Capillary Channels Research Articles

A backbone support structure and capillary effect-induced high-flux MOF-based mixed-matrix membrane for selective dye/salt separation

Metal-organic frameworks (MOFs) based mixed-matrix membranes (MMMs) exhibited an excellent promise for separating dye molecules and salt ions due to their permeability and selectability. However, controlling interfacial compatibility between the polymer and MOFs remained challenging, and MOF-based MMMs were poorly processable. Herein, the dual-bath coagulation method was developed to form the polyvinylidene fluoride (PVDF) and polyvinylpyrrolidone (PVP) polymer precursor layer, which provided a skeleton support structure. Inspired by the capillary effect, the nanosheet copper-ZIFs were in situ grown in the skeleton support structure, forming capillary microporous channels. The formation of capillary microporous channels endowed the MOF-based MMMs with high permeability of water molecules. Interestingly, the MOF@PVDF/PVP membrane exhibited excellent water permeance (ca. 635.7 L·m−2·h−1·bar) and selectivity for dye/salt separation (dye removal approaching 100 %, salt removal < 10 %). Even for the cationic small-molecule dyes, the MMMs showed an outstanding rejection (e.g., 96.3 % for malachite green and 96.7 % for methylene blue). The water permeance of the MOF@PVDF/PVP membrane was much higher than that of most membranes with similar rejection rates for dyes. Besides, the antifouling and antibacterial properties confirm that the MMMs could maintain long-term stability in practical dyeing wastewater treatment. This study provides an efficient approach to preparing MOF-based MMMs with capillary and skeleton support structures, which exhibited superior performance and outstanding stability for water purification.

Influence of industrial waste and mineral admixtures on durability and sustainability of high-performance concrete.

The present research explores the strength, durability, microstructure, embodied energy, and global warming potential investigations made toward cleaner production of high-performance concrete (HPC) using a new composition. For this, various mixes were considered by replacing cement with metakaolin (MK) and silica fumes (SF) while simultaneously altering fine aggregates with industrial waste, copper slag (CS) in 0%, 25%, 50%, 75%, and 100% at 0.23 w/b ratio. The observations on fresh properties show a decrease in the slump due to pozzolans MK and SF but get compensated by the inclusion of copper slag simultaneously. HPC mixes with 50% replacement of CS revealed the best outcomes in compressive and splitting tensile strengths. Upon testing the concrete mixes against resistance to sulfate exposure, chloride penetration, and water absorption, the durability performance results best for modified mixes having 50% CS substitution levels. Scanning electron microscopy and energy dispersive spectroscopy support a 25% substitution of CS, showing a thickset microstructure with an ample amount of C-S-H gel with negligible cracks and capillary channels resulting in having best-strengthening properties. Overall, decrement in embodied energies and global warming potential has resulted with a reduction in the usage of cement and river sand in modified concrete mixes, ultimately making the production sustainable as well as environment friendly.

Modelling the Impregnation of a Pressure-Tight Casting

AbstractPressure tightness is important for many die-cast aluminium castings, but the interconnected porosity formed in the aluminium alloy high-pressure die castings (HPDC) can form a network connecting several surfaces of the casting, especially as a result of machining after casting. The resulting potential leakage path can be imagined as a series of discontinuities varying in size over wide limits, where voids of different volumes are connected by channels of smaller cross section. Although vacuum impregnation is often used to ensure pressure-tightness is a robust technology, cavity systems through the casting can still cause leakage after impregnation. This study addresses the issues of the impregnation of discontinuities with a complex geometry. The elementary part of a complex cavity geometry consists of a cavity and a capillary channel. We estimated the limited permeability of the capillary section using a mathematical model based on the Hagen–Poiseuille equation. The calculation results prove that the degree of filling of the cavity with a capillary section is basically a function of the geometrical characteristics of the cavity, primarily the radius of the capillary. If the cavity system also contains a section with a diameter of less than 28 μm, impregnation does not necessarily ensure the gas tightness of the casting. Depending on the radius of the capillary, the impregnation of a cavity of a given volume and connected to the surface through a capillary section of a given length can produce three different results. Above a certain capillary radius, filling is complete, below a certain capillary radius only the capillary itself will contain resin, and between the two limit values, the partial impregnation of the cavity is expected but the capillary section is not filled.

