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We demonstrate a novel way for powdered hemostats to enter deep bleeding sites of complicated wounds and achieve targeted hemostasis.

This review summarizes the recent progress in interfacial solar vapor generation, which was first proposed in 2014. The technique shows great potential for decentralized desalination applications using ubiquitous and renewable solar energy.

75D/72f and 100D/96f polyester yarns were knitted to develop three kinds of fabrics with different densities (large, medium and small) on circular knitting machine (gauge E18). It was found that the strength of the fabrics increased with the increase of loop density, and the initial modulus of the fabrics also increased. In order to obtain better strength and lower initial modulus, the washing and heat setting process of the fabric were studied and explored. The strength and initial modulus of the three density fabrics of the two yarn specifications decreased after pre-washing. Finally, the lowest initial modulus (0.36 MPa at warp direction and 0.22 MPa at weft direction) were found in sample (sample no. 3–3) made of 75D filament in longest loop length under 140 °C heat setting. The weft strength and warp strength were 9.54 MPa and 5.28 MPa respectively.

Usually, traditional insulation materials have a constant thermal resistance value that cannot change within the ambient temperature and will decrease as ambient humidity or external stress increases. Humans heavily rely on heating, ventilation, and air conditioning (HVAC) systems to meet the thermal comfort requirements of their bodies, giving rise to energy waste and global warming. As an infinitely available natural resource, air is one of the most efficient thermal retaining substances known to science. Inspired by soft pneumatic robotics, we propose an architecture for air-driven thermoregulation fabrics called soft robotic fabrics (SRF). By changing the thickness of trapped air layer in fabric system through SRF, wearers could modify garments’ thermal insulation performance. A fabrication method is introduced to rapidly manufacture low-cost pneumatic structures using various types of construction and dimensions. With excellent ductility, elasticity, and compression resistance, the thickness of SRF increases by 12 times or more after inflation, and the fabric even can lift an object 270 times heavier than its weight. The excellent deformability can effectively increase stable air layer between clothing and skin. Based on the Predicted Mean Vote–Predicted Percentage of Dissatisfied model, the thermoregulation capability of SRF helps HVAC expand the temperature setpoint range by 3–8 times when compared with traditional fabrics, and has far-reaching significance in saving energy.

Warp-knitted spacer fabrics are generally used for sportswear, functional clothing, protective clothing, and other applications. This article studied the heat and mass transfer properties of polyester warp-knitted spacer fabrics from low thickness (2 mm) to high thickness (20 mm), from low mass (247.34 g/m2) to high mass (1,585.9 g/m2), and surface structure in plain or mesh construction. Water vapor permeability, air permeability, water absorption, and thermal insulation property were conducted to evaluate the spacer fabrics. The results revealed that with increasing volume density the water vapor permeability of spacer fabrics decreased, but the water absorption ratio increased. The water vapor permeability of fabrics increased when thickness decreased and volume density increased. It was further found that spacer fabrics with mesh worn nearby the skin and plain structure worn far from the skin could facilitate water vapor and air transmission. The difference of 8.82% for water vapor permeability and 14.19% for air permeability were found between testing mesh side up and down for the spacers (2.56 and 3.37 mm), respectively. Thermal insulation ratio was highly and significantly correlated with heat transfer coefficient at −0.958 and with thickness at 0.917. Thermal insulation ratio is highly and significantly correlated with air permeability at 0.941.

The six types of weft knitted fabric in plating structure were made by cotton, polypropylene and polyester with different number of filaments. The fabrics were tested by moisture management tester, vertical wicking and drying test. It was found that the large water absorption difference and the large filament density difference between the inner and outer side of fabric could result in the better water absorption and drying ability. The fabric with polypropylene yarn (75D/48f) at inner side and cotton yarn (40 Ne) at outer side had best moisture management property evaluated by MMT. While the fabric with polypropylene yarn (75D/48f) at the inner side and polyester yarn (75D/144f) at the outer side had highest vertical wicking and drying rate.

