Cassie-Baxter Model Research Articles

Theoretical investigation on microstructured hybrid surface heat transfer characteristics with Marangoni convection effect

The vapor condenses on the surface as a liquid film or distinct droplets, depending on the surface and fluid properties and the environmental conditions. The maximum droplet radius, micro-structure geometry, and Marangoni convection have proved to have a significant impact on the heat transfer characteristics. This paper investigates and compares the effects of different parameters on the heat transfer characteristics of three distinct hybrid surface models. The Marangoni convention effects originating from the temperature gradient are considered in all calculations. The results showed that an optimum maximum droplet radius exists in all three models in which the maximum hybrid heat flux can be obtained. The maximum hybrid heat flux is 133.94 (kW/m2) for the smooth model, 102.34 (kW/m2) for the Wenzel model, and 93.05 (kW/m2) for the Cassie-Baxter model. The results indicated that shorter micropillar heights result in higher complete dropwise and hybrid heat fluxes in the Cassie-Baxter and Wenzel models. The increase in the solid fraction is observed to increase the complete dropwise and hybrid heat fluxes as well.

Quantification and modeling of anisotropic wetting of textured surfaces

This work focuses on the study of anisotropic wetting on textured surfaces. Three stripe-like textures are studied with different texture heights. These textures are realized with three different polymers. With using a lab-built goniometer, the contact angle of each surface is studied in all directions. Two different behaviors are observed on these surfaces and correlated to the wetting state according to the Extrand model. The Wenzel state is characterized by a strong anisotropy. The contact angle as a function of direction follows a Gaussian evolution. This Gaussian evolution is modeled as a function of contact angles measured experimentally in a first step. In a second step, these experimental angles are computed according to a hierarchical approach based on a roughness parameter. The Cassie–Baxter state is characterized by an absence of anisotropy, the contact angle as a function of the direction remains constant with a value close to the contact angle predicted by the Cassie–Baxter model. Thus the understanding of the phenomenon of droplet anisotropy is deepened by linking this behavior to the textures and surface chemistry.

HYDROPHOBICITY IMPROVEMENTS OF POLYMERS USED IN BIOMEDICAL APPLICATIONS.

Improvement in hydrophobicity is important for polymers used in various applications such as biomedical applications, as it can delay their degradation due to long-term exposure to moisture environments. Although a number of surface modification techniques have been developed over the years to improve hydrophobicity, their specific influences on hydrophobicity enhancement as well as long-term mechanical and tribological performances are yet to be fully understood. In this study, surface textures, with variation in type and geometry, are introduced on Ultrahigh Molecular Weight Polyethylene (UHMWPE) and High Density Polyethylene (HDPE) surfaces to study the effect of surface modification on hydrophobicity and long-term mechanical and tribological performances. Based on the theoretical study using Wenzel and Cassie-Baxter models, surface textures of various types and dimension are introduced on UHMWPE and HDPE surfaces. The results show that introduction of surface textures significantly improves the hydrophobicity of polymers. Specific relationship between texture type and geometry, and improvement in hydrophobicity is explored. Based on the comparison between experimental results and theoretical models, transition state modeling seems to be more suitable in describing the change in hydrophobicity with the addition of surface texture. The study provides useful guidelines to improve hydrophobicity of polymers for biomedical applications.

Construction and anti-corrosion behavior study of silanol-modified Ni-WS2 superhydrophobic composite coating

In this study, a novel superhydrophobic composite coating is prepared by a two-step method. Firstly, the Ni-based deposition coating is fabricated via composite electrodeposition with WS2 granules worked as dispersed phase particles, and the roughness of deposit coating can be adjusted by changing the added amount of sulfide in the bath. The prepared Ni-WS2 deposit exhibits sufficiently high roughness and suitable micro-nano morphology. Then, hexadecyltrimethoxysilane (HDTMS) is used to modify the deposition coating with low surface energy to obtain silanol-modified Ni-WS2 coating with superhydrophobic properties. The superhydrophobic behavior of the composite coating conforms to the Cassie-Baxter model, with low adhesion to aqueous solutions. When the addition amount of WS2 particles in the plating solution during the electrodeposition process is 5 g·L−1, the prepared composite coating has the excellent superhydrophobic properties. The water contact angle of the composite coating is 154.7° ± 2 and the sliding angle can reach 6°. And the superhydrophobic composite coating exhibits excellent corrosion resistance.

