Nanofibrous adhesion: the twin of gecko adhesion

Functionalizing Surfaces by Physical Vapor Deposition To Measure the Degree of Nanoscale Contact Using FRET

Adhesion between solid materials is caused by intermolecular forces that only take place if the adhering surfaces are at nanoscale contact (NSC) (i.e., 0.1-0.4 nm. To study adhesion, NSC can be evaluated with Förster Resonance Energy Transfer (FRET). FRET uses the interaction of compatible fluorescence molecules to measure the nanometer distance between bonded surfaces. For this, each surface is labeled with one fluorescence dye, named the Donor or Acceptor. If these molecules are in NSC, a nonradiative Donor-Acceptor energy transfer will occur and can be detected using FRET spectroscopy. Here, for the first time, we introduce an innovative concept of a FRET-based NSC measurement employing dye-nanolayer films prepared by a physical vapor deposition (PVD). The dye nanolayers were prepared by PVD from the vaporization of the Donor and Acceptor molecules separately. The selected molecules, 7-Amino-4-methyl-cumarin (C120) and 5(6)-Carboxy-2',7'-dichlor-fluorescein (CDCF), present high quantum yields (QY, QYD = 0.91 and QYA = 0.64) and a low FRET distance range of 0.6-2.2 nm, adequate for the study of NSC. The produced dye-nanolayer films exhibit a uniform dye distribution (verified by atomic force microscopy) and suitable fluorescence intensities. To validate the NSC measurements, FRET spectroscopy experiments were performed with bonded dye-nanolayer films prepared under different loads (from 1.5 to 140 bar), thus creating different degrees of NSC. The results show an increase in FRET intensity (R 2 = 0.95) with the respective adhesion energy between the films, which is directly related to the degree of NSC. Hence, this work establishes FRET as an experimental technique for the measurement of NSC, and its relation to surface adhesion. Additionally, thanks to the FRET dye-nanolayer approach, the method can be employed on arbitrary surfaces. Essentially, any sufficiently transparent substrate can be functionalized with FRET compatible dyes to evaluate NSC, which represents a breakthrough in contact mechanics investigations of soft and hard solid materials.

Trichome-Like Biomimetic Air Filters via Templated Silicone Nanofilaments

Air pollution threats to human health have increased awareness of the role of filter units in air cleaning applications. As an ideal energy-saving strategy for air filters, the slip effect on nanofiber surfaces can potentially overcome the trade-off between filtration efficiency and pressure drop. However, the potential of the slip effect in nanofibrous structures is significantly limited by the tight nanofiber stacks. In this study, trichome-like biomimetic (TLB) air filters with 3D-templated silicone nanofilaments (average diameter: ≈74 nm) are prepared based on an in situ chemical vapor deposition (CVD) method inspired by plant purification. Theoretical modeling and experimental results indicate that TLB air filters make significant use of the slip effect to overcome the efficiency-resistance tradeoff. The selectable filter class (up to U15, ≈99.9995%) allows TLB air filters to meet various requirements, and their integral filtration performance surpasses that of most commodity air filters, including melt-blown cloth, ePTFE membranes, electrospun mats, and glass fiber paper. The proposed strategy directly transforms commercial filter media and filters into TLB air filters using a bottom-up, one-step approach. As a proof-of-concept, reusable N95 respirators and air purifiers equipped with TLB air filters are fabricated, overcoming the limitations of existing filter designs and fabrication methods.

Biodegradable and high-performance multiscale structured nanofiber membrane as mask filter media via poly(lactic acid) electrospinning

The usage of single-use face masks (SFMs) has increased since the outbreak of the coronavirus pandemic. However, non-degradability and mismanagement of SFMs have raised serious environmental concerns. Moreover, both melt-blown and nanofiber-based mask filters inevitably suffer from poor filtration performance, like a continuous decrease in the removal efficiency for particulate matter (PM) and weak breathability. Herein, we report a new method to create biodegradable and reusable fibrous mask filters. The filter consists of a true nanoscale bio-based poly(lactic acid) (PLA) fiber (an average size of 37 ± 4 nm) that is fabricated via electrospinning of an extremely dilute solution. Furthermore, we designed a multiscale structure with integrated features, such as low basis weight (0.91 g m-2), small pore size (0.73 μm), and high porosity (91.72%), formed by electrospinning deposition of true nanoscale fibers on large pore of 3D scaffold nanofiber membranes. The resultant mask filter exhibited a high filtration efficiency (PM0.3-99.996%) and low pressure drop (104 Pa) superior to the commercial N95 filter. Importantly, this filter has a durable filtering efficiency for PM and natural biodegradability based on PLA. Therefore, this study offers an innovative strategy for the preparation of PLA nanofibers and provides a new design for high-performance nanofiber filters.

