Cassie-Baxter State Research Articles

Superhydrophobic waveguide: Liquid-core air-cladding waveguide platform for optofluidics

In this paper, we present an optofluidic waveguide platform consisting of liquid as a core material and air as cladding, enabled by using a superhydrophobic channel featured with hydrophobized high-aspect-ratio sharp-tip nanostructures. The contact of the liquid core with the superhydrophobic channel wall is minimized with an air layer retained between them so that the effective refractive index of the cladding layer is close to that of air. Thus, when light is introduced through the core liquid having a higher refractive index than that of the cladding air at the incident angle parallel to the channel direction less than a critical angle, it is reflected at the liquid-gas interface by the total internal reflection. When the cladding layer is filled with water (i.e., Wenzel state), the waveguide losses for the incident angles of 0 and 10° were ∼3.9 and ∼6.8 dB/cm, respectively. In contrast, when the cladding layer is retained with air (i.e., Cassie-Baxter state), the waveguide losses for the same incident angles were as low as ∼0.1 and ∼1.8 dB/cm, respectively. The significantly lowered waveguide losses at the Cassie-Baxter state indicate that superhydrophobic channels can provide the effective waveguide platform for optofluidics, exploiting the air layer as the cladding material.

Synthesis of superhydrophobic surfaces with Wenzel and Cassie–Baxter state: experimental evidence and theoretical insight

We demonstrate surfactant- and template-free synthesis of superhydrophobic thin films by controlling surface morphology. The surface morphology evolution process was determined from time-dependent studies. The relationships between the water contact angle, sliding angle, water droplet size, and surface microstructures were investigated. It is found that structural parameters play an important role in determining the adhesion of a droplet on superhydrophobic surfaces and the liquid–solid adhesion can be effectively manipulated through tailoring the morphology or the size of the microstructures on the surface. We provide a theoretical explanation for the Cassie–Baxter state (water rolls) and the Wenzel state (sticky surface, water adheres) with droplet size on different microstructure surfaces. The new theoretical insight provided in this study matches well with experimental results.

Effect of the texture geometry on the slippery behavior of liquid-infused nanoporous surfaces

Slippery liquid-infused porous surfaces (SLIPS) achieve non-wetting properties through surfaces containing pockets of a lubricating liquid rather than of delicate air. Although studies have demonstrated the potential of SLIPS, such as self-cleaning, anti-icing, biomedical devices and non-fouling marine vessels, the effect of multiple phase structures on the motion of drops has not been thoroughly addressed. The present work was focused on the effect of texture geometry on the slipperiness of liquid-infused nanoporous surfaces. We fabricated a set of surfaces with liquid phase and solid phase coexistence using silicone oil as a lubricant and well-ordered nanoporous anodic alumina plates with various pore diameters and interpore distances. The non-wetting state on the fabricated SLIPS is a full Cassie–Baxter state. The results showed that static water contact angles on the SLIPS were mainly dependent on the non-wetting ability of the lubricant retained in the pores. However, a water droplet sliding angle (WSA) was sensitive to the texture geometry and exhibited a strong correlation with the area ratio of the lubricant phase on the SLIPS. A proposed equation in which the cosine of WSA was a negative linear correlation with the critical factor porosity (P) is a good model for the relationship between the WSA and texture geometry. This suggests that the water droplet slippery property can be altered by changing the porosity of the substrate for fabricating specific SLIPS.

Effects of Contact Angle and Flocculation of Particles of Oligomer of Tetrafluoroethylene on Oil Foaming.

