling in microchannels is an attractive cooling strategy for high-power density electronic/optoelectronic devices, while the exist of sub-millimeter "hot spots" characterized by ultra-high heat fluxes would degrade the reliability and safety for commercial and consumer applications. In this paper, the bubble nucleation and evolution characteristics of subcooled flow boiling in high aspect ratio flat microchannels were experimentally studied with the aid of high-speed visualization and infrared temperature measurement at localized heat fluxes exceeding 1.5 kW/cm2. Dielectric liquid HFE-7100 was used at mass flow rates ranging from 11.1 to 178.2 kg/(m2·s). Results show that the explosive boiling process was accompanied by an approximately linear bubble growth rate of about 20 m/s at about the first half of growth stage, and the time duration for overall bubble growth was less than 200 μs. The bubble nucleation was suppressed dramatically on the smooth bottom surface of the channel even at a localized "hot spot" heat flux above 2.5 kW/cm2. The introduction of staggered short micro-pin fin arrays at the bottom wall significantly lowered the threshold heat flux and onset temperature of bubble nucleation and reinforced the bubble growth to larger sizes. It is attributed to the reduction in the temperature gradient of thermal boundary layer adjacent to the heated wall, and it would be further reduced by increasing the micro-pin fin denseness. Even at quite high superheats on smooth and micro-structured surfaces, the heterogeneous boiling mechanism was proved to still work. [ABSTRACT FROM AUTHOR]

In this paper, the bubble nucleation of ultra-thin liquid argon film on a platinum surface is studied by molecular dynamics simulation. In the simulations, the liquid is heated by the middle part and cooled by the two sides of the platinum surface. Observation of bubble nucleation is performed after increasing the temperature of heating zone. The space and time evolution of argon density distribution and the size of bubble are monitored to investigate the nucleation processes above surfaces with and without nanostructures. The results illustrate that nanostructure surface increases the solid-liquid contacting area and the degree of surface temperature non-uniformity, which results in the enhancement of heating intensity and higher rate of nucleation. Comparisons are made among surfaces with different nanostructures, with which it is further found that the heat flux increases with the height of nanostructures. Higher nanostructure reduces nucleation duration. [ABSTRACT FROM AUTHOR]

Liquid cell electron microscopy is a useful technique for the observation of chemical, biological, and mechanical processes in liquids at nanometer-scale resolution. This study investigated the generation and growth of nanobubbles using the Fresnel fringe method, which enabled us to determine the location of bubble interface; the nanobubbles were induced in the 600-nm-thick water sample in the cell, by the electron beam. Nucleation occurred first at the solid–liquid interface in the upstream side of electron beam, and this was followed by second-group nucleation at the downstream-side interface; all of the stable nucleations occurred on the solid surfaces. The size of the nucleated bubbles at the moment they became visible depended on the magnification used in the electron microscope, and a higher-energy density in the electron beam induced larger bubbles. The underlying mechanism was also considered in this study. [ABSTRACT FROM AUTHOR]

• Modeling of super deep thermosyphons in artificial ground freezing. • Development of a novel cold heterogenous boiling model for thermosyphons. • Bubble nucleation site at thermosyphon wall only. • Formation of three unique zones at deep evaporators. • Observation of a dead-zone at large thermosyphon filling ratios due to hydrostatic pressure. Super-long thermosyphons exceeding 100 m are being employed more frequently in artificial ground freezing (AGF) applications. In this study, we develop a fully-conjugate computational-fluid-dynamics (CFD) model to fundamentally analyze the heat extraction capacity and profile of super-long thermosyphons in AGF. The CFD model couples a heterogeneous condensation/evaporation mass transfer model inside the thermosyphon with thermosyphon-pool hydrostatic pressure distribution and heat diffusion from the ground. The heterogeneous model also considers the kinetic energy required for bubble nucleation and has been validated against experimental studies from the literature. Three main parameters have been investigated: 1) the filling ratio, 2) the charge pressure inside the thermosyphon, and 3) the wind temperature. The results reveal a no-boiling-zone below 10–25 m of pool surface. Further, the charge pressure significantly affect the start-up of the thermosyphon. Lastly, lower wind temperature extracts more heat from the ground in a qualitatively similar manner (similar heat flux profile along thermosyphon wall) to that of higher wind temperature. Overall, the results of this study provide fundamental understanding of the performance of super-long thermosyphons in AGF. [ABSTRACT FROM AUTHOR]

