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The scarcity of drinking water and the contamination of water sources in underdeveloped countries are serious problems that require immediate low-tech and low-cost solutions. In this study, we fabricated polyacrylonitrile (PAN) porous membranes coated with silver nanoparticles (AgNP) and demonstrated their use for water filtration and water treatment applications. The membranes were prepared by electrospinning a PAN solution and treating in a hydroxylamine (NH2OH) aqueous solution to form &ndash, C(NH2)N&ndash, OH groups that were used for functionalization (Ag+ ions) of the membrane. The coordinated silver ions were then converted to silver nanoparticles. The microstructure of the membrane, water permeability, antimicrobial effect (using Escherichia coli), and particulate filtration capabilities were studied. This study verified that the membrane demonstrated a 100% reduction for Gram-negative bacteria with an effective filtration rate of 8.0 mL/cm2 min. Furthermore, the membrane was able to eliminate 60% of latex beads as small as 50 nm and over 80% of the 2 &micro, m beads via gravity filtration. This study demonstrated that PAN&ndash, AgNP membranes can be employed as antimicrobial membranes for the filtration of water in underdeveloped countries.

The aim of this study was to demonstrate the use of electrospinning to produce hierarchical fibrous structures for antibacterial and antifungal applications. Silver nanoparticles (Ag NPs) are produ...

This study aims to produce a hydrophobic polymer film by mimicking the hierarchical micro/nanostructures found on the surface of rose petals. A simple and two-step UV-based nanoimprint lithography was used to copy rose petal structures on the surface of a polyurethane acrylate (PUA) film. In the first step, the rose petal was used as a template, and its negative replica was fabricated on a commercial UV-curable polymer film. Following this, the negative replica was used as a stamp to produce rose petal mimetic structures on UV curable PUA film. The presence of these structures on PUA influenced the wettability behavior of PUA. Introducing the rose petal mimetic structures led the inherently hydrophilic material to display highly hydrophobic behavior. The neat PUA film showed a contact angle of 65°, while the PUA film with rose petal mimetic structures showed a contact angle of 138°. Similar to natural materials, PUA with rose petal mimetic structures also displayed the water pinning effect. The water droplet was shown to have adhered to the surface of PUA even when the surface was turned upside down.

Strong and tough structural materials occurring in nature have long fueled the search for an advanced class of strong biomimetic synthetic structural materials (SSMs). One such example in recent ye...

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Lightweight photovoltaics (PV) modules are important for certain segments of the renewable energy markets—such as exhibition halls, factories, supermarkets, farms, etc. However, lightweight silicon-based PV modules have their own set of technical challenges or concerns. One of them, which is the subject of this paper, is the lack of impact resistance, especially against hailstorms in deep winter in countries with four seasons. Even if the front sheet can be made sufficiently strong and impact-resistant, the silicon cells inside remain fragile and very prone to impact loading. This leads to cracks that significantly degrade performance (output power) over time. A 3D helicoidally architected fiber-based polymer composite has recently been found to exhibit excellent impact resistance, inspired by the multi-hierarchical internal structures of the mantis shrimp’s dactyl clubs. In previous work, our group demonstrated that via electrospinning-based additive manufacturing methodologies, weak polymer material constituents could be made to exhibit significantly improved toughness and impact properties. In this study, we demonstrate the use of 3D architected fiber-based polymer composites to protect the silicon solar cells by absorbing impact energy. The absorbed energy is equivalent to the energy that would impact the solar cells during hailstorms. We have shown that silicon cells placed under such 3D architected polymer layers break at substantially higher impact load/energy (compared to those placed under standard PV encapsulation polymer material). This could lead to the development of novel PV encapsulant materials for the next generation of lightweight PV modules and technology with excellent impact resistance.

Electrospun fibers have shown tremendous potential for advanced development of high-performance composites. Large surface areas afforded by the smaller-diameter fibers have the potential to promote close interaction with the matrix leading to the fabrication of stronger and tougher composites. A considerable amount of work has been carried out over the past decade or so to use electrospun fibers for composite reinforcement. This chapter reviews some of the recent advances made in using electrospinning for the development of next-generation fiber-reinforced composites. Additionally, the potential of electrospinning is also presented for mimicking helicoidal fibrous structures identified in the exoskeleton of some of the species such as mantis shrimp and lobsters. Most of these naturally occurring helicoidal composites are a combination of two phases, namely fibers and matrix. The fiber material and its properties play a very important role in enhancing the strength and toughness of resulting composites. Many attempts have been made in the past to produce such helicoidal architectures, mostly at a macroscopic range. In this chapter, the potential of electrospinning to fabricate such fiber layout is particularly highlighted. These helicoidal fibrous structures produced using electrospinning can then be embedded into a matrix to create bioinspired and impact-resistant composites.