Oil foams have been stabilized by using particles of oligomer of tetrafluoroethylene (OTFE). OTFE particles were dispersed in oil mixtures prior to aeration, to exclude the oil-repellency nature of the particles due to the formation of the metastable Cassie-Baxter state and properly evaluate the effects of contact angle on the foaming behavior. The particle contact angle (θY) against air/oil surfaces were controlled by changing a composition of two oils with different surface tension (n-heptane and methyl salicylate). The θY value increases with increasing a mole fraction of methyl salicylate, from 42° (for pure n-heptane) to 89° (for pure methyl salicylate). The air volume incorporated in the oils after aerating OTFE dispersions in the oil mixtures shows a maximum when θY = 55°. The flocculation of OTFE particles in bulk oils is responsible for the unexpected behavior of foaming observed when θY is relatively high. The increase in the degree of the flocculation reduces the effective concentration of OTFE particles in bulk oil, leading to the inefficient bubble stabilization. These findings suggest the efficient oil foaming using particles as a stabilizer is achieved by optimizing both the particle contact angle and the degree of flocculation in oils.

Molecular dynamics study on the hydrophobicity of a surface patterned with hierarchical nanotextures

Using all-atom molecular dynamics simulation, we investigated the wetting behavior of a water droplet on a surface texturized with nanoscale pillars. We studied the surfaces with nonhierarchical and hierarchical roughnesses by systematically varying the pillar height and the characteristic surface energy. A surface decorated with a hierarchical roughness showed an enhanced hydrophobicity in that the droplet on such a surface prefers the Cassie-Baxter state over the Wenzel state. A hierarchical roughness also gave an enhanced contact angle of a droplet. Even a hydrophilic surface (contact angle is < 90°) turned hydrophobic (contact angle > 90°) with a hierarchical roughness on it. We show the adhesion energy of a droplet to its underlying surface is correlated with the contact angle and therefore is a fundamental measure of the wettability of the surface.

Rates of cavity filling by liquids

Understanding the fundamental wetting behavior of liquids on surfaces with pores or cavities provides insights into the wetting phenomena associated with rough or patterned surfaces, such as skin and fabrics, as well as the development of everyday products such as ointments and paints, and industrial applications such as enhanced oil recovery and pitting during chemical mechanical polishing. We have studied, both experimentally and theoretically, the dynamics of the transitions from the unfilled/partially filled (Cassie-Baxter) wetting state to the fully filled (Wenzel) wetting state on intrinsically hydrophilic surfaces (intrinsic water contact angle <90°, where the Wenzel state is always the thermodynamically favorable state, while a temporary metastable Cassie-Baxter state can also exist) to determine the variables that control the rates of such transitions. We prepared silicon wafers with cylindrical cavities of different geometries and immersed them in bulk water. With bright-field and confocal fluorescence microscopy, we observed the details of, and the rates associated with, water penetration into the cavities from the bulk. We find that unconnected, reentrant cavities (i.e., cavities that open up below the surface) have the slowest cavity-filling rates, while connected or non-reentrant cavities undergo very rapid transitions. Using these unconnected, reentrant cavities, we identified the variables that affect cavity-filling rates: (i) the intrinsic contact angle, (ii) the concentration of dissolved air in the bulk water phase (i.e., aeration), (iii) the liquid volatility that determines the rate of capillary condensation inside the cavities, and (iv) the presence of surfactants.

Inversely designed micro-textures for robust Cassie–Baxter mode of super-hydrophobicity

The robust Cassie–Baxter mode of the wetting behaviour on a micro-textured solid surface, is a key topography element yielding stable super-hydrophobicity. To meet this purpose, we propose an inverse computational design procedure for the discovery of suitable periodic micro-textures, based on three different tilings of the plane. The symmetric tiles of the lattice are regular triangles, quadrangles, and hexagons. The goal of the inverse design procedure is to achieve the robust Cassie–Baxter state, in which the liquid/vapour interface is mathematically described using the Young–Laplace equation on the lattice, and a topology optimisation approach is utilised to construct a variational problem for the inverse design procedure. Based on numerical calculations of the constructed variational problem, underlying effects are revealed for several factors, including the Bond number, duty ratio, feature size, and lattice constant. The effects of feature size and lattice constant provide approaches for compromisingly considering the robustness of the Cassie–Baxter mode and manufacturability of the inversely designed micro-textures; the effect of the lattice constant permits the scaling properties of the derived patterns, and this in turn provides an approach to avoid the elasto-capillary instability driven collapse of the micro/nanostructures in the derived micro-textures. Further, a monolithic inverse design procedure for the periodic micro-textures is proposed in the conclusions, with synthetically considering the manufacturability as well as contact angle and surface-volume ratio of the liquid bulge held by the supported liquid/vapour interface.