Drop encapsulation and bubble bursting in surfactant-laden flows in capillary channels

Drop encapsulation and bubble bursting in surfactant-laden flows in capillary channels

Kinetic verification and Comsol modeling of the Diels-Alder reaction of conjugated pentadiene with maleic anhydride in a continuous flow capillary reactor

Synthesis of 3/4-methyltetrahydrophthalic anhydride was carried out by the Diels-Alder reaction in a continuous flow capillary reactor. The efficient liquid–liquid reaction of pentadiene with molten maleic anhydride provided a theoretical basis for replacing the conventional batch reaction process. Different inner diameter sizes of the channel tubes were adjusted in the range of 0.50 ∼ 2.00 mm to find the optimal size to match the strong exothermic reaction. Under the capillary channel size of 0.8 mm, the conversion rate of C5 and MA reached 100 % when nC5: nMA = 1.08:1 at 65 °C for 8 min. The D-A reaction process is accompanied by the autopolymerization of diolefin in the batch reactor. Our results showed that choosing a continuous flow capillary reactor of appropriate size can directly improve the selectivity of the reaction and the quality of the product. The accurate reaction rate constant and activation energy were obtained by establishing a kinetic model, and the effects of temperature, reaction time and pipe diameter were designed using Comsol software.

Does backflow occur in forced imbibition into a dual-permeability pore network?

Backflow is a typical flow phenomena in two-phase displacement, but it is rarely studied in the forced imbibition and it remains unknown if the backflow occurs in forced imbibition into a porous medium. In this paper, we focus on the forced imbibition and investigate the backflow behavior in pore doublet (PD) models and in dual-permeability pore networks. Inspired by Gu et al. (2021) and Shan et al. (2023), a general mathematical model is first developed to predict the evolution of the penetration length of wetting fluid in a PD with arbitrary initial interface locations. Based on the analytical predictions and numerical simulations, we then study the influence of initial interface location, capillary number, viscosity ratio, and channel width ratio on the backflow. Results show that the backflow in a PD is mainly attributed to high flow rate in narrow channel, driven by the dominant capillary force. To supplement the insufficiency of inlet flow rate, the fluid in wide channel has to flow backwards to fill the narrow channel. The backflow strengthens with the decrease of capillary number or viscosity ratio, but it exhibits non-monotonic variation with initial interface location and channel width ratio. Moreover, the phase diagrams showing the critical conditions for the occurrence of backflow are established in terms of capillary number, viscosity ratio and channel width ratio. Finally, numerical simulations are conducted to explore the occurrence of backflow in three different dual-permeability pore networks. It is found that the backflow does not occur in dual-permeability pore networks due to the low flow rate caused by overlap event and pronounced tortuosity in the low permeability zone, or occurs only in the early stage when the capillary valve effect does not work.

Rational Design of a Hydrophilic Core-Hydrophobic Shell Yarn-Based Solar Evaporator with an Underwater Aerophilic Surface for Self-Floating and High-Performance Dynamic Water Purification.

Interfacial solar vapor generation holds great promise for alleviating the global freshwater crisis, but its real-world application is limited by the efficiently choppy water evaporation and industrial production capability. Herein, a self-floating solar evaporator with an underwater aerophilic surface is innovatively fabricated by weaving core-shell yarns via mature weaving techniques. The core-shell yarns possess capillary water channels in the hydrophilic cotton core and can trap air in the hydrophobic electrospinning nanofiber shell when submerged underwater, simultaneously realizing controllable water supplies, stable self-flotation, and great thermal insulation. Consequently, the self-floating solar evaporator achieves an evaporation rate of 2.26 kg m-2 h-1 under 1 sun irradiation, with a reduced heat conduction of 70.18 W m-2. Additionally, for the first time, a solar evaporator can operate continuously in water with varying waveforms and intensities over 24 h, exhibiting an outdoor cumulative evaporation rate of 14.17 kg m-2 day-1.