The ever-increasing development of flexible and wearable electronics has imposed unprecedented demand on flexible batteries of high energy density and excellent mechanical stability. Rechargeable lithium (Li) metal battery shows great advantages in terms of its high theoretical energy density. However, the use of Li metal anode for flexible batteries faces huge challenges in terms of its undesirable dendrite growth, poor mechanical flexibility, and slow fabrication speed. Here, a highly scalable Li-wicking strategy is reported that allows ultrafast fabrication of mechanically flexible and electrochemically stable Li metal anodes. Through the rational design of the interface and structure of the wicking host, the mean speed of Li-wicking reaches 10 m2 min-1 , which is 1000 to 100 000 fold faster than the reported electrochemical deposition or thermal infusion methods and meets the industrial fabrication speed. Importantly, the Li-wicking process results in a unique 3D Li metal structure, which not only offers remarkable flexibility but also suppresses the dendrite formation. Paring the Li metal anode with lithium-iron phosphate or sulfur cathode yields flexible full cells that possess a high charging rate (8.0 mA cm-2 ), high energy density (300-380 Wh kg-1 ), long cycling stability (over 550 cycles), and excellent mechanical robustness (500 bending cycles).

A mask that creates a physical barrier to protect the wearer from breathing in airborne bacteria or viruses, reducing the risk of infection in polluted air and potentially contaminated environments, has become a daily necessity for the public especially as COVID-19 has exploded around the world. However, the use of masks often causes soaring temperatures and thick humid air, leading to thermal and wear discomfort and breathing difficulties for a number of people, and further increasing the elevated risk of heat illnesses including heat stroke and heat exhaustion. When wearers become highly active or work under high tension, the excess sweat generated negatively affects the functionality of masks. Here, we report on an innovative design of an air-conditioned mask (AC Mask) system, facilitating thermoregulation in the mask microclimate, ease of breathing, and wear comfort. The AC Mask system is developed by integrating a cost-effective and lightweight thermoelectric (TE) and ventilation unit in a wearable 3D printed mask device, compatible with existing disposable masks, to protect end users safely against toxic particles such as viruses. A wind-guided tunnel has been developed for quick and efficient ventilation of cooling air. Based on a human trial, reductions in the apparent microclimate temperature and the humidity by 3.5 °C and 50%, respectively, have been achieved under a low voltage. With the excellent thermal management properties, the AC Mask will find also wide application among professional end-users such as construction workers, firefighters, and medical personnel., Graphical abstract Image 1

A super breathable skin-like fabric with artificial sweating glands to excrete fluid droplets and repel external liquids., Personal moisture management fabrics that facilitate sweat transport away from the skin are highly desirable for wearer’s comfort and performance. Here, we demonstrate a “skin-like” directional liquid transport fabric, which enables continuous one-way liquid flow through spatially distributed channels acting like “sweating glands” yet repels external liquid contaminants. The water transmission rate can be 15 times greater than that of best commercial breathable fabrics. This exceptional property is achieved by creating gradient wettability channels across a predominantly superhydrophobic substrate. The flow directionality is explained by the Gibbs pinning criterion. The permeability, mechanical property, and abrasion resistance (up to 10,000 cycles) of the fabric were not affected by the treatment. In addition to functional clothing, this concept can be extended for developing materials for oil-water separation, wound dressing, geotechnical engineering, flexible microfluidics, and fuel cell membranes.

Silk biomaterials have been attracting significant attention in medical bioengineering because of their versatile function, morphologic flexibility, and excellent biocompatibility. The present study was inspired by the shielding and protective function of the silkworm cocoon shell (SCS) to the silkworm chrysalis, which is notably similar to the protective effect of skin on the body. SCS was treated and used as the base substrate for dressing materials to promote wound healing; for this, a facile approach to maintaining its original framework was proposed. By treatment with a ternary solution, the original compact 2D SCS was upgraded to a transparent 3D macro-chambered sheet (MCS) this was followed by the loading of gold nanoparticles (AuNPs) to form MCS-Au and by the self-assembly of methimazole (ME) over AuNPs, the new composite material MCS-Au@ME was obtained. This new material exhibited excellent antibacterial properties against four bacterial strains without the use of antibiotics. Animal testing indicated that the performance of MCS-Au@ME in the promotion of wound healing was superior to that of the commercial product, Mepitel® Soft Silicone Wound Contact Layer. The present study abandoned the conventional sophisticated and lengthy preparation process for silk biomaterials that involved dissolution and remodeling and also eliminated the inclusion of antibiotics to reduce the threat of antibiotic abuse—which has been an issue of concern in medical antibacterial applications. This study brings new insights into the selection and sustainable processing of silk for wound dressing.

