Nanoscale thin liquid films play a pivotal role in diverse natural phenomena and industrial applications, where their evaporation heat transfer characteristics and morphological evolution are critically influenced by disjoining pressure. While classical theories adequately describe this effect on smooth surfaces, the disjoining pressure of liquid films on nanostructured surfaces remains poorly understood. In this work, we propose a mesoscopic model to investigate disjoining pressure effects in nanoscale liquid films on nanostructured substrates, in which long-range solid-fluid interactions are directionally discretized on high-order lattice to enable the treatment of nanostructured surfaces. The model is validated in isothermal and nonisothermal systems, demonstrating its capability to capture disjoining pressure effects on both smooth and nanostructured surfaces. Furthermore, we reveal the fundamental interplay between surface tension and disjoining pressure in dictating the morphology of thin liquid films and provide insights into the Hamaker constants of nanostructured surfaces. A comprehensive stability analysis of thin liquid films on nanostructured surfaces is also presented. This work advances the understanding of microscale mechanisms in liquid-vapor phase change processes and offers a versatile tool for optimizing heat and mass transfer in nanoscale systems.

Nucleate boiling heat transfer on thin substrates plays a pivotal role in thermal management systems for advanced miniaturized electronics. Through high-fidelity numerical simulations of water boiling on silicon and copper substrates with thickness ranging from 25 to 500 & micro;m, we demonstrate that substrate's heat transfer process could significantly influence microlayer evaporation. Through detailed heat balance analysis, we demonstrate that lateral heat conduction within the thin heated substrate serves as the primary energy source for microlayer evaporation. Augmenting the substrate thickness or thermal conductivity improves lateral conduction, which intensifies microlayer evaporation and consequently modulates the overall boiling heat transfer performance and bubble dynamics. Furthermore, we define a thermophysical parameter, lambda s delta s (the product of substrate thermal conductivity and thickness), to delineate two distinct boiling regimes. In the substrate-dominant regime, lateral heat conduction dictates performance: the bubble departure diameter increases sharply, whereas the spatiotemporal-averaged heat transfer coefficient exhibits a sharp decrease with increasing lambda s delta s. Conversely, this influence diminishes in the hydro-dominant regime. These findings sharply contrast with the conventional understanding, which assumes that the influence of the heated substrate side on both bubble departure sizes and heat transfer coefficient is negligible. This study provides fundamental insights into the micro-scale mechanisms of boiling on thin substrates, offering critical guidelines for the thermal design of microelectronic cooling solutions.

In this work, we employ a high-fidelity numerical model that integrates microlayer evaporation and contact angle hysteresis to systematically investigate the influence of contact angle hysteresis on bubble dynamics and heat transfer during nucleate boiling. The model is rigorously validated against both analytical solutions and experimental data, showing very good agreement. Notably, comparison with experimental results demonstrates that it not only successfully predicts bubble dynamics, but also accurately captures the transient and local heat transfer characteristics on the heating surface. We explore the evolutions of bubble dynamics, temperature field, wall temperature/heat flux distribution, and microlayer distribution beneath the bubble with high temporal and spatial resolutions. Our results reveal a strong interdependence between bubble dynamics and nucleate boiling heat transfer. Contact angle hysteresis is shown to directly modulate contact line motion, thereby governing microlayer evolution and dictating heat transfer characteristics during nucleate boiling. We provide a quantitative assessment of the contribution of microlayer evaporation to bubble heat transfer. This work highlights the significant role of contact angle hysteresis in nucleate boiling simulations and offers comprehensive insights into its effects on phase change heat transfer processes.

This study presents three-dimensional pore-scale numerical investigations of thin liquid film evaporation on hydrophilic micro-pillar array wicks, utilizing the pseudo-potential multiple-relaxation-time lattice Boltzmann liquid–vapor phase-change method. The simulation captures both the steady-state meniscus morphology and the dynamic recession behavior during dryout, offering pore-scale insights into the evaporation-to-dryout transition. At the pore scale, we numerically capture the profile of the curved meniscus during steady-state evaporation, as well as its continuous recession during dryout after surpassing the capillary-driven dryout heat flux. A parametric study is conducted to systematically investigate the effects of wettability, pillar pitch, and pillar height on the dryout heat flux. A thermal-fluidic analytical model for predicting capillary-driven dryout heat flux is applied to verify the simulated dryout heat flux and wickabilities of the micro-structured wicks. We demonstrate that wickability—characterized by both the liquid front velocity and the volumetric wicking flow rate—governs the capillary-limited heat transfer performance. We find that the influence of both wettability and pillar pitch on the dryout heat flux is primarily attributed to wickability, as characterized by the liquid front propagation rate. While the liquid front propagation rate initially increases and finally flattens out with increasing pillar height, the wickability characterized by volumetric wicking flow rate continues to rise with the increasing pillar height. The analytical predictions for the capillary-limited dryout heat flux demonstrate good agreement with the simulation results. This work advances the understanding of pore-scale evaporation and offers design guidelines for optimizing capillary wick structures in thermal management applications.