Numerical study of laminar boundary-layer flows over a superhydrophobic plate

Liquid flow in the laminar boundary layer over a flat plate with a superhydrophobic surface formed by a square array of pillars is studied theoretically. Assuming the water surface on top of the pillars to be in the Cassie-Baxter state, asymptotic analysis is first carried out to separate the micro-scale flow in a typical cell surrounding a pillar and the macro-scale development of the laminar boundary layer of Blasius type. The 3-D cell problem and the 2-D boundary layer problem are solved together iteratively, yielding the slip length and the entire flow field. Numerical results are presented to examine the effect of solid fraction, pillar-to-pillar spacing, and the speed of the ambient flow on drag reduction. It is shown that the slip length is practically constant, while the boundary layer thickness grows monotonically downstream so that hydrophobicity affects drag reduction primarily over the leading portion of a long surface.

Tailoring of polycrystalline diamond surfaces from hydrophilic to superhydrophobic via synergistic chemical plus micro-structuring processes

The wettability properties of the surfaces of polycrystalline diamond films, grown by Hot Filament Chemical Vapor Deposition, with different grain sizes, ranging from microcrystalline diamond (MCD), to nanocrystalline diamond (NCD), to ultrananocrystalline diamond (UNCD), were successfully tailored from as-grown flat semi-hydrophobic to hydrophilic surfaces, via oxygen (O2) plasma treatment, to highly-hydrophobic by carbon tetrafluoride (CF4) plasma treatment, and to super-hydrophobic diamond surface, via synergistic tailoring of surface chemistry and 3-D texturing. Raman spectroscopy analysis and secondary electron microscopy revealed the Hydrogen-surface termination and increased grain size, respectively, of the diamond films. Atomic Force Microscopy imaging of the plasma-treated UNCD films revealed no significant change in the surface roughness after either O2 or CF4 treatments. Nevertheless, a surface-chemistry alteration was confirmed by X-ray Photoelectron Spectroscopy. Finally, the synergistic combination of 3-D surface structuring and plasma-induced fluorine atoms surface termination of UNCD films produced a superhydrophobic state as revealed by a Water Contact Angle (WCA) close to ∼174°, thus producing the highest superhydrophobic surface of diamond material demonstrated today. The metastable Cassie-Baxter state, for the superhydrophobic 3-D UNCD surface, was demonstrated via a close correlation between the mathematical calculation based on the Cassie-Baxter Model and the experimental measurement of the WCA.

Enabling Highly Effective Boiling from Superhydrophobic Surfaces.

A variety of industrial applications such as power generation, water distillation, and high-density cooling rely on heat transfer processes involving boiling. Enhancements to the boiling process can improve the energy efficiency and performance across multiple industries. Highly wetting textured surfaces have shown promise in boiling applications since capillary wicking increases the maximum heat flux that can be dissipated. Conversely, highly nonwetting textured (superhydrophobic) surfaces have been largely dismissed for these applications as they have been shown to promote formation of an insulating vapor film that greatly diminishes heat transfer efficiency. The current Letter shows that boiling from a superhydrophobic surface in an initial Wenzel state, in which the surface texture is infiltrated with liquid, results in remarkably low surface superheat with nucleate boiling sustained up to a critical heat flux typical of hydrophilic wetting surfaces, and thus upends this conventional wisdom. Two distinct boiling behaviors are demonstrated on both micro- and nanostructured superhydrophobic surfaces based on the initial wetting state. For an initial surface condition in which vapor occupies the interstices of the surface texture (Cassie-Baxter state), premature film boiling occurs, as has been commonly observed in the literature. However, if the surface texture is infiltrated with liquid (Wenzel state) prior to boiling, drastically improved thermal performance is observed; in this wetting state, the three-phase contact line is pinned during vapor bubble growth, which prevents the development of a vapor film over the surface and maintains efficient nucleate boiling behavior.