This study used melt-electrospinning writing to fabricate three-dimensional fiber constructs by embedding them in a polyvinyl alcohol (PVA) matrix to obtain thin composite films. Fourier transform infrared spectroscopy (FTIR) and dynamic scanning calorimetry (DSC) were used to demonstrate an interaction between the polycaprolactone (PCL) fibrous phase and the PVA matrix phase. Following this, the mechanical deformation behavior of the composite was investigated, and the effect of reinforcement with three-dimensional fibrous constructs was illustrated. The specific strength of the composite was found to be five times higher than the specific strength of the neat PVA matrix. Additionally, the specific toughness of the composite was determined to be roughly four times higher than the specific toughness determined for the neat PVA matrix. These results demonstrate the potential of using melt-electrospinning writing for producing three-dimensional fibrous constructs for composite reinforcement purposes.

There has been increased interest to develop protective fabrics and clothing for protecting the wearer from hazards such as chemical, biological, heat, UV, pollutants etc. Protective fabrics have been conventionally developed using a wide variety of techniques. However, these conventional protective fabrics lack breathability. For example, conventional protective fabrics offer good protection against water but have limited ability in removing the water vapor and moisture. Fibers and membranes fabricated using electrospinning have demonstrated tremendous potential to develop protective fabrics and clothing. These fabrics based on electrospun fibers and membranes have the potential to provide thermal comfort to the wearer and protect the wearer from wide variety of environmental hazards. This review highlights the emerging applications of electrospinning for developing such breathable and protective fabrics.

When silicon solar cells are used in the novel lightweight photovoltaic (PV) modules using a sandwich design with polycarbonate sheets on both the front and back sides of the cells, they are much more prone to impact loading, which may be prevalent in four-season countries during wintertime. Yet, the lightweight PV modules have recently become an increasingly important development, especially for certain segments of the renewable energy markets all over the world—such as exhibition halls, factories, supermarkets, farms, etc.—including in countries with harsh hailstorms during winter. Even in the standard PV module design using glass as the front sheet, the silicon cells inside remain fragile and may be prone to impact loading. This impact loading has been widely known to lead to cracks in the silicon solar cells that over an extended period of time may significantly degrade performance (output power). In our group’s previous work, a 3D helicoidally architected fiber-based polymer composite (enabled by an electrospinning-based additive manufacturing methodology) was found to exhibit excellent impact resistance—absorbing much of the energy from the impact load—such that the silicon solar cells encapsulated on both sides by this material breaks only at significantly higher impact load/energy, compared to when a standard, commercial PV encapsulant material was used. In the present study, we aim to use numerical simulation and modeling to enhance our understanding of the stress distribution and evolution during impact loading on such helicoidally arranged fiber-based composite materials, and thus the damage evolution and mechanisms. This could further aid the implementation of the lightweight PV technology for the unique market needs, especially in countries with extreme winter seasons.

Temperature triggered switchable cellulose acetate-poly(N-isopropylacrylamide) (CA-PNIPAM) core-shell and blend nanofibers are fabricated for controlled moisture harvesting applications. Core-shell fibers are fabricated using a co-axial electrospinning setup whereas the conventional electrospinning setup is employed for fabricating the blend fibers. Investigation of their wettability behaviour demonstrated that the blend fibers are superhydrophilic whereas the core-shell fibers are hydrophilic at ambient temperature. Furthermore, both the samples have an ability to switch between the two states viz. hydrophilic to hydrophobic state based on thermal stimulus. The core-shell fibers are shown to have higher moisture sorption ability compared to the blend fibers. This study investigates the mechanism behind the switchable wettability behaviour of the core-shell fibers and demonstrates the crucial role played by the functional groups present on the surface layer of fibers in governing their moisture collection efficiency.

