Hydrophobicity Gradient Research Articles

Construction of a gradient cobblestone pattern on pyrolytic carbon surface for improved anti-thrombus

Surface modification is believed to be one of the most effective solutions to prevent thrombus formation associated with mechanical heart valves. Herein, we report, for the first time, the fabrication of a gradient cobblestone pattern on the pyrolytic carbon (PyC) surface aiming to improve its anti-thrombotic properties. A unique cobblestone pattern with gradient changing interspacing was generated on the PyC surface through laser etching. After coating with fluorosilane, the surface exhibited a water contact angle gradient ranging from 110 to 173°. The morphological and physicochemical properties, mechanical durability, drag reduction, in vitro anti-thrombotic properties, and in vivo biocompatibility of the surface were investigated to evaluate its potential as a heart valve replacement. It was found that the hydrophobic gradient wettability surface exhibited an excellent self-driving effect and mechanical durability, significantly reduced hemolysis rate and platelet adhesion, and prolonged coagulation time compared to the counterpart without gradient wettability. Furthermore, the gradient wettability surface demonstrated excellent in vivo biocompatibility. These findings are important for the design of artificial heart valves with improved anti-thrombotic performance.

Bioinspired construction of petal-like regenerated PVDF/cellulose fibers for efficient fog harvesting

Water collection from atmospheric fog was deemed to be an efficient and sustainable strategy to defuse the freshwater scarcity crisis. Fog harvesting and trapping fibers, therefore, has aroused extensive interest due to their ease of preparation, weave, and use. However, the traditional fibers used in fog collector usually have a low fog collection capacity and efficiency because of their unreasonable morphology and structure design. Herein, we proposed a simple process to construct advanced fibers using a one-step wet spinning of hydrophobic polyvinylidene fluoride (PVDF) and hydrophilic cellulose mixture fiber for fog harvesting. The as-prepared fibers featured a petaloid structure and surface hydrophobic gradient, thus facilitating fog deposition, water droplet formation, and drainage. The unique longitudinal groove structure above enabled the hybrid fiber to achieve an excellent fog collection efficiency of 2750.26 mg/cm2/h per monofilament, which outstripped most of other fiber materials. When woven these fibers were in a longitudinal array network with an interval of 1 mm, and the fog collection efficiency can maintain at 10.30 L/m2/h. Therefore, this work provided a new strategy for further exploration of effective fog collection by cellulose-based fiber materials.

Patterned hydrophobic gas diffusion layers for enhanced water management in polymer electrolyte fuel cells

Flooding of the cathode due to water accumulation is one of the biggest limiting factors in the performance of polymer electrolyte fuel cells (PEFCs). This study therefore attempts to solve this issue by fabricating gas diffusion layers (GDLs) with differently patterned hydrophobic regions. The GDLs in three different patterns (triangular, diamond, and inverted-triangular) were prepared by brushing a Polytetrafluoroethylene (PTFE) solution onto commercial carbon papers through a mask and tested in PEFCs. The patterned GDLs results in superior performance in all cases compared to a uniformly PTFE-treated GDL. Notably, the oxygen transport resistance is significantly reduced, indicating that the water accumulation in the cathode is avoided. This is attributed to the patterned hydrophobicity gradient providing distinct pathways for water and oxygen. The GDL with triangular patterning displays the highest peak power density, due to the fact that the untreated less hydrophobic region is in direct contact with the cathode outlet in this case, facilitating the removal of excess liquid water. Overall, the study confirms that the GDLs with patterned hydrophobicity could be used to enhance the performance of commercial PEFC systems by facilitating water management, potentially leading to improved efficiency and durability.