Humans sweat to cool down and dissipate heat in indoor and outdoor environments. The evaporation of liquid sweat absorbs body heat and reduces skin temperature. However, when the wearer sweats a lot or is fully wet by external liquids such as rain, even the best moisture management fabrics can be soaked and saturated, becoming unbreathable, heavy, and clingy, and imposing significant reduction in wearer’s comfort and performance. Herein, inspired by liquid collection and release processes on the desert beetle’s skin, we develop an innovative moisture management fabric, transferring excessive sweat from the skin side to the outer surface of the fabric and dissipating it in terms of droplets, while blocking and repelling the external liquids. The completely unidirectional liquid transport is achieved in the developed nature-inspired fabric, which is treated with global hydrophobicity and discrete localized wettability gradient by a sustainable plasma treatment method. Moreover, the proposed fabric shows improved thermal comfort with high breathability, reduced clinginess, and dry skin surface. Specifically, a highly directional water transport capability of 1647.9% is obtained using a Moisture Management Tester, while the water resistance from the external side is equivalent to a water column of 15 mm height. The water flow rate from the inner skin side to the environment is equal to 91.44 g/min∙m2, which is much higher than that of the high sweating rate of an adult (i.e., 15 g/min∙m2). Regulation of liquid movement in the proposed fabric will attract wide potential applications, including sportswear, worker uniform, casualwear, protective clothing, firefighter apparel, and military clothing. Maintaining moisture and thermal comfort while adapting to the expanded ambient temperature range of the air-conditioning system will also be desirable in building energy conservation.

Textile manufacturing of silk generates unwindable raw silk fibers, which are treated as silk wastes for downcycling applications, thus unexpectedly demoting the luxury status of silk. As a primary trial seeking to reshape the destiny of silk wastes, the present study is focused on transforming natural silk nonwovens, as a typical model of artificial silk nonwovens, into bioadhesives for tissue repair. Herein, a technique is demonstrated for the preparation of a transparent and stretchable adhesive from a silkworm cocoon sheet (SCS), a typical natural silk nonwoven. This technique differs from the traditional method of completely dissolving silk fibers to obtain silk fibroin. Specifically, the technique entailed pretreatment of the SCS with a CaCl2-ethanol-H2O ternary solution to obtain a modified cocoon sheet (MCS), followed by surface modification with a CaCl2-formic acid (Ca-FA) solution to obtain MCS@Ca with controllable adhesion, which was achieved by adjusting the Ca2+ content in Ca-FA. The highly stretchable MCS@Ca firmly adhered to various substrates for loads as high as 54 kPa, and its performance in repairing an injured liver in vivo was superior to that of a commercial product, Sorbalgon®. Additionally, MCS@Ca effectively sealed a freshly punctured porcine heart and stomach ex vivo, thereby demonstrating its potential as a sealant. To our knowledge, this is the first trial study for on upcycling disqualified silk fibers using a convenient top-down approach to prepare robust bioadhesives for tissue repair, wherein MCS@Ca may serve to bridge the gap between advanced biomaterials and disqualified silk wastes.

Pulsating heat pipe (PHP) has drawn much attention due to its simple structure and high thermal performance. In this study, we proposed a novel PHP with separating walls within the flow channel. This could change the flow pattern of the liquid and vapor plugs, resulting the enhancement of the heat pipe thermal performance. To uncover the effect of the separating wall, we built a 2D numerical model of a single loop pulsating heat pipe to study the heat transfer mechanism of the proposed novel PHP. The effects of the separating wall locations and the PHP filling ratios were investigated at the bottom heating conditions. The results showed that the novel PHP had the best thermal performance at the filling ratio of 70% in our study and the thermal performance could be enhanced by 14% compared to the classical PHP without the separating wall. Besides, the separating wall located at the inner side of the heat pipe channel performed best at the filling ratios of 30% and 50%, whereas the separating wall located at the middle of the flow channel had the highest equivalent thermal conductivity at the filling ratio of 70%.