Microstructured Magnetoactive Elastomers for Switchable Wettability.

We demonstrate the control of wettability of non-structured and microstructured magnetoactive elastomers (MAEs) by magnetic field. The synthesized composite materials have a concentration of carbonyl iron particles of 75 wt.% (≈27 vol.%) and three different stiffnesses of the elastomer matrix. A new method of fabrication of MAE coatings on plastic substrates is presented, which allows one to enhance the response of the apparent contact angle to the magnetic field by exposing the particle-enriched side of MAEs to water. A magnetic field is not applied during crosslinking. The highest variation of the contact angle from (113 ± 1)° in zero field up to (156 ± 2)° at about 400 mT is achieved in the MAE sample with the softest matrix. Several lamellar and pillared MAE structures are fabricated by laser micromachining. The lateral dimension of surface structures is about 50 µm and the depth varies between 3 µm and 60 µm. A systematic investigation of the effects of parameters of laser processing (laser power and the number of passages of the laser beam) on the wetting behavior of these structures in the absence and presence of a magnetic field is performed. In particular, strong anisotropy of the wetting behavior of lamellar structures is observed. The results are qualitatively discussed in the framework of the Wenzel and Cassie–Baxter models. Finally, directions of further research on magnetically controlled wettability of microstructured MAE surfaces are outlined. The obtained results may be useful for the development of magnetically controlled smart surfaces for droplet-based microfluidics.

High Hysteresis Suspended Wetting State: A Wetting Regime for Controlled Trapping of Drops on Micro‐Trench Covered Surfaces

AbstractWetting of simple liquids on rough surfaces is usually described either by the Cassie–Baxter model where drops are suspended over the roughness features with air trapped below, or by the Wenzel model where the drops are impaled on the roughness features. According to the Wenzel model, drops can be trapped on the surfaces, whereas in the Cassie–Baxter model, the drops can be easily released upon adding a small tilt. Surfaces with very regular surface pattern, such as closely spaced micro‐trenches, can exhibit the so‐called High Hysteresis Suspended State (HySS). In this state the drops assume a spherical shape with air trapped below them, a characteristic feature of the Cassie–Baxter state, but show a very high contact angle hysteresis (up to >70°), typical for the Wenzel regime. The HySS regime is studied as a function of the surface geometry, applied pressure and the mass of the drop. Through roll‐off measurements it is shown that surfaces with controlled trapping conditions can be generated, where drops can be captured, however, upon addition of a small amount of energy (e.g., induced by tilt) can be released. The parameters that can be used to control the “depth” of the trap are elucidated.

Wettability of a hybrid PTFE/Kevlar fabrics at micro-, meso-, and macroscales

Hybrid PTFE/Kevlar fabric reinforced epoxy composites have widely been used as structural self-lubricating liners. To achieve the best working performance, the Kevlar rich side has to be well impregnated (good wettability), whereas the PTFE rich side must remain dry at the same time. Consequently, it is essential to study the wetting properties of both sides from the microscale (fibre) to the macroscale (fabric) via the mesoscale (tow). After characterization of the wetting properties of hybrid PTFE/Kevlar fabrics at each scale, a revised Cassie-Baxter model was developed, linking the three scales and taking into account the three phases, i.e. air, PTFE and Kevlar. Water contact angles served to characterize the wetting properties at every scale and to validate the predictions. This study aims at providing a better insight into the hierarchical wetting of fibrous materials, both from experimental and theoretical perspectives.

