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Submicron aerosols, especially nanoaerosols less than 100nm, are present in large number concentrations both indoors and outdoors, although they contribute very little to the PM2.5, which is based on mass and is biased toward the large above-micron aerosols. By virtue of their small size, submicron- and nanoaerosols can penetrate deep into our body through inhaling causing potential chronic and acute diseases. A well-known example of the latter is the SARS-CoV-2 virus, which is air transmittable. Improved filtration technologies for the submicron- and nanoaerosols at various fronts, including personal protection (facemasks and respirators), family protection (home filters), and protection in public transport (filters in airplanes, trains, buses, and ships) and public places (filtration systems in hospital wards, offices, auditoriums, shopping centers, and markets), are all necessary. The multilayer or multimodule charged nanofiber provides high capacity for filtration of aerosols under depth filtration as each charged fiber is fully utilized to capture aerosols around and along the fiber by electrostatic interactions, and all the fiber layers in the entire filter is being deployed. This contrasts with conventional single-layer charged/uncharged filter where the skin layer located upstream trumps the remaining filter from being fully utilized. Thus depth filtration can be prolonged with multilayer/multimodule charged nanofibers in the filter (i.e., delaying cake formation). The charged nanofiber technology is equally promising for personal protective equipment—facemasks, respirators to high-performance filters, such as with high-efficiency performance air/ultralow particulate air filters. Antimicrobial agents can be incorporated into the as-spun fibers so that both filtration and disinfection can be carried out simultaneously. Because of the large surface, the pressure drop across the nanofibrous filter can be very high. For depth filtration, this can be remedied with multilayer/multimodule nanofiber filters together with pleating under constant quality factor (QF) or iso-QF condition. For cake filtration, it is desirable to have a porous microfiber layer installed upstream of the nanofiber layer to form a composite filter. The cake formed on the microfiber layer is much more permeable resulting in lower pressure drop as compared to that formed on the nanofiber filter. Integrating the two layers conveniently during production or combining them conveniently during assembly into a filter when the two layers are produced separately would deserve future development. Cleaning of nanofiber filters by backpulse, backblow, or combination to remove preloaded aerosols can be quite effective. Ancillary equipment and disposing of the nano-particles from filter cleaning need to be developed. Although nanofibers are used in much smaller quantities in a filter, one limitation to widespread nanofiber filter applications has been the production rate that is significantly lower as compared to other nonwoven fibers. Thus increasing the production rate of nanofibers per production machine will be prudent in the future.

Aerosols are fine particles of liquid and/or solid, which are airborne. Aerosols can settle in reasonable distance when they are over 10µm. For aerosols less than 10µm, they take progressively longer time for the aerosol to settle. For aerosols less than 5 μm, they are practically unsettleable. For aerosols less than 1µm (i.e., submicron aerosols), they practically stay airborne indefinitely, and people can easily inhale these submicron aerosols through the respiratory system. By virtue of their small size, they can pass to the vascular circulation to different organs causing potential chronic and acute diseases. This is even more so for nanoaerosols (NAs), less than 100nm. Unfortunately, we are surrounded by submicron aerosols and nanoaerosols in our daily environment. In indoors, a common source of these tiny aerosols can be found in cooking simple breakfast, such as toasting bread, to more sophisticated broiling and frying. Tobacco smoking, burning candles and incense, using cleaning detergents, and wearing perspiration suppresser can release these tiny aerosols in the air. In outdoors, these aerosols can be present in high concentrations from emissions of road traffic, power plants, biomass plants, and commercial kitchen exhaust to incineration plants. Not only these submicron aerosols and nanoaerosols might have harmful chemical ingredients but they can also carry viruses and tiny bacteria. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus and its variants (such as Delta variant which are highly contagious and infectious) are good examples showing wide-spread human-to-human infection as a result of airborne transmission of the virus. Unfortunately, this aspect has escaped attention by the public authorities in early part of the coronavirus (COVID-19) pandemic and even to some extent as of today. In environment with low relative humidity, the liquid content from droplet aerosol can further evaporate resulting in much smaller size for the aerosols that can be airborne even farther. Nanofibers with diameter less than 1µm, and typically less than 300nm, have a large specific surface to capture these submicron aerosols and nanoaerosol. They are ideal and effective for filtering these tiny aerosols.