In this work, a simple but effective fractal model is proposed to predict the mechanical properties of complex porous composites. By extending the Mori-Tanaka and Double Inclusion models, we develop a computationally efficient algorithm based on the mean-field homogenization, to quickly determine the macroscopic elastic properties of the random porous media by the fractal theory. The random pore sizes and locations are properly characterized by a single fractal dimension, and the fractal model agrees well with the numerical results obtained by the finite element homogenization technique and the existing experimental data from literature. Moreover, the effect of pore size distribution has been investigated, indicating that the elastic properties decrease with the increase in the fractal dimension. In particular, the computation time can be reduced from 4 h for the finite element analysis to 15 s based on the fractal model. Based on the mathematically analogous governing equations, the present fractal modeling has great potential to determine the other significant physical properties such as thermal conductivity and coefficient of thermal expansion.

Water pollution, such as marine oil spill and organic chemicals pollution, is a worldwide challenge. To advance this emerging field, Janus membranes with asymmetric wettability, for oil-water separation, similitude of diode in electrocircuit, were fabricated by leveraging the buoyancy of Polyvinylidene fluoride (PVDF) nanofiber on water/air interface. ZnO nanowires are in situ grown on the surface of nanofibers immersed in the solution, mimicking a nanoscale Setaria viridis structure. Such Janus membrane showed a good hydrophilicity/underwater oleophobicity on ZnO nanowires modified side and exhibited opposite hydrophobicity on the other side. Due to the side-specific morphologies, this membrane can work either as “water-removing” type or “oil-removing” type filter as a diode in fluid. The membrane shows excellent permeability with the water and oil flux up to 1210 L m-2 h-1 and 7653 L m-2 h-1, respectively, only driven by gravity. The durability of functional membrane is excellent, and separation efficiency is higher than 97.24% after 10 cycling tests. Furthermore, the hydrophilic side demonstrated an excellent photocatalytic performance over the degradation of rhodamine B dye, as self-cleaning properties. This Janus membrane is to be an excellent candidate for wastewater remediation, both oil/water separation and organic pollutant decomposition.

The drying process of fibrous porous media such as textile, paper and filters is substantially significant to the manufacturing cost of these materials. Development of faster and inexpensive procedures for drying fibrous materials is an increasing need in modern industry, so understanding of the enhanced drying processes including the heat and mass transfer mechanisms is urgent. In particular, passive wicking-driven drying in fibrous porous materials has received increasing attention in broader applications including performance garments, heat pipes, electronics cooling and solar steam generation. In fibrous porous systems, the wicking or capillary filling is widely described by the Lucas-Washburn equation. However, the volatile fluid evaporates or dries up at the surface of fibrous porous structures, slowing and hindering the wicking process by deviating the flow dynamics from the Lucas-Washburn relationship. This chapter discusses the basic drying processes together with combined wicking and drying transport phenomena in fibrous porous materials. Optimization of fibrous porous structures for enhanced wicking and drying is also conducted and analyzed. The modeling and optimization results advance the understanding of heat and mass transfer in drying processes in fibrous porous media and will facilitate the engineering of fibrous architectures for various applications.

Personal thermal management, including warming, cooling, and adaptive thermoregulation of human body, is critical to the comfort and performance of individuals in daily life. Most people feel comfortable within a narrow range of temperature (i.e., between 20 °C and 27 °C) and relative humidity (between 35% and 60%). However, extremely hot or cold environments, dynamic climate changes, and excessive body heat and sweat produced from intensive human activities can overwhelm the coping mechanisms of physiological thermoregulation, causing serious heat or cold stress and even fatal conditions. Over the past decade, thermally conductive composites have stimulated remarkable attention to the facilitation of personal thermal management by integrating advanced composite materials and structures for enhanced and programmable warming and cooling effects. In this review, we discuss the developments in personal thermal management using thermally conductive composites, with a focus on mechanistic models and engineering strategies for thermal conduction. We explore the diverse implementations for improving the cooling and warming performance from fibers and yarns to fabrics and the latest wearable technologies. Current challenges and research gaps are summarized, in order to provide a road map for future research opportunities.