Morphology modification of electrodeposited superhydrophobic nickel coating for enhanced corrosion performance studied by AFM, SEM-EDS and electrochemical measurements

In this work, an electrodeposited superhydrophobic nickel film with apparent contact angle of 156° was fabricated using different contents of H3BO3 as a morphological modifier to effectively improve the corrosion resistance of the underlying copper substrate. Atomic force microscopy and scanning electron microscopy analysis allowed visualization of growth mechanism and morphology of the electrodeposited coating in the presence and absence of H3BO3. The results showed that H3BO3 considerably effects on morphology of coating by its hindering property that leads to form micro and micro-nanocones hierarchical structures with the mechanism of screw dislocation-driven growth. The spontaneous adsorption of airborne hydrocarbon contaminants on the nickel micro-nanostructures enhances the hydrophobicity of the surface. Cyclic voltammetry and X-ray diffraction provide other evidences of the boric acid hindering effect. The quantitative analysis of atomic force microscopy results showed that after about 5 months exposure to air, the roughness value has been decreased and apparent contact angle has been decreased to 127°. Finally, corrosion behavior of the superhydrophobic nickel film was investigated in 3.5 wt.% NaCl by means of AC/DC electrochemical methods. Regarding potentiodynamic polarization and electrochemical impedance spectroscopy, the superhydrophobic nickel film (Cassie-Baxter state) exhibited five times greater corrosion resistance in comparison with fresh nickel film (Wenzel state).

Superhydrophobic polymeric films with hierarchical structures produced by nanoimprint (NIL) and plasma roughening

The structuration of various polymeric films has been studied to create superhydrophobic surfaces. Nanoimprint lithography and/or plasma etching processes with CF4/Ar have been used on FEP, PMMA and PET polymer films. On the one hand, the effect of the CF4/Ar gases, the input power and the plasma treatment duration have been investigated in terms of etching and fluorination degree, and XPS analyses are precisely discussed. On the other hand, wettability performances were characterized. Relationships between the contact angle, the contact angle hysteresis and the surface structures have been investigated. The wetting behaviors and the transition between the Wenzel and the Cassie-Baxter states was discussed as a function of the roughness. We have prepared each studied polymer films in transparent and flexible superhydrophobic surfaces whose contact angle are ∼160° and hysteresis are ∼2°. A short plasma treatment time (10 s) is sufficient to obtain a superhydrophobic behavior on FEP and PMMA. Results indicate that hierarchical structures allow a more stable superhydrophobic state regarding inhomogeneities. Moreover, the use of plasma etching is suggested to overcome some limitations of the NIL in the case of structures with a high aspect ratio. Finally, a quick and large surface fabrication method for superhydrophobic films is detailed.

Gradient wetting state for droplet transportation and efficient fog harvest on nanopillared cicada wing surface

As a result of natural selection, cicadas can easily remove water droplets, pollen or dust from their wing surface. A gradient wetting state along the wing veins is revealed for the first time in this work. That is, the Wenzel state (hydrophilic) is transitioned to the Cassie-Baxter state (superhydrophobic) from foot to apex of the wing, which is attributed to different radius, height, and gap of the nanopillars on its surface. A fog harvesting test demonstrates that the cicada wing has a remarkable fog harvesting efficiency of 6.6 g m−2 s−1, which is comparable to the values reported for representative plant and animal surfaces. Superhydrophobic region exhibits a lower adhesion force to the droplets than hydrophilic region. So the accumulated droplets easily roll off from superhydrophobic to hydrophilic regions, and the light weight state on the wing can be maintained under its slight shaking. This work may direct the design of gradient wetting surfaces by mimicking the nanopillar structure on cicada wing and explore potential application in water harvesting.

Revisiting the Critical Condition for the Cassie-Wenzel Transition on Micropillar-Structured Surfaces.