The abundance of organic pollutants in environment has persuaded researchers to establish an advanced technology and address this global issue. We discovered that graphene incorporated p-type CuO/n-type TZB-Gr (Copper Oxide/Titanium dioxide-Zinc Oxide-Bismuth Oxide-Graphene) heterojunctions nanofibers (NFs) with band gap of (1.7eV) can effectively produce active radicals to degrade most of the organic pollutant to become CO2 and H2O. One-dimensional electrospun (NFs) of p-type CuO/n-type TZB-Gr heterojunctions with high visible and UV-light activity were successfully synthesized using sol-gel and a facile electrospinning technique. The results show that the CuO/TZB-Gr (NFs) were 75 nm in diameter and multi-micrometers in length. The as prepared electrospun (NFs) were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), UV–vis diffuse reflectance (DR) spectroscopy, resonant Raman spectroscopy and thermogravimetric analysis (TGA). Kinetic study of composite CuO/TZB-Gr (NFs) was carried out. Compared with commercial P25 particles, the as prepared p-type CuO/n-type TZB-Gr (NFs) disclosed a strikingly higher photocatalytic activity in the degradation of Methylene Blue (MB) and formaldehyde. The enhanced photocatalytic activity of MB under Visible and UV light and formaldehyde under Visible light could be regarded to the formed p–n heterojunction between CuO and TZB-Gr and their higher separation efficiency of photogenerated electrons and holes. Moreover, due to one–dimensional nature of p-type CuO/n-type TZB-Gr heterojunctions, these (NFs) could be easily reused without the decrease of photocatalytic efficiency, therefore rendering a novel strategy for environmental applications.

It is imperative to design and develop a technology which would completely remove pollutants from contaminated waters due to agricultural, industrial, and domestic uses. For the first time, we discovered that graphene incorporated composite nanofibers (TZB-Gr) (Titanium dioxide-Zinc Oxide-Bismuth Oxide-Graphene) with a band gap of (2.5 eV) can effectively activate organic dyes under visible-light and UV-light irradiation to produce active ·O2− and ·OH radicals. The produced radicals are powerful oxidizing species to degrade most of the organic pollutant to become CO2 and H2O. TZB-Gr demonstrated a higher activity than TZB (Titanium dioxide-Zinc Oxide-Bismuth Oxide) and P25 (Commercial TiO2 nanoparticles). Kinetic study of composite nanofibers (NFs) was carried out. Furthermore, a reasonable catalytic mechanism of the TZB-Gr (NFs) was proposed, based on electron-hole pairs and recombination of photogenerated charges. It was shown that graphene-based nanofiber photocatalysis is superior to that on transition metal oxide (TZB) and P25 in degradation of a dye (methylene blue, MB) and (rhodamine B, RhB) in water, therefore providing a novel strategy for environmental remediation.

