Hybrid Wettability Research Articles

Molecular dynamics study of the mechanism of explosive boiling on hybrid wettability surfaces

In this work, molecular dynamics (MD) simulation is applied to study the effect of heating surfaces with different hydrophobicity occupancy ratios (the ratio of the surface area of hydrophobic spots to the total area of the heating surface) on the boiling process of the liquid film explosion. At the same time, the mechanism is revealed from the trajectory of argon atoms. The simulation results showed that the onset of explosive boiling was later for purely hydrophilic surfaces than for hybrid wettability surfaces with a hydrophobicity percentage of <11%. The earliest onset of explosive boiling was observed for the heated surfaces with a hydrophobicity ratio of 6%. In addition, it was found that the superheat required for explosive boiling tended to decrease first and then increase with the gradual increase of the hydrophobicity ratio. Hydrophobic spots arranged on the surface provided bubble nucleation earlier for explosive boiling while enhancing convective heat transfer and thermal perturbation. The critical heat flux (CHF) of the heated surfaces with a hydrophobicity ratio of <11% were all greater than that of the purely hydrophilic surfaces, and all reached the CHF before the purely hydrophilic surfaces.

Biomimetic Fog Collector with Hybrid and Gradient Wettabilities.

Water scarcity is a global problem and collecting water from the air is a viable solution to this crisis. Inspired by Namib Desert beetle, leaf venation and spider silk, we designed an integrated biomimetic system with hybrid wettability and wettability gradient. The hybrid hydrophilic-hydrophobic wettability design that bionomics desert beetle's back can construct a three-dimensional topography with a water layer on the surface, expanding the contact area with the fog flow and thus improving the droplet trapping efficiency. The venation-like structure with wettability gradient not only provides a planned path for water transportation, but also accelerates water removal under the synergistic effect of gravity and wettability driving force, thus further improving the surface regeneration rate. The collector combines droplet capture, coalescence, transportation, separation, and storage capabilities, which provides new ideas for the design of future high-efficiency fog collectors.

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.

Bubble formation dynamics in aerospace applications

Under microgravity conditions, simulations of bubble formation in a T-junction microchannel with hybrid wettability surfaces allows for more precise control of multiphase flow in space exploration technologies

Bubble generation mechanisms in microchannel under microgravity and heterogeneous wettability

Advances in hybrid surfaces have revealed interesting opportunities for multiphase flow control under microgravity, as the surface tension force is dominant in this condition. However, a comprehensive investigation of bubble generation rates and slug flow parameters remains challenging. This research integrates hybrid wettability and modified dynamic contact angle models to address this important knowledge gap. Using the computational capabilities of the IsoAdvector multiphase method, we performed detailed simulations of complex multiphase flow scenarios with the OpenFOAM package. We then validated these simulation results through rigorous comparison with available experimental data, thereby strengthening the accuracy and reliability of our numerical simulations. Our comprehensive research demonstrates the profound effect of altering contact angle distribution patterns on several critical parameters. These results highlight the precise control that can be achieved through the strategic manipulation of these patterns, offering the possibility of adjusting factors such as bubble production rate, slug length, bubble diameter, the relationship of flow residence to bubble movement, bubble movement speed in the channel, and pressure drop. Interestingly, altering these patterns can also induce asymmetric behavior in bubbles under microgravity conditions, a phenomenon that has significant implications for various applications. Such insights are crucial for fields such as heat transfer in energy systems, reaction mechanisms in chemical processes, multiphase flow control in petrochemical industries, fluid dynamics in aerospace engineering, and cooling mechanisms in electronic devices. With the ability to modulate these fundamental parameters, we gain valuable insights into the design and optimization of microchannel systems. Consequently, this research presents a more efficient and innovative approach to multiphase flow control, promising improved operational performance, and efficiency in various engineering applications.