Hydrophobicity gradient optimization of fuel cell gas diffusion media for its application in vehicles

During Fuel Cell Vehicle (FCV) operation, the liquid water in gas diffusion media (GDM) prevents the reaction gas from reaching the reaction zone and lead to output power fluctuation and reduce the lifespan of FCV. In the present research, hydrophobicity gradient settings of micro-porous layer (MPL) and gas diffusion layer (GDL) are optimized to improve the water removal ability of GDM. Computational fluid dynamics (CFD) model is constructed for numerical simulations to analyze the fuel cell power output and the water content in the GDM with different hydrophobicity gradients. Experiments with different hydrophobicity gradients, which are specifically prepared with corresponding concentrations of polytetrafluoroethylene (PTFE) solutions, are conducted for validation of simulation results. It is shown that the positive hydrophobicity gradient of MPL and GDL provides a better capacity for water removal and oxygen transport. The contact angles of MPL and GDL are further optimized as 147.9°-138.6° by genetic algorithm integrated with the CFD simulations.

Self-propelled Leidenfrost droplets on femtosecond-laser-induced surface with periodic hydrophobicity gradient

The controllable transfer of droplets on the surface of objects has a wide application prospect in the fields of microfluidic devices, fog collection and so on. The Leidenfrost effect can be utilized to significantly reduce motion resistance. However, the use of 3D structures limits the widespread application of self-propulsion based on Leidenfrost droplets in microelectromechanical system. To manipulate Leidenfrost droplets, it is necessary to create 2D or quasi-2D geometries. In this study, femtosecond laser is applied to fabricate a surface with periodic hydrophobicity gradient (SPHG), enabling directional self-propulsion of Leidenfrost droplets. Flow field analysis within the Leidenfrost droplets reveals that the vapor layer between the droplets and the hot surface can be modulated by the SPHG, resulting in directional propulsion of the inner gas. The viscous force between the gas and liquid then drives the droplet to move.

Gas Diffusion Electrode with Microporous Layers of Hydrophobicity Gradient Distribution for CO2 Electrolysis in a Zero-Gap Electrolyzer Operated with Pure Water

Gas Diffusion Electrode with Microporous Layers of Hydrophobicity Gradient Distribution for CO₂ Electrolysis in a Zero-Gap Electrolyzer Operated with Pure Water

Combing of picogram level DNA equivalent to genomic DNA present in single human cell by self propelled droplet motion over a stable gradient surface

DNA combing is a powerful technique for studying replication profile, fork-directionality and fork velocity. At present, there is requirement of a methodology to comb DNA present in a single human cell for studying replication dynamics at early embryonic stage. In our study, a surface having dual characteristics i.e., affinity towards negatively charged single DNA molecules and a hydrophobic gradient for self propelled droplet motion of combing solution was developed. The surface was made by coating of TCOS (trichloro-octylsilane) by vapor diffusion on APTES (Aminopropyl-triethoxysilane) coated glass slides. A gradient surface having high deposition efficiency (DE) was developed on which 5 picogram DNA equivalent to genomic DNA present in one single human cell can be combed. The gradient surface was thermostable in nature having the ability to sustain boiling temperature for two hours and sustain anisotropy in 70 % ethanol for 80 h. Applicability for multiple runs was enhanced such that the surface can be used for 13–14 times. Factors associated with gradient surface are unidirectional movement of combing solution droplet over the gradient surface for combing straight DNA molecules and a longer gradient surface of more than 1 cm such that long size DNA molecules can be combed. Ellipsometry and contact angle hysteresis confirmed the presence of hydrophobic gradient. XPS (X-ray photoelectron spectroscopy) and FTIR (Fourier Transform Infrared Spectroscopy) confirmed the presence of characteristic affinity towards negatively charged DNA molecules on the gradient surface. Combing solution was optimized for increasing deposition efficiency and for increasing the applicability of gradient surface for multiple runs. High temperature of combing solution was found to increase Deposition Efficiency. Combing solution was also optimized for combing single DNA molecules over the gradient surface. Single DNA molecules were combed by reducing pH and lowering concentration of triton-X in the combing solution. Dye: bp ratio was optimized for high fluorescent intensity and low surface background.

