Wenzel Mode Research Articles

Investigation on dynamic characteristics of droplet impact on surfaces with different rough microstructures based on lattice Boltzmann method

Constructing microstructures on smooth surfaces is significant for 3D printing technology and surface self-cleaning effects in engineering applications. This paper uses the lattice Boltzmann method (LBM) to study the droplet impact behavior on different microstructured surfaces: right-triangle, semicircular, and rectangular microstructures. The formulas for calculating the surface wetting characteristics of different microstructures are derived theoretically, the difference between simulation results and those from theoretical analyses is less than 5 %. The increase of the microstructure column height h may eventually lead to the disappearance of the Wenzel mode. The droplet impact on the microstructured hydrophilic surface can accelerate the droplet to the stable stage and reduce the excessive impact effect. The droplet impacts on the superhydrophobic microstructured surface can accelerate the rebound separation and shorten the time to reach the stable stage. When droplets impact the superhydrophobic surface of the right-triangle microstructure, a secondary separation phenomenon will occur, resulting in the maximum rebound height and the maximum reduction of the near-wall density. The superhydrophobic right-triangle microstructured surface can more effectively promote self-cleaning.

Abrasion mechanisms of superhydrophobic coating surfaces wetted in Wenzel state

Under natural application conditions, water in the forms of condensates and high-speed raindrops tend to form Wenzel contact with the surface of superhydrophobic coatings, and in this case, the effect of the standing water on the abrasion mechanisms of superhydrophobic coatings is not clear. To investigate the issue, comparative tests of abrasion-induced aging on the surface of the superhydrophobic coating under dry, and water infiltration conditions are conducted. A theoretical prediction model of the friction magnitude on the surface of the superhydrophobic coating under the above two conditions is established, and the experimental phenomenon is analyzed based on the theoretical results of the model. The results show that once water wets superhydrophobic surfaces in Wenzel mode, the microscopic rough structures on the surfaces are more likely to fall off under external friction due to the presence of Laplace pressure, which means that the presence of water makes the superhydrophobic surfaces more prone to abrasion-induced aging in this condition.

Surface wettability of water and blood on diversified nanocone‐shaped ZnO films modified with n‐dodecyl mercaptan

How to understand the effect of the surface microstructures and chemical compositions on the anticoagulant properties is the key to develop novel anticoagulant materials. In view of the special surface microstructures and low surface free energy of superhydrophobic materials, it is expected that excellent anticoagulant properties can be obtained from superhydrophobic materials. Here, various nanocone‐shaped ZnO films were prepared by electrochemical deposition method followed by hydrothermal method and modified with n‐dodecyl mercaptan. Ordered nanocone‐shaped ZnO arrays are easier to obtain under suitable preparation conditions and can exhibit superhydrophobicity. The surface wettability is related to the ratio of the diameter and spacing (d/l) of ZnO nanocones. When the d/l value is 0.25–0.55, the contact angle (CA) is consistent with the Cassie mode; when the d/l value is 0.55–0.65, it corresponds to the Wenzel mode. Although the d/l value of the nanocones cannot directly determine the value of the surface free energy, which is strongly associated with the contact mode of the droplets on the film surface. Furthermore, the relationship among surface morphology, CA, and surface free energy is established, and the influence of surface morphology and surface free energy on blood contacting with foreign bodies is explored. It is found that the surface wettability of blood contacting with foreign bodies has some characteristics similar to that of general fluids. For example, under the same contact mode, although the CA of blood on the surface of ZnO nanofilms is about 20° smaller than that of water on the whole, the variation of CA with surface roughness is the same as that of general fluids. However, it also has some unique characteristics, such as the surface of ZnO nanofilms modified with hydrophobic material, the CA becomes larger. These characteristics are of special significance for the development of blood compatibility of superhydrophobic nanomaterials.

"Foam Marble" Stabilized with One Type of Polymer Particle.