Chitosan-based electrospun nanofibrous scaffolds have been selected as wound healing/tissue scaffolds because of their extracellular matrix nature and biocompatible properties. However, crosslinking of scaffolds is necessary to avoid lysozyme degradation in an aqueous environment, as a stable scaffold is crucial for the activities of fibroblasts, including adhesion and proliferation during wound healing. Autoclaving (physical) and genipin crosslinking (chemical) methods have been employed to stabilize chitosan-based scaffolds individually. However, the differences in scaffold microstructure induced by the individual or combined crosslinking methods have yet to be investigated systematically. In this study, autoclaving crosslinking improved mainly the structural properties (tensile strength and crystallinity), but it also expanded the chitosan and PEO network by hydrolysis, which enlarged the fiber diameter and caused chitosan chain degradation. Meanwhile, genipin crosslinking improved the physiochemical properties, primarily hydrophilicity. On the other hand, the combined crosslinking significantly improved both the structural and physiochemical properties through the unique reorganization of the polymeric network. The confined geometry of the nanofiber as well as the genipin crosslinks resulted in maximal crystallization of chitosan and amorphization of PEO chains. Unfortunately, the combined crosslinking resulted in the lowest antibacterial activity because of the consumption of amino and protonated amino groups in the crosslinking process. Despite this, the combined crosslinking scaffold achieved the best stability under lysozyme degradation and therefore it is preferred over autoclaving or genipin crosslinking alone. In conclusion, the results show that chemical and physical crosslinking methods induce different changes in crystallinity and hydrophilicity that affect the physicochemical properties. Therefore, crystallinity and hydrophilicity are significant considerations when designing a tissue scaffold.

Semiconductor driven photocatalysis has galvanized great attention as it holds tremendous promise to address the worldwide environmental and energy issues. Photocatalysis, in which photons are used for redox reactions, is at the central point to achieve this goal. The heterogeneous photocatalysts with integrated functional nano-composites can combine the advantages of different nano-composites to overcome the drawbacks of single nano-photocatalysts. Coupling of TiO2 with narrow band gap semiconductor nanocomposites has been a strategy used by researchers to obtain visible light active photocatalysts. In this work, graphene has been used to improve the performance of photocatalysts based on its great charge conductivity as well as other exciting properties. The edge effect has been removed by introducing the 2D graphene into circular rolls inserted in the 65–140 nm TiO2, TiO2-CuO (TC), and TiO2/ZnO/Bi2O3 (TZB) nanofibers (NFs) and free electrons can only travel in specific direction along the axis of the TiO2, TiO2-CuO (TC), and TiO2/ZnO/Bi2O3 (TZB) (NFs). The resulting (NFs) has less band-gap energy that facilitates harvesting of the visible light spectrum. The graphene incorporation helps to harvest more energy from the entire UV–vis spectrum and almost doubled the surface area of the (NFs) when maximum amount of graphene is embedded into the (NFs). The T-Gr, TC-Gr and TZB-Gr photocatalyst, after optimized with as much as 32.18%, 16.87% and 26.5% respectively by mass of graphene in the (NFs), has superior photoactivity in degradation of formaldehyde under solar irradiation. The kinetics and fundamental mechanism of formaldehyde degradation are also addressed. The graphene insertion controls the work function of photocatalysts, which is critical for photocatalytic reactions.