Fabricating Tunable Superhydrophobic Surfaces Enabled by Surface‐Initiated Emulsion Polymerization in Water

AbstractFabricating controllable superhydrophobic surfaces remains challenging in various fields ranging from chemical industries to biomedical engineering. Conventional methods commonly require volatile organic solvents and the assistance of special surface deposition and modification equipment, which are detrimental to environment and limit their applications in micro‐devices. Herein, an equipment‐free method is reported to directly transform fluorinated monomer micro‐droplets into hydrophobic polymer particles on flat substrate surfaces in water, simultaneously depositing hydrophobic coatings with tunable surface structures. The as‐prepared surfaces show superior superhydrophobicity and great stability in extreme conditions (e.g., varying acidity, basicity, and heating conditions), and excellent anti‐fouling property. Meanwhile, surface hydrophobicity can be manipulated by adjusting emulsion droplet number density and reaction time. Hence, superhydrophobic surfaces with tunable hydrophobicity gradients have been successfully fabricated in one pot. This study provides an equipment‐free method to facilely fabricate controllable superhydrophobic surfaces, with great potential in the development of smart superhydrophobic materials in various engineering and industrial applications.

Single-Side Superhydrophobicity in Si3N4-Doped and SiO2-Treated Polypropylene Nonwoven Webs with Antibacterial Activity

Meltblown (MB) nonwovens as air filter materials have played an important role in protecting people from microbe infection in the COVID-19 pandemic. As the pandemic enters the third year in this current global event, it becomes more and more beneficial to develop more functional MB nonwovens with special surface selectivity as well as antibacterial activities. In this article, an antibacterial polypropylene MB nonwoven doped with nano silicon nitride (Si3N4), one of ceramic materials, was developed. With the introduction of Si3N4, both the average diameter of the fibers and the pore diameter and porosity of the nonwovens can be tailored. Moreover, the nonwovens having a single-side moisture transportation, which would be more comfortable in use for respirators or masks, was designed by imparting a hydrophobicity gradient through the single-side superhydrophobic finishing of reactive organic/inorganic silicon coprecipitation in situ. After a nano/micro structural SiO2 precipitation on one side of the fabric surfaces, the contact angles were up to 161.7° from 141.0° originally. The nonwovens were evaluated on antibacterial activity, the result of which indicated that they had a high antibacterial activity when the dosage of Si3N4 was 0.6 wt%. The bacteriostatic rate against E. coli and S. aureus was up to over 96%. Due to the nontoxicity and excellent antibacterial activity of Si3N4, this MB nonwovens are promising as a high-efficiency air filter material, particularly during the pandemic.

Fabrication of cerium oxide films with thickness and hydrophobicity gradients

Versatile surfaces with hydrophobicity gradients have attracted intensive attention because of their tunable wettability. Here, cerium oxide films are prepared on tilted substrates by magnetron sputtering, the induced thickness gradient strongly influences the structural, morphological and wetting properties. It is found that the surface hydrophobicity is positively proportional to the root-mean-square (RMS) roughness and thickness variations, larger thickness gradient brings more obvious gradient hydrophobic surfaces. The results show that the RMS roughness gradient increases from 0 nm/cm, 0.08 nm/cm to 0.16 nm/cm and the thickness gradient increases from 0 nm/cm, 12.0 nm/cm to 13.2 nm/cm, with the tilt angle changing from 0°, 30° to 60°, respectively. Correspondingly, the hydrophobicity gradient increases from 0°/cm, 1.6°/cm to 2.7°/cm, respectively. Our study offers practicable method a method for developing hydrophobicity gradient surfaces, which can be used for droplet movement applications. • Cerium oxide films are deposited on tilted substrates by magnetron sputtering. • Tilt angle allows varying morphology and properties of deposited films. • Larger tilt angle brings more obvious gradient hydrophobic surfaces. • The hydrophobicity gradient ranges from 0°/cm, 1.6°/cm to 2.7°/cm.