Retracted: A New Fractal Permeability Model for Porous Media Based on Rough Capillary Channels

Retracted: A New Fractal Permeability Model for Porous Media Based on Rough Capillary Channels

An embedded-atom method interatomic potential for lithium-iron binary system and its applications in the liquid first wall system

Molecular dynamics simulations have been widely used to investigate the thermodynamic and structural properties of liquid lithium (Li) and solid substrates as the concept of liquid divertor of fusion devices becomes more and more popular. However, this kind of studies are limited by the complexity of constructing the interatomic potentials for alloy systems. In this work, a new interatomic potential based on the embedded-atom method (EAM) was developed for the Li-Fe (iron) binary system. The parameters of the potential were optimized based on the first-principles calculations and experimental results of the elemental and binary properties of Li-Fe system. Results show that the new potential provides good descriptions of the mechanical and thermal properties of solid α-Fe and liquid Li including the elastic moduli of α-Fe and the viscosity of liquid Li. Especially, the addition of the surface energies and interfacial adhesion work into the fitting database guarantees that the wetting properties of liquid Li on α-Fe surfaces are well predicted. The contact angle on {100} Fe surface was calculated to be 58.4° which agrees very well with the experimental results. The new potential was applied to simulate the tensile tests of Li-Fe alloy and the Li flow in an α-Fe capillary channel. Results suggest that this potential is suitable for simulations of a variety of processes such as the surface corrosion of α-Fe by liquid Li, and the wetting and spreading of liquid Li on α-Fe surfaces, which are of interests in the context of the development of liquid divertor in future fusion reactors.

Capillary efficiency study in leaf vein morphology inspired channels.

Inspired by the capillary transport function of plant leaf veins, this study proposes three typical leaf vein features by observing a large number of leaves, including wedge shape, branch asymmetry, as well as hierarchical arrangement, and investigates their capillary transport mechanism. Not only a preliminary theoretical analysis of capillary flow in the bio-inspired channels was carried out, but the COMSOL Multiphysics simulation software was also used to simulate gas-liquid two-phase flow in biomimetic channels. The results reveal the efficient transport mechanism of the leaf vein inspired structure and provide insight into the design of capillary transmission channels.

Comparative analyses of micro‐ and macro‐scale surface structures in the convergent evolution of rain‐harvesting behaviour in lizards

AbstractRain‐harvesting behaviour represents an adaptation for water collection that has evolved in some species of iguanian reptiles inhabiting arid environments. To date, such behaviour has been observed only in members of the families Agamidae and Phrynosomatidae. A common set of integumentary features characterizes these rain‐harvesting species, including scale surface microstructures (SSMs), capillary channels and hinge joints. The influence of variations in these features on rain‐harvesting has been a subject of discussion for many decades. Nevertheless, a comprehensive comparative study of similarities and differences between harvesting and non‐harvesting species on a broader scale remains lacking. In this study, we classify scale surface microstructures into three categories: large hexagons (SSM1), smaller nested hexagons (SSM2) and hinge pits (SSM2H). As the first two SSM types are widespread, they do not appear to be directly linked to the adaptation for rain‐harvesting. Conversely, the presence or absence of hinge pits distinguishes harvesters from non‐harvesters. Additionally, channel hierarchy, width and structure determine the effectiveness of the rain‐harvesting architecture. Only Moloch horridus exhibits distinct integumentary features in comparison to other agamids and rain‐harvesting species. Ancestral character state reconstruction suggests that rain‐harvesting behaviour was likely absent in the ancestor of Iguanians, even though overlapping scales and SSM1 were present. Our findings illustrate that rain‐harvesting species have independently converged upon similar structural solutions to address their water acquisition challenges, building upon shared pre‐existing features.