ZIF-8/PI Nanofibrous Membranes With High-Temperature Resistance for Highly Efficient PM0.3 Air Filtration and Oil-Water Separation

Air and water pollution poses a serious threat to public health and the ecological environment worldwide. Particulate matter (PM) is the major air pollutant, and its primary sources are processes that require high temperatures, such as fossil fuel combustion and vehicle exhaust. PM_0.3 can penetrate and seriously harm the bronchi of the lungs, but it is difficult to remove PM_0.3 due to its small size. Therefore, PM_0.3 air filters that are highly efficient and resistant to high temperatures must be developed. Polyimide (PI) is an excellent polymer with a high temperature resistance and a good mechanical property. Air filters made from PI nanofibers have a high PM removal efficiency and a low air flow resistance. Herein, zeolitic imidazolate framework-8 (ZIF-8) was used to modify PI nanofibers to fabricate air filters with a high specific surface area and filtration efficiency. Compared with traditional PI membranes, the ZIF-8/PI multifunction nanofiber membranes achieved super-high filtration efficiency for ultrafine particles (PM_0.3, 100%), and the pressure drop was only 63 Pa. The filtration mechanism of performance improvement caused by the introduction of ZIF-8/PI nanofiber membrane is explored. Moreover, the ZIF-8/PI nanofiber membranes exhibited excellent thermal stability (300 C) and efficient water–oil separation ability (99.85%).

High-performance anti-haze window screen based on multiscale structured polyvinylidene fluoride nanofibers

Indoor air quality (IAQ) has assumed new significance given the extensive amount of time spent indoor due to the coronavirus pandemic and particulate matter (PM) pollution. Accordingly, the development of window air filters to effectively intercept PM from outdoor air under natural ventilation conditions is an important research topic. However, most existing filters inevitably suffer from the compromise among filtration capability, transparency, and air permeability. In this study, we fabricate a high-performance transparent air filter to improve IAQ via natural ventilation. polyvinylidene fluoride (PVDF) superfine nanofibers of size 20-35 nm are prepared using extremely dilute solution electrospinning; a multi-scale nanofiber structure is then designed. By adjusting the ratio of PVDF superfine nanofibers (SNs) to PVDF coarse fibers (CNs), we balance the structure-performance relationship. Benefiting from the multiscale structural features that include a small pore size (0.72 μm) and high porosity (92.22%), the resulting filters exhibit excellent performance including high interception efficiency (99.92%) for PM_0.3, low air resistance (69 Pa), high transparency (∼80%) and stable filtration after 100 h of UV irradiation. This work describes a new strategy for the fabrication of nanofibers with true-nanoscale diameters and the design of high-performance air filters.

Electrospinning fabrication of polystyrene-silica hybrid fibrous membrane for high-efficiency air filtration

The development of new materials for air filtration and particulate matter (PM) pollution is critical to solving global environmental issues that threaten human health and accelerate the greenhouse effect. In this study, a novel electrospun polystyrene-SiO2 nanoparticle (PS-SNP) fibrous membrane was explored by a single-step strategy to obtain the composite multi-layered filter masks. In addition, the air filtration performance of this fibrous membrane for PM was evaluated. The effects of SiO2 on the composition, morphology, mechanical property, and surface wetting of PS-SNP membranes were studied. Allowing SiO2 to be incorporated into the PS polymer was endowed with promising superhydrophobicity and demonstrated excellent mechanical properties. As-prepared PS-SNP membranes possess significantly better filtration efficiency than pure PS membrane. Furthermore, a three-layered air filter media (viscose/PS-SNP/polyethylene terephthalate) used in this study has considerable performances compared to the commercial masks. Since this air filtration membrane has excellent features such as high air filtration and permeability, we anticipate it to have huge potential application in air filtration systems, including cleanroom, respirator, and protective clothing.