Comparative DCMD performance of hydrophobic-hydrophilic dual-layer hollow fibre PVDF membranes incorporated with different concentrations of carbon-based nanoparticles

High wetting resistance and high permeate flux are crucial for membrane distillation (MD) to achieve a high performance. To achieve both of these qualities, this work focused on the fabrication of co-extruded dual-layer hollow fibre polyvinylidene fluoride nanocomposite membranes with hydrophobicity gradients. The nanocomposite membranes were incorporated with hydrophobised carbon-based nanoparticles, i.e. multi-walled carbon nanotube (MWCNT) and graphene nanoplatelet (GNP). The effect of different concentrations of nanoparticles (0, 1 and 2 wt%) on the properties and MD performance of the membranes were investigated. The outer layers of the nanocomposites membranes were more hydrophobic than the neat membrane, and the membrane containing 2 wt% of hydrophobised GNP exhibited the highest contact angle of 111.1°. All the inner layers of the membranes were found to be hydrophilic with contact angle of about 60°, thus proving that hydrophobicity gradients were achieved in the membranes. All the membranes had pore sizes and porosities around 0.1 μm and 21.8–42.9% respectively, which are suitable to be used in MD. The surface roughness and wetting resistance of all the nanocomposite membranes, especially the ones with GNP, were higher than the neat membrane and the values increased with increasing concentration of nanoparticles. These membrane characteristics had effects on the direct contact MD (DCMD) performance where all the nanocomposite membranes showed better flux than the neat membrane. The membrane incorporated with 2 wt% of hydrophobised GNP achieved the highest flux of 8.27 kg/(m2h). All the membranes achieved salt rejections of more than 99.9%.

PTFE Content in Catalyst Layers and Microporous Layers: Effect on Performance and Water Distribution in Polymer Electrolyte Membrane Fuel Cells

This work describes the effects of catalyst layers (CLs) consisting of hydrophobic PTFE on the performance and water management of PEM fuel cells. Catalyst inks with various PTFE contents were coated on Nafion membranes and characterized using contact angle measurements, SEX-EDX, and mercury porosimetry. Fuel cell tests and electrochemical impedance spectroscopy (EIS) were conducted under varying operating conditions for the prepared materials. At dry conditions, CLs with 5 wt.% PTFE were advantageous for cell performance due to improved membrane hydration, whereas under humid conditions and high air flow rates CLs with 10 wt.% PTFE improved the performance in high current density region. Higher PTFE contents (≥20 wt.%) increased the mass transport resistance due to reduced porosity of the CLs structure. Operando neutron radiography was utilized to study the effects of hydrophobicity gradients within CLs and cathode microporous layer (MPLC) on liquid water distribution. More hydrophobic CLs increased the water content in adjacent layers and improved performance, especially at dry conditions. MPLC with higher PTFE contents increased the overall liquid water within the CLs and GDLs and escalated the water transfer to the anode side. Furthermore, the role of back-diffusion transport mechanism on water distribution was identified for the investigated cells.

Wing wettability gradient in a damselflyLestes sponsa(Odonata: Lestidae) reflects the submergence behaviour during underwater oviposition

The phenomenon of hydrophobicity of insect cuticles has received great attention from technical fields due to its wide applicability to industry or medicine. However, in an ecological/evolutionary context such studies remain scarce. We measured spatial differences in wing wettability in Lestes sponsa (Odonata: Lestidae), a damselfly species that can submerge during oviposition, and discussed the possible functional significance. Using dynamic contact angle (CA) measurements together with scanning electron microscopy (SEM), we investigated differences in wettability among distal, middle and proximal wing regions, and in surface nanostructures potentially responsible for observed differences. As we moved from distal towards more proximal parts, mean values of advancing and receding CAs gradually increased from 104° to 149°, and from 67° to 123°, respectively, indicating that wing tips were significantly less hydrophobic than more proximal parts. Moreover, values of CA hysteresis for the respective wing parts decreased from 38° to 26°, suggesting greater instability of the structure of the wing tips. Accordingly, compared with more proximal parts, SEM revealed higher damage of the wax nanostructures at the distal region. The observed wettability gradient is well explained by the submergence behaviour of L. sponsa during underwater oviposition. Our study thus proposed the existence of species-dependent hydrophobicity gradient on odonate wings caused by different ovipositional strategies.

