Cassie-Baxter Model Research Articles

Effect of micropatterning with nanowire-based microcavity array on bacterial enrichment and selective distribution

The micro-patterned topography can affect bacterial enrichment and selective distribution by modifying the characteristics of material surface. In this study, we fabricated microcavity patterned silicon surfaces, and quantitatively explored the amount and distribution of Escherichia coli (E. coli) cells attached on a series of defined topographies. The results showed that E. coli cells enrichment was significantly increased on silicon nanowires-based microcavity array when compared to nanowires patterned silicon wafer. Furthermore, the microcavity diameter can control bacterial distribution by changing hydrophilic performance of interface. With the microcavity diameter of 7 µm, the cells colonized into the microcavity almost in its entirety, whilst distributed around the microcavity completely with 10 µm diameter, regardless of centre distances. This phenomenon can be explained by the Cassie-Baxter model equation according to the wettability. The distribution of bacterial enrichment changed with the silicon surfaces turning from non-wetting status to wetting status when treated with oxygen plasma to modify the hydrophobicity. These results demonstrated that microcavity patterned surface could favor bacterial enrichment on silicon and strictly confined bacterial distribution inside/outside the microcavity. Moreover, the nanowires inside microcavity can also increase electron conductivity and reduce the internal resistance, thus providing scientific evidence for the design of wearable microbial fuel cell with rational optimization and integration of different components for electronic skin.

Influence of Surface Roughness on Phosphonation and Wettability of Nanosecond Laser‐Treated Titanium Surfaces

The duplex treatments of metal surfaces combining laser‐assisted functionalization and chemical modification attract a growing interest in view of better control of functional properties such as wettability. Patterned titanium oxides layers of different nature and roughness are produced in ambient atmosphere on a titanium surface using a nanosecond 532 nm Nd:YAG laser. Standalone lines and hatched surfaces obtained with different laser parameters are characterized by scanning electronic microscopy coupled with EDS analysis, micro‐Raman, XRD and X‐ray photoelectron spectroscopy (XPS). A set of surfaces having featureless and saw‐tooth morphologies of different roughnesses is modified with octylphosphonic acid (OPA). The effect of surface roughness on the efficiency of OPA grafting is investigated using XPS. It is found that the organic load on the chemically modified surfaces increases with surface roughness. The apparent thickness of the OPA layers is estimated using a standard uniform overlap model and considering the correction due to the corrugated appearance of the laser‐treated surfaces. The wetting properties of the surfaces before and after chemical modification are compared. The laser‐treated surfaces show superhydrophilic behavior consistent with Wenzel model. On the other hand, the chemically modified surfaces demonstrate a strong increase in hydrophobicity along with the roughness increase, in agreement with the Cassie–Baxter model.

Calculation of apparent contact angle for sessile droplets on structured hydrophobic surfaces based on dynamic of contact line during droplet formation

The sessile-droplet method is widely used to characterize surface wettability, however, the measured apparent contact angle (APCA) is usually a non-unique value between the advancing and receding contact angles, which seems hardly to be predicted by the classical Cassie-Baxter model. Thus, a reliable calculation model for APCAs of sessile droplets on structured hydrophobic surfaces is needed to guide the structural design of functional hydrophobic surfaces. Focusing on the force and motion analysis of the contact line during the formation process of sessile droplets, a theoretical model was developed to calculate the APCA of the droplet on hydrophobic surfaces featured with square micropillar arrays. The present model considers the synergistic effect of the droplet shape and the contact-line pinning on the APCA. Compared with the Cassie-Baxter model, the APCAs calculated by the proposed model agree better with the experimental data. This study shows that APCA is generally more susceptible to advancing contact angle, which is not significantly different on structured hydrophobic surfaces with different dimensions, resulting in the inability of APCA to accurately characterize surface wettability. As a complement, receding-contact-angle and adhesion-force measurements, which can reflect the effects of structural dimensions, are recommended to be adopted to characterize surface wettability.