Biological and engineering applications of superhydrophobic surfaces are limited by the stability of the wetting state determined by the transition from the Cassie-Baxter state to the Wenzel state (C-W transition). In this paper, we performed water droplet squeeze tests to investigate the critical conditions for the C-W transition for solid surfaces with periodic micropillar arrays. The experimental results indicate that the critical transition pressures for the samples with varying micropillar dimensions are all significantly higher than the theoretical predictions. Through independent measurements, we attributed the disparity to the incorrect assessment of the contact angle on the sidewall surfaces of the micropillars. We also showed that the theoretical models are still applicable when the correct contact angle of the sidewall surfaces is adopted. Our work directly validates and improves the theoretical models regarding the C-W transition and suggests a potential route of tuning superhydrophobicity using finer scale surface features.

Wetting properties of hybrid structure with hydrophilic ridges and hydrophobic channels

In the present study, we fabricated a hybrid structure where the upper surface of the ridge is hydrophilic and the inner surface of the channel is hydrophobic. Laser-induced backside wet etching (LIBWE) process was performed to machine the hybrid structure on a Pyrex glass substrate. Wetting properties were evaluated from static contact angles (CAs) measurement in parallel and orthogonal directions. The water droplet on the hybrid structure was in the Cassie-Baxter state and showed anisotropic wetting property along groove lines. Moisture condensation studies under humid condition indicated that water droplets grew and coalesced on the ridge with hydrophilicity. Furthermore, water–oil separation was tested using a microfluidic chip with the developed hybrid structure. In case of hybrid microfluidic chip, the water could not flow into channel but the hexadecane could flow due to the capillary pressure difference.

Localized Three-Dimensional Functionalization of Bionanoreceptors on High-Density Micropillar Arrays via Electrowetting.

In this work, we introduce an electrowetting-assisted 3-D biofabrication process allowing both complete and localized functionalization of bionanoreceptors onto densely arranged 3-D microstructures. The integration of biomaterials with 3-D microdevice components offers exciting opportunities for communities developing miniature bioelectronics with enhanced performance and advanced modes of operation. However, most biological materials are stable only in properly conditioned aqueous solutions, thus the water-repellent properties exhibited by densely arranged micro/nanostructures (widely known as the Cassie-Baxter state) represent a significant challenge to biomaterial integration. Here, we first investigate such potential limitations using cysteine-modified tobacco mosaic virus (TMV1cys) as a model bionanoreceptor and a set of Au-coated Si-micropillar arrays (μPAs) of varying densities. Furthermore, we introduce a novel biofabrication system adopting electrowetting principles for the controlled localization of TMV1cys bionanoreptors on densely arranged μPAs. Contact angle analysis and SEM characterizations provide clear evidence to indicate structural hydrophobicity as a key limiting factor for 3-D biofunctionalization and for electrowetting as an effective method to overcome this limitation. The successful 3-D biofabrication is confirmed using SEM and fluorescence microscopy that show spatially controlled and uniform assemblies of TMV1cys on μPAs. The increased density of TMV1cys per device footprint produces a 7-fold increase in fluorescence intensity attributed to the μPAs when compared to similar assemblies on planar substrates. Combined, this work demonstrates the potential of electrowetting as a unique enabling solution for the controlled and efficient biofabrication of 3-D-patterned micro/nanodomains.

Superhydrophobic structures on 316L stainless steel surfaces machined by nanosecond pulsed laser

In this paper nanosecond laser machining process was developed to improve the hydrophobicity of AISI 316L stainless steel surface. A geometrical model of laser machined Gaussian micro hole, together with constrain conditions, was established for the first time to predict surface contact angle and optimize structure geometries for maximizing its hydrophobicity. The effects of processing laser power and pitch of microstructures on the topography of the machined surface were investigated through laser machining experiment. Subsequently, the water droplet contact angle was measured to evaluate the hydrophobicity of different specimens. Results show that under the laser power of 10 W and 14 W, with the increase of the pitch of microstructures, the contact angle increases until it reaches its peak value then drops gradually. Under the large pitch of microstructure, the contact angle will increase with the increase of the processing laser power. Under the same pitch of microstructure, the contact angle will increase with the increase of ten-point height of surface topography, Sz which is a better parameter than Sa (arithmetical mean height) to characterise hydrophobicity of surface with Gaussian holes. This study shows that large Sz is an essential condition to form the stable and robust Cassie–Baxter state, i.e. a condition to achieve superhydrophobicity. The comparison between the predicted and measured contact angles in experiments shows that the proposed model can accurately predict contact angle and optimize the geometries of the microstructure to achieve maximum hydrophobicity.

