Mixed-wettability Surfaces Research Articles

A molecular dynamics study of thin water layer boiling on a plate with mixed wettability and nonlinearly increasing wall temperature

This paper introduces a molecular dynamics investigation of the influence of mixed-wettability on the boiling of a water layer over a flat plate surface with nonlinearly increasing wall temperature. The first-type Dirichlet temperature condition, which is considered for the first time in the analysis of the wettability influence on nucleation and boiling at the atomic scale, is consistent with the case of irradiation from photons, plasma, or high-temperature gas molecules. The simulation of the density evolution of water molecules in the nucleation zone shows that the surface with an optimal proportion (60 %∼70 %) of hydrophilic walls is most efficient for nucleation on the studied mixed-wettability substrates, attributing to the increased surface potential energy on the walls. Compared to the hydrophilic or hydrophobic surfaces, the mixed-wettability surfaces transfer more energy from the wall to the liquid. This is because the hydrophobic portion retards the forming of vapor layer and the hydrophilic portion induces an efficient liquid/solid interaction, with a lowered interfacial heat resistance and promoted nucleation. The results also indicate that there exists an appropriate area ratio that is most advantageous for nucleation and boiling intensification under such temperature-increasing boundary conditions. Based on this work, it may be possible to successfully manage boiling at the nanoscale by exploiting the coupled effects of hybrid wettability and irradiation.

Numerical study on heat and mass transfer characteristics of surface wettability

It has been shown that wettability has an important effect on the surface’s heat transfer performance. In this article, the effects of different wettability of the surfaces of the micropillar array on the heat transfer performance under the same working condition (single-wettability and mixed-wettability surfaces) are compared by numerical simulation, the effects of the surface’s geometrical structure on the flow state of the workpiece and the behavior of the bubbles are also described. It is found that the mixed wettability surfaces combine the characteristics of hydrophilic and hydrophobic surfaces can effectively reduce the wall temperature. The microcolumn array further enhances the boiling heat transfer effect by perturbing the flow process of the workpiece while increasing the heated area. The surface structure of the hydrophobic top combined with hydrophilic sidewalls can significantly improve the heat transfer performance by 10%∼40% during the stable boiling process.

Numerical investigation on pool boiling heat transfer with trapezoidal/inverted trapezoidal micro-pillars using LBM

Pool boiling with high-efficient heat transfer process is crucial to chemical engineering, nuclear engineering, electronic cooling, and other energy conversion applications. Surface modification technology has attracted more attention in recent years as a simple and reliable boiling enhancement technique. Most of the studies using surface modification technology have focused on the enhancement effect of rectangular micro-structures, only a few researchers have adequately investigated the enhancement effects on pool boiling heat transfer and mechanisms of other complex micro-structures (like circle, trapezoid, or inverted trapezoid). In the present study, the pool boiling processes on surfaces with trapezoidal/inverted trapezoidal micro-pillars were numerically investigated using lattice Boltzmann method (LBM). The bubble dynamics and heat transfer characteristics during the pool boiling processes on surfaces with different pillar bottom angles (30° ≤ θ ≤ 135°) were compared under different wall-superheat conditions. The orthogonal tests were adopted on micro-pillars to optimize the geometric parameters, including the bottom angle of the micro-pillar, the bottom width of the micro-pillar, the number of the micro-pillar, and the height of the micro-pillar. A new CHF correlation on the mixed wettability surface with trapezoidal/inverted trapezoidal micro-pillars was proposed. It can be found that the main mechanism of nucleate boiling heat transfer on the micro-pillars is the evaporation of the microlayer, and the pillar bottom angle reveals a remarkable effect on the pool boiling performance on the mixed wettability surfaces. The surface of θ = 135° has the minimum wall-superheat at the onset of nucleate boiling (ONB). Within a wide range of wall-superheat conditions, the heat transfer efficiency of pool boiling firstly increases with the increasing of the bottom angle of the micro-pillar and subsequently decreases. The surface of θ = 60° has the highest critical heat flux (CHF). The surface of θ = 90° has the highest heat transfer efficiency in the film boiling process. The results of orthogonal tests suggested that the bottom angle of the micro-pillar is the most significant influential factor, and the optimal geometrical combinations were proposed. The new CHF correlation has acceptable accuracy in predicting the CHF on surfaces with trapezoidal/inverted trapezoidal pillars, which provides guidance for the design and optimization of micro-structured surface modification in various pool boiling applications.