The evolution and retention of small amounts of gas bubbles in the porous electrodes of aqueous redox flow batteries (RFBs) significantly reduce the liquid phase’s relative permeability. This reduction disrupts electrolyte flow rates and compromises the uniformity of flow distribution—challenges that conventional single-phase flow models cannot adequately address. To overcome these issues, this study proposes a gas-liquid slip-flow model based on liquid-phase continuity. This model accounts for both the liquid’s sweeping effect on dispersed gas bubbles and the obstructive effect of gas bubbles on liquid flow. It integrates Darcy flow of the liquid phase in porous media with the movement of discontinuous gas bubbles and establishes empirical correlations between liquid-phase saturation and relative permeability using experimental data. This model’s accuracy is validated by comparing it with previous experimental results, and it is further applied to analyze the distribution of gas saturation and liquid permeability along the flow direction under various operating conditions.

The microscale effects are widely believed to have significant influences on bubble dynamics and heat transfer in nucleate boiling. In this paper, we propose a high-fidelity numerical approach for simulating bubble dynamics and heat transfer associated with a single nucleate boiling bubble by considering microscale effects including microlayer evaporation and contact angle hysteresis. Our simulated bubble shape, bubble growth rate and bubble departure size show very good agreement with experimental results. More importantly, our simulated transient and local heat transfer on the heating surface during the single bubble nucleate boiling also agrees well with experiments. This high-fidelity approach enables us to elucidate the multiscale heat transfer characteristics of a single nucleate boiling bubble and the thermal responses of the heated substrate with high spatial and temporal resolutions. We show the periodic heat storage and release inside the heated substrate and superheated liquid layer due to the nonuniform heat transfer in both space and time during the nucleate boiling bubble cycle. We demonstrate that a considerable amount of heat within the heated substrate is rapidly dissipated by microlayer evaporation owing to its ultrahigh heat transfer rate, resulting in a localized low-temperature region beneath the bubble. Our simulations also show that microlayer evaporation contributes 58 % to the bubble growth of water on silicon substrate under atmospheric pressure. This work provides a new avenue for elucidating the multiscale heat transfer mechanisms in nucleate boiling.

Electrical and electronic devices are producing a higher density of heat along with their elevated performance to meet the booming needs of industrials and human activities, and in response, more effective cooling technologies are always in demand. Liquid immersion is one of the most popular cooling methods developed due to its thermal efficiency and operational feasibility. However, conventional single-phase immersion cooling exhibits relatively lower thermal efficiency and heat capacity, and dual-phase immersion cooling occurs only when the heating surface temperature exceeds the liquid boiling point, constraining the selection of suitable coolants. This work presents a cooling method using guest noncondensable gas (NCG) microbubbles to enhance heat transfer in immersion cooling. Direct experimental results show that the heat transfer rate increases drastically, and surface superheating is suppressed or even eliminated. NCG microbubbles can disturb the thermal boundary layer, serve as evaporation/boiling nuclei, and collide with existing bubbles to facilitate their departure. This cooling method performs better at a large degree of pool subcooling, while the effect of bubble size is relatively minor. The fundamental understanding of bubble dynamics upon impinging onto the heating surface in subcooled liquid has promising applications in thermal management systems, such as microelectronic chips, nuclear reactors, steam generators, and among others.