There has been increasing interest in colloidal particles adsorbed at the air-water interface, which lead to stabilization of aqueous foams and liquid marbles. The wettability of the particles at the interface is known to play an important role in determining the type of air/water dispersed system. Foams are preferably formed using relatively hydrophilic particles, and liquid marbles tend to be formed using relatively hydrophobic particles. In this study, submicrometer-sized polystyrene particles carrying poly(N,N-diethylaminoethyl methacrylate) hairs (PDEA-PS particles), which are synthesized by dispersion polymerization, are demonstrated to work as a particulate stabilizer for both aqueous foams and liquid marbles. A key point for the hydrophilic PDEA-PS particles to stabilize both aqueous foams and liquid marbles, which have been generally stabilized with hydrophilic and hydrophobic particles, respectively, is the wetting mode of the particles with respect to water. The flocculates of PDEA-PS particles adsorb to the air-water interface from the aqueous phase to stabilize foam in a Wenzel mode, and the dried PDEA-PS particles adsorb to the interface as aggregates from the air phase to stabilize liquid marbles in a metastable Cassie-Baxter mode. On the basis of the difference in the wetting mode, stabilization of an air-in-water-in-air multiple gas-liquid dispersed system, named "foam marble", is realized. After the evaporation of water from the foam marble, a porous sphere is successfully obtained with pore sizes of a few tens of micrometers (reflecting the bubble sizes) and a few tens of nanometers (reflecting the gap sizes among the PDEA-PS particles).

Metal mesh-based special wettability materials for oil-water separation: A review of the recent development

Special wettability material is an important material for oily wastewater treatment. Among various wettability materials, the metal mesh material showed great advantages, such as high mechanical strength, high separation efficiency, and self-cleaning properties based on recent studies. This review article firstly describes the basic mechanism of surface wettability, including the contact angle in the air (e.g. Young's mode, Wenzel's mode, and Cassie-Baxter's mode), the contact angle hysteresis, and the rolling angle. The special wettability including superhydrophobic-oleophilic, hydrophilic-oleophobic, hydrophilic-underwater oleophobic, and switchable wettability and characteristic of the fabrication method and are discussed in this paper. The properties of the various metals that can be used for oil-water separation are summarized. Moreover, the challenges faced by metal mesh materials in oil-water separation are analyzed, and future development and application have prospected. • Review on the recent researches and developments of metal mesh oil-water separation materials. • Basic theories and application of special wetting material in oil-water separation. • The fabrication method and excellent properties of special wetting metal mesh with different substrates are reviewed. • Improving strategies and future prospect of metal mesh materials for oil-water separation.

3D Lattice Boltzmann simulations for water droplet's impact and transition from central-pointy icing pattern to central-concave icing pattern on supercooled surfaces. Part II: Rough surfaces

In this paper (Part II of a paper series), a 3D pseudo-potential lattice Boltzmann method in combination with a solid-liquid phase change model with volume expansion taken into consideration (the same model used in Part I) is applied to study a water droplet's impact and its subsequent freezing patterns on rough supercooled substrates, consisting of an array of micro-pillars in a saturated vapor environment. Freezing patterns of the droplet in three different modes on rough surfaces depending on the contact angle are investigated. In the Hemi-wicking Cassie-Baxter mode where the water droplet fills into grooves between micro-pillars completely, heat conduction rate increases because of the increasing contact area of the water droplet and the micro-pillars, and the contact angle becomes smaller which results in the central-concave icing pattern. When the droplets are in the Wenzel mode and the Cassie-Baxter mode on the rough surface, saturated vapor exists between the droplet and the micro-pillars, which brings additional thermal resistance between the supercooled substrate to droplets. This results in lower heat conduction rate which delays ice nucleation time and allows the droplet to complete its recoiling motion that leads to the formation of the central pointy icing pattern. Because static contact angles of rough substrates in these two modes are higher than those of the corresponding smooth substrates, droplets’ recoiling motions on rough substrates are stronger which lead to the formation of central-pointy icing patterns. Regime maps, showing effects of contact angle and Stefan number on formation of central-pointy icing or central-concave icing patterns of a water droplet (with D0 = 100 and Pr = 13.5) after its impact on rough substrates at supercooled temperatures at We= 320 and Re = 164.9 are presented.