Numerical Investigations on Enhancement of Pool Boiling Heat Transfer on a Mixed Wettability Surface Employing Lattice Boltzmann Method

Abstract In this paper, a systematic numerical study of pool boiling heat transfer on a mixed wettability heated surface is done using the lattice Boltzmann method (LBM) with a multiple relaxation time (MRT)-based collision operator. The effect of the design parameters, viz, size of the hydrophobic patch (D), spacing between hydrophobic patches (L), number of hydrophobic patches (N), and uneven-sized patches, on pool boiling was studied and results are explained through detailed analysis of bubble nucleation, growth, coalescence, and departure from the heated surface. The results show that mixed wettability surfaces with strategically sized and positioned hydrophobic patches on a hydrophilic surface can result in high heat flux for pool boiling across the entire range of surface superheat or Jacob number (Ja) by combining the advantages of hydrophobic surface in nucleate boiling and hydrophilic surface in transition and film boiling. Further, the mixed wettability surface can delay the onset of film boiling compared to a pure or superhydrophilic surface thereby resulting in higher critical heat flux (CHF). A hydrophobic to total surface area ratio of 30–40% was found to be optimal for all ranges of surface superheat or Jacob number (Ja), which agrees well with the experimental result of 38.46% reported by Motezakker et al. (2019, “Optimum Ratio of Hydrophobic to Hydrophilic Areas of Biphilic Surfaces in Thermal Fluid Systems Involving Boiling,” Int. J. Heat Mass Transfer, 135, pp. 164–174).

Effect of mixed wettability surfaces on flow boiling heat transfer at subatmospheric pressures

Subatmospheric flow boiling heat transfer is a promising method for electronics cooling due to lower saturation temperatures. However, pressure is a crucial parameter that affects surface tension and vapor density. In this study, the effect of surface mixed wettability configuration on bubble dynamics and flow boiling was investigated under atmospheric and subatmospheric pressure conditions. Superhydrophilic, superhydrophobic, and mixed-wettability surfaces were prepared and tested at various heat fluxes and three system pressures of 48 kPa, 68 kPa, and 101 kPa. The channel dimensions were 50 mm × 15 mm, and the channel had a depth of 1 mm. The results showed that biphilic surfaces enhanced the performance up to 28% compared to superhydrophilic surfaces at high heat fluxes for subatmospheric boiling. Flow visualization efforts reveal that mixed-wettability surfaces improve heat transfer by extending the efficient slug regime to higher heat fluxes by preventing dried spot formation. These surfaces benefit from high density nucleation sites at low and medium heat fluxes, resulting in a noticeable performance improvement compared to the superhydrophilic surface. The obtained experimental data in this study will be helpful for the development of thermal-fluid systems operating under subatmospheric conditions.

Research on preparation and water collection characteristics of bionic pattern surface for multi-order combination - multi-segment transport

Collecting water from the atmosphere is seen as a potential way to alleviate global water shortages. Although the large and complex water collecting pattern can quickly transport and collect droplets on the surface where the pattern is located, the collection of droplets is limited by the large-area transport distance and the preparation of complex patterns, and the water collecting pattern cannot completely cover the surface of the material and cannot collect droplets outside the pattern. Therefore, flexible application and amplification cannot be obtained. Aiming at the problem of large-area directional transportation, the characteristics of bionic triangular channel （BTC）and linear channel （LC） are studied in this paper. Combined with their characteristics, the water collection pattern surface of multi-order combination - multi-segment transport（MC-MT） is prepared by laser direct writing technology. This paper systematically describes the water collection process and explores the influence of parameters of BTC and LC on the water collection efficiency, and compares the water collection efficiency of single superhydrophobic, superhydrophilic and mixed wettability surfaces. The experimental results show that the BTC has the characteristics of the directional liquid collection, and the optimal interval of the linear channel is 0.3 mm, and the angle is 15°. The mixed wettability surface prepared by combining the two characteristics is 125.9 % and 242.9 % higher than a single superhydrophobic and superhydrophilic surface, respectively. The surfaces have strong self-transport and directional transport capabilities, and can still collect well into the designed water collection area even over large surface distances, which is expected to provide a reference for the design of efficient water collection systems and micro-titration directional manipulation.

