Hydrophobic Walls Research Articles

High Efficiency Shear-Driven Nanofluidic System for Energy Conversion/Harvesting.

In this work, we propose a shear-driven nanofluidic system for energy harvesting/conversion. The system consists of a nanochannel formed by two parallel walls, where the lower wall is negatively charged, while the upper wall is neutral. The motion of the upper wall caused by a shear force drives the solution in the fluidic system to move, which generates an ionic current due to the migration of excess cations in the system. Molecular dynamics simulations demonstrate that the efficiency of the system is affected by the wall charge density, shearing stress, channel height, and binding energy of the walls. The effects of these factors on the efficiency are studied. In particular, it is shown that a high binding energy for the upper wall (e.g., hydrophilic wall) can reduce the flow slip at the upper wall and effectively transfer energy from the wall to the fluid. For the lower wall, a low binding energy, which corresponds to a hydrophobic wall, can reduce the friction at the wall, enhance the flow velocity, and improve the energy conversion efficiency. By varying these parameters, it is found that the maximum energy conversion efficiency of the system reaches 65.8%, which is the highest compared with previous systems. The underlying mechanisms are explained using the slip length at the walls, wall velocity, and charge density profiles. The system proposed in this work provides insights into the design of nanofluidic systems for energy harvesting/conversion.

Impact and freezing characteristics of deionized water droplets on cold curved surfaces

Processes involving droplet impact and subsequent freezing occur widely in practical engineering applications. In the present study, a visualization experimental setup is utilized to investigate the effects of the impact of single millimeter-scale droplets on curved surfaces at room and low temperatures. The influences of the Weber number We, wall temperature, and wall wettability on the dynamics of droplet impact and the characteristics of ice formation are examined. The morphological evolution of droplet impact and the variations of the dimensionless spreading coefficient are analyzed. The results indicate that at high We (We = 277), droplets reach their maximum spread on cold walls in a shorter time than on room-temperature walls, and their peak spreading coefficient is smaller. Upon impact with a cold wall, droplets exhibit a spread–splatter behavior. Low temperatures suppress the oscillatory behavior of droplets on a curved wall. In the case of a hydrophilic wall surface, as the impact We increases from 42 to 277, the impact mode gradually transitions from spread–retract–freeze to spread–splatter–freeze. The maximum spreading coefficient first increases and then decreases with increasing impact We. At high We (We = 277), the wall wettability has a minimal effect on the dynamics of droplet impact and freezing, with a spread–splatter–freeze mode being exhibited for both hydrophobic and hydrophilic walls, and the final freezing morphology is similar.

Bifurcate migration of neutrally buoyant particles in unilateral slippery channel flows

As an important technique for manipulating particles in fluid–solid channel flows, inertial focusing encourages the design of the channel geometry to enhance particle radial aggregation. Traditional methods typically use exquisite sheathes or elbows to create constricted flows, which ultimately increase flow resistance and lower fluid–solid separation efficiency. This paper presents a slippery wall modification technique that, by regulating the channel flows, is expected to induce nontrivial particle lateral migrations. More specifically, interface-resolved simulations are performed using the lattice Boltzmann method. A slip boundary condition is applied to the redesigned hydrophobic bottom wall. It is observed that the typical bifurcate migration, i.e., particles moving divergently toward the upper and lower equilibrium positions around a crucial location (CL), does not occur along the channel centerline. The CL is always below the centerline, and it decreases consistently with an increase in Kn or Re. By increasing Re, particles are prone to approach the channel centerline. With larger Kn, particles in the higher equilibrium position are affected in the same way, but their lower counterparts are drawn to the bottom wall.

Oxidative Stability of Fish Oil-Loaded Nanocapsules Produced by Electrospraying Using Kafirin or Zein Proteins as Wall Materials.