A rod-like hollow graphitic carbon shell with nitrogen doping was fabricated via a template-assisted growth for enhanced capacitive deionization. The silica nanorods prepared as the templates have uniform structures, which can effectively avoid aggregation. Nitrogen-doped graphitic carbon is favourable for higher electrical conductivity. The hollow structure obtained after etching provides a large ion-accessible surface area. The resultant rod-like nitrogen-doped carbon hollow shells have a hierarchically porous architecture with high electrical conductivity and a beneficial porosity. In this work, the morphology, structure, specific surface area, and porosity were carefully investigated using SEM, TEM, XRD, Raman, BET and BJH methods. Electrochemical test and electrosorptive test were conducted to analyse the specific capacitance, electrosorption capacitance and cycle ability. The largest electrosorption capacity was achieved as 16.72 mg g−1 at 1.6 V in 250 mg g−1 solution. Moreover, the adsorption kinetics were investigated using Lagergeren’s model. Overall, the nitrogen-doped carbon hollow shell is demonstrated as a promising candidate of electrode material for water desalination via capacitive deionization.

In this paper, we used electrospinning to fabricate electret polyvinylidene fluoride (PVDF) membranes that show tremendous potential for its use as a high-performance air filter medium. A novel charge storage enhancer by means of nanoscale graphite platelets (NGP) dispersed within the fibrous matrix and the filtration performance were investigated. The surface potential of as-spun membrane was characterized to demonstrate the role played by NGP in imparting high electrostatic force. Air filtration tests were performed to evaluate the performance of our membranes under a high face velocity (1.26 m/s). The composite membrane demonstrated a high filtration efficiency of 98.989% with low pressure drop as 1279 Pa and 3.591 kPa−1 quality index. A critical loading of NGP within the fibers is determined that is required to improve both filtration efficiency and pressure drop. Similar loading of NGP within the fibers is also required to improve the surface potential of the composite membranes.

This study presents a simple and effective technique for fabricating hierarchical structures for dry adhesive applications. The study employed electrospinning and post-template wetting method to fabricate hierarchical structures. This technique enabled fabrication of nanometer-sized pillars on the surface of the fibers. The shear adhesion strength of these structures was investigated using a custom-designed adhesion measurement setup attached to a tensile tester. Shear adhesion measurements were performed by sliding the samples along the glass slide. A constant preload was applied during the measurement. The obtained load versus displacement curves were then used to determine the adhesion behavior of the samples. Samples with two different aspect ratio nanopillars were used to investigate the effect of aspect ratio on the adhesion performance of the samples. The shear adhesion strength was higher for samples with higher aspect ratio nanopillars on its surface. Adhesion strength as high as 0.161 N/cm2 was obtained demonstrating their use for dry adhesive applications.

Membrane-based capacitive deionization (MCDI) is a promising method for desalination that is expected to alleviate the problem of water shortage. Typical CDI processes show limited efficiency in salt removal and experimental results suggest that an improvement in electrode materials could be the cornerstone of MCDI. In this study, bimetallic metal–organic frameworks (BMOFs) have been used as precursors to develop porous carbons for MCDI electrodes. BMOFs are a unique class of MOFs that possess two types of metal ions with properties dependent on the ratio of the metal ions. In our work, we have synthesized a series of BMOFs with different molar ratios of Zn and Co based on ZIF-8 and ZIF-67. By controlling the molar ratio between Zn and Co, we are able to synthesize BMOF-derived porous carbons with different particle sizes and graphitization degrees. Furthermore, the porous carbon derived from a BMOF of Zn : Co = 3 : 1 exhibits a superior salt removal capacity of 45.62 mg g−1 in a 750 mg L−1 NaCl solution at 1.4 V. We attribute this performance to the large ion-accessible surface area and improved electrical conductivity of the carbon derived from the BMOF.

This paper presents the design, modeling, and analysis of the force behavior acting on a wheel-legs (whegs) type robot which utilizes bilayer dry adhesives for wall-climbing. The motion of the robot is modeled as a slider-crank mechanism to obtain the dynamic parameters of the robot during movement. The required forces and moment to maintain equilibrium as the robot is in motion is then extensively analyzed and discussed. Following the analysis, fundamental measures to attain an operative climbing robot, such as adhesive requirement and torque specification, are then identified. The outcomes of the analysis are verified through experiments and working prototypes that are in good agreement with the design guidelines.