Bouncing dynamics of impact droplets on bioinspired surfaces with mixed wettability and directional transport control

Kingfishers stand on a branch, and raindrops tumble translationally from feathers during raining, enlightening functional surfaces design and liquid transport control. Far-ranging studies on oriented transportation are confined to vertical impacting, which is, to date, in-depth philosophy of horizontal droplets transport on motionless surface deems to be rather serviceable. This study, employed mixed-wettability surface inspired by kingfishers’ feather, occurs on directional transportation issues, such as the synergies of wettability-controlled, driving force and transportation capability. Here we conduct both experimental testing and CFD-aided numerical modelling to reproduce the asymmetric bouncing and directional transport phenomena. We found that the anisotropic surface manipulates to convert normally vertical impacting to horizontal droplets transport. Law of the thrown droplet, on the other hand, is predominated by the wettability-controlled surface, while the coexistence of contact angle difference and surface offset location cooperatively dictates the intensity and patterns of the thrown droplet. Of all these factors, the post-optimized surfaces are designed first and then the regime map of transportation pattern is elaborated. Results manifest that the elements induce the maximum horizontal transport distance by up to 6.2D0, and first desorption time is only 7.8 ms. The findings shed light on engineering design principles that can pave the way for novel applications in anti-icing, lubrication, and spray cooling.

Split of droplets at the nanoscale using mixed-wettability surfaces: A molecular dynamics simulation

The present work is to investigate nanodroplets impacting mixed-wettability surfaces, which are hydrophilic surfaces decorated by hydrophobic stripes, via molecular dynamics (MD) simulations. Based on various impact conditions (W/D0 = 0.1 to 1.5, and We = 1.5 to 410), the different impacting regimes are identified, including the stable non-splitting, coexisting regime, stable splitting, and hybrid regime, where W is the width of hydrophobic stripes and D0 is the diameter of droplets. There shows a significant scale effect for droplets splitting due to the intermolecular force, i.e. existing stable non-splitting regime. And, a new type of droplet splitting pattern is found, namely hole splitting, which would produce some daughter fragments during the impact-splitting process. The splitting time acting as an important parameter for evaluating the performance of splitting dynamics is discussed. Two splitting regimes are distinguished by the dependence/independence of splitting time to the impact We. Finally, a criterion for separating the stable splitting regime by the increasing W/D0 is obtained using a simple energy model. This work paves the way to understanding how to manipulate droplet splitting at the nanoscale.

Lattice Boltzmann method for simulation of solid–liquid conjugate boiling heat transfer surface with mixed wettability structures

Based on the one-component multiphase lattice Boltzmann method, a novel solid–liquid conjugate boiling heat transfer pseudo-potential lattice Boltzmann (LB) model is tentatively proposed in this paper. By respectively introducing the physical property parameters of solids and liquids into the relaxation time τT of the temperature distribution equation, different energy transfer rates in solid, liquid, and vapor regions can be successfully predicted. After verifying the accuracy, stability, and reasonability of this model, the bubble detaching behavior and boiling heat transfer performance on the rectangular cavity structure are analyzed through setting different contact angles of the cavity surface and plane heating surface. The results show that the hydrophobic cavity surface can initialize bubble nucleation earlier and obviously increase the bubble detaching frequency because of its gas-bounding character, while the hydrophilic plane heating surface can restrict the expansion of bubbles and delay the appearance of film boiling. Moreover, for uniform wettability surfaces, the bubble detaching period varies in the quadratic equation with the surface contact angle due to the interaction of surface tension and buoyancy, and there is a minimum detaching period. While for the mixed wettability surfaces, the bubble detaching period also has a minimum value with the decrease in the contact angle the cavity surface, but the average bubble detaching diameter basically does not change with the cavity surface contact angle; moreover, the cavity surface contact angle corresponding to the minimum detaching period also increases with the increase in the plane heating surface contact angle. In addition, for the boiling heat transfer surface with cavity structure, the maximum heat flux and temperature gradient occur on the cavity surface, and the local heat flux of the hydrophobic cavity surface is higher than that of the hydrophilic cavity surface. This work will provide useful help for the further development of the conjugate boiling heat transfer LB model and clarify the mechanism of enhanced boiling heat transfer on a mixed wettability surface.