Nanoparticle deposition from a dried sessile microdroplet has important applications in printing and manufacturing. However, the various morphologies of the nanoparticle depositions in existing experiments could not be fully explained based on the traditional deposition criteria, which neglect the nanoparticle deposition kinetics effect. The mystery is resolved herein by simulating nanosuspension microdroplet evaporation based on the pseudopotential phase-change lattice Boltzmann method, incorporating the impact of the time integration of nanoparticle deposition kinetics and deposition front surface wettability. The simulated morphologies of the four typical deposition patterns ("coffee-ring", "coffee-eye", "dome-like" and uniform) on a hydrophilic surface and the tall conical "pillar" on a hydrophobic surface are shown in agreement with existing experimental data. The morphologies of these four typical nanoparticle deposition patterns formed on a hydrophilic surface are found to be governed by two dimensionless characteristic parameters: Peclet number and a new dimensionless parameter Ti that describes the effects of time-integration of nanoparticle deposition kinetics on the deposition morphologies. A decrease in Ti leads to a more apparent "dome-like" pattern formed inside the deposition; otherwise, a more "uniform" pattern is formed inside the deposition. On the other hand, an increase of Peclet number leads to a higher altitude deposition edge, and with a lower-altitude deposition edge on the contrary. This study also paves the way for adjusting evaporation-induced nanoparticles' self-assembly morphologies at a much lower cost and in an easier approach, without traditionally applying sophisticated external fields or multicomponent additives.

The Schrage equation has been extensively used to calculate the interfacial heat/mass transfer rate during liquid-vapor phase change processes including boiling, evaporation and condensation. A critical parameter in this equation is the mass accommodation coefficient (MAC). We demonstrate that as device miniaturization progresses, the interfacial evaporation thermal resistance becomes increasingly significant, making MAC a key factor in determining the overall heat transfer performance. Using a mesoscopic approach for nano-/microscale liquid-vapor phase change heat transfer, we determine MAC values for pentane, water and hydrofluoroether-7100 (HFE-7100) under diverse conditions. Our results demonstrate a remarkable consistency between MAC values obtained from temperature-driven and pressure-driven phase transitions, indicating that the MAC is unaffected by the phase change driving forces. Furthermore, we show that while disjoining pressure suppresses evaporation by reducing the equilibrium vapor pressure, it has no discernible effect on the MAC value itself. Based on MAC values determined by our approach, we predict heat transfer performance of the porous wick and identify the optimal porosity that maximizes the overall heat transfer coefficient. This study provides an effective tool for predicting the MAC and interfacial transport rates in various liquid-vapor phase change phenomena, which are widely used in thermal management of high-heat-flux electronics.

The morphology of evaporating menisci near the three-phase contact line region—whether characterized by a truncated adsorbed liquid film or an extended adsorbed liquid film—has been the subject of long-standing debate. In this Letter, we present a unified mesoscopic approach for predicting both the morphological evolution and heat transfer characteristics of evaporating menisci. This methodology provides a comprehensive framework capable of simultaneously characterizing the truncated-adsorbed liquid film regime, the extended-adsorbed liquid film regime, and their transitional behavior. We explain the stability mechanism and evaporation heat transfer characteristics of nanoscale liquid films, as well as the self-regulation mechanism governing the evaporating meniscus between the extended-adsorbed film regime and the truncated-adsorbed film regime. This unified framework provides a useful tool for investigating the nanoscale evaporation heat transfer characteristics and mechanisms in liquid–vapor phase change processes.

In recent years, the pseudopotential lattice Boltzmann method (LBM) has been extensively used to study nucleate boiling heat transfer. However, a critical issue persists in existing studies: the wall superheats in nucleate boiling simulations typically reach tens to over 100 K, significantly higher than the observed range of just similar to 10 K in experiments and practical engineering applications. These anomalously high superheats cast doubt on the reliability of numerical results. In this work, combining numerical simulations and theoretical analysis, we demonstrate that this discrepancy stems from the classical heterogenous nucleation mechanism assumed in existing studies, whereas bubble nucleation initiates from preexisting vapor/gas trapped in the surface crevices and defects in nucleate boiling experiments and real-world applications. Using LBM, we successfully simulate the nucleate boiling process governed by this nonclassical trapped-vapor nucleation mechanism, confirming that the required superheat for nucleate boiling can be as low as similar to 10 K, consistent with observations in experiments and engineering applications.