The encapsulation of fish oil by monoaxial electrospraying using kafirin or zein proteins as hydrophobic wall materials was investigated. Kafirin resulted in spherical fish oil-loaded nanocapsules (>50% of capsules below 1 µm), whereas zein led to fish oil-loaded nanocapsules with non-spherical morphology (>80% of capsules below 1 µm). Both hydrophobic encapsulating materials interacted with fish oil, successfully entrapping the oil within the protein matrix as indicated by Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy results. FTIR also suggested hydrogen bonding between fish oil and the proteins. Trapped radicals in the encapsulation matrix that were detected by electron paramagnetic resonance (EPR), indicated oxidation during electrospraying and storage. Results from isothermal (140 °C) differential scanning calorimetry (DSC) denoted that the encapsulation of fish oil by electrospraying using both kafirin or zein as wall materials protected fish oil from oxidation. In particular, the zein-based nanocapsules were 3.3 times more oxidatively stable than the kafirin-based nanocapsules, which correlates with the higher oil encapsulation efficiency found for zein-based capsules. Thus, this study shows that kafirin might be considered a hydrophobic wall material for the encapsulation of fish oil by electrospraying, although it prevented lipid oxidation to a lower extent when compared to zein.

Study of microfluidic heat transfer in 2D hydrophobic walls with different slip mechanisms

Numerous studies have shown that the gas at the solid-liquid interface in microflow can increase the slip length, reduce the flow resistance. However, the relationship between nanobubbles and fluid boundary slip is complicated, and the impact of nanobubble morphology must be considered. To investigate the combined effect of the bubble model on slip and heat transfer, we investigated the drag reduction and heat transfer effect of microfluidic flow on a 2D hydrophobic wall by theoretical derivation combined with numerical simulation. The comparison of the results revealed good agreement between the analytical solution and the numerical simulation results. It was shown that the gas slip generated by the rarefied gas significantly affected the flow heat transfer effect. Further analysis of the bubble meniscus shape revealed that minimizing the protruding angle of the meniscus is crucial for enhancing drag reduction. In addition, the simulation results also revealed that increasing the gas coverage fraction effectively reduces flow resistance while maintaining heat transfer performance. At the same time, the higher pressure enhanced the heat transfer effect while maintaining the stability of the drag reduction effect.

Nanoarchitectonics of S-Scheme Heterojunction Photocatalysts: A Nanohouse Design Improves Photocatalytic Nitrate Reduction to Ammonia Performance.

Photocatalytic nitrate reduction reaction (NitRR) is a promising route for environment remediation and sustainable ammonia synthesis. To design efficient photocatalysts, the recently emerged nanoarchitectonics approach holds great promise. Here, we report a nanohouse-like S-scheme heterjunction photocatalyst with high photocatalytic NitRR performance. The nano-house has a floor of plate-like metal organic framework-based photocatalyst (NH2-MIL-125), on which another photocatalyst Co(OH)2 nanosheet is grown while ZIF-8 hollow cages are also constructed as the surrounding wall/roof. Experimental and simulation results indicate that the positively charged, highly porous and hydrophobic ZIF-8 wall can modulate the environment in the nanohouse by (i) NO3 - enrichment/NH4 + discharge and (ii) suppression of the competitive hydrogen evolution reaction. In combination with the enhanced electron-hole separation and strong redox capability in the NH2-MIL-125@Co(OH)2 S-scheme heterjunction confined in the nano-house, the designed photocatalyst delivers an ammonia yield of 2454.9 μmol g-1 h-1 and an apparent quantum yield of 8.02 % at 400 nm in pure water. Our work provides new insights into the design principles of advanced photocatalytic NitRR photocatalyst.

Zafirlukast induces DNA condensation and has bactericidal effect on replicating Mycobacterium abscessus.

Mycobacterium abscessus infections are emerging in cystic fibrosis patients, and treatment success rate in these patients is only 33% due to extreme antibiotic resistance. Thus, new treatment options are essential. An interesting target could be Lsr2, a nucleoid-associated protein involved in mycobacterial virulence. Zafirlukast is a Food and Drug Administration (FDA)-approved drug against asthma that was shown to bind Lsr2. In this study, zafirlukast treatment is shown to reduce M. abscessus growth, with a minimal inhibitory concentration of 16 µM and a bactericidal concentration of 64 µM in replicating bacteria only. As an initial response, DNA condensation, a known stress response of mycobacteria, occurs after 1 h of treatment with zafirlukast. During continued zafirlukast treatment, the morphology of the bacteria alters and the structural integrity of the bacteria is lost. After 4 days of treatment, reduced viability is measured in different culture media, and growth of M. abscessus is reduced in a dose-dependent manner. Using transmission electron microscopy, we demonstrated that the hydrophobic multilayered cell wall and periplasm are disorganized and ribosomes are reduced in size and relocalized. In summary, our data demonstrate that zafirlukast alters the morphology of M. abscessus and is bactericidal at 64 µM. The bactericidal concentration of zafirlukast is relatively high, and it is only effective on replicating bacteria but as zafirlukast is an FDA-approved drug, and currently used as an anti-asthma treatment, it could be an interesting drug to further study in in vivo experiments to determine whether it could be used as an antibiotic for M. abscessus infections.