Nano-aerosols of size 100 nm and below are manifested everywhere in various urban micro-environments. In outdoor, the vehicular emission during traffic jam can release nano-aerosols with concentration above 200 million/m3 (200/cm3). Superimposed on vehicular emission are smog particles, around 10–50 nm, generated from the photochemical reaction of hydrocarbon and NOx in the presence of sunlight. These nano-aerosols, by virtue of their small sizes, can be inhaled readily into our bodies leading possibly to various chronic diseases. Air-borne viruses from influenza to epidemic viruses, which can lead to acute sickness and even death, are also in the same size range of 100 nm. Despite microfibers are being commonly used today in filters, there has been limited studies on filtration of real aerosols using microfiber filters, let alone nanofibrous filters, as most studies used simulated aerosols (e.g. sodium chloride) or test dust. Also, in standard sodium chloride test, only monodispersed aerosol size is allowed to challenge the filter. On the other hand, in reality aerosols of all sizes challenge simultaneously the filter. In this study, for the first time we have used nanofiber filter in a portable test filter set-up to filter polydispersed aerosols from the micro-environment near busy traffic area where aerosols comprise of both vehicular and atmospheric pollutants with size range between 10 and 400 nm. Real-time measurements are carried out with the portable test filter. The test results are compared to the theoretical correlation from Payet with diffusion correction at small Peclet number (Pe). We have found good comparison between test results with theoretical correlations using equivalent aerodynamic diameter for the aerosols with face velocity from 1 to 11 cm/s. A new dimensionless parameter, specific filter resistance (also established independently by Buckingham-π approach), is defined for the first time. It measures the flow drag on fibers to the amount of fibers present in the filter, the lower is the specific filter resistance the better is the filter. For our test condition, it is approximately 1.0–1.2. We have also improved the filtration efficiency of the filter by increasing the fiber basis weight by stacking up two layers of nanofibers. We have demonstrated that the efficiency increases while both the quality factor and specific filter resistance remain constant. We have also investigated the single fiber efficiency due to diffusion and found that at low velocity and low Peclet number (Pe 0.07 m/s) and large Peclet number (>10), there is small deviation from the theory, which is probably due to the smaller aerosols collecting by the larger aerosols upstream of the filter with an airstream consisting of polydispersed aerosol distribution challenging the filter. This has not been realized previously as tests conducted were mostly using monodispersed aerosol size distribution for which aerosol-aerosol interaction was absent.

Filtration of neutrally charged nano- and sub-micron aerosols (50–500 nm) using PVDF electret nanofiber filters based on dielectrophoretic effect was investigated experimentally for the first time. Dielectrophoretic effect is refers to inducing dipole charges on neutrally charged aerosols when they are in close proximity to the charged nanofibers; subsequently, the polarized aerosols are captured electrostatically by the charged fibers. Given the nanofiber diameter is less than 500 nm, the electrostatic attraction force in the proximity of the fiber was very strong, resulting in good aerosol capture without incurring higher pressure drop across the filter. We were able to charge a PVDF nanofiber filter using corona discharge and the performance of the filter remained relatively unchanged within 90 days of repeated testing with quality factor well reaching 0.14 Pa−1 or higher. Optimization of charging parameters was carried out and charge density on filters indicated by surface potential (SP) was found to increase with increasing charging voltage and decreasing charging distance. Correspondingly, filtration efficiency increased due to the enhanced electrostatic attraction between charged fibers and polarized aerosols. The influence of filter basis weight (W) on the filtration performance of electret filters was studied, which showed that the improvement in filtration performance was insignificant with limited increase in filtration efficiency and higher pressure drop. Although higher fiber amount led to higher surface potential, it also caused unsatisfactory charge density increment and higher electrical interference between adjacent fibers due to higher packing density. To enhance the performance of PVDF electret filters, a ‘multi-layering’ approach was proposed for the first time on electret nanofiber filter whereby the total charged fibers were distributed in thinner stack-up layers, with each layer being isolated by a porous permeable substrate, to reduce the electrostatic interference among fibers of adjacent layers. The latter has been the key culprit in reducing the performance when charged fibers were all stacked into a single layer, resulting in overlapping of intense electrostatic fields from different nanofibers. Moreover, through distributing fibers to multilayers, the filter porosity was increased, leading to lower air flow resistance and pressure drop. Therefore, compared with a filter with only one layer, a multi-layer filter with similar total basis weight of fibers had much better performance, which was further verified by halving the basis weight of each individual layer (WL) while maintaining the overall value. The single-fiber efficiency based on dielectrophoretic effect confirms larger aerosols got better charge induction resulting in stronger dipoles and better capture. The face velocity dictates the retention time and increasing retention time proves beneficial for dielectrophoretic capture. Multilayering could reduce electrostatic interference, thereby facilitating increasing basis weight of fibers to be used in a filter to achieve high capture efficiency. Moreover, filter durability/stability was investigated, which indicates the multi-layer PVDF electret filters could maintain high performance and are promising for long-term use. It was concluded that multilayer PVDF electret filters have a great performance in aerosol removal and a potential filtration enhancement mechanism was elaborated.