Effects of base angle and wettability of nanostructures on droplet wetting behaviors

The wetting modes of droplet on nanostructure surface including Cassie, Partial Wenzel, and Wenzel are of great importance in enhancing the condensation heat transfer, surface self-cleaning and oil-water separation. Previous studies focused mainly on the behaviors of droplets on the surface of nano-pillar structures. In this work, the wetting behaviors of argon nanodroplet on platinum surface is investigated by the molecular dynamics simulations. The effects of nanostructure geometry parameters and characteristic contact angle <i>θ</i><sub>e</sub> on the wetting mode and the transition between different modes are investigated. The three-dimensional simulation box includes a bottom wall containing trapezoid wires (TWs) with different geometry parameters and other five surfaces. The TWs are populated on the wall based on the array arrangement. The periodic boundary conditions are imposed on the four side surfaces of the simulation box. The base angles of the side surface of TW with respect to horizontal plane are chosen as 60° (inverted TW), 90° (rectangular pin fin) and 120° (TW), respectively. For all the three base angles, the nanostructure surface can be completely wetted by liquid, behaving as the Wenzel mode when <i>θ</i><sub>e</sub> < 118°, under which the gaps of nanostructures are filled with liquid. However, when the characteristic contact angle <i>θ</i><sub>e</sub> is in a range of 118°–145°, the base angles of nanostructures have different effects on wetting modes. The surface with inverted TWs (60° base angle) is conducive to keeping droplet in Cassie mode, in which the liquid does not penetrate into any gap of nanostructures. The surface with rectangular pin fins behaves as either Partial Wenzel mode or Cassie mode. The transition between the two modes takes place at <i>θ</i><sub>e</sub> ～130°. The surface with TWs (120° base angle) keeps the droplet in Partial Wenzel mode, in which the gaps of nanostructures are partially wetted by liquid. For <i>θ</i><sub>e</sub> larger than 145°, the dewetting process takes place on the surface of the nanostructure, in which the droplet leaves the solid surface. We conclude that the wetting modes on nanostructured surface satisfy the minimum surface energy principle. Our work discloses a new finding that the surface with inverted TWs is easy to maintain Cassie mode, which is good for dropwise condensation applications.

Fiber bundle topology optimization of hierarchical microtextures for wetting behavior in Cassie-Baxter mode

This paper presents the topology optimization of hierarchical microtextures for wetting behavior in the Cassie-Baxter mode, considering a structural unit of the hierarchical microtexture composed of base and secondary structures. The geometrical configuration of the considered structural unit can be described as a fiber bundle composed of an external surface of the base structure and the pattern of the secondary structures. Thus, two design variables are defined, one for the external surface of the base structure, and the other for the pattern of the secondary structures. The Young-Laplace equation, including a term depending on the mean curvature of the external surface, is used to describe the liquid/vapor interface imposed with a surface tension in the Cassie-Baxter mode. To overcome the difficulty of numerically computing the second-order derivative of the external surface, two partial differential equation filters are sequentially applied to the design variable of the base structure to ensure the numerical accuracy and feasibility of using an efficient linear-element-based finite element method to solve the Young-Laplace equation. To improve the performance of the hierarchical microtextures, the volume of the liquid bulges suspended at the liquid/vapor interface in the Cassie-Baxter mode, before the transition into the Wenzel mode, is minimized to optimize the match between the external surface of the base structure and the pattern of the secondary structures. In the topology optimization process, penalization of the material density of the surface tension is achieved by an artificial Marangoni phenomenon. In numerical examples, solid surfaces are tiled into textures with axial symmetry, radial symmetry, chirality, and quasiperiodicity; and structural units are derived consisting of base structures with peak shapes and dense secondary structures surrounding the crests of the peaks. The optimized performance of the derived structural units has been confirmed by comparisons.