We report the fabrication of hierarchical poly(vinylidene fluoride) (PVDF) fibrous structures using a unique fabrication technique based on electrospinning. Electrospinning was used to fabricate aligned PVDF fibrous membranes. These membranes were then brought in contact with anodized aluminum oxide (AAO) template and then heat-treated above the glass transition temperature (T g) of the polymer to assist the flow of polymer within the cylindrical pores of AAO template. Scanning electron microscopy images confirmed that this approach lead to the growth of nanopillars on the surface of PVDF fibers. Nanoindenter was used to measure the pull-off force that was required to completely detach the indenter from the samples. To investigate the effect of hierarchy, pull-off force required to detach the indenter from neat PVDF fibers was determined and compared with the pull-off force recorded for hierarchical fibers. The effect of indentation depth was also investigated on both PVDF fibers and PVDF fiber with nanopillars. Significant pull-off force recorded indicates that these PVDF hierarchical fibrous structures can be potentially used for dry-adhesive applications.

eparation of low viscous oil from water has attracted immense attention in recent times due to the ever-increasing amount of oily industrial wastewater discharge and frequent oil spill accidents. Hence, there is a persistent demand for the fabrication of robust oil–water separation membranes. Herein, robust oil–water separation membranes are successfully fabricated by direct electrospinning of poly(vinylidene fluoride-co-hexafluoropropylene) and fluorinated polyhedral oligomeric silsesquioxane composite mixture. These hybrid membranes exhibit differential wetting (highly hydrophobic/superoleophilic) behavior for water and oil. The contact angle made by water and low viscous oil (hexane) with the membrane are measured to be 145 and 0° respectively. The nanofiber membranes efficiently separate low viscous oil from water in a single-step with separation efficiency of nearly 100%. Furthermore, the results demonstrate that the membranes are robust and durable exhibiting differential wettability even after several oil–water separation cycles. The results reveal the potential of their use for real-time industrial wastewater treatment applications.

Poly(N-isopropylacrylamide) (PNIPAM) has been used extensively for numerous biomedical applications. However, there is not enough information in the literature on the wettability and hygroscopic properties of electrospun PNIPAM fibers, relevant for water harvesting applications. This study focuses on investigating the wettability and hygroscopic properties of electrospun PNIPAM fibers at room temperature and elevated temperature. The wettability properties of electrospun PNIPAM fibers were compared to spin-coated PNIPAM thin films. The wettability properties of the electrospun fibers were enhanced by 56% compared to spin-coated films. Water contact angle (WCA) measured on electrospun fibers was determined to be 137° at elevated temperatures while WCA on spin cast PNIPAM film was determined to be 81° at elevated temperatures. Furthermore, hygroscopic properties of the electrospun PNIPAM fibers were studied using thermogravimetric analysis (TGA). The PNIPAM fibers are seen to exhibit moisture absorption capacity of about 16.6 wt. % under humid conditions.

In this study, we used electrospinning combined with template wetting method to fabricate hierarchical poly(methyl methacrylate) (PMMA) fibrous structures. The adhesion performance of these samples was investigated using a nanoindenter. A flat circular diamond indenter of 10 μm diameter was brought in contact with the sample and then retracted back. The force required to detach the indenter from the sample was determined to be the pull-off force. The effect of indentation depth on the pull-off force was also investigated. Following this, an empirical relationship to predict the pull-off strength (σ) was established for a given fibril radius (r), fibril height (l), preload (p), and effective Young's modulus (E*). The pull-off force values recorded for hierarchical PMMA fibrous structure were also used to validate the empirical relationship. The empirical relationship demonstrated good correlation between the recorded pull-off strength and system parameters. We believe that this empirical relationship will be helpful in designing high strength synthetic dry-adhesives as the relationship can be used to predict the pull-off strength a priori.

This paper presents the design, modeling, and analysis of the force behavior acting on a wheel-legs (whegs) type robot which utilizes bilayer dry adhesives for wall-climbing. The motion of the robot is modeled as a slider-crank mechanism to obtain the dynamic parameters of the robot during movement. The required forces and moment to maintain equilibrium as the robot is in motion is then extensively analyzed and discussed. Following the analysis, fundamental measures to attain an operative climbing robot, such as adhesive requirement and torque specification, are then identified. The outcomes of the analysis are verified through experiments and working prototypes that are in good agreement with the design guidelines.