Mix wettability surface on solar still cover for freshwater productivity enhancement

Water purification using renewable energy sources is prevalent to produce clean water for human consumption and sanitation. Solar still is a water distillation method that uses solar energy as a heat source to produce fresh water. Selection of a solar still condensing cover material remains a challenge, particularly for enhancing the condensate formation and collection. Therefore, the application of a mixed wettability surface on the solar still condensing cover is examined and investigated. A commercial water-based silicone coating is used on a glass cover with different surface areas. The contact angle and surface profile for both coated area and uncoated area of the surface was examined to measure the wettability of the surfaces. The freshwater output was observed for the solar still at a steady-state condition in a laboratory experiment. The addition of coating showed improvement to the solar still productivity with the highest freshwater output at 30% coated surface area. However, by increasing the coating surface area above 50%, the inner cover temperature increased due to the increase in thermal resistance from the coating layer. A coating surface area of below 50% showed to be more cost-effective compared to a bare glass cover.

Experimental investigation of natural convection bubbly flows along a vertical heated plate with mixed wettability surfaces

Experimental investigation of natural convection bubbly flows along a vertical heated plate with mixed wettability surfaces

Molecular Insight into Bubble Nucleation on the Surface with Wettability Transition at Controlled Temperatures.

A surface with a smart wettability transition has recently been proposed to enhance the boiling heat transfer in either macro- or microscale systems. This work explores the mechanisms of bubble nucleation on surfaces with wettability transitions at controlled temperatures by molecular simulations. The results of the interaction energy at the interface and potential energy distribution of water molecules show that the nanostructure promotes nucleation over the copper surface and causes lower absolute potential energy to provide fixed nucleation sites for the initial generation of the bubble nucleus and shortens the incipient nucleation time, as compared to the mixed-wettability or hydrophilic nanostructure surface. An investigation on more nanostructured surfaces shows that a surface (F) with a wettability transition temperature of 620.0 K has the shortest average incipient nucleation time at 1672 ps with a wall temperature of 634.3 K. The surface with tunable wettability has also a high interfacial thermal conductance at low superheats, but it may not promote the critical heat flux at high superheats. The heat-transfer performance of the smart surface is better than the plate, the hydrophobic nanostructure, and the mixed-wettability surfaces, while it is lower than the hydrophilic nanostructure surface. This proposes a new method and provides insight for promoting bubble nucleation on a surface with temperature-dependent wettability.

Gradient mixed wettability surfaces for enhancing heat transfer in dropwise flow condensation

Steam condensation plays a pivotal role in a broad range of industrial applications. Dropwise condensation, which occurs on hydrophobic and superhydrophobic surfaces, has been proven to have a larger heat transfer coefficient than filmwise condensation because of enhanced droplet detachment. However, hydrophilic surfaces possess a better droplet nucleation rate compared to hydrophobic and superhydrophobic surfaces. These two characteristics (high droplet nucleation rate and better droplet removal) require counteracting properties. Many studies increased the nucleation rate by creating nanostructured surfaces. Wettability contrast mechanism is another approach, which could achieve both high nucleation rate and droplet removal, and have attracted much attention in enhancing a better heat transfer performance. In this study, gradient mixed wettability surfaces were developed on copper surfaces to improve heat transfer during steam flow condensation inside a minichannel. Circular hydrophobic islands were fabricated on a nanostructured superhydrophobic background surface. Various configurations of different island and pitch sizes are considered. Real-time images captured by a high-speed camera are used to examine the droplet dynamics and the influence of wettability on droplet size distribution. Thus, a parametric study is conducted to present the optimum conditions at different steam mass fluxes. According to the heat transfer analysis results, the heat transfer coefficient could be enhanced by 37% compared to a plain hydrophobic surface. A higher ratio of hydrophobic area provided better conditions for droplet nucleation and growth, while larger superhydrophobic regions in the downstream section was advantageous for droplet removal and sweeping. As a result, the role of mixed wettability in achieving the best performance was revealed for dropwise flow condensation.