(Invited) Characterization of Wetting Behavior on High Aspect Ratio Multilayer Structure

The miniaturization and complexity of semiconductor structures increase to improve device performance and functionality. In particular, the latest high-performance semiconductor architectures are formed of high-aspect-ratio structures composed of multilayer thin films. In the case of logic devices, it is reported that the gate-all-around(GAA) structure will become the mainstream, and in the 3D NAND flash memory, it will have a structure with more than 168 layers and a high aspect ratio of 50 or more. In order to form such a structure, the manufacturing process becomes complicated and various challenges to be solved occur. One of the challenges facing is efficiently removing contaminants generated while forming structures having a complex and high aspect ratio structure without damaging the structures. In the case of a wet cleaning process, a technology capable of removing only contaminants without pattern collapse and material loss is required.In order to develop a wet cleaning process for effective contaminant removal and precise control in cleaning process on a high aspect ratio multilayer structure, it is necessary to understand the chemical reaction characteristics between the material and the cleaning chemicals. In particular, it is important to understand the wettability of the cleaning liquid in the structures for damage-free cleaning process development.The wettability of HAR nanostructures was studied using the FTIR-ATR measurement technique. Studies have shown that FTIR-ATR is a reasonable method for studying the wettability properties in HAR structures made of Si. We performed similar experiments and similar experimental results were obtained. However, the FTIR-ATR analysis method is difficult to apply to a HAR multilayer structure. In this structure in which thin films with different optical properties are stacked, the IR signal did not reach the detector sufficiently, making measurement impossible. In this paper, we introduce a new measurement method for analyzing the wettability in the HAR multilayer structures.To evaluate the wettability of various chemicals in HAR multilayer structures, the trench pattern with a width of 200 nm and a depth of 9500 nm, the aspect ratio of 47.5 was used. This sample was immersed in a 20 nm silica particle solution to observe the arrival depth of the particles in the trench. To visualize the particle arrival depth, the sidewall of the trench was exposed by cutting and the sidewall surfaces were characterized by atomic force microscope (AFM, NX-20, Park Systems, Korea). Furthermore, an organic thin film which was carried out to form a hydrophobic surface was deposited on the samples to observe the wetting behavior according to the various wettability of the trench surface. A hydrophobic surface with a water contact angle of 95° was formed on the sample by depositing an organic thin film to observe the wetting behavior according to various wetting properties of the trench surface. The surface tensions of the particle solutions were varied by adding SDS surfactant.The change in the sidewall surface of the trench using AFM was visualized successfully, and the observation explained the wetting behavior of the liquid. The experimental results agreed with the values calculated by the Wenzel & Cassie-Baxter model. The visualization of the trench sidewall using AFM could help understand the wetting behavior in the HAR multilayer structure.

Theoretical analysis on condensation heat transfer on the hydrophobic–hydrophilic hybrid surfaces with the impact of the Marangoni convection

Condensation occurs in the forms of filmwise condensation and dropwise condensation. Over the past decade, hybrid hydrophobic–hydrophilic surfaces have received significant attention due to their potential to enhance the heat transfer rate compared to complete dropwise condensation. The maximum condensed droplet radius in the hydrophobic region has a considerable impact on the heat transfer characteristics. This paper discusses the effect of the maximum droplet radius on hybrid surface heat transfer characteristics. On top of that, Marangoni convection effects originated due to the temperature gradient as a heat transfer resistance on the heat transfer rate is considered, modelled and investigated. Three models were investigated in this paper. The results indicate that there is an optimum maximum droplet radius for each model that can improve the heat transfer rate. When the filmwise region width increases from 0.5 mm to 1.0 mm, the optimum maximum droplet radius increases from 0.25 mm to 0.3 mm. For a fixated droplet contact angle and filmwise and dropwise region widths, the hybrid heat flux of the smooth dropwise model can be up to 1.55 times higher than that of the Cassie–Baxter model, and the hybrid heat flux of the smooth dropwise model can be up to 1.38 time higher than that of the Wenzel state. Finally, considering Marangoni convection showed up to a 30% increase in the heat transfer coefficient in some cases.

Preparation and Hydrophobicity of Bionic Structures Based on Composite Infiltration Model.

