Artificial Nucleation Sites Research Articles

Effect of an artificial cavity on the microlayer and contact line dynamics during bubble growth in nucleate boiling

We present an experimental study on the near-wall phenomena during the growth of a single bubble in saturated pool boiling of water at atmospheric pressure. Our focus is on the dynamics of triple contact line and liquid microlayer that can form between the heater and the liquid-vapor interface of the bubble. The microlayer thickness, the wall temperature distribution and the bubble shape are measured simultaneously and synchronously at 4000 fps by white light interferometry, infrared thermography and sidewise shadowgraphy, respectively. To study the effect of cavities (artificial nucleation sites) we compare two experiments using different heaters. In the first experiment, the bubble grows on a smooth surface of nanometric roughness whereas in the second, the bubble grows over a cylindrical cavity of 25 µm diameter and 50 µm depth. We found that the cavity reduces three times the required wall superheating to trigger the bubble growth. Moreover, the radii of the bubble, microlayer and dry spot are smaller by half and the macroscale bubble dynamics is also slower. The microlayer is thinner and is measurable in a larger portion of its extent. Based on the absence of interference fringes near the contact line (due to high interface slopes) and on recent numerical simulations, we conclude that the microlayer consists in two regions: a dewetting ridge near the contact line that grows over time and a flatter and wider region that thins over time. The microlayer can be seen as a film deposited by the receding meniscus and its profile is controlled by the viscous and surface tension effects; its thinning over time is due to local evaporation only. The ridge is a result of liquid accumulation due to contact line receding and strong viscous shear in the film.

Vapor bubble on a single nucleation site : Temperature and heat flux measurements

This work aimed at gaining a detailed understanding of heat transfer when boiling occurs on a heated substrate. More specifically, this work focuses on thermal transfers occurring at wall-fluid interfaces at the transition between natural convection and nucleate boiling regimes in pool boiling experiments. To avoid parasitic effects, the vapor bubble is created at a single artificial nucleation site. The boiling cell is instrumented with sensors for temperature measurement, pressure control, and parietal heat flux measurement. The fluid tested is Fluorinert FC-72. For providing a boiling surface and measuring heat flux, a boiling-meter consisting of a heating resistance, heat flux sensors and two thermocouples is used. This boiling-meter enables heat flux generation, the measurement of both temperature and heat flux, and can be rotated through 360°, enabling the influence of inclination to be studied. The boiling-meter is indented at its center to create a single vapor bubble. Thermal measurements are obtained and studied for different inclination angles between 0° and 180° at the saturation conditions. The results showed the nucleation site’s recurrent pattern of being active and inactive for whatever is the boiling surface inclination. Preliminary results are presented and discussed.

Influence of subcooling conditions on pool boiling in microgravity conditions: Single artificial case

A series of boiling experiments on a single artificial nucleation site has been carried out on the International Space Station (ISS) in the framework of the Multiscale Boiling (RUBI) setup installed in space in 2019. The aim of these unique experiments is to understand the bubble nucleation-growth mechanisms involved in heat transfer during boiling under well-controlled operating conditions without being masked/impeded by gravity and natural convection. The bubbles can grow to large sizes that are inaccessible in the presence of gravity. They are observed by a side-view black-and-white camera and by an infrared camera through a transparent heated substrate. In this paper, we present the results of a single-bubble pool-boiling experiment with an emphasis upon the role of the surrounding liquid subcooling, several values of which are investigated. The experimental results are complemented by numerical simulations using a previously developed model. However, the model is slightly modified to account in a simplest way for possible non-condensable residuals, without which we would have a hard time to explain certain observed behaviors.

Experimental investigation of the impact of surface tension and artificial nucleation site geometry on the bubble growth

The present work provides an experimental assessment of the impact of liquid surface tension on the single bubble dynamics generated by artificial nucleation cavities during boiling in a quiescent liquid. De-ionized water is used with and without added non-ionic surfactant (Dynol 800) to modify the fluid’s surface tension while keeping the other thermo-physical fluid properties unchanged. Furthermore, two different cavity geometries are investigated: a vertical and a 30°-inclined cone produced via femtosecond laser texturing. This paper shows preliminary results related to the influence of surface tension and cavity geometry on the single bubble dynamics. A methodological baseline is provided to enable future studies aiming to answer still-open questions about vapour bubble nucleation phenomena.