The efficiency of dye-sensitized solar cells (DSSC) has been limited by the inherent slow transport of charges in the TiO2 photoanode. The trap-limited diffusion of charges in the semiconductor prevented photoanodes from having thickness greater than 15 µm, thus limiting the potential of DSSC. This is especially with more recent research moving away from iodine-based electrolyte that requires a thicker photoanode. Thicker photoanodes means higher potential efficiency due to better light harvesting without resorting to scattering or reflecting layers to trap light as with thin photoanodes. Graphene had been added to TiO2 photoanodes in other research but in the form of rGO and suffered from increasing electron recombination at higher concentration. In this study, shear-exfoliated graphene was successfully incorporated into TiO2 nanofibers via electrospinning to produce fiber with diameter 52–73 nm in the photoanode with graphene in roll-up form inside the nanofibers eliminating edge effect of 2D graphene sheet that causes electron recombination. The nanofiber-based photoanode showed significantly longer electron diffusion length with little impact on recombination, primary due to higher electron diffusion coefficient. As a result, a thicker optimal photoanode, 23 µm, can be obtained with device efficiency measured at 8.9%, which is 22% higher than that without graphene with optimal thickness of 13 µm. The graphene also showed no interference in dye loading density of N719 dye. For the first time, we have demonstated that the electrospun TiO2/graphene nanofibers is a potential starting point to thicker, more efficient DSSCs.

Highlights • Filtration of COVID-19 + airborne carrier, simulated by ambient aerosols 100 nm. • Ambient aerosol - Irregular shape, aerosol interactions, different properties. • Added electrostatic efficiency 100% – 180%X mechanical efficiency for aerosol >80 nm. • 6-layer stack-up charged nanofibers 525 nm fiber diameter, max electrostatic capture, reduce △p. • 50, 100, 300 nm. Ambient aerosol:88%, 88% 96%; NaCl aerosol:92%, 94%, 98%(N98) efficiency. • △P 2.65 mm water, 1/10 ≪ N95 respirator 25–35 mm water≪ N98, Quality Factor = 0.1 – 0.13/Pa., The novel coronavirus (COVID-19), average size 100 nm, can be aerosolized by cough, sneeze, speech and breath of infected persons. The airborne carrier for the COVID-19 can be tiny droplets and particulates from infected person, fine suspended mists (humidity) in air, or ambient aerosols in air. To-date, unfortunately there are no test standards for nano-aerosols (≤100 nm). A goal in our study is to develop air filters (e.g. respirator, facemask, ventilator, medical breathing filter/system) with 90% capture on 100-nm airborne COVID-19 with pressure drop of less than 30 Pa (3.1 mm water). There are two challenges. First, this airborne bio-nanoaerosol (combined virus and carrier) is amorphous unlike cubic NaCl crystals. Second, unlike standard laboratory tests on NaCl and test oil (DOP) droplets, these polydispersed aerosols all challenge the filter simultaneously and they are of different sizes and can interact among themselves complicating the filtration process. For the first time, we have studied these two effects using ambient aerosols (simulating the bio-nanoaerosols of coronavirus plus carrier of different shapes and sizes) to challenge electrostatically charged multilayer/multimodule nanofiber filters. This problem is fundamentally complicated due to mechanical and electrostatic interactions among aerosols of different sizes with induced charges of different magnitudes. The test filters were arranged in 2, 4, and 6 multiple-modules stack-up with each module having 0.765 g/m2 of charged PVDF nanofibers (mean diameter 525 ± 191 nm). This configuration minimized electrical interference among neighboring charged nanofibers and reduced flow resistance in the filter. For ambient aerosol size>80 nm (applicable to the smallest COVID-19), the electrostatic effect contributes 100–180% more efficiency to the existing mechanical efficiency (due to diffusion and interception) depending on the number of modules in the filter. By stacking-up modules to increase fiber basis weight in the filter, a 6-layer charged nanofiber filter achieved 88%, 88% and 96% filtration efficiency for, respectively, 55-nm, 100-nm and 300-nm ambient aerosol. This is very close to attaining our set goal of 90%-efficiency on the 100-nm ambient aerosol. The pressure drop for the 6-layer nanofiber filter was only 26 Pa (2.65 mm water column) which was below our limit of 30 Pa (3.1 mm water). For the test multi-module filters, a high ‘quality factor’ (efficiency-to-pressure-drop ratio) of about 0.1 to 0.13 Pa−1 can be consistently maintained, which was far better than conventional filters. Using the same PVDF 6-layer charged nanofiber filter, laboratory tests results using monodispersed NaCl aerosols of 50, 100, and 300 nm yielded filtration efficiency, respectively, 92%, 94% and 98% (qualified for 'N98 standard') with same pressure drop of 26 Pa. The 2–6% discrepancy in efficiency for the NaCl aerosols was primarily attributed to the absence of interaction among aerosols of different sizes using monodispersed NaCl aerosols in the laboratory. This discrepancy can be further reduced with increasing number of modules in the filter and for larger 300-nm aerosol. The 6-layer charged nanofiber filter was qualified as a 'N98 respirator' (98% capture efficiency for 300-nm NaCl aerosols) but with pressure drop of only 2.65-mm water which was 1/10 below conventional N95 with 25-mm (exhaling) to 35-mm (inhaling) water column! The 6-layer charged PVDF nanofiber filter provides good personal protection against airborne COVID-19 virus and nano-aerosols from pollution based on the N98 standard, yet it is at least 10X more breathable than a conventional N95 respirator.