Enhancing pool boiling heat transfer of modified surface by 3-D Lattice Boltzmann method

In this study, pool boiling from micro-pillar modified surface has been simulated numerically by a 3-D lattice Boltzmann method. Effects of geometries and wettability of micro-pillar on boiling heat transfer performance were also systematically evaluated. Result showed that compared within micro-pillar surface, heat flux of cubic micro-pillar surface was the highest with the lowest wall temperature. In addition, compared to hydrophilic condition, Heat flux of cubic micro-pillar surface with hydrophobic wettability increased by 98.3%. This is because hydrophobic wettability influenced nucleation site density, vapor-liquid-flow field and heat transfer performance much more than cubic shaped geometry. Finally, heat flux of cubic micro-pillar surface with hybrid wettability increased by 430.7% compared to pure hydrophilic wettability. That is due to optimal hybrid wettability surface could control nucleate site location, restrict bubble growth, and increase obviously heat transfer performance.

Bioinspired Graphene Aerogels with Hybrid Wettability for Solar-Driven Purification of Complex Wastewater.

Salt deposition and pollutant enrichment greatly hamper efficient and sustainable water production for a solar evaporator. Inspired by the desert beetle, a dual-region hydrophobic graphene/hydrophilic titanium dioxide (TiO2) aerogel (GTA) with internal hydrophilic-hydrophobic hybrid wettability structure is prepared via a facile freeze-drying and thermal reduction method. The evaporator shows adjustable wettability, optimized water content, and a low energy loss in the evaporation process. Simultaneously, the hybrid wetting structure in aerogel subjects salt to a dynamic crystallization-dissolution process to prevent salt deposition. The GTA solar evaporator achieves an evaporation rate of 1.52 kg·m-2·h-1 with a 91.02% efficiency under 1 sun irradiation. Furthermore, GTAs achieve a stable evaporation rate in high salinity brine (25 wt % NaCl) under 1 sun irradiation for 100 h, which could compete well with other most advanced photothermal evaporation materials. Moreover, the synergistic effect of graphene and TiO2 endows GTAs with excellent photocatalytic degradation and self-cleaning properties, which can effectively reduce the enrichment of contaminants on the evaporator. Therefore, GTA evaporators can efficiently and stably obtain clean water from seawater and wastewater, which provides a feasible strategy for the purification of complex wastewater.

Microscopic insight into mechanisms of heat and mass transfer improvement of dropwise condensation on a modified nanopillar surface

In this work, a series of molecular dynamics simulations are carried out to explore the mechanisms of heat and mass transport on textured surfaces during the initiation of condensation. The solid fractions of composite nanopillars on textured surface range from 0.13 to 0.35. The addition of nanopillars with φ=0.35 on bare surface results in a 50 % decrease of the total thermal resistance. Increasing surface wettability brings a higher overlap of phonon vibrational density of states between solid and near-surface atoms, and an order magnitude reduction of interfacial Kapitza resistance can be obtained. Also, enhancing temperature (Thot) gives rise to the formation of droplets that exhibits higher solid-liquid viscous force within multi nanostructure cells. The wetting map with respective to the temperature difference and nanostructure spacing shows that the increases of Thot and spacing l facilitates the formation of Wenzel droplets. For Cassie droplets, the highest condensation rate is found for the case of Thot =130 K and l = 30 Å. While both dense and sparsely arranged nanopillar surfaces have higher Wenzel droplet condensation rates. Compared to the cylindrical structure, the cylinder-hemisphere composite structure shows superior condensation performance after droplet approaches the hemisphere region. In addition, the condensation rate on hybrid wettability surface exhibits a 30 % enhancement than that on uniformly hydrophilic surface.