The wettability, surface energy, structure, and morphology of a material’s surface will affect the interaction process between the material and the organism. Moreover, these factors are not independent of each other, but will affect each other, which together determine the biological surface of the material. Although two classic theories of surface wettability control have been established, including the Wenzel model and the Cassie–Baxter model, the mechanism of the microstructure parameters on the surface wettability has not been considered. This paper established a two-dimensional mathematical model of the composite wetting pattern based on microstructure parameters, revealed the mechanism of the microstructure parameters on the surface wettability, and then used ultra-precision cutting and molding composite preparation methods to quickly and efficiently prepare bionic structures, and the hydrophobic character of the microstructure was characterized by the contact angle meter, which provides theoretical support and preparation technology for the modification of the hydrophobic character of the material.

3D Printed Parahydrophobic Surfaces as Multireaction Platforms.

Parahydrophobic surfaces (PHSs) composed of arrays of cubic μ-pillars with a double scale of roughness and variable wettability were systematically obtained in one step and a widely accessible stereolithographic Formlabs 3D printer. The wettability control was achieved by combining the geometrical parameters (H = height and P = pitch) and the surface modification with fluoroalkyl silane compounds. Homogeneous distribution of F and Si atoms onto the pillars was observed by XPS and SEM-EDAX. A nano-roughness on the heads of the pillars was achieved without any post-treatment. The smallest P values lead to surfaces with static contact angles (CAs) >150° regardless of the H utilized. Interestingly, the relationship 0.6 ≤ H/P ≤ 2.6 obtained here was in good agreement with the H/P values reported for nano- and submicron pillars. Furthermore, experimental CAs, advancing and receding CAs, were consistent with the theoretical prediction from the Cassie-Baxter model. Structures covered with perfluorodecyltriethoxysilane with high H and short P lead to PHSs. Conversely, structures covered with perfluorodecyltrimethoxysilane exhibited a superhydrophobic behavior. Finally, several aqueous reactions, such as precipitation, coordination complex, and nanoparticle synthesis, were carried out by placing the reactive agents as microdroplets on the parahydrophobic pillars, demonstrating the potential application as chemical multi-reaction array platforms for a large variety of relevant fields in microdroplet manipulation, microfluidics systems, and health monitoring, among others.

(Invited) Characterization of Wetting Behavior on High Aspect Ratio Multilayer Structure

The wetting behavior of solutions was successfully visualized in the high aspect ratio multi-stack trench pattern using the inclined-AFM methodology. The visualization allows the characterization of the wetting behavior of cleaning solutions on various hydrophobic surfaces. The experimental results agreed with the values calculated by the Wenzel & Cassie-Baxter model. The wetting characteristic analysis using inclined-AFM is an efficient method regardless of pattern dimensions.

Fabrication of nanostructured DLC coatings using anodic alumina films

Diamond-like carbon (DLC) films, deposited by dry processes, such as, physical vapor deposition and chemical vapor deposition, provide high hardness, lubricity, and wear resistance. With improvements in microfabrication technologies, tuning the microscopic surface morphology to control physical properties (such as wettability) of materials has attracted immense attention. However, the strengths of microstructures with uneven surfaces are insufficient for numerous applications. Therefore, nanoscale structures with uneven surfaces and enhanced strength were synthesized using DLC. Anodic porous alumina films with regularly aligned linear nanopores produced by anodic oxidation of Al were used as mask materials to deposit DLC films on Si substrates. Uneven structures (with uniform distributions of DLC nanoscale protrusions) were created on the substrates. Mask materials with large pore diameters and small thicknesses created DLC protrusions with large diameters and heights, respectively. The friction coefficients of nanostructured DLC surfaces with tall protrusions and normal DLC films were similar. Unevenly structured DLC surfaces were slightly more water-repellent than normal DLC films, with greater water repellency for taller protrusions. The Cassie-Baxter model was applied to analyze the surface wettability. This report describes a new, simple, and effective method of fabricating DLC nanoscale structures using anodic alumina films.

Self-Assembly of CsPbBr3 Perovskites in Micropatterned Polymeric Surfaces: Toward Luminescent Materials with Self-Cleaning Properties.