The World Health Organization declared the novel coronavirus (COVID-19) outbreak as a pandemic on March 12, 2020. Within four months since outbreak in December 2019, over 2.6 million people have been infected across 210 countries around the globe with over 180,000 deaths. COVID-19 has a size of 60–140 nm with mean size of 100 nm (i.e. nano-aerosol). The virus can be airborne by attaching to human secretion (fine particles, nasal/saliva droplets) of infected person or suspended fine particulates in air. While NIOSH has standardized N95, N99 and N100 respirators set at 300-nm aerosol, to-date there is no filter standards, nor special filter technologies, tailored for capturing airborne viruses and 100-nm nano-aerosols. The latter also are present in high number concentration in atmospheric pollutants. This study addresses developing novel charged PVDF nanofiber filter technology to effectively capture the fast-spreading, deadly airborne coronavirus, especially COVID-19, with our target aerosol size set at 100 nm (nano-aerosol), and not 300 nm. The virus and its attached aerosol were simulated by sodium chloride aerosols, 50–500 nm, generated from sub-micron aerosol generator. PVDF nanofibers, which were uniform in diameter, straight and bead-free, were produced with average fiber diameters 84, 191, 349 and 525 nm, respectively, with excellent morphology. The fibers were subsequently electrostatically charged by corona discharge. The amounts of charged fibers in a filter were increased to achieve high efficiency of 90% for the virus filter but the electrical interference between neighbouring fibers resulted in progressively marginal increase in efficiency yet much higher pressure drop across the filter. The quality factor which measured the efficiency-to-pressure-drop kept decreasing. By redistributing the fibers in the filter into several modules with lower fiber packing density, with each module separated by a permeable, electrical-insulator material, the electrical interference between neighboring charged fibers was reduced, if not fully mitigated. Also, the additional scrim materials introduced macropores into the filter together with lower fiber packing density in each module both further reduced the airflow resistance. With this approach, the quality factor can maintain relatively constant with increasing fiber amounts to achieve high filter efficiency. The optimal amounts of fiber in each module depended on the diameter of fibers in the module. Small fiber diameter that has already high performance required small amounts of fibers per module. In contrast, large diameter fiber required larger amounts of fibers per module to compensate for the poorer performance provided it did not incur significantly additional pressure drop. This approach was applied to develop four new nanofiber filters tailored for capturing 100-nm airborne COVID-19 to achieve over 90% efficiency with pressure drop not to exceed 30 Pa (3.1 mm water). One filter developed meeting the 90% efficiency has ultralow pressure drop of only 18 Pa (1.9 mm water) while another filter meeting the 30 Pa limit has high efficiency reaching 94%. These optimized filters based on rigorous engineering approach provide the badly needed technology for protecting the general public from the deadly airborne COVID-19 and other viruses, as well as nano-aerosols from air pollution which lead to undesirable chronic diseases.