Preliminary physical analysis of a single-bubble pool-boiling experiment in space: Effect of subcooling and possible non-condensable residuals

The purpose of this work is to study the boiling heat transfer mechanisms in microgravity. Single bubbles growing without departure up to a centimetric size on a single artificial nucleation site are investigated under well-controlled pool-boiling conditions in the framework of the RUBI experiment on the International Space Station. Subcooling is one of the important parameters defining the bubble growth process. In this work, we conduct a first physical analysis of the experiment. The bubble diameter is evaluated in time using a developed algorithm. Several stages are highlighted. It is shown that subcooling has a significant effect on all stages of bubble life and can even lead to a partial collapse. A numerical model is developed to help a better understanding of the phenomenon. Some observations couldn't be explained without evoking the presence of non-condensable residuals. The amount of those is estimated.

Comparative investigation of different types of expanding micro-pin-fin heat sinks at low mass flux

Expansion of cross-sectional area of flow passages in micro-pin-fin heat sinks provides advantages due to suppression of bubble-confinement problem. However, low mass flux in addition to orientation of fins and existence of artificial nucleation sites have great potential in terms of influencing performance of expanding heat sinks. Therefore, in this study, different types of expanding micro-pin-fin heat sinks at a low mass flux (G = 98 kg m−2 s−1) are experimentally investigated for saturated conditions. At a low fluid content, effectiveness of inline or staggered fin configuration and artificial nucleation sites are evaluated via three different expanding heat sinks. For the whole database, B-Type (expanding, having artificial nucleation sites, inline fins) increases the heat transfer coefficient up to 25.5% against A-Type (expanding, no artificial nucleation sites, inline fins). C-Type heat sink (expanding, having largest cross-sectional area at the downstream region, no artificial nucleation sites, staggered fins) is exposed to serious instabilities, early reaches critical heat flux (CHF). Due to CHF, first five data can be obtained for C-Type; therefore, up to 130 W, on first five data, A-Type increases heat transfer coefficient up to 1011% compared to C-Type; while B-Type increases heat transfer coefficient up to 1101.3% compared to C-Type.

Experimental study of bubble growth on novel fin structures during pool boiling

Boiling heat transfer associated with phase change is perhaps one of the most efficient cooling methodologies to manage extreme heat flux due to its large latent heat. Fin structures are used to further increase the magnitude of boiling heat transfer from the heated surface and have shown better performance than flat surface heat sinks. This work aims to experimentally investigate the heat transfer performance of two fin structures, namely regular and modified fins, in a pool boiling facility. The modified hollow fin structure is designed to enhance the regular fin's heat transfer performance by adding an artificial nucleation site. Heat transfer rates and heat transfer coefficients of the two fin structures are estimated in atmospheric pressure conditions using deionized water and compared with the literature. The results show that the regular fin heat sink shows a better heat transfer rate than the plane surface, while the modified fin structure shows higher heat transfer performance than the regular fin. This is attributed to the additional nucleation sites on the hollow fin, a better rewetting phenomenon, and therefore a favorable bubble growth and release mechanism. Also, a multilayer perceptron artificial neural network with a back-propagation training algorithm is applied for modeling the bubble departure diameter concerning wall superheat and subcooling level to predict the bubble behavior from the artificial nucleation site.