Nature has taught us fascinating strategies to design materials such that they exhibit superior and novel properties. Shells of mantis club have protein fibres arranged in a 3D helicoidal architecture that give them remarkable strength and toughness, enabling them to absorb high-impact energy. This complex architecture is now possible to replicate with the recent advances in additive manufacturing. In this paper, we used melt electrospinning to fabricate 3D polycaprolactone (PCL) fibrous design to mimic the natural helicoidal structures found in the shells of the mantis shrimp’s dactyl club. To improve the tensile deformation behavior of the structures, the surface of each layer of the samples were treated with carboxyl and amino groups. The toughness of the surface-treated helicoidal sample was found to be two times higher than the surface-treated unidirectional sample and five times higher than the helicoidal sample without surface treatment. Free amino groups (NH2) were introduced on the surface of the fibres and membrane via surface treatment to increase the interaction and adhesion among the different layers of membranes. We believe that this represents a preliminary feasibility in our attempt to mimic the 3D helicoidal architectures at small scales, and we still have room to improve further using even smaller fibre sizes of the modeled architectures. These lightweight synthetic analogue materials enabled by electrospinning as an additive manufacturing methodology would potentially display superior structural properties and functionalities such as high strength and extreme toughness.

Miniaturization of robotic systems has led to a demand for an alternative adhesive for use as the footpad of robots, with primary requirements of minimizing energy expenditure and satisfying performance and operational scenarios such as surveillance and reconnaissance. Inspired by nature, the dry adhesive concept as seen in climbing lizards such as the gecko has drawn significant interest from researchers. Adhesion in geckos is attributed to micro/nano fibrils found on its feet that rely on van der Waals forces to adhere to a surface, hence the terminology of dry adhesive. While immense progress has been made in the design and fabrication of multiscale hierarchical adhesive structures, the robustness, durability and endurance (ability to adhere to surfaces for an extended period of time) of gecko-foot mimetic dry adhesives still lags behind their biological counterparts. In this review article, we highlight the design considerations for the development of robust and durable bio-inspired synthetic adhesives. Current challenges and future directions are also highlighted for the design and development of robust and durable dry adhesive structures.

The unusual ability of geckos to climb vertical walls underlies a unique combination of a hierarchical structural design and a stiffer material composition. While a dense array of microscopic hierarchical structures enables the gecko toe pads to adhere to various surfaces, a stiffer material (β-keratin) composition enables them to maintain reliable adhesion over innumerable cycles. This unique strategy has been seldom implemented in engineered dry adhesives because fabrication of high-aspect-ratio hierarchical structures using a stiffer polymer is challenging. Herein, we report the fabrication of high-aspect-ratio hierarchical arrays on flexible polycarbonate sheets (stiffness comparable to that of β-keratin) by a sacrificial-layer-mediated nanoimprinting technique. Dry-adhesive films comprising the hierarchical arrays showed a formidable shear adhesion of 11.91 ± 0.43 N/cm2. Cyclic adhesion tests also showed that the shear adhesion of the adhesive films reduced by only about 20% after 50 cycles and remained nearly constant until about 200 cycles. Most importantly, the high-aspect-ratio hierarchical arrays were integrated onto the feet of a miniature robot and the locomotion on a 30° inclined surface was demonstrated.

We employed a novel picoindenter (PI)/scanning electron microscopy (SEM) technique to measure the pull-off force of an individual electrospun poly(vinylidene fluoride) (PVDF) fibers. Individual fibers were deposited over a channel in a custom-designed silicon substrate, which was then attached to a picoindenter. The picoindenter was then positioned firmly on the sample stage of the SEM. The picoindenter tip laterally pushed individual fibers to measure the force required to detach it from the surface of substrate. SEM was used to visualize and document the process. The measured pull-off force ranged between 5.8 ± 0.2 μN to ~17.8 ± 0.2 μN for individual fibers with average diameter ranging from 0.8 to 2.3 μm. Thus, this study, a first of its kind, demonstrates the use of a picoindenter to measure the pull-off force of a single micro/nanofiber.