Personal thermal management systems (PTMS) are highly desirable for improving individual thermal comfort and reducing indoor HVAC energy consumption. Although there have been many attempts in developing PTMS, existing PTMS are generally bulky, heavy or immobile. Besides, previous analysis on additional personal cooling and heating power required for maintaining thermal comfort in changing environments did not consider the individual difference in metabolism and the variability of comfort skin temperature with environmental conditions. Here, we report on the development of a novel lightweight (

The clothing air gap between the skin surface and clothing layer plays an important role in the body heat dissipation. In this study, we built a numerical model to investigate the heat transfer phenomenon through the clothing air gap in a single layer loose-fitting garment with large air gaps. Firstly, we investigated the thermal performance of the static air gaps with closed ends and open ends, respectively, against their thickness and height at different environmental temperatures. Then, the dynamic air gaps with oscillating clothing layers were studied to examine the influence of the so called pumping effect (viz. the oscillation of clothing layer) on the heat transfer through the air gaps. The results showed that closing the ends of the static air gap (i.e. simulation of closing the openings of clothing) can double the thermal resistance when the air gap thickness is sufficiently large. On the other hand, the pumping effect, which may be caused by body motion or external forces, can halve the thermal resistance of the air gap with open ends in some conditions. This study is of great importance for the design of functional clothing for human body thermal management.

On the basis of pulsed thermography, this article presents the development of a quantitative detection method for sub-surface defects. A two dimensional gapped smoothing algorithm (GSA) applied in this method to process the surface thermal distribution and no reference information is needed. A new thermal contrast is defined and a damage index based on the thermal contrast is proposed to estimate the probability of location of a defect, and differential curves of thermal contrast are applied for further quantification of defect size. One aluminium plate and one glass fibre reinforced composite laminate containing different depths of flat-bottom holes, delaminations and impurities are introduced to assess the performance of proposed method. Numerical and experimental results indicate that the influence of non-uniform heating is clearly suppressed. Compared with the differentiated absolute contrast approach, distinct enhancement of thermal contrast is achieved, and the location and size of sub-surface insta...

In this work, the longitudinal permeability of squarely packed dual-scale fiber preforms is studied theoretically. These fiber preforms are composed of aligned porous tows and the tows are tightly packed. The effective permeability is calculated as a parallel-like network of intra-tow permeability and inter-tow permeability, which are quantified by Darcy’s law and the inscribed radius between tows, respectively. The jump velocity at the interface between inter-tow fluids and porous tows is considered, as derived by substituting Beavers and Joseph’s correlation into Brinkman’s equation. We further examine the effects of intra-tow permeability on the effective permeability of the fibrous system with three interface conditions: (1) interface velocity = 0, (2) interface velocity = mean intra-tow velocity, and (3) interface velocity = jump velocity. The jump-velocity-based model is found to be closest to numerical data. The influence of the fiber volume fraction of tows on the effective permeability is also analyzed.

Cell encapsulation has been shown to hold promise for effective, long-term treatment of type 1 diabetes (T1D). However, challenges remain for its clinical applications. For example, there is an unmet need for an encapsulation system that is capable of delivering sufficient cell mass while still allowing convenient retrieval or replacement. Here, we report a simple cell encapsulation design that is readily scalable and conveniently retrievable. The key to this design was to engineer a highly wettable, Ca2+-releasing nanoporous polymer thread that promoted uniform in situ cross-linking and strong adhesion of a thin layer of alginate hydrogel around the thread. The device provided immunoprotection of rat islets in immunocompetent C57BL/6 mice in a short-term (1-mo) study, similar to neat alginate fibers. However, the mechanical property of the device, critical for handling and retrieval, was much more robust than the neat alginate fibers due to the reinforcement of the central thread. It also had facile mass transfer due to the short diffusion distance. We demonstrated the therapeutic potential of the device through the correction of chemically induced diabetes in C57BL/6 mice using rat islets for 3 mo as well as in immunodeficient SCID-Beige mice using human islets for 4 mo. We further showed, as a proof of concept, the scalability and retrievability in dogs. After 1 mo of implantation in dogs, the device could be rapidly retrieved through a minimally invasive laparoscopic procedure. This encapsulation device may contribute to a cellular therapy for T1D because of its retrievability and scale-up potential.

