Inspired by a few phenomena in nature such as the lotus leaf, red rose petal, gecko’s feet and Nepenthes Alata plant, much attention has been paid to use simple and feasible means to achieve remarkable wetting behaviour for many applications in various areas including self-cleaning for building exteriors and windshields, oil/water separation, anti-icing, liquid collecting, anti-fogging and anti-corrosion. Based on the established theoretical models, wetting behaviour of a liquid droplet obtained by molecular dynamics simulation method is generally in good agreement with the experimental results. In macro and micro scale, the previous theories can explain and predict the wetting behaviors well. However, these theories are invalid for nanoscale. It is essential to reveal the underlying physical mechanism of the wetting behaviors of the droplet on solid surface with nanoroughness. Extensive studies on nanosale wettability focus on the effect of nano structures on wettability state. Desired wetting behavior of rough material surface achieved by nanosize reentrant geometry like “T” or mushroom shape and other variant geometry with solid overhangs has been widely used in self-cleaning surfaces, heat exchange and many applications. For example, “T” shape groove with different depths and widths under nanoscale has been considered to confer superhydrophobicity to hydrophilic surfaces gradually. In this paper, wettability transition of a liquid droplet on geometrically heterogeneous solid substrate with nanoscale structures of inverted triangular grooves is investigated by using molecular dynamics simulation method under the parameter space spanned by structure geometry and solid-liquid molecular interaction potential strength. Three wettability states, namely Cassie nonwetting state, Cassie-to-Wenzel transition state and Wenzel wetting state, are identified with various geometries and potential strength. For Cassie nonwetting state, increasing height of the triangles has less effect on wettability transition with weak solid-liquid molecular interaction. Besides, the Cassie nonwetting state is less sensitive to different interval between the triangles as solid-liquid molecular interaction is weak. For Cassie-to-Wenzel transition state, increasing height of the triangles and decreasing interval between the triangles decrease wettability. For Wenzel wetting state, increasing interval between the triangles with low height increases wettability. With strong solid-liquid molecular interaction, different interval between the triangles results in wetting state transition from Wenzel to transition state. What’s more, liquid droplet changes its state from Wenzel wetting state to Cassie-to-Wenzel transition state with increasing height of the triangles or decreasing interval between the triangles. Three wettability transition regions are identified in the parameter space.

Boiling heat transfer is one of the most effective methods to meet the challenge of heat dissipation of high heat flux devices. Wetting hybrid surface has been demonstrated to have better performance than hydrophilic or hydrophobic surface. But this kind of wetting hybrid modification is always carried out on plain or flat surface. In this paper, we coated PTFE powders on superhydrophilic micro copper dendrite fin surface to build wetting hybrid surface. The pool boiling experimental results showed that after coating, the wall superheat dramatically decreased to 8 K, which is 9K lower than that on original surface at 250 W·cm −2 . Besides, it also has much better performance than Silicon pin fin based wetting hybrid surface.

A simultaneous visualization and heat performance of oscillating heat pipes (OHPs) were performed. Experiments were performed under different surface wetting characteristics. Results showed that the start-up performance was improved on hydrophilic OHP as opposed to the copper OHP. A small bubble grew quickly and became a vapor plug in the evaporation section with hydrophilic surface. The process of vapor expansion and contraction accompanying liquid slug movement upward and backward continued to occur as a spring, and the OHPs started up. However, the hydrophobic OHP failed to start up. For the superhydrophobic OHP, nucleate boiling took place in the evaporation section, and the bubble expansion and contraction phenomenon were not observed. Heat transfer results showed that wall temperature fluctuations were observed at the start-up stage. The start-up time for the hydrophilic OHP was lowest and the amplitudes of temperature oscillations were increased in hydrophilic OHP compared to the copper OHP.

The wetting, spreading and drying of pure liquid and nanofluid sessile droplets on a patterned solid surface were investigated systematically in terms of liquid and surface property. The patterned nickel surface was characterized with diamond, circular, hexagon and rectangular pillars. The size ratio between interval and pillars varies from 1.0 to 5.0. The study was firstly carried out for the effect of pure water droplet size on liquid spreading and droplet evaporation process on diamond-shape micro structured substrate with L Interval /L Pillar =1.0. Larger amount of liquid leads to a larger wetting area. With fixed substrate (diamond, L Interval /L Pillar =1.0) and droplet size (1 μm), mixture of DI water and Ethanol (volume ratio varies from 0.5 to 2.0) was used for generating droplets with different surface tension and evaporation coefficient. Fingering shape would generate on the contact line. With higher concentration of ethanol, the fingering effect is stronger and appeared in a shorter time. The contact area shrinks when increase the size ratio of interval and pillar. This would reduce the length of the contact line, and thus slow down the liquid evaporation. The role of pillar shape was examined based on time for complete evaporation. The effect of surface material on evaporation process was conducted on nickel and PMMA substrate fabricated with the same design. Additionally, investigations were conducted with solutions consisted with nanoparticles and DI water. The mixture were made at different weight ration to achieve concentration of nanoparticles varies from 0.02% to 0.18% with an interval at 0.04%.