Novel flat plate pulsating heat pipe with ultra sharp grooves

• Diffusion bonding was applied for the fabrication of flat plate PHPs. • Ultra-sharp grooves in the evaporator improved the heat transfer capacity of a PHP. • The proposed PHPs perform effectively for heat fluxes up to 1200 W (20.9 W/cm 2 ). • The gravity influence becomes negligible for powers beyond 600 W (10.4 W/ cm 2 ). • The PHPs appear to be a good alternative for temperature control of electronics. Pulsating heat pipe (PHP) is a very efficient solution for electronics cooling. Several strategies can be applied to improve the thermal performance of PHPs. In this context, the heat transfer enhancement of a flat plate pulsating heat pipe with a channel modification in the evaporator region, resulting in ultra sharp lateral grooves, was investigated experimentally. Chamfers were machined in the lateral walls of thirteen semi-circular cross section U-turn channels, drilled in flat copper plates. To form the sharp-grooved circular channels, two plates were faced against each other and diffusion bonded, resulting in a monolithic piece with high quality channels. The ultra sharp grooves had an angle of 29.1 ± 2.9°. The lateral grooves work as artificial nucleation sites, helping in the bubble formation, and act as a capillary medium, spreading the liquid over the evaporator region, delaying the dry-out. Therefore, the device could be less dependent on gravity, enabling it to be considered for applications in microgravity environments. To ascertain the efficiency of the proposed device, its performance was compared with another similar PHP with the same external geometry and with round ordinary cross-section channels of the same 2.5 mm channel diameter. Distilled water was selected as the working fluid, which, as predicted by literature models, worked at confinement conditions. As usual, the thermal behaviors of PHPs were characterized by their temperatures and pressures, depending on the operation conditions. The best filling ratio for each PHP was experimentally determined, considering heat loads from 20 up to 350 W (from 0.35 up to 6.1 W/cm 2 ). The influence of the ultra sharp grooves on the thermal performance of the PHP was investigated for a large range of power inputs, reaching up to 1200 W (20.9 W/cm 2 ), for the best filling ratio. The gravity influence in the PHP operation was evaluated by tests in three orientations: gravity-assisted, horizontal and against-gravity. Both cross-section profile PHPs performed effectively well for heat fluxes up to 20.9 W/ cm 2 , even in the against-gravity position, showing that the devices are suitable for temperature control of electronics, including those with high heat fluxes. Besides, the gravity effect could be neglected for heat powers beyond 600 W (10.4 W/ cm 2 ), which make them adequate for microgravity applications. The presence of ultra sharp grooves in the evaporator section of the PHP reduced by 2.1 °C the average evaporator temperature, decreased the temperature variations among sections and improved the thermal performance by 12% in the horizontal and gravity-assisted orientation.

Isolated bubbles growing and detaching within an electric field in microgravity

The experimental outcomes of single bubbles nucleated and growth from a heated surface immersed in an electric field in high-quality microgravity level are presented. Data were obtained between September 2019 and January 2021 from the European experiment known as Reference mUltiscale Boiling Investigation (also multiscale boiling project), in which single bubbles of FC-72 were nucleated on a heated surface, on-board the International Space Station. In the experiments reported here, which are only a part of the complete dataset, an electrostatic field is imposed in the boiling region by means of a washer-shaped electrode, centred above the nucleation site. The experimental apparatus is designed to initiate a single bubble from an artificial nucleation site machined in the surface. Such bubbles are heavily distorted by the electric stresses; in particular, contact angles and contact line length increase with electric field intensity. While in the absence of electric field no detachment occurs, when the electric field is applied the bubbles are continuously and regularly sucked towards the electrode as they are attracted to regions of weaker electric field; a detailed analysis of the electric forces is provided as well. The significant contribution of electro-convection is highlighted by the bubbles growth rate. These experimental observations contribute to the insight of the basics of boiling and the effects of an electric field on the ebullition cycle and show promising opportunities for practical application of electric fields in space.

Heat flux partitioning and macrolayer observation in pool boiling of water on a surface with artificial nucleation sites

In this study, the heat transfer mechanisms in pool saturated boiling of water on a sapphire heat transfer surface with a controlled nucleation site density (NSD) were observed using a high-speed infrared camera. Artificial nucleation sites were arranged on the wall surface by ink-jet printing of a superhydrophobic agent, and NSD was varied between 24 and 318 sites/cm2. The heat transfer characteristics of each fundamental heat transfer process and its contribution to the total wall heat transfer were evaluated through the heat flux partitioning analysis. There was an upper limit to the heat transfer enhancement by increasing the NSD. The boiling heat transfer coefficient (HTC) did not increase monotonically with increasing NSD but had a maximum value at an NSD between 89 and 130 sites/cm2. When NSD was increased excessively, the microlayer formation was inhibited by bubble–bubble interaction. The contribution of the microlayer evaporation to the wall heat transfer also decreased, resulting in a decrease in the boiling HTC. At all tested NSDs, liquid-phase heat transfer dominated the wall heat transfer, as in the case of the bare surface. The contribution of the microlayer evaporation that occupies a small area to the total wall heat transfer was less than 35%. In the high heat flux region, the macrolayer, a liquid layer with suppressed liquid motion, was successfully observed at the bottom of the coalesced bubble.