PDMS Soft Skin Device with Deformable Micro-Diaphragm Array Fabricated with Rapid Substrate-Releasing Process

This paper describes the fabrication and characterization of a prototype wettability switching soft skin device that dynamically switches its surface morphology between flat and rough states. The device, which consists of a 1-μm-thick polydimethylsiloxane (PDMS) deformable diaphragm on a PDMS substrate with a micro-bump arrays, was successfully formed with a high fabrication yield by a novel method of device releasing from a dummy substrate. In buffered hydrofluoric acid (BHF) solution, a sacrificial layer of a novolak-resin-based resist was able to be rapidly released from the OH-terminated SiO2 surface of the dummy substrate, probably due to the breaking of hydrogen bonds at the interface. The wettability of the fabricated device was reversibly switched using micro-diaphragm deformation by varying the inner pressure. When a droplet was placed on the surface in the rough state, a large contact angle of approximately 140° was obtained, close to the Cassie mode with air in the concave-deformed PDMS micro-diaphragms, which indicated a high surface hydrophobicity. During cyclic switching between the rough and flat states after second switching, the contact angle reversibly changed between 106° and 120°, in good agreement with the Wenzel mode, where the micro-diaphragm surfaces were fully wet. Additionally, we observed that the droplet did not move even on the tilted device.

Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces.

We experimentally and theoretically examine the influence of a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), on the wetting states and contact angles on superhydrophobic (SH) surfaces made of hydrophobic microcylinders. We use two types of micropatterns of different surface roughness, r, and packing fraction, ϕ, and vary nine dimensionless surfactant concentrations (CS), normalized by the critical micelle concentration (CMC), in the experiments. At low CS, some of the surfactant-laden droplets are in a gas-trapping, Cassie-Baxter (CB) state on the high-roughness microstructures. In contrast, some droplets are in a complete-wetting Wenzel (W) state on the low-roughness microtextures. We found that the contact angle of CB drops can be well predicted using a thermodynamic model considering surfactant adsorption at the liquid-vapor (LV) and solid-liquid (SL) interfaces. At high CS, however, all of the DDAB drops wet in a Wenzel mode. Based on a Gibbsian thermodynamic analysis, we find that for the two types of superhydrophobic surfaces used, the Wenzel state has the lowest thermodynamic energy and thus is more favorable theoretically. The CB state, however, is metastable at low CS due to a thermodynamic energy barrier. The metastable CB wetting state becomes more stable on the SH microtextures with greater ϕ and r, in agreement with our experimental observations. Finally, we generalize this surface-energy analysis to provide useful designs of surface parameters for a DDAB-laden surfactant droplet on the SH surface with a stable and robust CB state.

Gibbsian Thermodynamics of Wenzel Wetting (Was Wenzel Wrong? Revisited).

When a drop is in contact with a rough surface, it can rest on top of the rough features (the Cassie-Baxter state) or it can completely fill the rough structure (the Wenzel state). The contact angle (θ) of a drop in these states is commonly predicted by the Cassie-Baxter or Wenzel equations, respectively, but the accuracy of these equations has been debated. Previously, we used fundamental Gibbsian composite-system thermodynamics to rigorously derive the Cassie-Baxter equation, and we found that the contact line determined the macroscopic contact angle, not the contact area that was originally proposed. Herein, to address the various perspectives on the Wenzel equation, we apply Gibbsian composite-system thermodynamics to derive the complete set of equilibrium conditions (thermal, chemical, and mechanical) for a liquid drop resting on a homogeneous rough solid substrate in the Wenzel mode of wetting. Through this derivation, we show that the roughness must be evaluated at the contact line, not over the whole interfacial area, and we propose a new Wenzel equation for a surface with pillars of equal height. We define a new dimensionless number H = h(1 - λsolid)/R to quantify when the drop's radius of curvature (R) is large enough compared to the size of the pillars for the new Wenzel equation to be simplified (h is the pillar height; λsolid is the line fraction of the spherical cap's circumference that is on the pillars). Our new line-roughness Wenzel equation can be simplified to cos θW = ρ cos θY when H ≪ 1, where ρ is the line roughness. We also perform a thermodynamic free-energy analysis to determine the stability of the equilibrium states that are predicted by our new Wenzel equation.