The speed of capillary flow is a key bottleneck in improving the performance of nanofluidic and microfluidic devices for various applications including microfluidic diagnostics, thermal management heat pipes, micromolding devices, functional fabrics, and oil–water separators. Here, we present a novel nanofibrous or microfibrous hollow-wedged channel (named as W-Channel), which can significantly speed up the capillary flow. The capillary flow in the initial 100 s in the nanofibrous W-Channel was shown to be 8 times faster than that in the single-layer strip of the same material when placed vertically and over 20 times faster when placed horizontally. The enhanced flow under gravity is attributed to the adaptive interplay of capillary pressure and flow resistance within the triangular hollow wedge between the fibrous layers. The W-Channel can be fabricated following a simple procedure using inexpensive materials such as electrospun nanofibers or microfibrous filter papers.

In this work, we theoretically revisit the longitudinal permeability of aligned fiber arrays from ordered configuration (i.e. rectangular and staggered arrangements) to random pattern, based on a geometrical scaling rule. The scaling model on the basis of the characteristic length and the characteristic ratio quantifies the flow behaviors more accurately than the widely applied Kozeny-Carman equation. The model for actual fiber arrays composed of randomly located fibers is extended from the ordered case, and the randomness of fiber distribution is realized by the Voronoi tessellation method and quantified by a single parameter. The proposed compact and easy-use model is verified by experimental and numerical results throughout the range of fiber volume fractions (FVFs). The structural parameters, including FVF, fiber packing angle, and randomness of fiber distribution are also extensively analyzed.

Although the basic principles of gas flow through unidirectional fibers have been widely studied and well understood since the 1950s, questions arise when these principles are applied to electrospun polymer nanofibers. Classic theories based on orderly packed coarse fibers are inadequate in accounting for the influences of random fiber distribution and slip flow. In this work, a mechanistic model in terms of fiber volume fraction and fiber radius is presented to determine the through-plane permeability of electrospun nanofiber layers. The fibrous system is subdivided into a series of cells of orthogonal fibers with random volumes. A single factor is proposed to quantify the effect of randomness of fiber distribution on flow behaviors. When the fiber radius is comparable with the mean free path of air molecules, the slip flows in the nanoscale fibrous media are particularly explored. The solutions obtained are successfully validated through comparison with experimental and numerical results. It is demonstrated that the through-plane permeability of electrospun nanofibers is enhanced by the slip effect and randomly distributed fibers are more permeable than ordered structures.

When capillary flow occurs in a uniform porous medium, the depth of penetration is known to increase as the square root of time. However, we demonstrate in this study that the depth of penetration in multi-section porous layers with variation in width and height against the flow time is modified from this diffusive-like response, and liquids can pass through porous systems more readily in one direction than the other. We show here in a model and an experiment that the flow time for a negative gradient of cross-sectional widths is smaller than that for a positive gradient at the given total height of porous layers. The effect of width and height of local layers on capillary flow is quantitatively analyzed, and optimal parameters are obtained to facilitate the fastest flow.

Porous materials engineered for rapid liquid absorption are useful in many applications, including oil recovery, spacecraft life-support systems, moisture management fabrics, medical wound dressings, and microfluidic devices. Dynamic absorption in capillary tubes and porous media is driven by the capillary pressure, which is inversely proportional to the pore size. On the other hand, the permeability of porous materials scales with the square of the pore size. The dynamic competition between these two superimposed mechanisms for liquid absorption through a heterogeneous porous structure may lead to an overall minimum absorption time. In this work, we explore liquid absorption in two different heterogeneous porous structures [three-dimensional (3D) circular tubes and porous layers], which are composed of two sections with variations in radius/porosity and height. The absorption time to fill the voids of porous constructs is expressed as a function of radius/porosity and height of local sections, and the absorption process does not follow the classic Washburn's law. Under given height and void volume, these two-section structures with a negative gradient of radius/porosity against the absorption direction are shown to have faster absorption rates than control samples with uniform radius/porosity. In particular, optimal structural parameters, including radius/porosity and height, are found that account for the minimum absorption time. The liquid absorption in the optimized porous structure is up to 38% faster than in a control sample. The results obtained can be used a priori for the design of porous structures with excellent liquid management property in various fields.