Quantification and Imaging of Nanoscale Contact with Förster Resonance Energy Transfer

Adhesion is caused by molecular interactions that only take place if the surfaces are in nanoscale contact (NSC); i.e., the distance between the surfaces is in the range of 0.1-0.4 nm. However, there are several difficulties measuring the NSC between surfaces, mainly because regions that appear to be in full contact at low magnification may show no NSC when observed at higher magnifications. Thus, the measurement area of NSC is very small with imaging techniques, and an experimental technique to evaluate NSC for large contact areas has not been available thus far. Here, we are proposing Förster resonance energy transfer (FRET) spectroscopy/microscopy for this purpose. We demonstrate that NSC in a distance range of 1-10 nm can be evaluated. Our experiments reveal that, for thin films pressed under different loads, NSC increases with the applied pressure, resulting in a higher FRET signal and a corresponding increase in adhesion force/energy when separating the films. Furthermore, we show that local variations in molecular contact can be visualized with FRET microscopy. Thus, we are introducing a spectroscopic technique for quantification (FRET spectroscopy) and imaging (FRET microscopy) of NSC between surfaces, demonstrated here for the application of surface adhesion. This could be of interest for all fields where adhesion or nanoscale surface contact are playing a role, for example, soft matter, biological materials, and polymers, but also engineering applications, like tribology, adhesives, and sealants.

2D and 3D Bulk Materials for Environmental Remediation: Air Filtration and Oil/Water Separation

Air and water pollution pose an enormous threat to human health and ecosystems. In particular, particulate matter (PM) and oily wastewater can cause serious environmental and health concerns. Thus, controlling PM and oily wastewater has been a great challenge. Various techniques have been reported to effectively remove PM particles and purify oily wastewater. In this article, we provide a review of the recent advancements in air filtration and oil/water separation using two- and three-dimensional (2D and 3D) bulk materials. Our review covers the advantages, characteristics, limitations, and challenges of air filters and oil/water separators using 2D and 3D bulk materials. In each section, we present representative works in detail and describe the concepts, backgrounds, employed materials, fabrication methods, and characteristics of 2D and 3D bulk material-based air filters and oil/water separators. Finally, the challenges, technical problems, and future research directions are briefly discussed for each section.

Spider-Web-Inspired PM0.3 Filters Based on Self-Sustained Electrostatic Nanostructured Networks

Particulate matter (PM) pollution has become a serious public health issue, especially with outbreaks of emerging infectious diseases. However, most present filters are bulky, opaque, and show low-efficiency PM0.3 /pathogen interception and inevitable trade-off between PM removal and air permeability. Here, a unique electrospraying-netting technique is used to create spider-web-inspired network generator (SWING) air filters. Manipulation of the dynamic of the Taylor cone and phase separation of its ejected droplets enable the generation of 2D self-charging nanostructured networks on a large scale. The resultant SWING filters show exceptional long-range electrostatic property driven by aeolian vibration, enabling self-sustained PM adhesion. Combined with their Steiner-tree-structured pores (size 200-300 nm) consisting of nanowires (diameter 12 nm), the SWING filters exhibit high efficiency (>99.995% PM0.3 removal), low air resistance (<0.09% atmosphere pressure), high transparency (>82%), and remarkable bioprotective activity for biohazard pathogens. This work may shed light on designing new fibrous materials for environmental and energy applications.

Polyacrylonitrile/polyimide composite sub-micro fibrous membranes for precise filtration of PM0.26 pollutants