Electrically modulated adhesive hydrophobicity for patterning various microstructures

In this study, we present an approach for patterning various microstructures using modulated electric-field. Experimentally, electrical potential was applied to the water droplet on the micropillar or microhole array textured dielectric surfaces, contact state transition could occur from Cassie-Baxter mode to Wenzel mode, and therefore enhance the adhesive hydrophobic property. The liquid droplet would spread over the microstructure dielectric surface and fill up the asperities around the micropillar or microhole array structures, which was indicated clearly by UV-curable polymer deformation under electric-field and solidification under UV irradiation. By this means, microstructures of UV-curable polymer with various morphologies including microlens, microbump and micropillar array structures could be generated by tuning the applied voltage.

Tuning nanostructured surfaces with hybrid wettability areas to enhance condensation.

Vapor condensation is widespread in natural and industrial applications. Rapid and efficient condensation plays an essential role in improving energy efficiency. Despite numerous efforts over the past few decades, the fundamental mechanism of condensation and the microscopic features of condensed droplets are not well understood. Moreover, designing a nanostructured surface with wetting contrast to enhance dropwise condensation remains unclear. Herein, through molecular dynamics simulation, we characterized the condensation processes on various nanopillar surfaces, including the nucleation, growth and coalescence of nanodroplets. During condensation, the droplet size grows linearly with time as V ∝ t, and the coalescence between small droplets can affect the resultant wetting mode of large droplets. The results indicate that the cooperation between spatially ordering nucleation and dropwise growth endows hybrid nanopillar surfaces with better heat and mass transfer performance compared with other homogeneous nanopillar surfaces. Moreover, an interesting dewetting transition occurring on hydrophobic nanopillar surface was observed during droplet growth, the nucleation site and dewetting transition were analyzed based on potential energy field of surface. By varying the geometric parameters of the nanopillar, we found that the condensation rate of the hybrid nanopillar surface increases with the increase of surface solid fraction. The dense nanopillar array can not only restrain the formation of Wenzel mode droplet, but also enhance the condensation rate, which provides a guidance for the design of hybrid nanostructured surfaces.

Preparation of superhydrophobic and anti-resin-adhesive surfaces with micro/nanoscale structures on high-speed steel via laser processing

In order to solve problems of high viscous resin adhering on machine or tool surfaces in manufacturing resin based composites, a superhydrophobic and anti-adhesive surface was prepared and investigated. Micro/nanoscale hierarchical structures in shape of volcanic vents were fabricated on high-speed steel surfaces via laser processing. After being hydrophobized by fluoroalkylsilane solution, the microprotruding surfaces showed great hydrophobicity and even superhydrophobicity, with a water contact angle of 157.2° and sliding angle of 2.8°. Besides, the surfaces also displayed low wettability and excellent anti-adhesivity for highly viscous epoxy resin, with a best resin contact angle of 149.7° and sliding angle of 7.3°. As microprotruding diameters and spacings decreased from 500 to 100 μm, and heights rose from 14 to 54 μm, both water and resin contact angles increased while sliding angles decreased. The decrease of microprotruding diameter and spacing as well as the raise of height could increase surface roughness, enlarge air-liquid interface fraction and reduce actual contact area between liquid and the steel surface. Thus the wetting mode of the microstructured surface changed from Wenzel's mode to Cassie's mode, leading to a reduction of wettability and an enhancement of anti-resin-adhesivity on the surface.