In the present study, the interfacial dynamics of displacement of three dimensional spherical droplet on a rectangular microchannel wall considering wetting effects are studied. The two-phase lattice Boltzmann Shan-Chen model is used to explore the physics. The main focus of this study is to analyse the effect of wettability, low viscosity ratio and capillary number on the displacement of spherical droplet subjected to gravitational force. The hydrophobic and hydrophilic nature of wettabilities on wall surface are considered to study with capillary number, Ca=0.1, 0.35 and 0.66 and viscosity ratio, M ≤ 1. The results are presented in the form of temporal evolution of wetted length and wetted area for combined viscosity ratios and wettability scenario. In the present study, it is observed that in dynamic droplet displacement, the viscosity ratio and capillary number play a significant role. It is found that as viscosity ratio increases, both the wetted area and the wetted length increase and decrease in the case of hydrophilic and hydrophobic wettable wall respectively.

Gravity-driven displacement of droplet on an inclined micro-grooved surface is studied using Pseudo-potential model of lattice Boltzmann method. To validate the numerical method, we find good agreement of the LB simulations with the pressure difference by Laplace’s law. The equilibrium contact angle of a droplet wetting on a smooth horizontal surface is studied as a function of the wettability, finding good agreement with an empirical scheme obtained with Young’s equation. The dynamic behavior of a droplet wetting on micro-grooved horizontal surface is found to be complex and greatly affected by the fraction of the grooved area and the groove width, the results indicate that the effect of grooves on contact angle is dependent on the fraction of the grooved area and the groove width has not a consistent effect on contact angles. For an inclined nonwetting micro-grooved surface, in given range, higher fraction of the grooved area and smaller groove width lead to greater benefit for droplet sliding down. What’s more, the results indicate that higher gravity value leads to a higher decrease of movement resistance of the droplet by decreasing the contact area between the droplet and solid surface.

The dynamic wetting of water nano-droplet with evaporation on the heated gold substrate was examined using molecular dynamics simulation. Various substrate and droplet pre-heated temperatures were calculated to obtained different evaporating rates. Water molecules attachment-detachment details were traced near the contact line region to show the microscopic details and evidences for the spreading-evaporating droplet. The increasing substrate temperature greatly affected the dynamic wetting process, while the initial temperature of water droplet had very limited effects. The effects of free surface evaporation on wetting kinetics for both hydrophobic and hydrophilic substrates were examined. The radius versus time curves agree well with the Molecular kinetic theory (MKT) for spreading without evaporation and deviate from the MKT for the spreading with evaporation. The enhancement on wetting kinetics due to evaporation can be attributed to the reducing of liquid-vapor surface tension and the strengthening in water molecules transport in contact line region and bulk droplet.

Flooding caused by excessive droplet feeding on heat dissipation area periodically occurs for droplet-based thermal management, including spray cooling and electro-wetting. The conventional highly wettable texture of surfaces, which is designed for thin film evaporation, has negligible effect on improving thermal performance during flooding. This work examines a combination of micro-pillar structures and engineered wettability that aims to improve the liquid-vapor phase change intensity and heat dissipation rate during flooding. Numerical simulation has been made to investigate the thermal and dynamic impact of the proposed combination structure on boiling and evaporation, with control variables of pillar height and pillar array density. A transient 3-D volume-of-fluid (VOF) model has been developed to analyze behaviors of bubble growth, coalescence, and departure processes. Parameters including volumetric liquid-vapor mass transfer rate, heat source temperature and heat transfer coefficient are examined. The results indicated the structured surface can reduce bubble sizes and enhance bubble departure rates. The optimized value of pillar height exists. The pillar height has more impact on cooling enhancement than pillar array density when the increased solid-liquid interface area was kept the same.