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

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

Manipulating the heat transfer of pool boiling by tuning the bubble dynamics with mixed wettability surfaces

Boiling heat transfer is widely involved in various energy conversion and utilization systems owing to its high heat transfer ecoefficiency even at low temperature differences. It is still a great challenge to control the boiling behaviors due to their multiple influencing factors and their randomness of bubble behaviors. The wettability greatly affects the boiling heat transfer. Hydrophobic surfaces can promote bubble nucleation and increase the heat transfer coefficient of nucleate boiling, while hydrophilic surfaces can promote bubble detachment, inhibiting film boiling, and hence increasing the critical heat flux. In order to meet such mixed requirements of wettability at different boiling stages, we propose a scalable and affordable method to fabricate the hydrophilic / hydrophobic mixed wettability surface. Compared with pure hydrophilic or superhydrophobic surfaces, the mixed surfaces exhibit higher heat transfer coefficient than the hydrophilic surface at low superheat degrees but larger critical heat flux than the hydrophobic surface at high superheat degrees. Therefore, the mixed wettability surfaces are more adaptable for a wider range of inputted heat flux. Such boiling heat transfer characteristics is the resultant of the unique bubble dynamics. The bubbles are likely to pinned on the hydrophobic spots but more easily detach from hydrophilic area. Tuning the hydrophilic / hydrophobic area ratio can manipulate the bubble dynamics. A numerical model is used to mimic the boiling on the hydrophilic / hydrophobic mixed surface, which agrees with the experimental results. And then the optimal area ratio is suggested by a boiling phase diagram based on a large number of simulations with various area ratios. Besides proposing a scalable and affordable way to fabricate the hydrophilic / hydrophobic mixed wettability surface, we also provide details of bubble dynamics on the pure or mixed wettability surfaces. Such details facilitate to understand the enhancement mechanisms of boiling heat transfer on mixed surface, which is still an open issue. The effects of the area ratios on the heat transfer coefficient are also discussed, which would provide a guideline to fabricate the mixed boiling enhancement surfaces.

Effect of Functional Surfaces with Gradient Mixed Wettability on Flow Boiling in a High Aspect Ratio Microchannel

Flow boiling is one of the most effective phase-change heat transfer mechanisms and is strongly dependent on surface properties. The surface wettability is a crucial parameter, which has a considerable effect on the heat transfer performance, particularly in flow boiling. The contact angle determines the number of nucleation sites as well as bubble dynamics and flow patterns. This study introduces three new generation mixed wettability surfaces and compares them with a wholly hydrophobic surface reference sample, in flow boiling in a high aspect ratio microchannel. The mixed wettability substrates have five regions as fully Al2O3, (hydrophobic zone) region, three different patterned configurations with various A* values, and fully SiO2 (hydrophilic zone) region, where A* is defined as A Al2O3/A total (hydrophobicity ratio). Boiling heat transfer results were obtained for each surface at various wall heat fluxes and three different mass fluxes. According to the obtained results, significant enhancements in heat transfer (by up to 56.7%) could be obtained with biphilic surfaces compared to the reference sample (hydrophobic surface). Performed flow visualization proves that the tested biphilic surfaces enhance heat transfer by reducing the bubbly flow regime and extending the slug regime.

Lattice Boltzmann study of bubble dynamics and heat transfer on a hybrid rough surface with a cavity-pillar structure

In this work, a two-dimensional phase-change lattice Boltzmann model is used to investigate the pool boiling from a hybrid rough surface with a single cavity and one or two pillars on the superheated plate for an entire ebullition cycle. The bubble growth and departure dynamics on the hybrid pillared cavity surfaces, namely a single-pillared cavity surface (a surface with one cavity and one neighbouring pillar) and a double-pillared cavity surface (a surface with one cavity and one pillar on its both sides), are also compared with a simple cavity surface (a surface with only one cavity). The results showed that the incorporation of the pillar to the simple cavity surface increases the surface area and distance between the pinning locations of the three-phase contact line (TCL), and thus, enhances the bubble departure diameter and heat flux. It is found that bubble departure diameter and heat flux increase with cavity-pillar spacing up to a certain value, in this case sp∗ = 0.50, beyond which they start to decease and approach the simple cavity surface case at sp∗ = 1.0, while the opposite trend is observed for bubble departure frequency. The bubble departure diameter, frequency and heat flux also increase with pillar width. Compared to uniformly hydrophilic surfaces, the mixed wettability surfaces exhibit large heat flux with relatively small increase in bubble departure diameter.