Double laser two-step process for creating interconnected frame microstructures' superhydrophobic surface with dual scales

Superhydrophobic surfaces have garnered significant attention for their unique wettability and extensive applications across various critical fields. In this paper, a two-step laser processing technique, involving infrared nanosecond laser patterning and ultraviolet picosecond laser surface modification, was employed to fabricate interconnected frame surfaces with dual-scale microstructures on Ti6Al4V substrates. After that, the microstructure surface was chemically modified with 1H,1H,2H,2H-perfluorodecyl trimethoxy-silane to achieve a superhydrophobic surface. An L32 (47) orthogonal experiment was used to analyze the effect of seven main factors (including the structure size, scanning velocity and the fluence, number of scans of infrared nanosecond laser and ultraviolet picosecond laser) on the wettability of the interconnected frame microstructures surface. After optimization, the surface strongly corresponded with the Cassie-Baxter model and demonstrated a water contact angle of 153.22±1.64° and roll-off angle of 4.43±1.70°. The experiments demonstrated that the optimized surface possesses excellent mechanical stability and corrosion resistance. Additionally, the optimized surface exhibits anti-adhesion, effective self-cleaning, and anti-fouling properties, which hold promise for addressing issues of liquid adhesion and dust pollution in industrial machinery.

Surface properties of laser-woven magnesium alloys based on dot-matrix processing

Abstract This paper investigates the enhancement of surface properties of magnesium alloys through infrared nanosecond laser processing with a focus on creating hydrophobic and wear-resistant surfaces. By manipulating dot-matrix microgroove sizes on the magnesium alloy surfaces, the research evaluates the influence on hydrophobicity and mechanical durability. The experimental results reveal that larger microgrooves contribute to increased hydrophobicity due to decreased liquid-solid contact, confirmed by static contact angle measurements which align with predictions from the Cassie-Baxter model. Mechanical durability tests, involving sandpaper abrasion, demonstrate that the surface integrity is maintained despite wear, attributed to the effective laser-texturing approach. The hydrophobic surfaces were achieved through a low-surface-energy modification using fluorosilanes, followed by a thermal treatment to ensure durability. The findings highlight the potential of this processing method in applications requiring robust, hydrophobic materials such as biomedical and engineering materials, offering a cost-effective and efficient solution for industry-scale problems. This work not only extends the understanding of surface engineering via laser texturing but also provides a novel approach for optimizing surface properties crucial for advanced material applications.

Selective etching-induced surface modifications of FeCrAl alloy bipolar plates: Mechanisms for enhanced corrosion resistance and hydrophobicity

The FeCrAl alloy has been considered a potential candidate material for bipolar plates (BPs) in fuel cells, emphasizing the improvement of corrosion resistance through surface modification. This study involves a selective etching process that facilitates the preferential dissolution of iron ions to establish a high-chromium surface, enhancing both corrosion resistance and hydrophobic properties. This crucial etching process alters the surface roughness and enhances the surface energy. The transformed rough surface due to surface polishing and etching changes from hydrophilic to hydrophobic, which facilitates corrosion management and moisture control based on the Cassie-Baxter model. The modified micro/nanostructure of the FeCrAl alloy surface and the reconstituted protective film exhibit excellent corrosion inhibition and long-term stability. The study elucidates how the correlation between surface free energy (SFE) and the formation of hydrophobic and rough surfaces acts as a mechanism for enhancing the corrosion resistance of FeCrAl surfaces. Utilizing the Van Oss-Chaudhury-Good (Van Oss) method based on the Lewis acid/base theory further clarifies this corrosion inhibition process, proposing a novel approach that offers significant advantages over traditional methods. Furthermore, this method offers economic and efficient benefits compared with traditional coating methods, providing a promising avenue for reliable corrosion protection in fuel cell applications.

Experiment of contact angle hysteresis of a droplet on hydrophobic fibrous membrane

Fibrous membranes are widely used in liquid-gas filtration and separation, porous structures formed by randomly cross-layered fibers have a significant influence on interfacial free energies, showing complex changes in contact angle. The present paper aims to measure the pore structures of a hydrophobic ((C2F4)n, PTFE) fibrous membrane by Confocal Laser Scanning Microscope (CLSM), and analyze the droplet contact angle hysteresis along its circumference. The capillary force of the solid-liquid interface is calculated by the surface integral of Laplace pressure on half droplet surface, and the solid-liquid and liquid-vapor interface area ratios are obtained according to the Cassie-Baxter model. The liquid penetration depth in pore structures is calculated by the quasi-equilibrium of the interface free energy. The experimental result shows that for the PTFE fibrous membrane, the liquid-vapor interface area ratio (f2The experimental result shows that for the PTFE fibrous membrane, the liquid-vapor interface area ratio (f2