The bubble nucleation temperatures of several organic liquids (methanol, ethanol, butanol, n-heptane) on stress-minimized platinum (Pt) films supported by SiN membranes is examined by pulse-heating the membranes for times ranging from 1 μs to 10 μs. The results show that the nucleation temperatures increase as the heating rates of the Pt films increase. Measured nucleation temperatures approach predicted superheat limits for the smallest pulse times which correspond to heating rates over 10 8 K/s, while nucleation temperatures are significantly lower for the longest pulse times. The microheater membranes were found to be robust for millions of pulse cycles, which suggests their potential in applications for moving fluids on the microscale and for more fundamental studies of phase transitions of metastable liquids. [ABSTRACT FROM AUTHOR]

Cahn–Hilliard phase-field method is used to numerically simulate subcooled water boiling in which nucleation occurs from an artificial cavity on the inner surface of a microchannel. The boiling initiates from a cavity and the growth and departure of the nucleated bubbles from the cavity are simulated. The velocity, temperature and pressure distributions inside the microchannel have been analyzed. The bubble generation frequency increases by increasing inlet mass flux. For this case of study, the rising trend continues up to the mass flux of 64 kg/m 2 s. Further increase in the inlet mass flux does not accelerate the bubble formation. The radius and the shape of the generated bubble compared well with experimental data available in the literature. [ABSTRACT FROM AUTHOR]

This study describes the performance of stress-minimized platinum (Pt) films supported by thin membranes (200 nm thick) to promote bubble nucleation of water near its theoretical superheat limit. The membrane configurations consist of Pt films deposited on 200 nm thick SiN films over bulk Si, with membranes being formed by etching Si from the back side of the films. Results are compared with more conventional Pt films supported by SiO 2 and Si substrates. The average metal temperature is monitored by a bridge circuit with capacitive and inductive filtering to reduce noise in the output signal. Voltage pulses with durations ranging between 0.5 and 10 μs are imposed on the bridge to electrically heat the Pt film. The paper includes discussion of fabrication of the films, their treatment prior to using them as temperature sensors, the bridge circuit design for monitoring the change in electrical resistance during the power pulse, the calibration process of the films, and results of the bubble nucleation temperatures for the range of pulse durations examined. The results show that significantly less power is needed to trigger bubble nucleation on a membrane-supported platinum film compared to a platinum film on a bulk Si substrate. The nucleation temperatures which were closest to the theoretical limit of water were realized at heating rates of nearly 10 9 C/s. The potential for employing back-side etched devices is suggested for fundamental studies of phase transitions of highly superheated liquids and in applications where bubble nucleation is an important process such as for ink-jet printers and microscale bubble pump concepts. [ABSTRACT FROM AUTHOR]

Abstract: A single channel microdevice (Dh =230μm) was used in this study to measure the local wall temperature in a flow boiling configuration using thin-film (70nm titanium) thermistors. The device and experimental configuration allows the measurement of surface temperature with high spatial resolution and a temporal resolution in the kHz range with synchronized flow regime observation. A steady heat-flux was supplied to the working fluid (de-ionized water) using a thin-film heater (100nm titanium) to study the transient temperature evolution accompanying several flow patterns, such as bubbly flow, elongated bubble (slug) flow, partial wall dryout, and critical heat flux (CHF). Transient local convective heat transfer coefficients are computed for phenomena such as bubble nucleation and bubble growth through the use of a 3-D finite element model to account for the transient conjugate heat transfer effects in the substrate. [Copyright &y& Elsevier]

• A theoretical model for bubble nucleation in liquid film boiling is proposed. • Bubble nucleation during liquid film boiling is much more difficult than that in pool boiling and flow boiling with the minimum size of active microcavities being several times larger. • The model is well validated by existing experimental data with theoretically predicted critical thickness for liquid film boiling is larger than 100 µm. Many industrial applications such as electronic cooling and thermal desalination have adopted the heat dissipation technologies of capillary evaporation and liquid film boiling, with bubble nucleation being the transition criterion between them. In this paper, a theoretical model for bubble nucleation in liquid film boiling is developed, which mainly considers three aspects: transient heat conduction inside the liquid film supported with porous wick, evaporation at the liquid-vapor interface, and superheat requirement for the stable existence of a bubble nucleus. The criterion for the onset of nucleate boiling (ONB) in liquid film boiling is established, and size range of active microcavities for bubble nucleation is obtained, as well as comparisons being made with that during pool boiling and flow boiling. The results indicate that the size range of active microcavities in liquid film boiling is narrower with the minimum size being several times larger, and bubble nucleation in liquid film boiling is much more difficult. Although superhydrophilic surface with small contact angle is disadvantageous to bubble nucleation, it will decrease the minimum size of effective microcavities radius within the size scope of a few micrometers, which is beneficial for ONB in liquid film boiling. Moreover, critical thickness to trigger ONB in liquid film boiling is found to decrease with increasing contact angle, augmenting superheat and strengthening effective thermal conductivity of the wick, and its theoretical prediction is validated with experimental data in the literature. Our model offers a new avenue for understanding the ONB in liquid film boiling, and therefore can serve as a guideline for future enhancement design. [ABSTRACT FROM AUTHOR]