Molecular dynamics study of explosive boiling on hybrid wettability concave and convex surfaces

This paper uses the molecular dynamics (MD) method to study the boiling mechanism of water film explosion on purely wettability surfaces and the effect of hybrid wettability concave and convex surfaces on boiling heat transfer. The results suggest that the water molecules on the strong wettability surface adhere to the solid surface and collide with the solid atoms at high frequencies. However, the water molecules on the weak wettability surface move away from the surface in a disorderly manner. The difference in the motion behavior of water molecules on various wettability heating surfaces makes water films nucleate earlier on hydrophobic surfaces and boil off faster on hydrophilic surfaces. Meanwhile, the explosive boiling process of pure and different hybrid wettability concave and convex surfaces was studied, which indicates that the best heat transfer efficiency of B6 (The convex nanostructures have hydrophobic surfaces on the sides and hydrophilic surfaces on the other parts). B6 has an initial maximum heat flux, maximum initial HTC, and a maximum average heat flux, as well as a lower nucleation temperature and an earlier nucleation time. The reasonable setting of the combination of surface wettability and concave-convex nanostructures benefits the enhancement of explosive boiling heat transfer.

Origami-like 3D Fog Water Harvestor with Hybrid Wettability for Efficient Fog Harvesting.

Collecting water in fog has also become a breakthrough to solve the hidden danger of water shortage in some arid areas. The three-dimensional (3D) structure fog collection material can increase the surface area in direct contact with the fog flow and reduce the quick flow of fog, which can effectively improve the fog collection efficiency. Imitating the three-dimensional structure of corrugated paper, the 3D fog collecting material with hybrid wettability was prepared by chemical and physical means. We discuss the influence of different wettability combinations on the fog collection efficiency of 3D structures and study the influence of spraying times and illumination times on the surface wettability during the construction of wettability. We also study the influence of the concavity and tip as well as the bending angle on the fog collection in the 3D structure and obtain the most reasonable concavity and convex ratio and bending angle. The superhydrophilic and superhydrophobic 3D structure fog harvesting material prepared by us performs well in the fog harvesting process, and the fog harvesting efficiency reaches 1.442 g cm-2 h-1. The fog collection efficiency is 418% of the original zinc sheet. At the same time, compared with the superhydrophilic and superhydrophobic hybrid two-dimensional (2D) plane, the increase is 168%, and compared with the superhydrophobic 3D structure, the increase is 150%.

Channel flow boiling on hybrid wettability surface with lattice Boltzmann method

Channel flow boiling on the hybrid wettability surface is studied numerically using a two-dimensional pseudo potential lattice Boltzmann method (LBM) coupled with a thermal phase transition model. Heat transfer performance between single and hybrid wettability surfaces is compared first, and it is shown that the heat flux on the hybrid surface is higher than that of its single wettability competitor. After analyzing 36 groups of numerical results, it is found that the impacts brought by contact angle and geometrical parameters of the hybrid surface on heat flux is complex. By fixing the hydrophilic contact angle, the heat flux increases first but then decreases with the increase of hydrophobic contact angle. However, the heat flux decreases monotonously with increasing the hydrophilic contact angle under a fixed hydrophobic contact angle. The 30°/120° hybrid wettability surface shows the best heat transfer performance due to its better three-phase contact line spreading and accelerated bubble departure frequency. Aside from the contact angle, the flow boiling heat transfer performance dominated by bubble dynamics is also closely related to the widths of hydrophilic/hydrophobic regions, and there exist optimal widths under various wall superheats. Finally, the pressure drop inside channel is investigated, and it is shown that the hybrid surface is favorable to reduce pressure drop fluctuation on the premise of providing high vapor yield.

Nature-inspired hybrid wettability surface to enhance water management on bipolar plates of PEMFC

Proton exchange membrane fuel cell (PEMFC) requires both sufficient water in proton exchange membrane (PEM) and fast water drainage in flow channel to achieve a desirable performance, which creates a dilemma for water management. Herein, we have fabricated a nature-inspired bionic multifunctional surface (BMS) consisting of superhydrophobic coating and superhydrophilic groove. The characterizations of water contact angle, morphology and chemical composition have verified the successful preparation of hybrid wettability surface. The effects of groove width on droplet capture, storage and drainage have been investigated, which yields an optimal width of 600 μm. More importantly, BMS has been fabricated inside the flow channel of graphite bipolar plate (BMS@BP). The peak power density of BMS@BP cell is 26.3% higher than that of the cell without BMS. Under a drought-inclined condition of low current and humidity, BMS@BP mitigates the increase of internal resistance caused by PEM dry-out, whereas under a condition prone to flooding (high current and humidity), BMS@BP can reduce the voltage fluctuation and pressure drop variation incurred by flooding. Hence, BMS has demonstrated a capacity to balance the opposite needs of moisturizing PEM and draining channel water. Our work may afford an innovative hybrid-wettability-based approach to enhance water management for PEMFC.