In this study, we used core-shell electrospinning to fabricate cellulose acetate-poly(N-isopropylacrylamide) (CA-PNIPAM) fibrous membranes and demonstrated the ability of these fibers to capture water from a high humid atmosphere and release it when thermally stimulated. The wettability of the fibers was controlled by using thermoresponsive PNIPAM as the shell layer. Scanning electron and fluorescence microscopes are used to investigate the microstructure of the fibers and confirm the presence of the core and shell phases within the fibers. The moisture capturing and releasing ability of these core-shell CA-PNIPAM fibers was compared with those of the neat CA and neat PNIPAM fibers at room temperature as well as at an elevated temperature. At room temperature, the CA-PNIPAM core-shell fibers are shown to have the maximum moisture uptake capacity among the three samples. The external temperature variations which trigger the moisture response behavior of these CA-PNIPAM fibers fall within the range of typical day and night cycles of deserts, demonstrating the potential use of these fibers for water harvesting applications.

Here, we combine electrospinning and replica-molding to produce hierarchical poly(methyl methacrylate) structures and investigate its adhesion behavior. Normal and shear adhesion of these biomimetic hierarchical structures was measured using nanoindentaton and a custom-built apparatus attached to Zwick tensile testing machine, respectively. Shear adhesion was measured by sliding the samples along the glass slide under a predefined normal preload. Normal adhesion was measured by indenting the surface of the sample with the help of a diamond indenter tip and retracting it back to determine the pull-off force needed to detach it from the sample. These experiments were also conducted on neat PMMA fibers to investigate the effect of hierarchy on the adhesion performance of the samples. Our results show that the shear adhesion strength and pull-off forces recorded for the hierarchical samples are higher than those recorded for neat fibers.

Electrospinning is increasingly used as a simple and straightforward technique to fabricate one-dimensional fibers from both organic and inorganic materials. These one-dimensional fibers with controlled sizes possess some unique features such as large surface area to volume ratio, high porosity, and low density. Compared to other conventional materials, these features make them attractive for applications such as energy harvesting, energy storage, super-hydrophobic membranes, and sensors. This chapter provides an overview on the synthesis of inorganic fibers through polymer-mediated electrospinning. Some of the common techniques employed by many researchers, such as solgel combined with electrospinning, emulsion electrospinning, and electrospinning combined with solid–gas reaction, to fabricate metal oxide fibers are discussed. In addition, techniques to fabricate ceramic and metal oxide fibers having different morphologies and hierarchical structures are described. Recent applications of electrospun metal oxide fibers are finally highlighted with a focus on filtration, sensors, photocatalysis, and energy.

Natural extracellular matrix (ECM) provides a model for designing and developing synthetic tissue scaffolds intended for tissue repair and regeneration. Among all processing techniques, electrospinning has received considerable amount of interest due to its simple and versatile approach to produce nonwoven membranes that resemble the ECM. The ability to produce micro/nano materials with various compositions, fiber sizes and fiber arrangements make electrospinning a versatile technique for fabricating biomimetic structures for tissue engineering applications. These features ensure that the biological, chemical, and mechanical properties of the scaffolds can be controlled. Through this review, we illustrate some common electrospinning techniques used for producing composite fibers and highlight their use for tissue scaffold applications.

The micro- and nano-scale fibrillar structures found on the feet of spiders and geckos function as adhesion devices which allow them to adhere to both molecularly smooth and rough surfaces. This adhesion has been argued to arise from intermolecular forces, such as van der Waals (vdW) force, acting at the interface between any two materials in contact. Thus, it is possible to mimic their adhesion using synthetic nanostructured analogs. Herein, we report the first successful pull-off force measurements on a single electrospun fiber and show the potential of using electrospinning to fabricate adhesive analogs. A single fiber is glued to the atomic force microscope cantilever, and its adhesion to a metal substrate is studied by recording the pull-off force/displacement curves. The measured adhesive force of ~18 nN matches closely that of their biological counterparts. Similar to natural structures, the adhesive mechanism of these electrospun structures is controlled by vdW interactions.