• Surface morphology influences the Leidenfrost and transitional boiling regime. • An unconstrained droplet shows a non-monotonic variation in the total evaporation time with temperature in the transitional boiling regime. • An unconstrained droplet exhibits trampolining in the transitional boiling regime. • The gain in momentum of the droplet resulting in trampolining is attributed to the bubble nucleation and escape prior to droplet take-off. Droplet dynamics on a heated surface plays an important role in applications ranging from spray cooling to solidification and pharmaceuticals. Here, we show that in the transitional boiling regime, an unconstrained droplet exhibits trampolining where the droplet bounces to higher heights with each consecutive contact with the substrate. A water droplet of volume 1 μ L bounces to a height as high as 6-7 times of its diameter and the height of bounce increases to 18–20 times the diameter as the volume reduces to ∼ 0.1 μ L. The dynamics of the droplet in the transitional boiling regime results in a non-monotonic variation in the total evaporation time of the droplet with an increase in substrate temperature – an observation that is absent for a constrained droplet. We develop a force-based model to explain the trampolining behavior of the droplet and propose that the gain in momentum of the droplet that results in trampolining is caused by bubble nucleation and escape at the liquid-substrate interface prior to droplet takeoff. [ABSTRACT FROM AUTHOR]

• It was found that the position of the nucleate site varies with the spacing of structures. Then the evolutions of the three-phase contact line were also different between the cases. The mechanism of this phenomenon was analyzed by kinetic-potential energy theory, which was mainly related to the spacing of structures. • It was found that the relationship between liquid-solid contact area and the heat transfer enhancement of bubble expansion was not monotonically increasing. The evaporation of the microlayer around the three-phase contact line was more critical to CHF, which was not necessarily related to the liquid-solid contact area. • Liquid retention induced by the various height of structures also had significant effects on heat transfer of bubble expansion. The increased rate of CHF with the increase of liquid retention gradually slowed down, which was due to the limited effect of larger liquid retention on the liquid at bottom of structures. As the main factor limiting the performance of microprocessors, how to improve its heat dissipation capacity is critical. Nucleate boiling on the structured surface is a method that can improve heat transfer significantly due to two-phase flow, and bubble behaviors have been attracted much attention because of its capability of enhancement on critical heat flux(CHF). Although a large number of studies have proved that the structured surface can improve CHF by increasing the number of nucleation sites, bubble departure frequency and reducing bubble departure radius, the mechanism of solid-liquid-gas interface evolution on the structured surface is still unclear, which has significant effects on nucleate boiling. Therefore, a model of nucleate boiling of liquid on the structured surface was constructed, the processes of nucleate generated and bubble growth were observed directly by using molecular dynamics simulation. A novel phenomenon was observed in which the position and number of interfaces are mainly dependent on the restriction of potential energy generated by structures. This phenomenon affects CHF significantly due to the area of the microlayer beneath the bubble was different. In addition, the effects of liquid retention on interface and CHF were also explored, the result showed that CHF increased with the increase of liquid retention, and the increase rate of CHF was slowed down due to the more retained liquid in the structures. We believe that this research can improve the existent theory of nucleate boiling and provide a more effective theoretical basis for the design, processing and application of surface with the nanostructure. [ABSTRACT FROM AUTHOR]

Abstract: The effect of ambient pressures below atmospheric, liquid subcooling and heating rate on bubble dynamics associated with rapid evaporation in highly superheated water is investigated. Platinum films of various aspect ratios are electrically heated under partial vacuum and the average metal film temperature monitored during a power pulse of duration of several tens of microseconds. At high liquid subcooling, the surface temperature of the pulse-heated thin platinum strips is shown to oscillate with a frequency on the order of 0.5MHz. The oscillation frequency increases with ambient pressure in the range 0.02MPa and 0.101MPa, and decreases with increasing ambient temperature at a fixed pressure. An unusual dynamic instability is observed in which the surface temperature is constant before relaxing into oscillations, and the delay to oscillations increases with decreasing pressure. High speed imaging shows that the bubble growth phase of the oscillation is associated with surface heating and the collapse phase with surface cooling in the cyclic temperature history. These effects are explained by a bubble growth theory that includes a heat loss mechanism to the surrounding liquid. The nucleation temperature is unaffected by pressure in the sub-atmospheric range examined and it approaches the theoretical superheat limit for water at a heating rate of about 109 °C/s. [Copyright &y& Elsevier]