Combined influence of artificial nucleation site and expanding cross section on flow boiling performance of micro pin fins

Present paper experimentally studies combined influence of artificial nucleation sites and expanding cross section on saturated flow boiling performance of heat sinks with micro pin fins. Thermal and visualization-based flow analysis is conducted. A novel micro pin fin heat sink having artificial nucleation sites and enlarging flow passages is designed; and compared with two different conventional heat sinks (parallel channel and uniform pin fin distribution) at different heating powers (from 90 W to 180 W or 132–272 kW m−2 heat flux range), constant mass flux (approximately 287 kg m−2 s−1) and inlet temperature (nearly 75 °C). Novel heat sink leads to improvement in heat transfer coefficient up to 557.8% against plain walled parallel channel heat sink, and up to 463.8% against uniformly distributed finned one. Compared to heat sink with uniform-distributed fins, a reduction in pressure drop up to 27.7% is achieved via the novel one. Liquid trapping phenomenon by artificial nucleation sites (ANSs), contribution of ANSs to nucleation, vacuum effect created by gradually increasing cross section, and effective prevention of bubble blockage are revealed to be basic underlying physical reasons of thermal and flow-based success.

Numerical study of interfacial dynamics in flow boiling of R134a inside smooth and structured tubes

A numerical study of flow boiling has been performed with R134a refrigerant flowing inside a horizontal tube. Flow pattern has been captured using VOF methodology in OpenFoam software up to mass flux of 150 kg/m2s. The various stages of bubble dynamics like nucleation, growth, merging and departure has been observed for ranges of heat and mass fluxes. The phase fraction variations during flow boiling have also been observed at two different saturation temperatures and the differences in interfacial features have been reported. An attempt has been made to estimate the number of nucleation sites and the bubble departure diameter for R134a in the case of flow boiling inside the smooth tube. With the help of numerical simulations, the study has also been made by providing artificial nucleation sites in the form of indentations/cavities inside the tube. An increase in nucleation sites has been observed for the dimpled tube as compared to the plain tube and the corresponding change in bubble departure phenomenon has been reported. The heat transfer coefficients at different heat flux, mass flux, saturation temperature and tube configurations have been evaluated to link observed interfacial phenomenon with enhancement factor.

Experimental Investigation of Artificial Cavities Effect of Single-Phase Fluid Flow and Heat Transfer in Single Microchannel

In this paper, an experimental study has been conducted to investigate the influence of artificial cavities (artificial nucleation sites, ANS) in a single microchannel on the characteristics of flow and heat transfer at a single-phase flow. The experiments were performed with deionized water as a working fluid at 30OC inlet temperature with a range (108.6-2372) of Reynolds numbers. Three models of the straight microchannel (model-1, model-2, and model-3) were manufactured of brass having a rectangular shape with a hydraulic diameter of (0.42 mm). Model-1 has a smooth surface, while model-2 has artificial cavities with a number of 40 ANS located on the base of the microchannel; along a line adjacent to one of the sidewalls. Also, the artificial cavities of model-3 exist at the base of the microchannel; along a line that is nearest to each sidewall for the microchannel. The number of ANS at each sidewall is 40 (i.e. the overall number of artificial nucleations is 80). The results manifested the enhancement of heat transfer by the presence of ANS for model-2 and model-3 as compared to model-1 by 15.53% and 16.67%, respectively. Also, the results proved that the fanning friction factor correlation for laminar and turbulent flow can predict very well the results (MAE=6.6-7.2%) and (MAE=4.1-7.7%), respectively. Also, the Nusselt number increases with increasing Reynolds number. However, the conventional correlation that predicted the experimental results is lower than the correlations (MAE=30.1%, 13.2% and 12.6%) for Model-1, -2 and -3, respectively.