Experiment of droplet contact angle hysteresis on PTFE fibrous membrane in pore-scale

The present paper aims to study the influence of pore-scale fiber structure on the Cassie-Baxter contact angle of droplets on PTFE fibrous hydrophobic membranes. The fiber structure is obtained by the Confocal Laser Scanning Microscope (CLSM), and an optical test bench is built to measure the Cassie-Baxter contact angles of a droplet on a PTFE membrane. The free energy of the solid-liquid interface is measured by the surface tension integral on the droplet profile. An iterative algorithm for liquid-vapor interface shrinking is brought forward to analyze the liquid-vapor interface area between the droplet and the membrane, and the interface parameters of the Cassie-Baxter model are recognized. The experiment shows that the Cassie-Baxter contact angles and the contact line radii along the droplet circumference are in a normal distribution due to random fiber structures, and the variance of the normal distribution increases with the membrane pore diameter. The normal distribution of Cassie-Baxter’s contact angle indicates the constraint of the solid-liquid interface to the liquid-vapor interface. On the same normal contact pressure condition, the liquid-vapor area ratio of the droplet and membrane interface changes little, and Cassie-Baxter’s contact angle increases with the solid-liquid interface area ratio.

Sodium alginate/epoxy resin was separated by emulsion phase to fabricate a strong superhydrophobic coating

It has been recognized that good mechanical robustness and chemical stability are the keys to superhydrophobic coatings. Here, we used epoxy resin/sodium alginate emulsion phase separation method to prepare the coating body, and polydimethylsiloxane/vapor phase silicon dioxide as the superhydrophobic material to prepare a superhydrophobic silicon dioxide coating. Sodium alginate is enriched on the coating surface by phase separation process, which greatly improves the interaction force between the superhydrophobic material and the coating body. The water contact angle of the EP/SA/SiO2 superhydrophobic coating is 157.6° and the sliding angle is 2.3° The EP/SA/SiO2 superhydrophobic coating has good mechanical robustness, chemical stability and self-cleaning performance. Thanks to the micro-nano structure of the surface Cassie-Baxter model, air can be trapped on the solid-liquid contact surface of the coating. The impedance value of the EP/SA/SiO2 coating is two orders of magnitude higher than that of the epoxy coating, which has excellent anti-corrosion properties. It is believed that this study can provide a new idea for preparing strong superhydrophobic coatings.

Role of hydrophobic sand particle granularity on water droplet post-impact dynamics

This paper investigates, experimentally and theoretically, the effects of sand particle size and hydrophobicity of a water drop impacting horizontal and inclined surfaces within a context of post-wildfire mudflow precursors. Although previous research established that wildfires often turn hillslope surficial soil hydrophobic, little research shows how wildfire-altered soil surfaces affect raindrops' impact behavior. Specific findings point to a significant effect of sand type on the drop impact dynamics. After spreading, the drop gathers back more on finer than coarser hydrophobic sands due to the substrate air-entrapped grooves, as in the Cassie-Baxter model, leading to drop retraction, rebound, splash, and fragmentation into droplets. The results indicate that if the hillslope dominant median grain size is fine, alteration to hydrophobicity would multifold enhance drop downhill mobility. In contrast, coarse sand surfaces exhibit minimal effects on drop mobility after hydrophobization. In addition, the paper proposes a modified Tang et al. model for the maximum spread factor as a function of the median grain size and hydrophobicity. This research offers essential findings from the perspective of the previous limiting understanding that only drop infiltration prevents erosion in hydrophilic slopes.