In this work, we present a series of porous, honeycomb-patterned polymer films containing CsPbBr3 perovskite nanocrystals as light emitters prepared by the breath figure approach. Microscopy analysis of the topography and composition of the material evidence that the CsPbBr3 nanocrystals are homogeneously distributed within the polymer matrix but preferably confined inside the pores due to the fabrication process. The optical properties of the CsPbBr3 nanocrystals remain unaltered after the film formation, proving that they are stable inside the polystyrene matrix, which protects them from degradation by environmental factors. Moreover, these surfaces present highly hydrophobic behavior due to their high porosity and defined micropatterning, which is in agreement with the Cassie–Baxter model. This is evidenced by performing a proof-of-concept coating on top of 3D-printed LED lenses, conferring the material with self-cleaning properties, while the CsPbBr3 nanocrystals embedded inside the polymeric matrix maintain their luminescent behavior.

Numerical study of the wetting dynamics of droplet on laser textured surfaces: Beyond classical Wenzel and Cassie-Baxter model

The classical wetting models, such as the Wenzel and Cassie-Baxter have been extensively used to quantify the wettability of laser-textured surfaces. However, these models do not provide any description of the corresponding droplet dynamics. In this work, we propose a detailed continuum-level modelling to study the wetting dynamics of a water droplet on Ti-6Al-4V alloy. The calculations are performed for flat surfaces and surfaces with various reliefs. The calculated evolutions of the droplet spreading parameter for flat surfaces, surfaces with triangular reliefs and one with two different periods and heights not only provide explanations of several experimental results but also underline the perspectives of using complex reliefs for efficient wettability control. Thus, such simulations are shown to be useful in relief design and laser texturing for a wide range of applications, for example, laser treatment of artificial implants and prostheses.

A universal strategy to prepare hot liquid super-repellent surfaces for anti-scalding and anti-scaling

Durably and fully repel hot liquid (representative by water and oil) is difficult to achieve because of the high-flux evaporation–condensation behavior, which caused a lot of trouble in life and production, such as scalding and scaling. In this work, we report a universal strategy to realize hot liquid super-repellency (HLSR) on various solid surfaces by structuring fluorine-modified micro-nano binary topography (F-MNBT). Cassie-Baxter model is still applying to the F-MNBT surfaces even it contacting to 95℃ water, that because nanostructures (with size below 50 nm) design provides hot liquid repellency and microcavities (with size of over 10 μm) design remains in their non-wetting state. Through this strategy, we demonstrate the unique advantage of the HLSR on various cloths for anti-scalding. In addition, we also develop a F-MNBT composite coating that can durably sustain HLSR after enduring various representative mechanical, thermal and chemical stresses, that can continuously perform self-cleaning and anti-scaling at high temperatures. This work promises new solutions for surface applications related to hot liquid, expecting to serve in heat insulation protection, thermal management system, pipeline transport, marine petroleum exploitation, and so on.

Effect of spray powder particle size on the bionic hydrophobic structures and corrosion performance of Fe-based amorphous metallic coatings

Fe-based amorphous metallic coatings (AMCs) with different spraying particle size powders were prepared on a 316 L stainless steel by the activated combustion high-velocity air fuel (AC-HVAF) method. The bionic hydrophobic structural characteristics and corrosion behavior of the AMCs after chemical etching and surface modification were studied by micro morphology observation and electrochemical testing. Results show that all AMCs have completely dense structure and a good combination with the 316 L stainless steel. In addition, the particle size shows a significant effect on the surface hydrophobic structure. The number of unmelted particles on the surface decreased with the decreasing particle size. As a result, micro/nanoscale hierarchical hydrophobic structures are constructed according to the Cassie–Baxter model, and an enhanced corrosion resistance is obtained given the particle size of 400 mesh. The TEM results showed that many nanoscale Cr-rich particles are randomly distributed in the AMCs. These Cr-rich particles built a nanoscale hydrophobic structure on the surface and improved the corrosion resistance of the AMCs. With the increase in the temperature and the concentration of the NaCl solution, the corrosion resistance of the hydrophobic AMCs decreased, and the water contact angle of the hydrophobic AMC with 400 mesh particle spraying powder reached 152.11°. Changing the particle size of spraying powder is an effective method to prepare the optimum bionic hydrophobic interfaces for AC-HVAF AMCs.