Effect of drop volume and surface statistics on the superhydrophobicity of randomly rough substrates

In this paper, a simple theoretical approach is developed with the aim of evaluating shape, interfacial pressure, apparent contact angle and contact area of liquid drops gently deposed on randomly rough surfaces. This method can be useful to characterize the superhydrophobic properties of rough substrates, and to investigate the contact behavior of impacting drops. We assume that (i) the size of the apparent liquid–solid contact area is much larger than the micromorphology of the substrate, and (ii) a composite interface is always formed at the microscale.Results show apparent contact angle and liquid–solid area fraction are slightly influenced by the drop volume only at relatively high values of the root mean square roughness hrms, whereas the effect of volume is practically negligible at small hrms. The main statistical quantity affecting the superhydrophobic properties is found to be the Wenzel roughness parameter rW, which depends on the average slope of the surface heights. Moreover, transition from the Cassie–Baxter state to the Wenzel one is observed when rW reduces below a certain critical value, and theoretical predictions are found to be in good agreement with experimental data.Finally, the present method can be conveniently exploited to evaluate the occurrence of pinning phenomena in the case of impacting drops, as the Wenzel critical pressure for liquid penetration gives an estimation of the maximum impact pressure tolerated by the surface without pinning occurring.

Surface roughness effects on contact line motion with small capillary number

In this work, we investigate how surface roughness influences contact line dynamics by simulating forced wetting in a capillary tube. The tube wall is decorated with microgrooves and is intrinsically hydrophilic. A phase-field method is used to capture the fluid interface and the moving contact line. According to the numerical results, a criterion is proposed to judge whether the grooves are entirely wetted or not at vanishing capillary numbers. When the contact line moves over a train of grooves, the apparent contact angle exhibits a periodic nature, no matter whether the Cassie-Baxter or the Wenzel state is achieved. The oscillation amplitude of apparent contact angle is analyzed and found to be inversely proportional to the interface area. The contact line motion can be characterized as stick-jump-slip in the Cassie-Baxter state and stick-slip in the Wenzel state. By comparing to the contact line dynamics on smooth surfaces, equivalent microscopic contact angles and slip lengths are obtained. The equivalent slip length in the Cassie-Baxter state agrees well with the theoretical model in the literature. The equivalent contact angles are, however, much greater than the predictions of the Cassie-Baxter model and the Wenzel model for equilibrium stable states. Our results reveal that the pinning of the contact line at surface defects effectively enhances the hydrophobicity of rough surfaces, even when the surface material is intrinsically hydrophilic and the flow is under the Wenzel state.

Atomistic dewetting mechanics of Wenzel and monostable Cassie-Baxter states.

Water adhesion underlies wettabilities, and thus hydrophobicities, and defines surface properties like self-cleaning, icephobicity and many others. The nanomechanics of water adhesion, especially in the dynamic dewetting processes, has not been fully investigated. Here in this article, atomistic modeling and molecular dynamics simulations were utilized to probe the adhesion mechanics of water droplets on nanopillars and flat surfaces, covering dewetting in the Wenzel and the newly discovered monostable Cassie-Baxter states. The simulations were able to identify intermediate dewetting states on rough surfaces, and resolve the transition between wetting states under force. The results revealed characteristic features of dynamic water adhering stress underpinning dewetting on the nanoscale, which provided deeper knowledge on surface dewetting mechanics. This work complements nanoscale dewetting experiments for new fundamental insights in studies including nanoroughness design, enhanced oil recovery, anti-icing and others.