Discovery and characterization of a novel LysinB from F2 sub-cluster mycobacteriophage RitSun

The escalating antibiotic resistance in mycobacterial species poses a significant threat globally, necessitating an urgent need to find alternative solutions. Bacteriophage-derived endolysins, which facilitate phage progeny release by attacking bacterial cell walls, present promising antibacterial candidates due to their rapid lytic action, high specificity and low risk of resistance development. In mycobacteria, owing to the complex, hydrophobic cell wall, mycobacteriophages usually synthesize two endolysins: LysinA, which hydrolyzes peptidoglycan; LysinB, which delinks mycolic acid-containing outer membrane and arabinogalactan, releasing free mycolic acid. In this study, we conducted domain analysis and functional characterization of a novel LysinB from RitSun, an F2 sub-cluster mycobacteriophage from our phage collection. Several key properties of RitSun LysinB make it an important antimycobacterial agent: its ability to lyse Mycobacterium smegmatis from without, a higher than previously reported specific activity of 1.36 U/mg and its inhibitory effect on biofilm formation. Given the impermeable nature of the mycobacterial cell envelope, dissecting RitSun LysinB at the molecular level to identify its cell wall-destabilizing sequence could be utilized to engineer other native lysins as fusion proteins, broadening their activity spectrum.

Revealing the structural and transport properties in the narrow channels of porous carbon for fuel cells

Porous carbon is a promising catalyst support due to its high specific surface area and suppressing catalyst poisoning by ionomer. However, the underlying transport phenomena in narrow channels of porous carbon is still unclear. We employ molecular dynamics (MD) simulations to reveal the structural and transport properties in the narrow pore and crack channels. The results show that the smaller narrow channel (<6 nm) can effectively prevent the PFSA chains from penetrating the channel and covering the catalyst inside the porous carbon. The narrow channel is accessible for water, proton, and oxygen. The PFSA chains prefer to aggregate along the hydrophobic wall of narrow channel, while the water channel is formed in the middle of narrow channel. The protons migrate via the water channel that is more easily formed in a wider channel and at a higher hydration. The oxygen molecules preferentially migrate along the wall of narrow channel. The transport resistances of protons and oxygen molecules will decrease as the channel width increases. The findings demonstrate that the opening width of narrow channels needs to be optimized by balancing the blockage effect on PFSA chains and the transport resistances of protons and oxygen.

Lattice boltzmann investigation of droplet interactions with non-uniform chemically patterned surfaces

In this study, we employed a non-orthogonal, three-dimensional, multi-relaxation-time pseudo-potential lattice Boltzmann method, and investigate the behaviors of droplets impacting chemically patterned surfaces. We considered two interfaces: a hydrophobic/neutral strip on a hydrophilic wall (surface A), and a hydrophilic strip on a hydrophobic wall (surface B). The dynamics of impact, including the evolution of the morphology and the droplet spreading factor, were investigated under the influence of the difference in wettability, Weber number (We), and strip width. An increase in the wettability difference of surface A delayed droplet detachment and reduced the amplitude of oscillations, where this can be attributed to the surface tension and viscous dissipation, which had more time to weaken the strength of the jet. The magnitude of droplet detachment time initially increased with We but eventually decreased. As We is further increased, the ratio of viscous loss to the initial kinetic energy of the droplet is decreased and resulted in a shorter detachment time. The unbalanced Young's force significantly affected the evolution of the droplet on surface B. The mass of the droplet accumulated near the borderline of the strip and expanded along the y-axis under the influence of the inertial force, where this led to a larger spreading factor along the y-axis. In addition, the mass of the droplet on the hydrophobic wall affected the strength of the unbalanced Young's force. As the strip width increased, the spreading factor initially increased but then decreased along the y-axis owing to the combined action of inertial forces.