Particulate matter (PM) pollution has enormously threatened ecosystem and public health. Among various air filtration medium, fibrous ones are very attracting and promising, with an array of advantages such as high specific surface area, and good internal connectivity. Even so, the large-scale fabrication of fibrous filtration materials still remains challenging. Here, three-dimensional polyacrylonitrile/polyimide (PAN/PI) composite sub-micro fibrous membranes were fabricated facilely via free surface electrospinning for precise filtration of PM_0.26 pollutants, where the waste PI short fibers were utilized as raw material. The resultant composite fibrous membranes, featuring thin fiber diameter (~150 nm), low areal density (<0.8 g m^-2), large porosity, and highly tortuous airflow channels with uniform poresize distribution, possessed excellent mechanical property with tensile strength of 4.95 MPa (twice that of pristine PAN), high thermal durability as well as remarkable filtration performance for ultrafine NaCl aerosol particles (≤0.26 µm) even after multiple filtration tests at high airflow velocity of 14.1 cm s^-1. The deepened aperture channels inside three-dimensional sub-micro fibrous membranes are tortuous enough for capturing ultrafine PMs from the airstream mainly via diffusion, interception, and impaction mechanisms, and the reported large-scale fabrication of cost-effective homogeneous PAN/PI fibrous filter media is promising for industrial production and commercial applications.

Progress on particulate matter filtration technology: basic concepts, advanced materials, and performances

The PM (particulate matter)-induced haze problem has caused serious environmental and health concerns. It is still a huge challenge to control PM pollution because of the complex structure, diverse sources and intricate evolution mechanism of the particles. In recent years, there has been increasing efforts to develop advanced strategies for PM treatment. Herein, we wish to provide a systematic summary of recent progress in air filtration. The review covers the definition of PM, the characterization of PM, the mechanism of PM capture, advanced purification materials, and special multifunctional performances. As for characterizing PM particles, removal efficiency, pressure drop, flow rate, quality factor and optical transparency are the basic parameters. For the advanced filters with excellent filtration performance, some special properties such as thermal stability, antibacterial property, flame retardancy, recyclability and special wettability are in great need under certain extreme conditions. Finally, some future prospects for filtration materials, like material choice and structural design, are also discussed.

Highly Efficient, Transparent, and Multifunctional Air Filters Using Self-Assembled 2D Nanoarchitectured Fibrous Networks

Particulate matter (PM) pollution is a significant burden on global economies and public health. Most present air filters are heavy, bulky, and nontransparent and typically have inevitable compromise between removal efficiency and air permeability. We report a scalable strategy to create ultralight, thin, rubbery, self-assembled nanoarchitectured networks (nanonetworks) with high-efficiency and transparency (ULTRA NET) as air filters using capacitive-like electronetting technology. By controlling the ejection, deformation, and phase separation of charged droplets from a Taylor cone, our approach allows continuously welded two-dimensional nanonetworks (∼20 nm fiber diameter) to assemble into filters on a large scale. The resulting ULTRA NET filters exhibit integrated properties of desirable pore structure yet maintaining strikingly low thickness (∼350 nm) and free-standing capability, 99.98% removal efficiency, and <0.07% of atmosphere pressure for PM_0.3 filtration at ∼85.6% transmittance, which enable them to serve as a multifunctional filter against PMs either in rigid solid or in soft oil forms and even biohazard pathogens. This work should serve as a source of inspiration for the design and development of high-performance fibrous materials for various filtration and separation applications.

Electrospun Polyimide/Metal-Organic Framework Nanofibrous Membrane with Superior Thermal Stability for Efficient PM2.5 Capture

Particulate matter (PM) pollution is a serious threat to human health. Zeolitic imidazolate framework-8 (ZIF-8) is a kind of metal-organic framework, and ZIF-8 not only can capture PM_2.5 efficiently but also possesses excellent chemical and thermal stability. In this study, ZIF-8-modified soluble polyimide (PI) nanofibrous membranes were prepared via an electrospinning process. As a result, the PI-ZIF membrane shows high PM_2.5 filtration efficiency (up to 96.6 ± 2.9%), superior thermal stability (up to 300 °C), good transmittance, excellent mechanical properties, and low pressure drop. The prepared PI-ZIF membrane with excellent comprehensive property shows a promising application in PM_2.5 capture, especially in harsh conditions.