Effect of roughness on droplet motion in a capillary channel: A numerical study

This study presents droplet dynamics in a rough capillary channel. Prior studies investigating the effect of roughness on fluid flow have mainly considered a continuous phase whose behavior is different from a discontinuous phase, i.e., an oil slug. To explore the dynamic behavior of droplet motion across a rough channel, a direct numerical simulation of in a three-dimensional channel is performed. Three models have been considered: model A had a rough surface only on the bottom walls, model B on both the bottom and top walls, and model C on all walls. The results show that in contrast with common observations, roughness promotes droplet mobility in comparison with smooth walls. The presence of roughness results to an additional energy required to move the droplet, and the degree of confinement increases with the roughness; thus, the difficult of mobilization increases with the increase in roughness. Different roughness parameter effects have been investigated. The results have shown that the critical pressure increases with the increase in the pillar's height and decreases with the pillars spacing. The offset leads to a decrease in flow resistance for larger contact angles. We noted also that it is more difficult to mobilize a discontinuous phase in a neutral-wet surface condition. Furthermore, discontinuous pillars in the lateral direction led to much higher resistance. Through our comprehensive numerical study, we provide valuable insights into the impact of roughness in capillary channels. These findings can be used as guidelines for designing droplet flow on complex and rough surfaces, such as microfluidic devices, and hold significant relevance in the optimization of droplet control strategies in enhanced oil recovery methods.

Microfluidic systems with a pulsating heat pipe

This research addresses a critical issue in modern microelectronics, which arises from increased miniaturization and heat generation, necessitating effective temperature control. The study focuses on pulsatile heat pipes, offering a passive and highly efficient heat transfer solution by utilizing fluid and vapor phases within a closed capillary channel. To enhance temperature regulation, microfluidics are employed with integrated separation barriers to improve capacity and efficiency. Altering the flow pattern of liquid and vapor plugs through droplet generation may enhance thermal performance. The study demonstrates the accuracy of the heat transport model through mathematical and empirical data comparison, achieving a remarkable 90.9% accuracy and efficiency. Pulsatile flows, especially in microfluidic systems, exhibit advantages over steady flows, promising avenues for future physics-based research.

Alleviation of the necessity for supernatant prefiltering in the protein a recovery of Monoclonal antibodies from Chinese hamster ovary cell cultures

Protein A (ProA) chromatography is a mainstay in the analytical and preparative scale isolation/purification of monoclonal antibodies (mAbs). One area of interest is continuous processing or continuous chromatography, where ProA chromatography is used in the large-scale purification of mAbs. However, filtration is required prior to all ProA isolations to remove large particulates in cell culture supernatant, consisting of a mixture of cell debris, host cell contaminants, media components, etc. Currently, in-line filters are used to remove particles in the supernatant, requiring replacement over time due to fouling; regardless of the scale. Here we demonstrate the ProA isolation of unfiltered Chinese hamster ovary (CHO) cell media using capillary-channel polymer (C-CP) fiber stationary phases modified with S. aureus Protein A (rSPA). The base polymer of the analytical scale C-CP columns costs ∼$5 per 30 cm column, and when modified with ProA, the base cost is ∼$25 per 30 cm column, a cost-effective option in comparison to analytical-scale commercial columns. To directly sample unfiltered media, a 5 cm gap was created at the head of the C-CP column, where the large particulates are trapped, while molecular solutes flow through the capillary channels without sacrifice in analytical performance, mAb loading capacity, or backpressure increases. The binding capacity of the gap ProA C-CP column was ∼ 2 mg mL−1 of IgG per bed volume. The same analytical column could be operated after processing a total of ∼ 56 column bed volumes of supernatant (>25 analytical cycles) without the need for caustic clean-in-place processing.

Soft Injectable Sutures for Dose-Controlled and Continuous Drug Delivery.