Nano-aerosols are airborne aerosols in the range of 100 nm. They are present abundantly in both pollutants and viruses, both of which affect human health. Filtering these nano-aerosols leads to high-pressure drop in cabin filters due to their small sizes forming impermeable dust cake, and prior to cake formation the capture efficiency is rather poor with conventional microfiber filters. We have investigated a composite filter with a microfiber layer positioned immediately upstream of a nanofiber layer. Such composite filter provides high efficiency and low-pressure drop during all phases of filtration of nano-aerosols: initial depth filtration, transition, and final cake filtration. The behavior of the composite filter together with various filter benchmarks (single and composite filters of microfiber/nanofiber filters) has been investigated by monitoring the pressure drop, especially during cake filtration. Using the measurements together with Ergun/Kozeny-Carman equation and material balance we come up with a new method to determine the solid volume fraction and permeability of the cake. The grade efficiency for the most penetrating particle is also measured concurrently to define the incipient point to cake formation and provide material balance. Most importantly, for the first time we have confirmed that the cake ultimately forms on the composite filter is due to the cake that originates from the upstream microfiber layer and not from the downstream nanofiber layer. Therefore, the pressure drop for the composite filter can be sustained relatively low throughout the filtration. On the other hand, the aerosol capture efficiency stays very high from start-to-end of filtration, with high initial filtration efficiency courtesy of the nanofiber filter installed downstream of the composite filter. The price to pay is a slightly higher pressure drop for the composite filter in the clean unloaded state, but this is insignificant during aerosol loading as most pressure drop is attributed to that of the cake. For the first time we have introduced, also independently using Buckingham-Pi analysis, a brand new dimensionless parameter, Beta, which measures the equivalent “thickness” of additional cake deposition (in molten form) to the viscous flow resistance “path length” due to the additional cake deposit. A high-capacity filter aimed for heavy aerosol loading should have large Beta that results in low solidosity, high porosity, and high permeability for the cake. In our study, the microfiber-nanofiber composite filter has the highest Beta of nearly 4 and also highest efficiency. Despite high efficiency the single nanofiber filter has much lower Beta of 2, while the single microfiber filter has Beta of 3, yet much lower efficiency, especially at initial filtration. Using the microfiber-nanofiber composite filter design, we can “engineer the cake” deposited on the filter to be more permeable and with least flow resistance during aerosol loading to attain high Beta values. This is an innovative approach to increase the badly needed aerosol storage capacity with low pressure drop for the nanofiber filter while maintaining high efficiency throughout the aerosol loading for extended filtration applications. The highly permeable aerosol cake can also be cleaned by backpulsed-and-backblow readily for filter reuse.