We use electrospinning to obtain polyvinylidene fluoride (PVDF) fibers and demonstrate simultaneous improvements in β-crystal microstructure and in tensile properties of fibers with reduction of their diameter. PVDF fibers with average diameters ranging from 70 to 400 nm are obtained by controlling the concentration of the polymer in the electrospinning solution. The amount of β-crystals present is found to be greater for finer diameter fibers, yielding a maximum β-phase fraction of 0.86 in the 70-nm fibers. Moreover, the deformation behavior of the fibers reveals that the tensile modulus and strength improve with reductions in fiber size. Sharp increases in tensile properties are demonstrated when the size of the fibers is reduced below 175 nm. We attribute the enhanced concentration of β-crystals and the tensile behavior of finer diameter fibers to the extensional forces experienced by the material during electrospinning. POLYM. ENG. SCI., 55:1812–1817, 2015. © 2014 Society of Plastics Engineers

Inspired by the extreme wetting states displayed by the natural materials, various techniques have been widely investigated to fabricate superhydrophobic and superhydrophilic surfaces. Electrospinning has gained huge amount of interest as fibers with suitable combination of surface chemistry and surface roughness can be easily obtained. This study provides a comprehensive overview of the progress that has been made on electrospun fibers that display superhydrophobicity, superhydrophilicity or a combination of both. The article discusses various modification techniques that can be implemented to obtain fibers with surface heterogeneity for improving its hydrophobicity or hydrophilicity. Both nanometer size of the fibers and secondary nanoscale structures ensure that the fibers have suitable surface topography to exhibit extreme wetting states. Additionally, for the first time, we critically review and identify the role of intrinsic structures such as crystallinity and chain orientation on the wettability of the fibers. We highlight some new emerging application areas that are being explored using superhydrophobic and superhydrophilic fibers. Further, methods for fabricating smart materials with special wettability are also discussed. Such fibers with special wettability show tremendous promise for water harvesting, unidirectional water collection and oil-water filtration applications.

Smart functional polymers have gained a huge amount of interest in recent times due to their innumerable applications in areas including sensors, actuators, switchable wettability, bio-medical and environmental applications. Numerous intensive research studies have been carried out to develop smart functional polymers using stimuli responsive polymeric moieties. This review article encapsulates recent developments in the area of smart functional polymers with a focus on various (physical, chemical and biological) stimuli responsive systems and their applications. Furthermore, this review also provides useful insights and in-depth analysis on the feasibility of utilizing stimuli responsive polymeric materials/composites in anti-fouling and water harvesting applications which hold tremendous potential to create a sustainable environment.

The fabrication of lubricants-infused textured surfaces has opened up a new route towards omniphobicity. However, achieving a homogeneous thin film of lubricating material on a flat/smooth surface still remains a challenge. This work shows the successful fabrication of a thin, transparent, and homogeneous coating of perfluoropolyether (PFPE, a lubricating material) on a smooth glass surface by the electrospraying technique. The sol–gel solution for electrospraying was prepared by adding a small amount of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (FTS) with PFPE which was subsequently electrosprayed on a glass substrate. After curing the coated samples at 80 °C, a transparent, homogeneous, and slippery coating with a low surface energy (12.5 mN m−1) was obtained. It was observed that the presence of FTS with PFPE, assisted significantly in the stacking of PFPE on the substrate resulting in the formation of smooth, uniform blended (PFPE + FTS) films. The surface nature of the blended films was characterized by spectroscopy and microscopy. The blended surface exhibited omniphobic properties. The surface contact angles and slipping angles made by water and acetone droplets were measured to be (116°, 40.8°) and (6°, 10°), respectively. Furthermore, the coating showed good optical (transmittance – 91%) and mechanical properties with strong adherence to glass surfaces, thus revealing the potential for applications in windows and solar modules.

In this study, we use a versatile sol–gel based electrospinning technique to fabricate nanostructured barium titanate (BaTiO3)/cobalt ferrite (CoFe2O4) composite fibers and analyze their magnetoelectric response. Scanning and transmission electron microscopy images indicate that the obtained fibers are composed of fine grains which are self-assembled and arranged. X-ray diffraction (XRD) studies of the composite fibers revealed the presence of perovskite and spinel structures corresponding to BaTiO3 and CoFe2O4 phases, respectively. The magnetic hysteresis loops of the resultant fibers showed that the fibers were ferromagnetic with magnetic coercivity of 650 Oe and saturation magnetization of 31 emu g−1. Moreover, the magnetoelectric coefficient developed on the surface of the fibers was measured as a function of the applied external DC magnetic field. A maximum magnetoelectric coefficient of 13.3 mV cm−1 Oe−1 was determined for the composite fibers, which is much higher than the commonly reported values for bulk or thin film counterparts. The large magnetoelectric coefficient of the composite fibers was attributed to the nano-sized grains and their arrangement within the fibrous geometry. The intimate contact and the large interfacial area presented by the nanostructures ensured that the composites displayed strong magnetoelectric behavior. Such composite fibers show tremendous potential for magnetic field sensor applications and for magnetoelectric devices.