Enhancing the permeability and selectivity of graphene oxide membrane through polyhedral oligomeric silsesquioxane intercalation for textile wastewater treatment

Graphene oxide-based loose nanofiltration membranes have shown significant potential in nanofiltration processes due to their unique 2D mass transfer nanochannels. However, they still suffer from poor structural stability and overly compact laminar stacking. To address these issues, a novel AP-POSS was intercalated into GO-based membrane to construct a tuned interlayer structure and mass transfer characteristics. The aromatic ring structure of aminophenyl groups on AP-POSS act as hydrophobic side walls of the mass transfer channels between adjacent GO nanosheets, reducing the interaction between water molecules and channel structure. With an optimal amount of AP-POSS as an intercalator, the permeation flux of the prepared POSS-GO membrane could be improved by 30 wt% while maintaining nearly complete rejection of Congo Red (CR). Moreover, the prepared membrane exhibited enhanced structural stability, long-term stability, and a separation factor above 91.0 for CR/NaCl in binary solutions. Our study highlights the excellent performance of the POSS-GO membrane and demonstrates its potential application in dye/salt wastewater treatment.

Lattice Boltzmann simulation of flow boiling heat transfer process in a horizontal microchannel with rectangular cavities

Bubble nucleation, growth and separation from cavities on the bottom of a microchannel for subcooled flow boiling are investigated by pseudo-potential lattice Boltzmann method. The influence of subcooling temperature, wall superheat, wettability, cavity size, and cavity number on the flow boiling heat transfer is systematically studied. The results show that the bubble equivalent diameter is 1.9 times larger at subcooling temperature 0.05Tc than that at 0.15Tc, and the heat flux is also 8 % higher at subcooling temperature 0.05Tc than that at 0.15Tc. It is found that the flow boiling changes from nucleate boiling to film boiling with the increase of wall superheat. When the wall wettability changes from the hydrophobic wall (θ = 120°) to the hydrophilic wall (θ = 30°), the average Nusselt number (Nuav) is reduced by 23 %. We also optimize cavity height and the uniformly distributed cavity number in the microchannel. It is found that the Nuav is increased by 9.7 % when the cavity height changes from h = 20lu (lattice unit) to h = 60lu. However, there exists an optimal cavity height about h = 60lu, where the heat transfer performance cannot be improved with the cavity height over this value. In addition, the number of cavities in the microchannel can improve the boiling heat transfer. When the cavity number changes from 1 to 4, the Nuav is increased by 10 %.

Simulating Two-Phase Seepage in Undisturbed Soil Based on Lattice Boltzmann Method and X-ray Computed Tomography Images.

The two-phase seepage fluid (i.e., air and water) behaviors in undisturbed granite residual soil (U-GRS) have not been comprehensively studied due to a lack of accurate and representative models of its internal pore structure. By leveraging X-ray computed tomography (CT) along with the lattice Boltzmann method (LBM) enhanced by the Shan-Chen model, this study simulates the impact of internal pore characteristics of U-GRS on the water-gas two-phase seepage flow behaviors. Our findings reveal that the fluid demonstrates a preference for larger and straighter channels for seepage, and as seepage progresses, the volume fraction of the water/gas phases exhibits an initial increase/decrease trend, eventually stabilizing. The results show the dependence of two-phase seepage velocity on porosity, while the local seepage velocity is influenced by the distribution and complexity of the pore structure. This emphasizes the need to consider pore distribution and connectivity when studying two-phase flow in undisturbed soil. It is observed that the residual gas phase persists within the pore space, primarily localized at the pore margins and dead spaces. Furthermore, the study identifies that hydrophobic walls repel adjacent fluids, thereby accelerating fluid movement, whereas hydrophilic walls attract fluids, inducing a viscous effect that decelerates fluid flow. Consequently, the two-phase flow rate is found to increase with then-enhanced hydrophobicity. The apex of the water-phase volume fraction is observed under hydrophobic wall conditions, reaching up to 96.40%, with the residual gas-phase constituting 3.60%. The hydrophilic wall retains more residual gas-phase volume fraction than the neutral wall, followed by the hydrophobic wall. Conclusively, the investigations using X-ray CT and LBM demonstrate that the pore structure characteristics and the wettability of the pore walls significantly influence the two-phase seepage process.