Condensation on superhydrophobic nanostructured surfaces offers new opportunities for enhanced energy conversion, efficient water harvesting, and high performance thermal management. Such surfaces are designed to be Cassie stable, which minimize contact line pinning and allow for passive shedding of condensed water droplets at sizes smaller than the capillary length. In this work, we investigated in situ water condensation on superhydrophobic nanostructured surfaces using environmental scanning electron microscopy (ESEM). The “Cassie stable” surfaces consisted of silane coated silicon nanopillars with diameters of 300 nm, heights of 6.1 μm, and spacings of 2 μm, but allowed droplets of distinct suspended (S) and partially wetting (PW) morphologies to coexist. With these experiments combined with thermal modeling of droplet behavior, the importance of initial growth rates and droplet morphology on heat transfer is elucidated. The effect of wetting morphology on heat transfer enhancement is highlighted with observed 6× higher initial growth rate of PW droplets compared to S droplets. Consequently, the heat transfer of the PW droplet is 4–6× higher than that of the S droplet. To compare the heat transfer enhancement, PW and S droplet heat transfer rates are compared to that of a flat superhydrophobic silane coated surface, showing a 56% enhancement for the PW morphology, and 71% degradation for the S morphology. This study provides insight into importance of local wetting morphology on droplet growth rate during superhydrophobic condensation, as well as the importance of designing CB stable surfaces with PW droplet morphologies to achieve enhanced heat transfer during dropwise condensation.

Lipid bilayers form nanopores on the application of an electric field. This process of electroporation can be utilized in different applications ranging from targeted drug delivery in cells to nano-gating membrane for engineering applications. However, the ease of electroporation is dependent on the surface energy of the lipid layers and thus directly related to the packing structure of the lipid molecules. 1,2-dipalmitoyl- sn -glycero-3-phosphocholine (DPPC) lipid monolayers were deposited on a mica substrate using the Langmuir-Blodgett (LB) technique at different packing densities and analyzed using atomic force microscopy (AFM). The wetting behavior of these monolayers was investigated by contact angle measurement and molecular dynamics simulations. It was found that an equilibrium packing density of liquid-condensed (LC) phase DPPC likely exists and that water molecules can penetrate the monolayer displacing the lipid molecules. The surface tension of the monolayer in air and water was obtained along with its breakthrough force.

This paper quantifies the influence of Al 2 O 3 nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a Turbo-BII-HP boiling surface. An Al 2 O 3 nanolubricant (a lubricant containing dispersed nano-size particles) was made by suspending nominally 10 nm diameter Al 2 O 3 particles in a synthetic polyolester to roughly a 1.0% volume fraction. The nanoparticles caused, on average, a 12% degradation in the boiling heat transfer relative to that for R134a/polyolester mixtures without nanoparticles for the three lubricant mass fractions that were tested. The degradation was nearly constant for heat fluxes between 20 kW/m 2 and 120 kW/m 2 . It was speculated that the boiling heat transfer degradation was primarily due to a combination of (1) film boiling in the reentrant cavity rendering the nucleate boiling enhancement mechanism of the nanoparticles ineffective and (2) a reduction in bubble frequency due to the increased surface wetting as caused by the nanoparticles. In addition, these degradation factors might be mitigated with increased nanoparticle loading.

An experimental study was conducted to investigate the effects of tube row and a micro-scale porous-layer coating on solution fluid wetting and heat transfer of a horizontal-tube, falling-film heat exchanger using an inline tube arrangement. A uniform layer of micro-scale copper particles was directly bonded onto plain copper tubes by sintering to create a porous-layer coating on the tubes. Distilled water was used as solution and heating fluids. The visual observation performed in open ambient condition revealed that when the solution was dripped onto horizontal tubes from a solution dispenser, the conventional plain tubes were always partially wetted while the porous-layer coated tubes were completely wetted due to capillary action, even at low solution flow rates. It was shown from the comparison of the evaporation heat transfer results of the plain and porous-layer coated tubes tested in a closed chamber under saturated conditions that the porous-layer coated tubes exhibited a superior evaporation heat transfer rate (around 70% overall improvement at low solution flow rates) due to the complete solution wetting and thin solution liquid film on the evaporator tubes. It was also observed that the heat transfer and surface wetting of the horizontal-tube, falling-film heat exchanger are greatly affected by both the flow mode of the solution fluid between the tubes and tube wall superheats. The effect of the tube row of the falling-film heat exchanger on the solution wetting and heat transfer was significant.

The wettability of patterned silicon microchannels with tunable surface free energy through coating hydrophilic TiO 2 nanoparticles using layer-by-layer (LbL) nano self-assembly technique is presented in this paper. Wettability of microchannels is tested by measuring the contact angle of a water droplet on the substrate. The capillary rise rate is tested by measuring the front location of liquid on the silicon microchannel surface laid on a 45 degree inclined platform. It is found that the silicon microchannels with tunable surface free energy have super-hydrophilic wettability, and demonstrate powerful capillary. For the silicon microchannels 200 μm wide, the liquid front can move up 40 mm in approximately 3 second. Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) reveal the generation of -OH radicals after coating TiO 2 nanoparticals, verifying that the -OH radicals have a strong effect on the hydrophilicity. The super-hydrophilic patterned microchannels suggest potential applications to microfluidic systems and heat diffusion systems based on liquid surface tension.