One-dimensional multiferroic nanostructured composites have drawn increasing interest as they show tremendous potential for multifunctional devices and applications. Herein, we report the synthesis, structural and dielectric characterization of barium titanate (BaTiO3)-bismuth ferrite (BiFeO3) composite fibers that were obtained using a novel sol-gel based electrospinning technique. The microstructure of the fibers was investigated using scanning electron microscopy and transmission electron microscopy. The fibers had an average diameter of 120 nm and were composed of nanoparticles. X-ray diffraction (XRD) study of the composite fibers demonstrated that the fibers are composed of perovskite cubic BaTiO3-BiFeO3 crystallites. The magnetic hysteresis loops of the resultant fibers demonstrated that the fibers were ferromagnetic with magnetic coercivity of 1500 Oe and saturation magnetization of 1.55 emu/g at room temperature (300 K). Additionally, the dielectric response of the composite fibers was characterized as a function of frequency. Their dielectric permittivity was found to be 140 and their dielectric loss was low in the frequency range from 1000 Hz to 107 Hz.

Electrospinning was used to process polyvinylidene fluoride (PVDF)/carbon nanotube (CNT) fibers. Aided by high electrostatic and extensional forces, the CNTs were uniformly dispersed within the fibrous matrix and aligned along the fiber axis. The dispersed CNTs served as nucleating agents and provided confinement to the segmental motion of neighboring chains. The structural developments induced by the presence of the CNTs not only improved the content of ferroelectric β-phase within the fibers but also improved the dielectric properties of the fibers and served to strengthen and stiffen the fibers. The conformal changes were determined using X-ray diffraction (XRD) and infrared (IR) spectroscopy methods. The content of β-crystals within the fibers increased consistently with CNT concentration. Compared to the neat fibers, the content of β-crystals for fibers filled with 3 wt% CNT was increased by 5.5%. This was attributed to the CNTs, which converted the molecules of the α-phase to an extended chain conformation. The improvement in the polar crystalline (β) content within the fibers increased the dielectric permittivity of the fibers. Additionally, the dielectric loss of the CNT reinforced fibers was as low as the neat PVDF fibers, which is significant for dielectric applications. The oriented β-crystalline structure also improved the tensile properties of the fibers. Strength and stiffness of the fibers were improved by 28% and 32%, respectively, when 3 wt% CNT was incorporated within the fibers. Understanding the effect of CNTs on crystal polymorphism and tensile strength of PVDF fibers has provided new insights for future development of high-performance ferroelectric fibers.

Herein, the fabrication of poly(vinylidene fluoride) (PVDF) fibrous membrane using electrospinning is reported and its use for dry-adhesive applications is demonstrated. The shear and normal adhesion performance of the samples was investigated using an Instron tensile tester and an atomic force microscope (AFM) respectively. For shear adhesion measurements, the electrospun membrane was finger pressed on to a glass slide and pulled in shear mode using a tensile tester. The thickness of the electrospun membrane was varied and the effect of thickness on shear adhesion was investigated. The shear adhesion strength increased when the thickness of the samples was reduced. Shear adhesion strength of a 200 µm thick sample was determined to be approximately 0.165 N/cm. For normal adhesion measurements, a flat tipless cantilever was used to indent the sample and then retract back to measure the pull-off force. High shear adhesion strength and normal pull-off force recorded are attributed to the fine size of the fibers that conform to the asperities present on the surfaces of the glass slide and the AFM cantilever. The durability of the adhesive was also verified by repeating the AFM adhesion measurements over 1000 consecutive attachment–detachment cycles. The pull-off force was seen to be constant over 1000 attachment–detachment cycles. Our results indicate that these electrospun fibrous membranes can potentially be used as reusable dry-adhesives. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44393.