In Situ Active Poling of Nanofiber Networks for Gigantically Enhanced Particulate Filtration

Enhancing the filtration efficiency of air filtering material without increasing its airflow resistance is a major challenge and of great significance. In this work, we report a type of active-poled nanofiber onto which in situ active poling is applied. It results in significantly enhanced filtration efficiency as well as dust holding capacity while keeping the airflow resistance constant. Owing to the in situ applied electric field, the nanofibers as well as the particulates are polarized. As a result, at a poling voltage of 2 kV, the removal efficiency and the quality factor for PM_2.5 are enhanced by 17% and 130%, respectively. More importantly, the dust holding capacity represents a 3.5-fold enhancement over normal nanofibers. The approach reported in this work has the potential of being practically utilized in air purification purposes because it can bring about not only promoted filtration performance but also lowered noise and reduced power consumption.

In Situ Investigation on the Nanoscale Capture and Evolution of Aerosols on Nanofibers

Aerosol-induced haze problem has become a serious environmental concern. Filtration is widely applied to remove aerosols from gas streams. Despite classical filtration theories, the nanoscale capture and evolution of aerosols is not yet clearly understood. Here we report an in situ investigation on the nanoscale capture and evolution of aerosols on polyimide nanofibers. We discovered different capture and evolution behaviors among three types of aerosols: wetting liquid droplets, nonwetting liquid droplets, and solid particles. The wetting droplets had small contact angles and could move, coalesce, and form axisymmetric conformations on polyimide nanofibers. In contrast, the nonwetting droplets had a large contact angle on polyimide nanofibers and formed nonaxisymmetric conformations. Different from the liquid droplets, the solid particles could not move along the nanofibers and formed dendritic structures. This study provides an important insight for obtaining a deep understanding of the nanoscale capture and evolution of aerosols and benefits future design and development of advanced filters.

Free-Standing Polyurethane Nanofiber/Nets Air Filters for Effective PM Capture

The filtration performance and light transmittance of nanofiber air filters are restricted by their thick fiber diameter, large pore size, and substrate dependence, which can be solved by constructing substrate-free fibrous membranes with true nanoscale diameters and ultrathin thicknesses, however, it has proven to be extremely challenging. Herein, a roust approach is presented to create free-standing polyurethane (PU) nanofiber/nets air filters composed of bonded nanofibers and 2D nanonets for particular matter (PM) capture via combining electrospinning/netting technique and facile peel off process from designed substrates. This strategy causes widely distributed Steiner-tree structured nanonets with diameters of ≈20 nm and bonded scaffold nanofibers to assemble into ultrathin membranes with small pore size, high porosity, and robust mechanical strength on a large scale based on ionic liquid inspiration and surface structure optimization of receiver substrates. As a consequence, the resulting free-standing PU nanofiber/nets filters exhibit high PM_1-0.5 removal efficiency of >99.00% and PM_2.5-1 removal efficiency of >99.73%, maintaining high light transmittance of ≈70% and low pressure drop of 28 Pa; even achieve >99.97% removal efficiency with ≈40% transmittance for PM_0.3 filtration, and robust purification capacity for real smoke PM_2.5 , making them promising high-efficiency and transparent filtration materials for various filtration and separation applications.

Nanofiber Air Filters with High-Temperature Stability for Efficient PM2.5 Removal from the Pollution Sources

Here, we developed high-efficiency (>99.5%) polyimide-nanofiber air filters for the high temperature PM2.5 removal. The polyimide nanofibers exhibited high thermal stability, and the PM2.5 removal efficiency was kept unchanged when temperature ranged from 25-370 °C. These filters had high air flux with very low pressure drop. They could continuously work for >120 h for PM2.5 index >300. A field-test showed that they could effectively remove >99.5% PM particles from car exhaust at high temperature.

Roll-to-Roll Transfer of Electrospun Nanofiber Film for High-Efficiency Transparent Air Filter

Particulate matter (PM) pollution in air has become a serious environmental issue calling for new type of filter technologies. Recently, we have demonstrated a highly efficient air filter by direct electrospinning of polymer fibers onto supporting mesh although its throughput is limited. Here, we demonstrate a high throughput method based on fast transfer of electrospun nanofiber film from roughed metal foil to a receiving mesh substrate. Compared with the direct electrospinning method, the transfer method is 10 times faster and has better filtration performance at the same transmittance, owing to the uniformity of transferred nanofiber film (>99.97% removal of PM2.5 at ∼73% of transmittance). With these advantages, large area freestanding nanofiber film and roll-to-roll production of air filter are demonstrated.