Patterned Hybrid Wettability Surfaces for Fog Harvesting.

The scarcity of fresh water resources has become increasingly serious in recent years, posing threats to the survival of mankind. The ability of the animals and plants in arid areas to collect water from moisture and fog has drawn attention worldwide. Inspired by the synergistic fog harvesting mode of natural organisms with superhydrophilic and superhydrophobic patterning, a composite membrane with a concave-convex morphology and hybrid wettability was prepared aiming at efficient fog harvesting. The hybrid wettability surface was obtained by chemically modifying the superhydrophilic PAN substrate with 1H,1H,2H,2H-perfluorooctyltrichlorosilane using iron mesh as the mask. The porous PAN substrate was prepared by the non-solvent-induced phase separation (NIPS) method. Fog harvesting is a three-step process: condensation, coalescence, and rapid transportation of water droplets. The area and ratio of the hydrophilic/hydrophobic regions were tuned by adjusting the mesh number of the iron meshes. Under the optimal condition, the fog harvesting efficiencies of 40.3 and 74.2 mg·cm-2·min-1 were obtained when the fog yields were 0.05 and 0.1 L·min-1, respectively. The present work provides an alternative strategy for addressing the shortage of fresh water resources.

On the effects of hybrid surface wettability on the structure of cavitating flow using implicit large eddy simulation

BackgroundSupercavitation flow is characterized by the presence of a supercavity, which can be unstable and cause unwanted motion or even loss of control of an object. To address this issue, flow control techniques are often employed. These techniques aim to manipulate the flow of liquid around the object and improve stability and control. Some of these methods include using control surfaces, injecting fluids, or more advanced techniques such as active flow control or smart materials. MethodThe aim of this study is to use computational methods to investigate the behaviour of supercavitation flow over a Clark-Y hydrofoil with different contact angle patterns (hybrid wettability) inspired by the Namib desert beetle. The hybrid wettability consists of six distinct patterns with a 160-degree contact angle for superhydrophobic surfaces and a five-degree contact angle for superhydrophilic surfaces. To predict the dynamic and unsteady behaviours of the supercavitation flow with a cavity number of 0.4, the implicit large eddy simulation (ILES) technique and Kunz mass transfer model are used. The compressive volume of fluid technique is used in conjunction with dynamic adaptive mesh refinement to track the interface between the vapour and liquid phases. The simulations are conducted using the interPhaseChangeFoam two-phase flow solver within the OpenFOAM framework. This study analyses the time-averaged and instantaneous fluid dynamic properties of the supercavitation flow of the hydrofoil with a hybrid surface, including pressure, velocity, vorticity fields, wake flow, recirculation zone, and liquid volume fraction. In addition, it examines the instantaneous cavity leading edge and flow separation, the vortical flow structure, vorticity stretching and dilatation, spanwise flow details, the creation of a low-pressure zone behind the hybrid hydrofoil, streamwise velocity fluctuations, and the cavity dynamics over an entire cycle. FindingsThe results show that the contact angle has a significant effect on the structure of the supercavitation flow. Specifically, the use of a superhydrophobic surface on the pressure side and trailing edge of a hydrofoil, along with a superhydrophilic surface on the leading edge, can reduce the thickness of the wake zone, the length of the cloud cavity, and flow instability. Additionally, this approach can delay the onset of cavitation.