Effects of surface nanostructure and wettability on CO2 nucleation boiling: A molecular dynamics study

Nanostructured tubes hold great potential for enhancing heat transfer in refrigeration/heat pump systems. Therefore, it is essential to study the effects of nanostructured surface characteristics on refrigerant boiling heat transfer. In this paper, the nucleation boiling behavior of CO2 on the nanostructured surface is simulated using molecular dynamics. The effect mechanism of nanostructure size and surface wettability on CO2 bubbles nucleation and growth is investigated. At first, the nucleation boiling processes of both smooth surfaces and nanostructured surfaces featuring three different wide grooves are simulated. The results show that the local thermal aggregation effect is the key for nanostructures to promote CO2 bubble nucleation. The bubble nucleation efficiency is highest on the nanostructured surface with 5 ​nm wide groove. Then, based on a 5 ​nm wide nanostructured wall surface, the wettability effect on nucleation boiling is investigated by adjusting the potential energy factor α. The results show that the hydrophilic walls enhance the solid-liquid heat transfer and the collision of atoms within the liquid, resulting in boiling heat transfer capacity improvement between CO2 and the walls. The average temperature, average heat flux and critical heat flux in the liquid phase are also improved. A significant temperature gradient between the layers of CO2 liquid is noted on hydrophilic wall, where intermolecular forces and molecular advection dominate the heat transfer mechanism. In contrast, on hydrophobic wall, intermolecular forces dominate the heat transfer process.

Dynamic behavior of floating ferrofluid droplet through an orifice with a magnetic field

In this study, we utilize the simplified lattice Boltzmann method (SLBM) to investigate numerically the motion of buoyancy-driven deformable ferrofluid droplets through the orifice of varying widths and depths in two-dimensional (2D) space. Positioned directly beneath a plate with a central hole, the magnetic fluid droplets undergo acceleration to meet the plate under the influence of buoyancy and magnetic forces. We investigate the impact of magnetic field strength (Bom), pore ratio (PR), plate thickness ratio (WR), droplet viscosity (Re), and the plate's wettability (contact angle) on the dynamic behavior of ferrofluid droplets ascending through the orifice. Our results reveal significant effects on the efficiency and morphology of ferrofluid droplets passing through the hole. The introduction of a magnetic field facilitates a larger volume of liquid droplets passing through the hole at PR = 0.25. Moreover, increasing magnetic field intensity leads to the generation of secondary droplets during passage through the orifice. In practical applications, to prevent the generation of secondary droplets, we recommend Bom < 3 when the pore ratio falls within 0.35 < PR < 0.45 and plate thickness ratio WR = 1. Additionally, with increasing obstacle thickness, ferrofluid droplets on the hydrophobic wall can pass through the orifice more easily. Furthermore, when the magnetic field strength exceeds a certain threshold (Bom = 6.08), the droplets can pass through the orifice regardless of the wall's hydrophilicity or hydrophobicity. For practical applications with the pore ratio PR = 0.25 and plate thickness ratio WR > 1, we suggest Bom > 3.

Editor's Note for article entitled “Numerical investigation of heat transfer and entropy generation in serpentine microchannel on the battery cooling plate using hydrophobic wall and nanofluid”

Editor's Note for article entitled “Numerical investigation of heat transfer and entropy generation in serpentine microchannel on the battery cooling plate using hydrophobic wall and nanofluid”

Dynamic characteristics of droplets impacting an inclined wall based on a front-tracking method

In the current study, the dynamic characteristics of droplets impacting a hydrophilic/hydrophobic inclined wall are investigated using the front-tracking method. The morphological and kinematic features of the droplets are examined across various contact angles (θ), low Weber numbers (We≤10), and inclination angles (α). A sequence of events, including deposition, spreading, contraction, rebound, and subsequent re-spreading, is observed when a droplet impacts a hydrophobic wall, according to the results. The droplet exhibits both a small dimensionless spreading area (A*) and significant fluctuation, along with a noticeable slip on the wall. As We increases, the spread and rebound effects become more pronounced, and the slipping velocity also increases. As α increases, the slipping velocity becomes greater, and both A* and the dimensionless tangential spreading diameter (β) decrease. When a droplet impacts a hydrophilic wall, continuous deposition, spreading, and a subtle slip with inconspicuous rebound are observed. As We increases, A* expands at an accelerating rate. Furthermore, the wetting effect of hydrophilic surfaces on droplets cannot be overlooked. Consequently, it also shows that as α increases, there is a larger diffusion range along the wall with a longer diffusion film and a larger β. Such behaviors stand in contrast to a droplet impacting a hydrophobic wall.