Construction of highly hydrophobic and fast endothermic surfaces using candle soot and nano-TiO2

Abstract This work combines hydrophobic and heat-absorbing materials to solve the problem of water and frost resistance in industrial and domestic scenarios in harsh environments. Here, highly hydrophobic and fast endothermic (HHFE) surfaces were prepared by applying candle soot (nanocarbon) as the template, slides as the carrier and nano-TiO2 as a backbone and a connecting layer. The resulting HHFE surface exhibited a coral-like porous structure, which is beneficial to hydrophobic performance. The contact angle between the water droplet and the glass with the HHFE coating was about 120°, thus implying that the prepared HHFE surface with a TiO2 skeleton layer has excellent hydrophobicity. The hydrophobic mechanism of the HHFE surface can be explained by the Cassie–Baxter model. Infrared thermography and thermometry were used to record the thermal capacity and heat-absorbing rate of the HHFE surface. The temperature of the glass covered with the HHFE coating rose from 16°C to 38°C within 5 minutes, which is 46.2% higher in capacity and 2.2 times faster in rate than ordinary glass under the same solar irradiation. The resulting HHFE thin film consists of nanocarbon materials, and nano-TiO2 particles were hydrophobic and good heat absorbers. They have great potential for anti-freezing and water-proofing applications, especially in harsh environments.

NIR-Reflective and Hydrophobic Bio-Inspired Nano-Holed Configurations on Titanium Alloy.

Near-infrared (NIR) radiation plays an important role in guided external stimulus therapies; its application in bone-related treatments is becoming more and more frequent. Therefore, metallic biomaterials that exhibit properties activated by NIR are promising for further orthopedic procedures. In this work, we present an adapted electroforming approach to attain a biomorphic nano-holed TiO2 coating on Ti6Al4V alloy. Through a precise control of the anodization conditions, structures revealed the formation of localized nano-pores arranged in a periodic assembly. This specific organization provoked higher stability against thermal oxidation and precise hydrophobic wettability behavior according to Cassie-Baxter's model; both characteristics are a prerequisite to ensure a favorable biological response in an implantable structure for guided bone regeneration. In addition, the periodically arranged sub-wavelength-sized unit cell on the metallic-dielectric structure exhibits a peculiar optical response, which results in higher NIR reflectivity. Accordingly, we have proved that this effect enhances the efficiency of the scattering processes and provokes a significant improvement of light confinement producing a spontaneous NIR fluorescence emission. The combination of the already favorable mechanical and biocompatibility properties of Ti6Al4V, along with suitable thermal stability, wetting, and electro-optical behavior, opens a promising path toward strategic bone therapeutic procedures.

Dramatically reducing the critical velocity of air cavity generation via biomimetic microstructure effects.

Control over the generation of air cavities in liquids is vitally important in numerous underwater marine systems, with behaviors ranging from largely reducing the fluid resistance to minimizing the energy consumption. However, reducing the critical velocity of air cavity generation simultaneously on both original and hydrophobic materials can be mutually exclusive. In strong contrast to the view that the surface wettability is what determines the critical velocity of air cavity entrainment, we report a facile method to dramatically reduce the critical velocity of air cavity generation by etching a kind of biomimetic microstructure on the sphere surface. It is worth noting that the generation of air cavities induced by the microstructure effect is almost independent of surface wettability; even for the hydrophilic spheres, the critical velocity of air cavity generation is reduced by over 86.3%. The physical mechanism is mainly related to the pinning of the moving contact line and the regulable transition of the wetting state from the Wenzel model to the Cassie-Baxter model at the solid-liquid-air interface. We also investigated the stability of the microstructured spheres to induce underwater air cavities and found that a continuous and robust air cavity could still be produced even after 1200 repeated impacts. This research provides a simple, economical and energy-efficient method for minimizing the critical velocity for generating air cavity entrainment to reduce hydrodynamic drag.