Transdermal drug delivery offers a promising alternative to traditional methods such as oral ingestion and hypodermic injection. Hypodermic injections are painful, while oral ingestion requires higher doses due to enzymatic degradation and poor absorption. While microneedles address the pain issue, they are limited to delivering small amounts of drugs and can be impractical due to peeling off with motion and sweat. Herein, this work proposes soft injectables using drug-carrying sutures for painless and localized sustained delivery in the dermis. These sutures can remain in place during delivery and are suitable for all skin types. Surgical sutures can also serve as open capillary microfluidic channels carrying drug from a wearable drug reservoir to enable long-term (weeks to months) transdermal drug delivery. The experiments focus on delivering 5-fluorouracil (5-FU), a cancer drug, and rhodamine B, a drug model. A fixed-length suture of 60 cm delivers 0.43 mg of 5-flurouracil in 15 min. The experiments also demonstrate a continuous drug delivery of rhodamine B for over 8 weeks at a rate of 0.0195 mL h-1 . The results highlight that soft injectable sutures are promising candidates for long-term sustained delivery of varying quantities of drugs over weeks period compared to hypodermic injection, oral ingestion, or microneedles.

Evaporation Driven Hydrovoltaic Generator Based on Nano-Alumina-Coated Polyethylene Terephthalate Film.

Collecting energy from the ambient environment through green and sustainable methods is highly expected to alleviate pollution and energy problems worldwide. Here, we report a facile and flexible hydrovoltaic generator capable of utilizing natural water evaporation for sustainable electricity production. The generator was fabricated by coating nano-Al2O3 on a twistable polyethylene terephthalate film. An open circuit voltage of 1.7 V was obtained on a piece of centimeter-sized hydrovoltaic generator under ambient conditions. The supercapacitor charged by the hydrovoltaic device can power a mini-motor efficiently. Moreover, by expanding the size or connecting it in series/parallel, the energy output of the generator can be further improved. Finally, the influence factors and the mechanism for power generation were primarily investigated. Electrical energy is produced by the migration of water through charged capillary channels. The environmental conditions, the properties of the solution and the morphology of the film have important effects on the electrical performance. This study is anticipated to offer enlightenment into designing novel hydrovoltaic devices, providing diverse energy sources for various self-powered devices and systems.

Heat transfer of uncoated and nanostructure coated commercially micro-enhanced refrigeration tubes under pool boiling conditions

The heat transfer performance of commercially produced micro-enhanced tubes with and without a nanocoating was investigated under pool boiling of saturated refrigerant. These multiscale enhancements were on the outside of 19 mm horizontal copper tubes heated by water to determine the effectiveness of this multiscale enhancement technique on industrially relevant tubes and geometry. The tubes tested were a plain tube roughened by sandpaper, a low finned GEWA-KS tube and two micro-enhanced re-entrant cavity tubes, the GEWA-B5 and EHPII. The tubes were tested in R134a at saturation temperatures of 5 °C and 25 °C across a range of heat fluxes from 20 kW/m2 to 100 kW/m2 under pool boiling conditions. The nanocoating applied to the tubes produced a forest of copper oxide nanostructures on the surface, increasing wickability of the surface. A Scanning Electron Microscopy showed that copper oxide nanocoatings coated all micro-enhanced tubes evenly without impeding the surface features or significantly blocking the re-entrant cavities. For the uncoated tubes in pool boiling, the heat transfer coefficients of the EHPII was up to 519 % greater than those of the plain roughened tube, the GEWA-B5 was up to 539 % higher than the roughened tube and the GEWA-KS was at best 64 % higher than those of the plain roughened tube. Increases in the saturation temperature to 25 °C produced minor improvements in heat transfer coefficients. The application of the copper oxide nanocoating resulted in generally decreased heat transfer performance by approximately 40 % on average compared to the uncoated tubes, with the GEWA-B5 tube the worst affected. Degradation of the heat transfer is thought on plain surfaces to be due to the flooding of nucleation sites, while re-entrant cavity style enhanced surfaces were thought to experience degraded sensible and latent heat transfer due to impeded flow in the microstructure capillary channel, as bubbles were noted to be trapped in the channels. It is recommended that the heat transfer at much higher heat flux ranges be explored for possible HTC enhancement.