Characterizing the thermal transport and kinetics of droplet evaporation on a solid surface with hybrid wettability

To investigate the kinetics and energy transport during droplet evaporation on a heterogeneous solid surface with hybrid wettability, a series of molecular simulations are conducted in this work. Results indicate that droplet evaporation with constant contact radius mode almost dominates the whole process. The depinning can be initiated when the droplet contact angle declines to the value at which the droplet is stable on a homogeneous hydrophilic surface. Most of the heat conducted from the hybrid solid surface is transferred along adsorption layer to contact line, while only a small part of the energy is transported to the bulk droplet. When the heterogeneous substrate temperature is >115 K, the heat flux transferred along the adsorption layer always decreases first and then increases as droplet volume decreases, otherwise, it gradually decreases. Increasing the substrate temperature not only increases the evaporation rate but also enlarges the proportion of heat flux transferred to the bulk droplet. The statistical number of atoms in the liquid N and evaporation time τ satisfy the relationship of N2/3∝τ on the hybrid wettability surface. The lifetime for droplets evaporating on a heterogeneous wettability surface is reduced by 50% compared to that on a homogeneous hydrophobic surface.

Defect-Density-Controlled Phase-Change Phenomena.

Juxtaposing hydrophilicity and hydrophobicity on the same surface, known as hybrid surface engineering, can enhance phase-change heat transfer. However, controlling hydrophilicity on hybrid surfaces in a scalable fashion is a challenge, limiting their application. Here, using widely available metal meshes with variable dimensions and by controlling the patterning pressure, we scalably fabricate hybrid surfaces having spot and gridlike patterns using stamping. Using fog harvesting in a controlled chamber, we show that optimized hybrid surfaces have a ∼37% higher fog harvesting rate when compared to homogeneous superhydrophobic surfaces. Furthermore, condensation frosting experiments reveal that, on grid-patterned hybrid surfaces, frost propagates at ∼160% higher velocity and provides ∼20% less frost coverage when compared to homogeneous superhydrophobic surfaces. During defrost, our hybrid surfaces retain more water when compared to superhydrophobic surfaces due to the presence of hydrophilic patterns and melt water pinning. We adapt our fabrication technique to roll-to-roll patterning, demonstrating wettability contrast on round metallic geometries via atmospheric water vapor condensation. This work provides guidelines for the rapid, substrate-independent, and scalable fabrication of hybrid wettability surfaces for a wide variety of applications.

Nanoscale investigation of surface wettability distribution on bubble nucleation with variable temperature boundary condition

Surface wettability plays an essential role in bubble nucleation. In this article, the molecular dynamics simulation method is conducted to study the bubble nucleation of liquid argon film on a copper substrate with hybrid wettability. Unlike the constant temperature boundary condition, the variable temperature boundary condition is adopted to simulate the bubble nucleation process. The temperature gradually increases from 86 K to 200 K, so the substrate temperature at the bubble nucleation time can be obtained. The potential energy, kinetic energy, total energy, and temperature during bubble nucleation are analyzed. The results show that bubble nucleation occurs when the kinetic energy exceeds the potential energy barrier. A method is used to capture the bubble nucleation process, described from the perspective of total energy. The initial nucleation time of the hybrid wettability surface gradually reduces with the rise of the proportion of the hydrophilic part. Compared with the pure hydrophilic surface, the hybrid wettability surface can achieve faster initial nucleation time and lower nucleation temperature by altering the proportion of wettability. The potential energy and temperature distribution reveal the microscopic mechanism of bubble nucleation. This study provides theoretical guidance for the design of cooling surfaces of micro/nanoelectronic devices.