Electrochemical CO2 Reduction: Tailoring Catalyst Layers in Gas Diffusion Electrodes

AbstractThe electrochemical conversion of CO2 into commodity chemicals or fuels is an attractive reaction for sustainable CO2 utilization. In this context, the application of gas diffusion electrodes is promising due to efficient CO2 mass transport. Herein, a scalable and reproducible method is presented for polytetrafluoroethylene (PTFE)‐bound copper gas diffusion electrodes (GDEs) via the dry‐pressing method and compositional parameters are emphasized to alter such electrodes. The assembly of the catalytic layer plays a critical role in the electrode performance, as elevated bulk hydrophobicity coupled with good surface wettability is observed to offer highest performance in 0.5 m KHCO3. With optimized electrodes, formate, CO, and H2 are obtained at a current density of 25 mA cm−2 as main products in 1 m KOH in faradaic efficiencies (FEs) of 27%, 30%, and 36%. At 200 mA cm−2, an altered product composition with ethylene (33% FE) and ethanol (9% FE) along with H2 (33% FE) is observed. In addition, n‐propanol is observed with 7% faradaic efficiency. The results indicate that the composition of the GDE has a severe influence on the electrode performance and setting proper hydrophobicity gradients within the electrode is key toward developing a successful electrochemical CO2 reduction.

Self-Assembly of Porphyrin Nanostructures at the Interface between Two Immiscible Liquids

One of the many evolved functions of photosynthetic organisms is to synthesize light harvesting nanostructures from photoactive molecules such as porphyrins. Engineering synthetic analogues with optimized molecular order necessary for the efficient capture and harvest of light energy remains challenging. Here, we address this challenge by reporting the self-assembly of zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrins into films of highly ordered nanostructures. The self-assembly process takes place selectively at the interface between two immiscible liquids (water|organic solvent) with the kinetically stable interfacial nanostructures formed only at pH values close to the pKa of the carboxyphenyl groups. Molecular dynamics simulations suggest that the assembly process is driven by an interplay between the hydrophobicity gradient at the interface and hydrogen bonding in the formed nanostructure. Ex situ X-ray diffraction (XRD) analysis and in situ UV–vis and steady-state fluorescence indicate the formation of chlathrate type nanostructures that retain the emission properties of their monomeric constituents. The self-assembly method presented here avoids the use of acidic conditions, additives such as surfactants, and external stimuli, offering an alternative for the realization of light-harvesting antennas in artificial photosynthesis technologies.

Nutrient diffusion control of fertilizer granules coated with a gradient hydrophobic film

A gradient hydrophobic film was prepared to improve the controlled release performance of film-coated fertilizers. A polyurethane (PU) was copolymerized with hydroxypropyl-terminated polydimethylsiloxane (HP-PDMS) at different ratios to produce a film with different levels of hydrophobicity. A film with a gradient hydrophobicity was prepared by successively coating a copolymer of PDMS/PU at different ratios. The urea diffusion rate via an inner hydrophilic and outer hydrophobic (InHL-OutHB) gradient hydrophobic film was reduced significantly, compared with that via a uniform hydrophilic or hydrophobic film. For the InHL-OutHB gradient hydrophobic film, the release period of the coated urea with a coating amount of 4 % was over 60 days, which was significantly longer than that of the uniform film.

Biophysical studies on the antimicrobial activity of linearized esculentin 2EM

Linearized esculentin 2 EM (E2EM-lin) from the frog, Glandirana emeljanovi was highly active against Gram-positive bacteria (minimum lethal concentration ≤ 5.0 μM) and strongly α-helical in the presence of lipid mimics of their membranes (>55.0%). The N-terminal α-helical structure adopted by E2EM-lin showed the potential to form a membrane interactive, tilted peptide with an hydrophobicity gradient over residues 9 to 23. E2EM-lin inserted strongly into lipid mimics of membranes from Gram-positive bacteria (maximal surface pressure changes ≥5.5 mN m−1), inducing increased rigidity (Cs−1 ↑), thermodynamic instability (ΔGmix < 0 → ΔGmix > 0) and high levels of lysis (>50.0%). These effects appeared to be driven by the high anionic lipid content of membranes from Gram-positive bacteria; namely phosphatidylglycerol (PG) and cardiolipin (CL) species. The high levels of α-helicity (60.0%), interaction (maximal surface pressure change = 6.7 mN m−1) and lysis (66.0%) shown by E2EM-lin with PG species was a major driver in the ability of the peptide to lyse and kill Gram-positive bacteria. E2EM-lin also showed high levels of α-helicity (62.0%) with CL species but only low levels of interaction (maximal surface pressure change = 2.9 mN m−1) and lysis (21.0%) with the lipid. These combined data suggest that E2EM-lin has a specificity for killing Gram-positive bacteria that involves the formation of tilted structure and appears to be primarily driven by PG-mediated membranolysis. These structure/function relationships are used to help explain the pore forming process proposed to describe the membranolytic, antibacterial action of E2EM-lin.