Effect of entry geometry on droplet dynamics in contraction microchannel

In droplet-based microfluidic systems, the movement and control of liquid droplets are primarily governed by microchannel geometry. The aim of this study is to examine droplet dynamics in contraction microchannels by using three-dimensional simulation and theoretical modeling. This work specifically describes three regimes of the droplet dynamics, including the trap, squeeze, and breakup regimes, and investigates the effects of capillary number (Ca), entry angle (α), and contraction channel ratio (C). Additionally, a theoretical model is proposed to describe the transition between squeeze and trap regimes, which depends on the entry angle. The critical value of capillary number (Ca1c) for this transition is observed to be Ca1c=a(CM−b/α), where a and b are fitted parameters. Meanwhile, the entry angle is found to have no influence on the transition from squeeze to breakup regime. The droplet deformations, retraction, and/or breakup position are quantitatively investigated for a wide range of capillary number and entry angle. The aforementioned findings would provide valuable recommendations for designing contraction micro channels.

Controlling the Motion of Interfaces in Capillary Channels with Non-uniform Surface Wettability

The use of self-driven flows in microfluidic devices attracts many researchers as the external flow-driving mechanism is diminished or eliminated. One of the mechanisms providing such motions is generating a pressure difference across interfaces as in the case of the motion in capillary tubes. The capillarity, namely, the pressure difference across the interface due to its curvature drives the motion. This pressure depends on the interaction with the capillary walls and is controlled if one varies the surface energy of the walls. In this study, we search for the effects of surface energy on the motion of interfaces in capillary-driven flow. To this end, we model the motion of fluid particles in a capillary channel and integrate the governing equations using the binary lattice Boltzmann method for the two-phase flow. We, first, validate our solver for canonical static and dynamic problems. We, then, discuss two main contributions; we show how to deviate the interface speed from the ones moving in channels with uniform wall energies and discuss the conditions under which such an interface stagnates (like a passive valve in a channel). Tuning the wettability of the channel walls, we provide a simple condition for stopping the interface: the summation of the equilibrium contact angles interface make with the channel walls at the bottom and top wall need to satisfy $\theta_{eq}^{top}+\theta_{eq}^{bot} \geq \pi$. Configurations and wetting properties of different wettability regions play major roles together

Dynamics of two unequal micron-sized viscous emulsion drops flowing in a two-dimensional constricted microcapillary channel: A numerical study

Emulsions containing unequal-sized droplets are ubiquitous. Compared to the equal-sized emulsion droplet coalescence process in porous media, unequal-sized emulsion droplets show distinctly different dynamics and features when flowing through the capillary channel. In this paper, we numerically studied the flow of two unequal small micron-size emulsion drops flowing in a two-dimensional (2D) half-sine pulse shape constricted tube. The finite element method solves the momentum, continuity, and level set equations. The oil–water interface is tracked using the level set method. Effects of drop size, capillary number (interfacial tension), viscosity ratio, and geometrical shape constriction effects on the droplets' flow behavior across the throat, its flow velocities, and average viscous pressure across the tube were investigated. It is found when there is a significant difference in diameters of the two un-equalized droplets, they will flow separately through the throat, whereas when the two un-equalized droplets’ size is close to each other, they coalescence with each other after passing through the constriction. Higher interfacial tension leads unequal-sized droplets to coalescence more readily while droplets pass without coalescence at low interfacial tension cases. The highest pressure drop is recorded at the critical point where the two unequal-sized droplets begin to merge in the constriction throat. Increasing droplet viscosity reduces the chances of coalescence and increases the average viscous pressure drop hence leading to mobility reduction. In the case of geometrical constrictions, circular-shaped contracted tubes offer the least resistance to fluid flow and are more appropriate for enhanced oil recovery processes. We hope our numerical findings will provide more information to facilitate the enhanced oil recovery process.