Nanoengineered surfaces have received significant interest for the manipulation of liquids in microfluidic systems. In this work, we present three-dimensional nanostructures that can control liquid film thickness and the directionality of the liquid spreading. We fabricated silicon nanopillars ranging from 200 nm to 800 nm in diameter and heights of approximately 5 μm. In the presence of notches on the pillars, the liquid separates into multiple layers of liquid films. The thicknesses of the liquid layers subsequently increase as the film propagates, which is determined by the specific position and geometry of the notches. In the presence of asymmetric nanopillars, where the pillars have deflection angles ranging from 0–50 degrees, directional spreading of water droplets can be achieved. The liquid spreads only in the direction of the pillar deflection and becomes pinned on the opposite interface. We performed detailed measurements and developed models to predict the behavior based on pillar geometries. The insights gained from this work offer promise for enhanced control of liquids in microfluidic systems.

Superhydrophobic surfaces are surfaces with fluid contact angles larger than 150°. Superhydrophobicity can be achieved by chemically modifying the surface or introducing texturing which increases the real or effective area of the surface. In this work we focus on the latter approach. If the texturing leads to a Cassie non-wetting state, the surface can also exhibit drag reduction characteristics. Thus, for the same energy input drag reducing surfaces lead to higher flow velocities or, conversely, in order to achieve the same flow rates and velocities, drag reducing surfaces require less energy input. In order to optimize the surface topography of the superhydrophobic surface, a stratified two-phase model of flow between flat plates was developed to simulate the friction reduction characteristics of the surface as a function of varying ‘fluid to gas’ ratios. The Stokes flow equation was used to derive velocity profiles with appropriate slip/no-slip conditions within the flow. Non-dimensional formulations were used to optimize the liquid flow rate as a function of the gas layer thickness. Based on these formulations, a pressure drop reduction of 72% is achieved when the air layer height to the total channel height is 7%. The results of the theoretical model were also compared against experimental measurements of microfluidic channels with different substrate surface topographies. Two different types of silicon substrates were used: one with flat plane topography and one with a micropillar array. The substrates were irreversibly bonded to PDMS (poly-dimethylsiloxane; Dow Corning) microfluidic channel replicas and were silanized to further enhance hydrophobicity. Flow was induced using a constant pressure source and the flow rate was measured using a high-precision scale. As expected, the experimental results deviated somewhat from the expected theoretical model due to the presence of the micropillar obstruction in the air layer. It was also observed that there was a certain ‘pillar-to-channel height’ ratio that minimized the pressure drop for a given flow rate.

Superhydrophobic hierarchical structures can not only enhance stability against wetting transition but, if well designed, also produce a larger slip compared to single-scale microstructures. Selective, rather than unilateral, roughening of the microstructure surfaces is desirable to realize increased slip and to study the role of nanoscale roughening; our superhydrophobic hierarchical structures are microscale posts and grates whose top surfaces are kept smooth while sidewall surfaces are etched into nanostructures. To obtain such two-scale micro-nano structures, we have combined two recent microfabrication techniques — gold coating by galvanic displacement and gold-assisted porous etching of silicon. As a result, we have obtained a slip length as large as ∼400 μm on micro-nano structures, which is a ∼100% increase from the previous record of ∼200 μm obtained on micro-smooth structures. Preservation of the microscale geometric parameters was what differentiated our hierarchical samples from others, which showed enhanced stability but would not result in an increased slip. The paper presents the process details developed to fabricate the micro-nano hierarchical structures.