Heat Flux Partitioning and Macrolayer Observation in Pool Boiling of Water on a Surface with Artificial Nucleation Sites

Heat Flux Partitioning and Macrolayer Observation in Pool Boiling of Water on a Surface with Artificial Nucleation Sites

Influence of hydrodynamic interactions among multiple bubbles on convective heat transfer in nucleate boiling

To clear the effects of the hydrodynamic interactions among neighboring vapor bubbles on the saturated pool boiling heat transfer, artificial nucleation sites with several cavity arrangements were manufactured on the substrate surface and measured liquid temperatures near heated substrate by a miniature thermocouple probe with 10 kHz. The surface temperature of heated substrate was measured by an infrared camera with 1 kHz and bubble dynamics were simultaneously visualized by a high-speed video camera with 5 kHz and 10 kHz, respectively. The liquid temperature fluctuations, measured with micro thermocouple 100 μm and 10 μm above the substrate surface, were compared among each other and with the transient temperature fields obtained through IR measurements. Results showed that the hydrodynamic interaction among neighboring bubbles causes the liquid convection, that was only detected 100 µm above the boiling surface. Generally, the degree of liquid temperature fluctuations at 10 μm height and different arrangements of nucleation sites was under 1 K. Here the superheated sublayer in the vicinity of the boiling surface was generally not affected by the convection. Only when the coalescence from two neighboring nucleation sites occurred and the liquid-vapor interface nearly reached the substrate, a steep temperature drop over 2 K was detected. It was concluded that the viscous sublayer stably exists during boiling of water in lower heat flux conditions. To enhance the convective heat transfer in nucleate boiling, coalesced bubble needs to peel off the viscous sublayer during its deformation. Based on the measurements in this study, this peel-off effect only happens when the coalescence point is less than 1 mm above the heated surface.

Experimental Investigation of Single Bubble Nucleate Boiling in Microgravity

The multiscale boiling project, also known as RUBI, is an experimental project supported by the European Space Agency (ESA) focusing on the nucleate boiling within the framework of the utilization of the International Space Station (ISS). In order to investigate the operational behavior of the experimental components and determine a promising parameter range, an experimental setup with the same heater design is constructed and tested within the similar parameter range in the framework of the ESA 70th parabolic flight campaign. The present work introduces the experimental setup as well as the measurement methodologies. It concentrates on the results obtained for the nucleate boiling in the absence of external forces such as electric field or shear flow, which are also part of the multiscale boiling project. A barium fluoride glass sputtered with chromium on the top of a chromium nitride layer served as the heater. The manufacturing process of the heater substrate is presented in detail. In order to generate a vapor bubble in the subcooled liquid pool, a laser beam is used to provide the necessary heat at an artificial nucleation site situated at the center of the glass. The experimental setup allows for time adjustment between switch-on of the heater and shooting the laser which is defined as preheating time. The effect of the preheating time on the nucleation and behavior of the bubbles is investigated. Furthermore, by selecting a long laser pulse duration the influence of the laser thermal energy on the growth of the bubbles is studied. It is observed that the preheating time has a significant effect on the bubble growth. However, no influence of the laser shot is detected on the bubble growth, even for a long pulse duration.

Artificial nucleation sites with stable SEI for Li metal anodes by aggressive Al pulverization

Uneven nucleation and fragile solid electrolyte interphase (SEI) restrict the practical application of lithium (Li) metal anode. A versatile strategy to simultaneously overcome these two drawbacks is critical for Li metal batteries (LMBs). Herein we propose an “aggressive aluminum (Al) pulverization” strategy to form abundant artificial nucleation sites and robust SEI films adjacently, leading to homogeneous Li deposition/stripping and high Li Coulombic efficiency. A core-shell structure with Al core and alumina-silica (Al 2 O 3 –SiO 2 ) shell, Al@Al 2 O 3 –SiO 2 (AAS), could be easily synthesized, by modulating the chemical diffusion of silicon (Si) element towards Si–Al alloy surface. The active SiO 2 sites not only contribute to form the robust SEI layer, but also facility the lithiation of the AAS, accelerating the conversion of reversible Li–Al alloy into inert Li 3 Al 2 phase. The as-generated AAS with Li 3 Al 2 and Li 12 Si 7 phases possess negligible Li nucleation over-potential (~0.7 mV), serving as an optimal structure for dendrite-free Li plating/stripping with a prolonged life-span of 6 times longer than that of the unprotected counterpart in carbonate electrolytes. Uniform Li deposition and robust SEI films have been enabled by an Al@Al 2 O 3 -SiO 2 core-shell structure via an aggressive Al pulverization strategy for stable Li metal anodes. • A core-shell structure Al 2 O 3– SiO 2 by modulating the chemical diffusion of Si element towards Si–Al alloy surface. • A structure with abundant artificial nucleation sites and robust SEI films for Li metal protection. • Stable cycling of Li||LiNi 0.88 Co 0.09 Al 0.03 O 2 full cells with a high NCA loading of 5 mAh cm −2 .