Water droplet's impact and subsequent freezing on inclined supercooled surfaces, having micro-pillar structures with three different types of wettabilities (hydrophilic, neutral, or hydrophobic) are investigated numerically by a three-dimensional, multi-relaxation time, pseudo-potential liquid-vapor phase-change lattice Boltzmann method. Effects of volume expansion of water at 0 degrees C as well as the advancing contact angle of the droplet on real rough surfaces are considered in this model. Temporal variations of droplet morphological changes after impact and subsequent freezing on the supercooled inclined surfaces under various wall temperatures are illustrated. Simulation results indicate that after impacting on the supercooled rough inclined surfaces at a given Weber number, a water droplet can form fully penetrated elliptical ice morphologies and stretched stream-like ice morphologies attached on the supercooled surface, or partially and fully rebounding from the supercooled surface depending on the wettability of the surface, the surface inclination, and the degree of supercooling. It is shown that elliptical ice patterns are formed on hydrophilic substrates where droplets do not recoil after spreading, while stream-like ice patterns are mostly formed on neutral substrates where droplets are gradually stretched due to the gravity because droplets are attached on the inclined surface. On the other hand, partial and full rebounds of droplets occur on hydrophobic surfaces having high inclination angle where droplets undergo strong recoiling and rebounding after spreading on rough surfaces. At low degrees of supercooling (i.e., higher substrate temperatures), partial rebounds occur because a portion of the water droplet freezes on the substrate during contact with the supercooled substrate; otherwise, fully rebounding occurs. These results elucidate the effects of wettability, roughness, temperature and inclined angle of the wall on ice morphologies. Additionally, effects of Stefan number (substrate temperature) on the four different ice morphologies of a water droplet (D0 = 100 and Pr = 13.5) after its impact on supercooled inclined rough surfaces at a Weber number of We = 112 (Reynolds number of Re = 164.9) are illustrated.

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The uniform delivery of electrolyte to a parallelly fed redox flow battery stack might be impaired due to the maldistribution of evolved gas bubbles in each single electrode. Entrapped gas bubbles bring about flow choking, resulting in the decrease of liquid velocity, which in turn weakens bubble sweeping and aggravates bubble trapping. Thus, a vicious cycle between bubble trapping and flow choking occurs in porous electrodes. To quantitatively illustrate the development and hazard of this vicious cycle, we establish analytical models based on previous experimental data and present the correlation among gas saturation, liquid velocity, pressure drop and hydrogen evolution rate. When the liquid velocity drops from 10 mm s-1 to 1 mm s-1, the gas saturation exhibits more than fourfold increase at a given hydrogen evolution rate, showing that more bubbles tend to reside in the electrode and the flow path of liquid phase become narrower. With the decrease in the pressure drop, the liquid velocity becomes lower and more sensitive to the gas evolution rate and the electrode wettability. As the pressure drop further drops below a critical value, the liquid flow is very likely to be blocked, resulting in severe maldistribution of electrolyte feeding.

Bubble dynamics significantly impact mass transfer and energy conversion in electrochemical gas evolution reactions. Micro-/nanostructured surfaces with extreme wettability have been employed as gas-evolving electrodes to promote bubble departure and decrease the bubble-induced overpotential. However, effects of the electrodes' wickability on the electrochemical reaction performances remain elusive. In this work, hydrogen evolution reaction (HER) performances are experimentally investigated using micropillar array electrodes with varying interpillar spacings, and effects of the electrodes' wettability, wickability as well as bubble adhesion are discussed. A deep learning-based object detection model was used to obtain bubble counts and bubble departure size distributions. We show that microstructures on the electrode have little effect on the total bubble counts and bubble size distribution characteristics at low current densities. At high current densities, however, micropillar array electrodes have much higher total bubble counts and smaller bubble departure sizes compared with the flat electrode. We also demonstrate that surface wettability is a critical factor influencing HER performances under low current densities, where bubbles exist in an isolated regime. Under high current densities, where bubbles are in an interacting regime, the wickability of the micropillar array electrodes emerges as a determining factor. This work elucidates the roles of surface wettability and wickability on enhancing electrochemical performances, providing guidelines for the optimal design of micro-/nanostructured electrodes in various gas evolution reactions.

Capillary wicking characteristics play an important role in two-phase thermal management devices including heat pipes and vapor chambers, yet three-dimensional (3D) pore-scale simulations of the dynamic capillary wicking process on various micro-structured surfaces have been rare. In this paper, we conduct 3D pore-scale simulations of capillary wicking on three commonly used micro-structured wicks including micro-pillar array, micro-channel, and sintered particles. The micro-scale liquid propagation dynamics and the “stick-slip” behavior of the propagating liquid front are captured using a 3D pseudo-potential multiple-relaxation-time lattice Boltzmann method. Based on the Lucus–Washburn approach and a work-energy approach, we theoretically analyze wickabilities of different micro-structured wicks. Effects of wick geometry and structural parameters on the capillary wicking characteristics are discussed. We demonstrate that an optimal pillar pitch distance exists, which maximizes the wickability of the micro-pillar array. We show that when the porosity is relatively low, the wickability of the micro-channel is higher than that of the micro-pillar array and the sintered particles. When the porosity is large, however, the sintered particles exhibit higher wickability than the micro-pillar array and the micro-channel. We also demonstrate that the capillary pressure of the sintered particles is always higher than that of the micro-pillar array and the micro-channel throughout the porosity range investigated. The numerical simulation results are compared with theoretical predictions. Findings in this work provide guidelines for the designs of porous wick in various two-phase thermal management systems for high heat flux devices.