Hydrophobicity and Leidenfrost point of ZnO nanorod array combined with nanoscale roughness on the topmost surface

Zinc oxide nanorods (ZnO-NR) were prepared using hydrothermal processing on a Si substrate. The rod diameter depended on the precursor solution concentration. Subsequent hydrothermal treatment using 3-dimethylaminopropylamine under appropriate conditions provided nanoscale roughness onto {0001}, the topmost surface of the ZnO-NR. After coating with hydrophobic fluoroalkylsilane, the ZnO-NR with the roughness on its topmost surface exhibited higher contact angles and sliding angles than the normal ZnO-NR, suggesting the contribution of Wenzel mode from the provided roughness and resultant petal surface formation. The Leidenfrost point against water droplets for normal ZnO-NR exhibited a decreasing trend concomitantly with an increasing contact angle. However, it was found to depend on the solid–liquid contact area ratio. Providing nanoscale roughness on the topmost surface of ZnO-NR decreased the Leidenfrost point.

Dual-Functional Superhydrophobic Textiles with Asymmetric Roll-Down/Pinned States for Water Droplet Transportation and Oil-Water Separation.

Superhydrophobic surfaces with tunable adhesion from lotus-leaf to rose-petal states have generated much attention for their potential applications in self-cleaning, anti-icing, oil-water separation, microdroplet transportation, and microfluidic devices. Herein we report a facile magnetic-field-manipulation strategy to fabricate dual-functional superhydrophobic textiles with asymmetric roll-down/pinned states on the two surfaces of the textile simultaneously. Upon exposure to a static magnetic field, fluoroalkylsilane-modified iron oxide (F-Fe3O4) nanoparticles in polydimethylsiloxane (PDMS) moved along the magnetic field to construct discrepant hierarchical structures and roughnesses on the two sides of the textile. The positive surface (closer to the magnet, or P-surface) showed a water contact angle up to 165°, and the opposite surface (or O-surface) had a water contact angle of 152.5°. The P-surface where water droplets easily slid off with a sliding angle of 7.5° appeared in the "roll-down" state as Cassie mode, while the O-surface was in the "pinned" state as Wenzel mode, where water droplets firmly adhered even at vertical (90°) and inverted (180°) angles. The surface morphology and wetting mode were adjustable by varying the ratios of F-Fe3O4 nanoparticles and PDMS. By taking advantage of the asymmetric adhesion behaviors, the as-fabricated superhydrophobic textile was successfully applied in no-loss microdroplet transportation and oil-water separation. Our method is simple and cost-effective. The fabricated textile has the characteristics of superhydrophobicity, magnetic responsiveness, excellent chemical stability, adjustable surface morphology, and controllable adhesion. Our findings conceivably stand out as a new tool to fabricate functional superhydrophobic materials with asymmetric surface properties for various potential applications.

Wettability evolution of different nanostructured cobalt films based on electrodeposition

The wettability evolution of eight different types of nanostructured cobalt films is investigated using galvanostatic electrodeposition. The morphology, which evolves from a plump, pea-like structure to cone, pyramid, shell, fluffy cone, fluffy shell and flower structures can be controlled by changing the anion type in the electrolyte or the deposition current density. A growth mechanism is proposed based on cathodic anomalous absorption theory, which explains both the influence of the anion type on the surface structure and the current-dependent morphology evolution. The contact angle and sliding angle of all films are measured, and fractions of water drop contact area with the surrounding air are calculated to illustrate the roughness induced wettability. The hierarchical (nano and micro) structures exhibit eminent superhydrophobicity compared with single dimensional surfaces, indicating this as an effective way to prevent transition from Cassie–Baxter mode to Wenzel mode.