Impact of surface parameters on confined boiling heat transfer in a Hele-Shaw cell

Confined boiling and bubble dynamics of pure water at atmospheric pressure were experimentally investigated in a Hele-Shaw cell established by two plates with a small spacing. The vapor bubble motion was visualized by means of a high-speed camera for different plate spacing and heating power levels. Four different kind of heating surfaces (copper/alumina and smooth/rough) and one surface with an artificial nucleation site were investigated. The copper and alumina surfaces were polished and grinded in order to investigate the impact of the surface parameters on the confined boiling system. Significant fluctuations of the heat transfer characteristics were observed during low heat flux level boiling. This observation was attributed to an intermittent boiling regime, when confined boiling was interrupted by flooding of the cell. The flooding effect substantially affected the boiling behavior, but it was found that even a single artificial nucleation site was able to stabilize the system at low heat flux level. This effect vanished during moderate heat flux level boiling, and then minor influence of the surface roughness was observed for small spacing values. Micro-layer evaporation was identified as major mechanism for the confined boiling in a Hele-Shaw cell by a computer-based post processing image analysis. Based on the new experimental observations, a theoretical model resting on a modified capillarity number was proposed.

Experimental investigation on bubble departure diameter in pool boiling under sub-atmospheric pressure

The objective of this research is to compare the nucleate boiling characteristic of the calcium chloride aqueous solution with distilled water at sub-atmospheric pressure by analyzing the growing bubble dynamics in order to provide some data for optimizing the design of the dehumidification system. The experiments were carried out with water and calcium chloride solution on the upper surface of a polished stainless steel plate, under sub-atmospheric pressure ranging from 3.6 kPa to 22.0 kPa. An artificial nucleation site was created in the center of the plate to generate the isolated bubbles. A high-speed camera was used to capture the images of dynamic bubbles, and the relevant parameters on bubble dynamics were measured and calculated by frame-by-frame image treatment.Experimental results show that bubble diameter tends to increase with the pressure decrease which means the lower vapor density and stronger surface tension force at sub-atmospheric boiling. The influence of superheat and sub-cooling degree were also analyzed. Additionally, a complex boiling regime of calcium chloride solution with irregular bubble dynamic parameters was observed. Finally, bubble growth dynamics under sub-atmospheric were analyzed and the force balance equation were established. It is shown that the dynamic effect especially the inertial force dominated the growth stage under sub-atmospheric boiling. A new bubble departure diameter correlation within ±20% deviation was proposed.

Realizing highly coordinated, rapid and sustainable nucleate boiling in microchannels on HFE-7100

Flow boiling in microchannels using dielectric fluids is one of the most desirable cooling solutions for high power electronics. However, it is challenging to promote the flow boiling performances, particularly critical heat flux (CHF), due to their unfavorable thermophysical properties. Flow boiling in parallel and isolated microchannels has been extensively studied. In this study, five parallel microchannels (W = 200 µm, H = 250 µm, L = 10 mm) are interconnected by 4 × 28 micro-slots (20 µm wide and 250 µm deep) starting from the middle section to the channel outlet. Our visualization study shows that these micro-slots designed as artificial nucleation sites can enable high frequency nucleate boiling by drastically reducing the bubble waiting time and remaining fully activated, simultaneously. More importantly, such rapid switch on–off harmonically coordinated nucleate boiling in the neighboring channels creates a highly desirable periodic rewetting mechanism to substantially delay CHF conditions and enhance heat transfer rate. Flow boiling in this innovative microchannel configuration has been systematically characterized with mass flux ranging from 462 kg/m2∙s to 1617 kg/m2∙s. Compared to plain-wall microchannels with inlet restrictors (IRs), flow boiling heat transfer coefficient (HTC) is enhanced up to ∼172% at a mass flux of 462 kg/m2·s primarily owing to the enhanced latent heat transfer including nucleate boiling and thin film evaporation. The peak value of effective HTC is ~60 kW/m2·K in the fully developed boiling regime. Moreover, CHF is substantially enhanced by ∼76% at a mass flux of 1155 kg/m2·s owing to the rapid and periodic rewetting enabled by these micro-slots. Such drastic enhancements have been achieved without compromising two-phase pressure drop.