Probing photothermal superhydrophobic behaviors of graphene&SiO2 hybrid coating for anti-freeze application

An active @ passive anti-icing surface coating was successfully prepared using graphene&SiO2 hybrid structure toward photothermal superhydrophobic performance. The coating consists of polydimethylsiloxane (PDMS) with E51 epoxy resin linked to fumed silica (nano-SiO2) to provide a multistage rough structure and graphene as the primary photothermal nanoparticle for various surface anti-freeze application. The surface morphology of this photothermal superhydrophobic coating is validated to be as a uniformly rough cauliflower structure with a large number of “air valley”. More importantly, the addition of 0.2 g graphene could exhibit the best photothermal effect (2 sun (≈ 200 mW/cm2), stable average temperature of 49.56 °C) along with superhydrophobic effect (162.2°), excellent chemical stability properties. In order to improve the organic-inorganic interfacial bonding and the mechanical durability of photothermal superhydrophobic coatings, using micro-arc oxidation technology to establish the microporous structure on the surface of substrate for exerting interlocking effect, after pushing and pulling under applied load of 300 g for 200 cm, the outstanding wear resistance of the composite coating was verified. Thanks to the interlocking effect, the excellent superhydrophobic properties provided by the construction of the typical Cassie-Baxter model of the surface and the use of multiple layers of graphene increasing the probability of π-π* electron leap, the multi-stage photothermal superhydrophobic composite coating is able to show a long-lasting anti-icing and de-icing effect.

Biomimetic surfaces with patterned wettability for high-efficiency dehumidification and fog harvesting

In this paper, by studying the characteristics of the bionic surface with pattern wettability, the process of its efficient dehumidification and atomization is explored. This paper briefly introduces the importance of dehumidification and physicochemical and the market demand, and also introduces the properties of superhydrophobic materials. The theoretical part mainly describes the basic principle of superhydrophobicity and the Wenzel and Cassie-Baxter models. Subsequently, natural and synthetic superhydrophobic surfaces are introduced, involving Namib beetles, butterfly wings, and so on. The final article points out that superhydrophobic surfaces can be used in numerous fields, especially in fog collection. However, the improvement of durability is the focus of attention for superhydrophobic surfaces. In future research, attention needs to be paid to the use of materials and the efficiency of fog collection.

Fabrication of superhydrophobic PVDF membrane via catechol/polyamine co-deposition and SiO2 nanoparticles for membrane distillation

Due to the societal development and population growth, the crisis regarding fresh water resources has become increasingly prominent. As a membrane separation technology, Membrane distillation (MD) has emerged as a widely used in the recovery and utilization of fresh water. However, membrane wetting and membrane fouling have hindered the further application of MD. To address these challenges, this work provides a method for fabricating a superhydrophobic composite PVDF membrane using catechol(CA)/polyethyleneimine(PEI) co-deposition, sol-gel growth of SiO2 nanoparticles, and the grafting of long chain perfluorosilane. The surface average roughness of the superhydrophobic membrane was up to 113.04 nm under the optimum conditions. The contact angle of the modified membrane was up to 157.5°, and a sliding angle is 8.0°, confirming its conformity to the Cassie-Baxter model. In addition, the modified membrane not only improved fouling and wetting resistance but also maintained a water flux equivalent to that of the PVDF membrane membrane when treating feeds containing surfactant, organic pollutants, and high salinity. In conclusion, this study provides a simple and feasible method for preparing superhydrophobic modified membrane, which further improves the adaptability of MD process for complex feed systems.

Near-infrared photodynamic antibacterial enhanced superhydrophobic electrospun membrane surfaces

Superhydrophobic surfaces have attracted great attention as effective antimicrobial platforms due to the superb repellency against various microbial biofouling. However, due to the lack of intrinsic antimicrobial activity, some bacteria still have the opportunity to adhere and survive on superhydrophobic surfaces, inevitably causing serious bacterial contamination. Herein, a poly(vinylidene) fluoride (PVDF) based nanocomposite membrane with both superhydrophobic property and near-infrared (NIR) light triggered photodynamic antibacterial activity has been prepared by a facile electrospinning method. Among those, the silicon dioxide nano-scale particles were introduced in the membrane to increase the roughness of the electrospun fibres, while the zinc phthalocyanine (ZnPC) as the NIR photosensitizer was also involved. Owing to the stable Cassie-Baxter model superhydrophobic properties, the resultant membrane demonstrated strong bacterial repellent performances for both Gram-positive and Gram-negative bacteria (~8.9 % reduction for S. aureus and ~9.4 % for E. coli). After being exposed to NIR light illumination (808 nm, 2.0 mW) for 10 min, the membrane surface showed further enhanced antimicrobial efficiency and no surviving bacteria were observed, which is extremely difficult to achieve by relying on either superhydrophobic repellency or NIR photodynamic sterilization alone. Furthermore, this nanocomposite membrane displays no detectable cytotoxicity to L929 cells, even suffered from NIR light irradiation, and the surface also shows no significant activation on red blood cells as well as good liquid-repellency against whole blood adhesion. These results conclusively show that the nanocomposite membranes exhibit great potential applications in a wide range of antimicrobial-related fields.