Hydrophilic/hydrophobic surface of Al 2 O 3 thin films grown by thermal and plasma-enhanced atomic layer deposition on plasticized polyvinyl chloride (PVC)

Thermal ALD (T-ALD) and plasma-enhanced ALD (PE-ALD) processes were employed to prepare the Al2O3 thin films on plasticized polyvinyl chloride (PVC) surfaces at a low deposition temperature of 80 °C. The water contact angle test shows that the T-ALD Al2O3 films exhibited a hydrophobic surface, while the PE-ALD Al2O3 films were hydrophilic. The difference in the surface hydrophobicity is attributed to the carbon residue in Al2O3 films, as revealed by the X-ray photoelectron and Fourier transform infrared spectroscopies. The trend of hydrophobicity follows with the evolution of surface roughness of the T-Al2O3 films, indicating that the surface roughness contributed to the hydrophobicity as predicted by the Wenzel's mode. The result indicates that the surface hydrophilicity/hydrophobicity of the Al2O3 thin films can be modified and controlled by the deposition conditions of the ALD process.

Kinetics of droplet wetting mode transitions on grooved surfaces: forward flux sampling.

The wetting configuration of a liquid droplet on a rough or physically patterned surface is typically characterized by either the Cassie wetting mode, in which the droplet resides on top of the roughness, or the Wenzel mode, in which the droplet penetrates into the roughness. For a fixed surface topology and droplet size, one of these modes corresponds to the global free-energy minimum. However, the other state is often metastable and long-lived due to a free-energy barrier that hinders the transition between the two wetting states. Metastable wetting states have been observed experimentally, and we also observe them in molecular dynamics (MD) simulations of a droplet on a grooved surface. Using forward flux sampling, we study the kinetics of the Cassie to Wenzel and Wenzel to Cassie transitions for two-dimensional droplets on periodically grooved substrates. The global-minimum wetting states that emerge from our nanoscale MD approach are consistent with those predicted by a macroscopic model based on free energy minimization. We find that the free-energy barriers for these transitions depend on the droplet size and surface topology. A committor analysis indicates that the transition-state ensemble consists of droplets that are on the verge of initiating/breaking contact with the substrate at the bottom of the grooves.

Superhydrophobic stability of nanotube array surfaces under impact and static forces.

The surfaces of nanotube arrays were coated with poly(methyl methacrylate) (PMMA) using an imprinting method with an anodized alumina membrane as the template. The prepared nanotube array surfaces then either remained untreated or were coated with NH2(CH2)3Si(OCH3)3(PDNS) or CF3(CF2)7CH2CH2Si(OC2H5)3 (PFO). Thus, nanotube arrays with three different surfaces, PDNS, PMMA (without coating), and PFO, were obtained. All three surfaces (PDNS, PMMA, and PFO) exhibited superhydrophobic properties with contact angles (CA) of 155, 166, and 168°, respectively, and their intrinsic water contact angles were 30, 79, and 118°, respectively. The superhydrophobic stabilities of these three surfaces were examined under dynamic impact and static pressures in terms of the transition from the Cassie-Baxter mode to the Wenzel mode. This transition was determined by the maximum pressure (p(max)), which is dependent on the intrinsic contact angle and the nanotube density of the surface. A p(max) greater than 10 kPa, which is sufficiently large to maintain stable superhydrophobicity under extreme weather conditions, such as in heavy rain, was expected from the PFO surface. Interestingly, the PDNS surface, with an intrinsic CA of only 30°, also displayed superhydrophobicity, with a CA of 155°. This property was partially maintained under the dynamic impact and static pressure tests. However, under an extremely high pressure (0.5 MPa), all three surfaces transitioned from the Cassie-Baxter mode to the Wenzel mode. Furthermore, the lost superhydrophobicity could not be recovered by simply relieving the pressure. This result indicates that the best way to maintain superhydrophobicity is to increase the p(max) of the surface to a value higher than the applied external pressure by using low surface energy materials and having high-density binary nano-/microstructures on the surface.