Enhancement of nucleate boiling by combining the effects of surface structure and mixed wettability: A lattice Boltzmann study

The combination of microstructures and mixed wettability for enhancing nucleate boiling has attracted much attention in recent years. However, in the existing experimental and numerical studies, the tops of microstructures are entirely subjected to wettability modification, which makes the influences of mixed wettability dependant on the characteristic length of microstructures. In order to disclose the joint effects of surface structure and mixed wettability on nucleate boiling, in this work we propose an improved type of pillar-textured surface with mixed wettability, in which the tops of square pillars are partially subjected to wettability modification. Numerical investigation of the boiling heat transfer performance on the improved mixed-wettability surface is carried out using a three-dimensional thermal multiphase lattice Boltzmann model. The numerical results show that the width of the wettability-modified region plays an important role in the boiling performance of the improved mixed-wettability surface and the best boiling performance is achieved in the situation that the width of the wettability-modified region is sufficiently large but the bubble nucleated on the pillar top still does not interfere with the coalescence-departure mechanism of the bubbles nucleated around the pillar, which optimizes the joint effects of surface structure and mixed wettability for enhancing nucleate boiling. The influences of the shape of the wettability-modified region are also studied. Among the investigated shapes, the square is found to perform better than the other two shapes.

Deterioration of boiling heat transfer on biphilic surfaces under very low pressures

Surface wettability engineering has attracted growing attention in recent years as an effective tool to enhance boiling heat transfer. On wettability-patterned (so-called biphilic) surfaces in particular, subatmospheric boiling has been shown to be nearly free of the severe degradation of heat transfer rate that tends otherwise to prevail on plain surfaces. The surprisingly consistent performance under reduced-pressure conditions can be attributed to the rather strong pinning of the three-phase contact line (TPCL) at the border between the hydrophobic and hydrophilic surfaces, which essentially eliminates the waiting period between bubble cycles. Only when the pressure is decreased sufficiently low does the transition to the undesired mode of intermittent boiling eventually occur on the biphilic surface. The purpose of the present study is to investigate the physical mechanism for the heat transfer deterioration on a mixed-wettability surface at very low pressures. To that end, we performed high-speed visualization experiments of the process of bubble nucleation and growth on a smooth copper surface coated with a single hydrophobic polytetrafluoroethylene (PTFE) spot, under different surface superheats and system pressures. The results show an interesting correlation between the TPCL behavior and the bubble growth dynamics. Specifically, under some certain threshold of pressure, it would become increasingly likely for the TPCL to be dislodged from its pinned position on the biphilic surface under a particularly rapid bubble expansion. As a result, full flooding of the hydrophobic surface might ensue, which is deemed responsible for temporary deactivation of the hydrophobic spot as a viable nucleation site. Furthermore, based on the diffuse-interface simulations of TPCL propagation across heterogeneous wettabilities, a comparison of cases with different bubble expansion rates offered qualitative evidence supporting the critical role of such accelerated bubble growth rate in driving the TPCL to overcome the energy barrier raised at the wettability divide.

Molecular dynamics study of rapid boiling of thin liquid water film on smooth copper surface under different wettability conditions

This work aims to present theory of heat transfer on rapid boiling for pure wettability surfaces and mixed wettability surfaces based on molecular simulation. The simulation results showed that the temperature of water increased obviously and the maximum evaporation rate of water reduced significantly with the increase of hydrophilic for pure wettability surfaces. Furthermore, a microfluidic layer which was formed on all cases except hydrophobic surface (surface 4) was conducive to critical heat flux (CHF) improvement. Meanwhile, the attraction of water molecules upon hydrophobic region was smaller than that on hydrophilic region which was conducive to boiling heat transfer coefficient (HTC) improvement. A mixed wettability surface enhances boiling heat transfer by regulating vapor spreading behaviors over the heating copper surface compared with pure wettability surfaces.