In this paper, nanofluid droplets (fluid containing metal nanoparticles) were subjected to evaporation on a nanoporous superhydrophobic surface to study the effects of nanoparticles on evaporation kinetics, wetting dynamics, and dry-out patterns. Metal nanoparticles (gold chloride) of three different sizes (10, 100, and 250 nm) at three different concentrations (0.001, 0.01, and 0.1% wt) were tested as nanofluids, uniformly dispersed in deionized water. Anodized alumina membranes (200 nm in pore diameter) were tested as nanoporous superhydrophobic surfaces, coated with a self assembled monolayer (SAM). During the course of evaporation in a room condition, the change of a contact angle, contact diameter, height, and volume was measured by a goniometer and compared with that of the base fluid (water) taken as a control. The initial equilibrium contact angle of the nanofluids was significantly affected by the nanoparticle sizes and concentrations. During evaporation, the evaporation behavior for the nanofluids exhibited a complete different mode from that of the base fluid. In terms of a contact angle, nanofluids showed slower decrease rate than base fluid. Nanofluid contact diameter remained almost a constant throughout evaporation with a slight change only at the very end of evaporation stage, whereas the base fluid showed a sequence of constant, increase, and mixed states of increase/decrease behavior. The nanofluids also showed a clear distinction in the evaporation rates, resulting in slower rate than base fluid. The variation of the nanoparticle sizes and concentrations did not make significant difference in the evaporation rate within the tested conditions. No abrupt change in a contact angle and diameter was observed during the evaporation, suggesting that no remarkable wetting transition from Cassie (de-wetting) to Wenzel (wetting) state occurred. The scanning electron microscope (SEM) images of the deposited nanoparticles after complete evaporation of solvent showed unique dry-out patterns depending on nanoparticle sizes and concentrations, e.g., a thick ring-like pattern with larger particle sizes while a uniformly distributed pattern with smaller particles at higher concentrations.

Wicking materials with tunable wettability are of great importance for both fundamental research and practical applications such as heat pipes. In this work, we adopt recently developed titanium bulk micromachining[1] techniques to fabricate pillar arrays. Then we modify the micromachined pillars to form micro- & nano-textured (bitextured) titania structures (BTS). Further, we investigated how to plate gold on the modified surfaces to tune the wettability. A wicking material for heat pipe requires super wetting by common fluids such as water. We show theoretical studies and experimental work to investigate the wetting behavior of two different designs/samples. For heat pipe applications the BTS and plating gold not only increases the capillary pressure which enhances liquid pumping from condenser to evaporator, but also increases the heat transfer performance by extended surface and smaller pore sizes[2]. Testing results show that water can completely wet the micromachined Ti pillars (Design A: 5μm in diameter/5μm gap). The BTS helps increase the wetting speed by over 100% for this design. A second design with much larger diameter and gap (Design B: 100μm in diameter/50μm in gap) is also tested to compare with design A for wetting speed. Results show that Design B gives a wetting speed twice of Design A. Plating method is used to decrease pillar gap (from 50μm to 5μm) by growing gold on surfaces. This will help increase thermal conductivity of wicking material which is preferred for the evaporator and condenser regions of heat pipes. Wetting experiment is done on Sample B after plating with gold. Wetting results after Au plating show that wetting velocity decreases but is still significantly large.

Wettability of the solid surface is a very important property in the micro/nanoscale thermal fluidic systems. In this study, molecular dynamics simulations were carried out to study the microscopic wetting characteristics at the nanostructured surface and the contacting mode in the vicinity of the solid-liquid-vapor interface. A pure liquid nanodroplet was placed on a solid surface in a shape of molecular-scale unevenness with different height and spacing. The wettability of the solid-liquid interface was examined with evaluating the contact angles at the three-phase interface and the liquid-solid contacting area ratio. The results of the measured contact angles demonstrated that the nanostructures could strengthen the hydrophobic properties for a partial wetting condition, while it was insignificant in a completely wetting case. Furthermore, we compared the results of molecular dynamics (MD) simulations to the classical descriptions of Wenzel’s model and Cassie-Baxter’s model. Significant discrepancy among the results was found and a new model of contact angle in consideration of liquid filling ratio among nanostructures was proposed.

Surface wettability is an important factor for micro/nanoscale thermal fluidic systems and it has attracted much interest for both fundamental research and practical applications. As one of the most attractive materials with controllable wettability, porous silicon is easy to be produced by the electrochemical etching. In this study, the effects of the microstructures of porous silicon on the wetting behavior of a pure water droplet were investigated experimentally. The solid-liquid contacting surface of the porous silicon substrate was prepared by varying both the geometrical microstructure and the chemical composition. The anodic etching was applied to the n type silicon substrate of orientation (100) and the geometrical microstructures of porous silicon were controlled by varying the fabrication conditions of the electrochemical etching. The pores of diameter ranging from 1–6 micrometers and the porosity up to 0.8 were obtained. Also, the surface chemical composition was controlled by coating the SiO 2 layer or the CYTOP fluoropolymer layer directly on the porous silicon surface. The contact angle of the pure water droplet was measured at the prepared porous silicon surface in a room with constant temperature and humidity. The effects of the microstructures on the contact angle were discussed and the results were compared to both the classical theoretical models and a modified model based on the molecular dynamics simulations.