Efficient fabrication of desert beetle-inspired micro/nano-structures on polypropylene/graphene surface with hybrid wettability, chemical tolerance, and passive anti-icing for quantitative fog harvesting

Obtaining fresh water from the atmosphere by fog collectors is a promising way to meet the challenge of the water crisis. Bioinspired fog collectors have attracted widespread attention, but their application is limited by complex preparation processes and weak environmental adaptability. Herein, an efficient method that combines extrusion compression molding and surface modification is proposed for the large-scale fabrication of united superhydrophobic-hydrophilic polypropylene/graphene nanosheets (UPP/GNS) films for quantitative fog harvesting. Specifically, the desert beetle-inspired micro-pillars on the UPP/GNS surface exhibit a hydrophilic top and a superhydrophobic bottom and sidewall. A micro-nano/structure-induced hybrid wettability is acquired with a CA of 160 ± 3°. The hybrid wettability is excellent in accelerating droplet condensation and aggregation, which is beneficial for mass transfer in a fog harvesting system. Importantly, excellent chemical tolerance, passive anti-icing, active photothermal anti-icing, and sufficient durability enable the UPP/GNS film to maintain a stabilized fog collection efficiency under various external disturbances, such as salt, acid-/alkali solutions, and high temperature. Finally, the combination of hydrophilic and superhydrophobic wettability endows the UPP/GNS film with a fog collection efficiency of 1251 mg cm-2h−1. The economical and mass production method for the efficient fabrication of high-efficiency UPP/GNS fog collectors is a promising solution for mitigating the global water shortage.

Multiscale numerical research on nucleate boiling enhancement from microscale to nanoscale

Nucleate boiling enhancement from surfaces in energy industries is of fundamental significance for improving energy conversion and thermal management efficiency. This study investigates the microscale and nanoscale boiling processes on the hybrid wettability micropillar structured surface (MPS) to continuously enhance nucleate boiling. The microscale and nanoscale boiling processes and underlying enhancement mechanisms are studied by Lattice Boltzmann Method (LBM) and Molecular Dynamics (MD) simulations, respectively. It is revealed that MPS designed with a hydrophilic bottom surface and a hydrophobic top surface decorated with nanopillars can improve nucleate boiling throughout the microscale and nanoscale boiling process. The microscopic boiling process of the hybrid wettability MPS shows that the synergistic effect of wettability and surface structure is attributed to nucleation boiling enhancement due to higher bubble nucleation density and higher bubble departure frequency. The nanoscale boiling process on hydrophobic nanopillar structured surfaces (NPS) shows further nucleate boiling enhancement. The NPS with a nanopillar height of 1.6268 nm is in the Wenzel state with higher heat flux and lower thermal resistance, resulting in faster boiling onset and shorter separation time, while the hydrophobic NPSs in the Cassie state leads to a deterioration in the nucleate boiling.

Nucleate boiling of thin liquid films on nanostructured surfaces with hybrid wettability using molecular dynamics simulation

The development in nanotechnology and bionics has increased the application of hybrid wettable nanostructured surfaces in engineering and science research. This study employed molecular dynamics to investigate the phase changes of thin liquid films on nanostructured surfaces with hybrid wettability. The liquid film and nanostructured surfaces were consisted of Lennard–Jones (LJ) fluid and LJ solid atoms, respectively. The simulation results indicate that the microscopic mechanism of nucleate boiling is facilitated by the original residual vapor nuclei. Additionally, the competition mechanism between the hybrid surface wettability and nanostructure is identified. The initial nucleation temperature for a solid wall with a hydrophobic nanocavity is 1.09ε/kB, which is 0.04ε/kB less than that for a hydrophilic nanocavity. This phenomenon can be attributed to the original residual vapor nuclei generated by hydrophobic nanocavities, which induce a piston-like effect at the vapor–liquid interface. As the nanocavity hydrophobicity increases, the piston-like effect increases the internal vapor pressure within the cavity and reduces the initial waiting time for bubble nucleation. For the hydrophilic nanocavities with contact angles greater than 18°, the bubbles are initially generated on the smooth surface rather than the nanocavity. Moreover, the smooth surface wettability is instrumental in bubble nucleation and growth. When the contact angle of the nanocavity is less than 18°, the nanostructures are dominant, and the bubbles are generated only in the nanocavity. These findings can aid the design, manufacture, and performance optimization of micro/nano-systems and devices.