Microstructure regulation and enhanced VOC removal performance of carbon aerogels by surface carbon nanotube grown

Carbon aerogels were newly employed in adsorption for volatile organic compounds (VOCs) as an emerging material. In contrast, the microstructure and high carbon consumption are the primary factors restricting their application scenarios. Carbon nanotubes, recognized for their controllable cylindrical hollow structure and hydrophobic walls, generally possess higher adsorption capacities than typical carbon adsorbents. In this study, carbon nanotubes were grown on the surface of carbon aerogels using the chemical vapor deposition method by controlling different deposition conditions. The results showed that the modified samples displayed the maximum adsorption capacity of 145.0 mg/g and 178.3 mg/g for toluene and 1,2- dichlorobenzene, respectively. After ten regeneration cycles, the performance decreased by 7.9 % and 5.6 %, respectively. Meanwhile, the carbon replenishment was about 0.2 g/g, which is an excellent complement for carbon consumption. Various characterization patterns showed that deposition temperature was reflected in its deposition rate, deposition times influenced the formation of multi-walled carbon nanotubes, and deposition concentration affected the length of carbon nanotubes. This study offers valuable insight into the growth patterns of carbon nanotubes and the microscale regulation of carbon material surfaces, and this method is expected to be an effective means of carbon replenishment, carbon addition to carbon-poor materials, and enhancement of VOCs removal performance, and the growth mechanisms investigated are instructive for practical applications.

Artificial Hydrophilic Organic and Dendrite-Suppressed Inorganic Hybrid Solid Electrolyte Interface Layer for Highly Stable Zinc Anodes.

Aqueous zinc-ion batteries (AZIBs) have gained significant attentions for their inherent safety and cost-effectiveness. However, challenges, such as dendrite growth and anodic corrosion at the Zn anode, hinder their commercial viability. In this paper, an organic-inorganic coating layer (Nafion-TiO2) was introduced to protect the Zn anode and electrolyte interface. Briefly, Nafion effectively shields against the corrosion from water molecules through the hydrophobic wall of -CF3 and guided zinc deposition from the -SO3 functional group, while TiO2 particles with a higher Young's modulus (151 GPa vs 120 GPa from Zn metal) suppress the zinc dendrite formation. As a result, with the protection of Nafion-TiO2, the symmetrical Zn∥Zn battery shows an improved cycle life of 1,750 h at 0.5 mA cm-2, and the full cell based on Zn∥MnO2 shows a long cycle life over 1,500 cycles at 1 A g-1. Our research offers a novel approach for protecting zinc metal anodes, potentially applicable to other metal anodes such as those in lithium and sodium batteries.

Facilitating Water Permeation in Graphene Oxide Membranes via Incorporating Sulfonato Calix[n]arenes.

Graphene oxide (GO) with its atomic thickness and abundant functional groups holds great potential in molecular-scale membrane separation. However, constructing high-speed and highly selective water transport channels within GO membranes remains a key challenge. Herein, sulfonato calix[n]arenes (SCn) molecules with a cavity structure, hydrophilic entrance, and hydrophobic wall were incorporated into GO interlayer channels through a layer-by-layer assembly approach to facilitate water permeation in a water/ethanol separation process. The hydrophilic entrance enables preferential access of water molecules to the cavity over ethanol molecules, while the high hydrophobicity of the cavity wall confers low resistance for water diffusion. After incorporating SCn molecules, the membrane shows a remarkable increase in the water/ethanol separation factor from 732 to 1260, while the permeate flux also increases by about 50%. In addition, the strong electrostatic interactions between the building blocks endow the membrane with excellent swelling resistance even under a high water content. This work provides an effective strategy of constructing high-efficiency water transport channels in membrane.