The massive emission of greenhouse gas CO2 has significant effects on global climate and environment, causing worldwide concern. It has been proposed to store CO2 in clathrate hydrates as a possible strategy for reducing atmospheric CO2 emission. Nevertheless, the CO2 hydrate formation rate is slow and its associated heterogeneous nucleation mechanisms are not well understood, which limit commercial applications of this technology. This study developed a thermodynamic model based on the change in availability function approach, considering effects of surface characteristics (including surface topography, roughness, curvature radius, and wettability), and CO2 dissolution on the solution's activity. The critical nucleation radius and energy barrier of CO2 hydrate nucleation are determined. Effects of pressure, temperature, wall characteristics and solution activity on the nucleation radius of CO2 hydrate are discussed. The results of this paper contribute to advancing carbon capture and storage technology based on hydrate methods.

We propose a mesoscopic approach for investigating steady/transient nanoscale evaporation heat transfer characteristics, whose kinetic nature makes it an ideal tool to model the evaporation kinetics at the liquid-vapor interface and the microscopic wall-fluid interactions. Simulation of the evaporation of a flat nanoscale thin liquid film demonstrates its capability of resolving kinetically limited evaporation and liquid film adsorption. For the simulation of an evaporating meniscus in a nanochannel, our proposed mesoscopic approach considers the conservation of mass, momentum and energy of both liquid and vapor phases, thereby naturally incorporating both the multi-dimensional heat transfer effect and vapor transport resistance in nano-confined space. We demonstrate that solid-fluid interaction plays dominant roles on the interfacial transport during nanoscale evaporation, and vapor transport resistance has nonnegligible influences on the evaporation heat/mass transport in nano-confined spaces. By handling statistical behavior of molecule ensembles, this mesoscopic approach not only preserves microscopic physics but also has high computational efficiencies, enabling the simulation of space and time domains on the practical application level. This work paves the way for quantitative investigations of steady/transient nanoscale evaporation heat transfer characteristics using mesoscopic approach.

AbstractSurface design by tailoring topographical features and interface function groups to modulate dynamic or kinetic behaviors of liquid droplets, has been an increasing hotspot due to its broad spectrum of applications in biochemical diagnosis, microfabrication, and energy conversion systems. Here we report an engineered surface decorated by packed nanosized caltrops resulting from two perfectly articulated oxidation processes, where self-assembled nanoislands generated in the 1st plasma oxidation serve as protective masks in the 2nd chemical oxidation. As caltrops per design can effectively block lateral motion, the present surface can anchor contact lines of advancing water films when being hydrophilic and selectively capture impinging droplets when being hydrophobic. Furthermore, biphilic patterns can be readily obtained by integrating nanocaltrops with other surface asperities, engendering directional droplet maneuvering and designated droplet arraying. This work provides guidelines in designing nanostructures that achieve on-demand manipulation of droplets and flow patterns for multifunctional applications.

Considering the urgent need for enhancing critical heat flux (CHF) on the outer surface of the reactor pressure vessel (RPV) by in-vessel retention (IVR) on Generation III pressurized water reactors (PWRs), we prepare a porous coating of 316 L stainless steel on the surface of SA508 Grade 3 (SA508-3) steel by flame spraying in this study. Using a small-scale pool boiling experimental setup, we first investigate boiling heat transfer characteristics on bare surfaces and flame-sprayed porous coating surfaces of SA508-3 steel in deionized water at atmospheric pressure, and obtain pool boiling curves and CHFs at different inclination angles. Furthermore, we carry out the IVR engineering verification tests for a full-height RPV bottom head on the REPEC-II test facility built at Shanghai Jiao Tong University. Our experimental results demonstrate that the flame-sprayed porous coatings can significantly enhance the CHF by 21-50 % at different inclination angles of the RPV bottom head. Our full-height facility tests demonstrate that the CHF in the high-inclination angle region, which normally has very high heat flux due to the focusing effect, exceeds 2 MW/m2 on the flame-sprayed porous coating surface. Therefore, our flame-sprayed porous coating has the potential to improve the safety margin of RPV.