In gas diffusion layers (GDLs) of proton exchange membrane fuel cells (PEMFCs), effective permeability is a key parameter to be determined and engineered. In this study, through-plane (TP) and in-plane (IP) flow behaviors of GDLs are investigated analytically based on a scaling estimate method. The TP permeability and IP permeability of unidirectional fibers are determined first, based on that the minimum distance and the inscribed radius between fibers are adopted as the characteristic lengths for normal and parallel flows, respectively. The permeabilities of two-dimensional (2D) and three-dimensional (3D) GDLs are estimated by a proper mixture of the local TP and IP permeabilities of fiber alignments. The mechanistic model agrees closely with experimental and numerical results over a wide porosity range. With the new model, the influences of porosity and fiber orientation on flow behaviors are analyzed.

Gas diffusion in nanofibrous and microfibrous materials is of great interest in microfluidics. In this work, an analytical model is proposed, based on fractal theory, to quantify gas diffusion across fibrous media composed of nanofibers and microfibers. The fractal model is expressed in terms of pore area and tortuosity fractal dimensions, allowing statistical quantification of the geometrical structures of fibrous media. Knudsen diffusion in nanoscale pores is considered. To validate this model, moisture vapor diffusion rate through electrospun nanofibrous webs was measured using the inverted-cup method. The diffusivities predicted from the proposed model agree well with the experimental measurements in the present investigation and those reported in the literature for effective diffusivities of gas diffusion layers in fuel cells. Based on the model, the effect of porosity, fiber radius, and the ratio between the minimum and the maximum pore sizes on the effective diffusivity is analyzed.

In gas diffusion layers (GDLs) of proton exchange membrane fuel cells (PEMFCs), effective gas diffusivity is a key parameter to be determined and engineered. Existing theoretical models of effective diffusivity are limited to one-dimensional (1D) regular fiber arrays. Numerical simulations were carried out to simulate gas diffusion through more realistic fibrous materials like GDLs, in which fibers are randomly distributed in a two-dimensional (2D) plane or three-dimensional (3D) space, but they could not fully reveal the underlying mechanisms. In this paper, we propose an analytical model to predict the effective diffusivities of 1D, 2D and 3D randomly distributed fiber assembles. The present model is established by extending the model of 1D regular fiber alignments to 1D random fiber arrangements through Voronoi Tessellation method, and using the 1D local diffusivities to determine the 2D and 3D diffusivities based on mixing rules. The predicted effective diffusivities agree well with experimental results and numerical data. With the new model, the influences of porosity, fiber distribution, and fiber orientation are analyzed in this study.

Transverse flow through aligned yarns with two length scales is theoretically studied. Darcy’s law and Stokes equation are employed to describe flow behaviors inside the porous yarns and in the open channels between yarns, respectively. Beavers and Joseph’s semi-empirical model and Brinkman’s extension of Darcy’s law are used to characterize the jump-velocity and continuum-stress boundary condition at the interface layer between the clear fluid and the porous yarn. The analytical model for predicting permeability is developed as a function of porosity, fiber radius, fiber cross-sectional shape, and fiber packing pattern, which would help improve the physical understandings of dual-scale flows in fibrous media. Additionally, a simple but effective semi-analytical model is provided for easy use. Permeability predictions calculated from our model agree fairly well with experimental and numerical results in literature.

In this study, the hydraulic permeability for viscous flow through fibrous media of high porosity is investigated theoretically. Fibrous media in one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D) structure are approximated as consisting of repetitive unit cells based on Voronoi Tessellation approximation and volume averaging method. In the new model, the hydraulic permeability of fibrous media is described as a function of porosity and fiber radius as well as geometrical formation factors including the degree of randomness (viz. randomness of fiber distribution) and fiber orientation. In particular, the slip effect for flow through superfine fibrous media is analytically studied. The prediction of the new model for fibrous media with porosity greater than 0.7 is highly consistent with the analytical, experimental and numerical results found in the literature. It is further demonstrated that randomly packed fibrous media have larger permeability than regular ones, the hydraulic permeability of fibrous media increases with increasing of through-plane orientation, but is less dependent on in-plane fiber orientation, and the slip effect on the longitudinal hydraulic permeability is greater than the perpendicular one.