Effect of Hydrophobicity Gradients within MPL and MEA on PEM fuel cell performance

The water inventory of PEMFC components strongly influences the performance under two-phase operating conditions. Within this work, the influence of hydrophobicity variation within electrode and microporous layer (MPL) on cell performance will be presented. Different weight percentages of PTFE is used in the catalyst and MPL inks as an additive in order to increase the hydrophobicity. The fabrication method for the membrane electrode assembly (MEA) is based on spraying the catalyst ink onto the membrane and coating MPL ink onto the GDL substrate using a doctor blade method. Fuel cell tests are carried out with a 25 cm² active area single cell at different relative humidities (RH) and oxygen flow rates to investigate the effect of PTFE on the ability of MEA and MPL on water management. Electrochemical impedance spectroscopy is performed as a diagnostic tool to investigate the reaction kinetics and mass transport. In order to assess the wettability of MEAs contact angle measurements were carried out. The materials are characterized by mercury porosimetry and scanning electron microscopy. Figure 1 shows the performance curve for cell 1 (conventional MEA +Conventional MPL), cell 2( MEA with 5 wt.% PTFE +Conventional MPL) and cell 3 (Conventional MEA+ MPL with 40 wt.% PTFE ) at different relative humidities. At dry conditions (RH=70%) cell 2 has a higher limiting current density compared to the other cells, which could be the indication that the hydrophobic electrode keeps the membrane material in the catalyst layer better hydrated. At over humidified conditions (RH=120%) the limiting current density value has decreased for cell 1 and 2 compared to the standard conditions (RH=100%) as a result of mass transport limitations. On contrary, Cell 3 shows an improved performance in high current density region at humid conditions, illustrating the ability of hydrophobic MPL to prevent the accumulation of liquid water at the interphase of MPL and catalyst layer at cathode side. In the presentation of the ongoing work, additional results, including HFR data will be reported. Figure 1

Neutron Tomographic Investigation of the Effect of Hydrophobicity Gradients within MPL and MEA on the Spatial Distribution and Transport of Liquid Water in PEMFCs

Tomographic neutron imaging was used to visualize water distribution inside polymer electrolyte membrane fuel cell (PEMFC) investigating influence of hydrophobicity gradients within cathode micro porous layer (MPL) and catalyst layer. Water content inside a volume of interest was studied, comparing water content inside layers in a region under flow field land and in the adjoining region under flow field channels. Polarization curves were attained, investigating influence of water distribution and content on the performance of these three cells. Hydrophobizing the cathode MPL increased the water content inside the cell, which resulted to a performance decrease obtained from the respective polarization curve.

Neutron Radiographic Investigations on the Effect of Hydrophobicity Gradients within MPL and MEA on Liquid Water Distribution and Transport in PEMFCs

In-Plane neutron radiography imaging technique was performed to investigate the effect of hydrophobicity gradients within catalyst layers (CLs) and cathode microporous layer (MPL) on water dynamics of polymer electrolyte membrane fuel cell (PEMFC) during operation. Effect of varying operating conditions (current density and relative humidity) of the inlet gases on the water content of the cells were evaluated. Hydrophobization of catalyst layers improved the membrane humidification, especially at more dry conditions. Implementation of more hydrophobic cathode MPL resulted in an increment of overall water content within the CLs and GDLs on anode and cathode side. Furthermore, the role of back diffusion transport mechanism on the water distribution was identified for the constructed cells in this study.