Abstract: High-speed video and infrared thermometry were used to obtain time- and space-resolved information on bubble nucleation and heat transfer in pool boiling of water. The bubble departure diameter and frequency, growth and wait times, and nucleation site density were directly measured for a thin, electrically-heated, indium–tin-oxide surface, laid on a sapphire substrate. These data are very valuable for validation of two-phase flow and heat transfer models, including computational fluid dynamics with interface tracking methods. Here, detailed experimental bubble-growth data from individual nucleation sites were used to evaluate simple, commonly-used, but poorly-validated, bubble-growth and nucleate-boiling heat-transfer models. The agreement between the data and the models was found to be reasonably good. Also, the heat flux partitioning model, to which our data on nucleation site density, bubble departure diameter and frequency were directly fed, suggests that transient conduction following bubble departure is the dominant contribution to nucleate-boiling heat transfer. [Copyright &y& Elsevier]

Abstract: In this investigation, boiling incipience and bubble dynamics on a microheater with a geometry of 100μm×100μm fabricated with MEMS technology are evaluated using a high-speed digital camera. For the purpose of comparison with conventional boiling heat transfer, boiling incipience and bubble dynamics are also studied on a carefully selected microheater with a fabricated defect (i.e., a microcavity on the heater surface). Of industrial interest are the effects of dissolved gases on boiling incipience and bubble dynamics, which are also discussed in detail. The possible nucleation temperature (or incipience temperature) is analyzed and discussed from the perspective of the measured bulk temperature of the microheater and a 3D heat conduction numerical model. The time-resolved bubble dynamics (i.e., the bubble size evolution, interface velocity and interface acceleration) are all presented along with high-speed digital images. Based upon this investigation, it is clear that explosive boiling can take place on a smooth surface no matter how slow the heating rate, and dissolved gases have a significant influence on the incipience temperature and bubble behavior. Furthermore, this study illustrates that the classical kinetics of boiling can explain the explosive boiling occurring on a smooth surface in principle and can provide a useful guide for the design of microscale heat transfer and/or MEMS devices. Although unexpected, due to the gravitational effects, Marangoni flow on the vapor–liquid interface induced by the temperature gradient was also observed. [Copyright &y& Elsevier]

Abstract: With thermal and mechanical nonequilbrium taken into consideration, the classical kinetic theory of boiling is modified to study unsteady-state homogeneous nucleation processes. Based on this newly developed model, the degree of superheat and the maximum nucleation rate corresponding to different rates of temperature rise in water are calculated and presented. For the first time, the initial nonequilibrium vapor pressure and the initial growth rate of bubble nuclei with different initial embryo sizes and different rates of temperature rise are accurately modeled. The resulting algorithm provides a method by which the details of bubble nucleation in a superheated liquid can be predicted, leading to a better understanding of the kinetics of boiling. Model validation, accuracy and application are also presented and discussed. [Copyright &y& Elsevier]

• Effects of patterned wettability on bubble nucleation over grooved surfaces were investigated. • A transient temperature boundary condition was used to approximate radiative heating. • The bubble growth rate on the all-hydrophilic groove is found to be the fastest. • BphoSphi surface can mostly facilitate the nucleation of bubble. • BphiSpho surface presents an apparent enhancement of the maximum heat flux. Mechanisms of bubble nucleation need a better understanding, especially for heterogeneous surfaces with different wettability. In this study, the bubble nucleation and the heat transfer performance on grooved surfaces with different hybrid wettability under a transient temperature boundary condition was investigated by characterizing the contact-line movement using molecular dynamics simulation. The bubble growth rate on the all-hydrophilic groove is faster than the other grooved surfaces. The grooved surfaces with patterned wettability, especially the B pho S phi (hydrophobic bottom wall and hydrophilic sidewall) surface, can facilitate the nucleation of the bubble. Meanwhile, an apparent enhancement of the maximum heat flux was found for the B phi S pho (hydrophilic bottom wall and hydrophobic sidewall) surface. This work provides insights into the mechanisms of enhancing the boiling heat transfer under irradiation condition by the contact line movement during bubble nucleation on structured surfaces. [ABSTRACT FROM AUTHOR]