Wetting Characteristics of Laser-Ablated Hierarchical Textures Replicated by Micro Injection Molding

Texturing can be used to functionalize the surface of plastic parts and, in particular, to modify the interaction with fluids. Wetting functionalization can be used for microfluidics, medical devices, scaffolds, and more. In this research, hierarchical textures were generated on steel mold inserts using femtosecond laser ablation to transfer on plastic parts surface via injection molding. Different textures were designed to study the effects of various hierarchical geometries on the wetting behavior. The textures are designed to create wetting functionalization while avoiding high aspect ratio features, which are complex to replicate and difficult to manufacture at scale. Nano-scale ripples were generated over the micro-scale texture by creating laser-induced periodic surface structures. The textured molds were then replicated by micro-injection molding using polypropylene and poly(methyl methacrylate). The static wetting behavior was investigated on steel inserts and molded parts and compared to the theoretical values obtained from the Cassie–Baxter and Wenzel models. The experimental results showed correlations between texture design, injection molding replication, and wetting properties. The wetting behavior on the polypropylene parts followed the Cassie–Baxter model, while for PMMA, a composite wetting state of Cassie–Baxter and Wenzel was observed.

Fabrication and Properties of Hydrophobically Modified ZnO–SiO2 Nanocomposite with Polysiloxane

Studies on the wettability properties of solid surfaces are very important in any of the scientific and industrial fields. The most common principle for a superhydrophobic self-cleaning surface is the lotus effect induced by surface roughness. In this study, silicate compounds have been used to produce hydrophobic surfaces. In this way, firstly, SiO2–ZnO nanocomposite was produced, and then vinyl trimethoxy silane was used to increase the water contact angle (WCA). The structure and morphology of nanocomposites were investigated by infrared spectroscopy (FT-IR), X-ray diffraction pattern (XRD), scanning electron microscopy (SEM) and energy-dispersion spectrometer (EDS) analyses. The thermal stability of nanocomposite coatings was examined by thermogravimetric analysis (TGA). In order to investigate the wetting properties, the surface roughness was measured using an atomic force microscope (AFM), where the subsurface roughness average was obtained at 37.79 nm. The WCA of the coated surfaces with ZnO–SiO2 and ZnO–SiO2 @Polysiloxane nanocomposites were measured at 69 and 160°, respectively, indicating the surface superhydrophobic properties of ZnO–SiO2 @Polysiloxane nanocomposites. Finally, superhydrophobic properties of nanocomposites were investigated by the Cassie-Baxter model. The value of the f2 parameter in the model was estimated at 0.9556. This means that air occupies about 95.56% of the contact area between the water droplet and nano-coating, which is responsible for the superhydrophobic property of the surface.

Cold gelation of whey protein isolate with sugars in an ultrasound environment

Dispersions of whey protein isolate (9.5% protein) with 1.5% of sugar (glucose, mannose or xylose) were preheated, and ions-induced gelation was performed in an ultrasound environment at room temperature. Depending on the used sugar, sonication increased, decreased or did not change the rheological properties of gels (storage modulus, hardness and ultrasound viscosity). That was contrary to the formation of Maillard reaction compounds, which probably by occupation of active sides of proteins, induced the formation of a weaker three-dimensional gel network. For most samples, sonication increased the value of the contact angle, switching their surface character from hydrophilic to hydrophobic one. The surface free energy of the obtained gels decreased after sonication, and such surfaces were less susceptible to wetting. The increase in the value of the contact angles corresponded to the increase in the roughness of the gels after sonication. The relationship between the surface roughness and the wettability can be attributed to the surface wetting state (Wenzel state, Cassie-Baxter state, and transition state). For the gels surfaces with lower roughness, the liquid was more susceptible to penetration to the surface due to large adhesive forces (Wenzel model) and for higher roughness, the liquid was more susceptible to resting on the “air micro cushions” (Cassie-Baxter model). The ultrasound-induced cold-gelation for the preparation of natural protein/sugar gels can offer a potential approach to develop new functional products with tailor-made attributes.