• Different bubble nucleation phenomena happen on the grooved substrates with different wettability. • A "PK" norm is developed to illustrate bubble nucleation mechanism. • The CNT about the liquid in the groove obtaining more thermal energy to achieve nucleating is verified by the "PK" norm. • The advantage of strongly hydrophilic groove on bubble nucleation is revealed by the theoretical approach and "PK" norm. • The CNT about some residual gases in the groove becoming an initial bubble nucleus is explained by "PK" norm. • The mechanism for growth of initial bubble nucleus from the strongly hydrophobic groove is explained by "PK" norm. • The "PK" norm is available to explain the existence of solid-like liquid atoms on the hydrophilic substrate surface. • The "PK" is a good tool to determine the incipient nucleation time. The classical heterogeneous nucleation theory explains that the groove in the substrate is a desirable place to breed a bubble nucleus. However, the existing research method cannot reproduce the nucleation process. Therefore, in the present study, the molecular dynamics simulation method is conducted to investigate the bubble nucleation on grooved substrates with different wettability. The simple L-J liquid argon is heated by the platinum grooved substrate, whose temperature is controlled by Langevin thermostat. Results show that the groove has significant impacts on bubble nucleation from two aspects: improve thermal energy transfer efficiency and support an initial bubble nucleus. For the substrate with a hydrophilic groove, a visible bubble nucleus generates on the groove region from nothing because of liquid in there obtaining more thermal energy than that on the smooth region within the same time. Moreover, the nucleation rate is improved with the increase of groove hydrophilicity. On the other hand, for the substrate with a hydrophobic groove, some residual gases form an initial bubble nucleus at the initial moment of the nonequilibrium simulation stage, and it takes some time to grow up. Furthermore, a method based on the competition between atomic potential energy and atomic kinetic energy is used to explain the formation of the bubble nucleus on the different wetting substrates. The present simulation study of bubble nucleation on the grooved substrate is another support for the classical heterogeneous nucleation theory. [ABSTRACT FROM AUTHOR]

• This paper presents a new boiling activation technique using electrolysis of water. • Electrolytically generated hydrogen bubbles were confirmed to act as boiling bubble nuclei through the high-speed visualization. • Electrolytic boiling activation resulted in heat transfer enhancement and reduction/control of boiling incipient superheat. • The heat transfer coefficient was more than doubled on supplying an electrolytic current of 0.08 A at 0.2 MW/m2. The effects of the electrolytic activation of bubble nucleation on heat transfer in the saturated boiling of water were experimentally investigated. A copper heated surface is used as a cathode in the electrolysis circuit in this study. The hydrogen bubbles electrolytically generated at the heated surface are used as boiling bubble nuclei to increase the nucleation site density. This paper is the first report on the heat transfer characteristics in boiling activated by electrolysis. In the experiment, the generated hydrogen bubbles were confirmed to actually act as boiling bubble nuclei through the visualization of bubble behavior. The electrolytic supply of boiling bubble nuclei significantly enhanced the boiling heat transfer, as intended. The electrolytic boiling activation was found to be more effective at a lower heat flux with fewer bubbles at the heated wall. Finally, it was demonstrated that this method is also useful for reducing/controlling incipient boiling superheat. [ABSTRACT FROM AUTHOR]

• Boiling process above realistic rough surfaces are studied by molecular dynamics simulations. • Randomly distributed rough surface profiles are generated by multivariable Weierstrass-Mandelbrot function. • The effects of nanoscale surface roughness on boiling are investigated in two aspects. • Rough structures advance the boiling inception time and improve the energy transfer process. The boiling process of argon above solid copper substrates with different surface roughness levels are investigated using molecular dynamics simulations. The randomly distributed rough surface profiles are generated by a multivariable Weierstrass-Mandelbrot function. With the application of the thermostat on the solid atoms, bubble nucleated at the solid-liquid interface after experiencing a period of evaporation process. Afterwards, the bubble grew into vapor film and pushed the bulk liquid cluster away from the solid wall. It was found that the rough structures advanced the boiling inception time and increased the evaporation mass flux. The existence of cavities is favorable for bubble nuclei appearance. Besides, the heat transfer between the solid and fluid atoms become more efficient with the help of surface nanostructures. The energy exchange rate is further improved with higher surface roughness. [ABSTRACT FROM AUTHOR]