Freezing as a Path to Build Micro‐Nanostructured Icephobic Coatings

AbstractSuperhydrophobic photothermal materials with the micro‐nano structure are considered to be promising icephobic surfaces. Unfortunately, converting micro‐nano hierarchical structure concepts into genuine synthetic materials has proven to be exceedingly expensive and difficult, partially because their sophisticated structures need construction at several length scales. Herein, a facile strategy of employing ice crystals to construct sophisticated hierarchical micro‐nanostructured anti‐icing composites with photothermal, self‐healable, and self‐cleaning properties is presented. The composites are covered with interconnected microscale pores replicated from ice crystals, which facilitates the construction of the hydrophobic or superhydrophobic properties based on the Cassie–Baxter model, endowing the coating with self‐cleaning ability. Besides, by adding solar‐to‐heat conversation nanomaterials, the coating can implement in situ solar anti‐/deicing. The abundant micropores caused by ice templates can further improve the photothermal conversion capability through multiple reflections of light. Importantly, the coating is endowed with the self‐healing capability to repair hydrophobicity under sunlight. Additionally, it is demonstrated that the self‐cleaning and self‐healing abilities are mutually reinforcing, synergistically improving anti‐/deicing performances. Overall, the presented ice‐templated coating shows great potential and broad impacts owing to its inexpensive component materials, simplicity, eco‐friendliness, and high energy efficiency.

Pulse Electrodeposited Super-Hydrophobic Ni-Co/WS2 Nanocomposite Coatings with Enhanced Corrosion-Resistance

The hydrophobicity and corrosion resistance of composite coatings can be effectively improved by changing the electrodeposition method and adding inorganic nanoparticles. In this work, the incorporation of WS2 nanoparticles significantly increased the surface roughness of Ni-Co coatings. The best hydrophobicity and corrosion resistance of the Ni-Co/WS2 nanocomposite coatings (water contact angle of 144.7°) were obtained in the direct current electrodeposition mode when the current density was 3 A/dm2 and the electrodeposition time was 50 min. Compared with direct current electrodeposition, the pulsed current electrodeposition method was more conducive to improving the electrodeposition performance of the nanocomposite coatings. Under the conditions of a current density of 3 A/dm2, pulse duty cycle of 70%, and pulse frequency of 1000 Hz, the nanocomposite coatings reached a superhydrophobic state (water contact angle of 153.8°). The nanocomposite coatings had a slower corrosion rate and larger impedance modulus in this state, and thus the corrosion resistance was superior. The wetting state of the Ni-Co/WS2 nanocomposite coating surface was closer to the Cassie–Baxter model. The protective air layer formed by the layered rough microstructures significantly reduced the actual contact area between the liquid and the substrate, achieving excellent hydrophobic and corrosion resistance properties.

Critical parameters controlling wettability in hydrogen underground storage - An analytical study

The large-scale implementation of hydrogen economy requires immense storage spaces to facilitate the periodic storage/production cycles. Extensive modelling of hydrogen transport in porous media is required to comprehend the hydrogen-induced complexities prior to storage to avoid energy loss. Wettability of hydrogen-brine-rock systems influence flow properties (e.g. capillary pressure and relative permeability curves) and the residual saturations, which are all essential for subsurface hydrogen systems. This study aims to understand which parameters critically control the contact angle for hydrogen-brine-rock systems using the surface force analysis following the DLVO theory and sensitivity analysis. Furthermore, the effect of roughness is studied using the Cassie-Baxter model. Our results reveal no considerable difference between H 2 and other gases such as N 2 . Besides, the inclusion of roughness highly affects the observed apparent contact angles, and even lead to water-repelling features. It was observed that contact angle does not vary significantly with variations of surface charge and density at high salinity, which is representative for reservoir conditions. Based on the analysis, it is speculated that the influence of roughness on contact angle becomes significant at low water saturation (i.e. high capillary pressure).