Although the application of graphene-derived nanomaterials in the electron transport layer (ETL), hole transport layer (HTL), or top electrode of perovskite solar cells (PSCs) has been thoroughly studied, the effects of inserting such materials into the perovskite layer of PSCs is not well understood. In this study, pristine graphene nanofibers were introduced into the perovskite layer of PSCs for the first time. The quality of the electrospun graphene nanofibers was optimized by controlled centrifugation of graphene sheets in the precursor suspension. Under optimized conditions, the device power conversion efficiency increased from 17.51 % without graphene to 19.83 % with graphene nanofibers, representing a 13 % increase. The introduction of graphene nanofibers into the perovskite layer led to a dramatic increase in the grain size of the perovskite layer to over 2 μm, owing to improved nucleation and crystallization at the nanofiber interface, which led to much higher FF and Jsc values. The significant increases in Jsc and Voc are attributed to the improved charge-transport properties of the graphene nanofibers with superb charge conductivity introduced into the perovskite layer. The latter was independently verified by the measured electron transport time. The stability of the device was also improved. In summary, an effective approach has been developed to improve the performance of PSCs by using pure graphene nanofibers for the first time.

Nano-aerosols, less than 100 nm, are present in high concentration in air in the millions per cubic meter both indoor and outdoor. By virtue of their small size, they are easily inhaled into our body and can lead to chronic diseases. Nanofiber filter is effective in capturing these nano-aerosols suspended in air. Understanding the pressure drop behavior during aerosol loading of nanofiber filter is important for the operation of these filters. Given these filters are highly efficient for capturing nano-aerosols, over aerosol loading they can change from depth filtration, for which aerosols are captured inside the filter, to surface filtration, for which a cake forms on the filter surface that ultimately becomes the effective filter media. This transition, which is important in operation of nanofiber filters, is largely not understood. This study fulfills such unmet needs by investigating experimentally and theoretically the transition from depth- to surface-filtration of nanofiber filter challenged by nano-aerosols. Initially, filtration occurs across the filter that quickly leads to most filtration taking place in the upstream thin skin layer facing the incoming flow. As flowable pores in the skin layer get blocked, aerosols “bridge” across the pore openings and subsequently build-up above the filter surface. A new bridging model is developed that characterize this behavior. The bridge of aerosols among adjacent pores starts to interact at the filter surface that eventually leads to a continuous aerosol cake layer forming across the filter. Experiments are conducted using 50–400 nm sodium chloride aerosols challenging high-efficiency nanofiber filters (>50% for 300 nm sodium chloride challenging aerosol) that exhibit transition change from depth- to surface-filtration under accelerated loading condition. The new model not only matches well the test data in the depth-surface transition regime, it also predicts correctly the concave downward behavior of the pressure-drop test data. It also provides a smooth transition to the cake filtration for which the pressure drop varies linearly with deposited aerosol mass.

Photocatalytic removal of Dairy effluent (DE) was studied by using TiO2-GeO2 and TiO2-CdO nanofibers (NFs), produced by electrospinning method. These NFs were characterized by SEM, TEM and XRD studies. The TiO2-GeO2 and TiO2-CdO NFs were smooth and continuous, with an average diameter of about 273 nm and 256 nm respectively, and held their nanofibrous morphology even after more than 9 h of photocatalytic removal of DE under visible light irradiation. TiO2-GeO2 and TiO2-CdO NFs were effective materials for removal of DE, even after many runs and cycles. TiO2-GeO2 and TiO2-CdO NFs showed a maximum removal of 65% and 75%, respectively, after 3 h. The TiO2-GeO2 and TiO2-CdO NFs also showed excellent results in hydrogen release.

A thin, highly porous, nanofiber filter with low-pressure drop start out in “depth filtration” in a clean state with aerosols mostly captured by the fibers in the filter. For nanofibers with high capture efficiency, the fibers in the region upstream of the filter facing the challenging aerosol flow tends to capture more aerosols. The capture aerosols in turn can capture more incoming aerosols in this region leaving much less aerosols escaping downstream of the filter. Furthermore, these captured aerosols, forming dendritic structures, reduce the size of flowable pores in the region. This “skin region” upstream of the filter can be quite thin as compared to the entire filter thickness, yet it accounts for the large fraction of the aerosols captured in the filter as well as large fraction of the pressure drop across the filter. With more aerosols loading, the openings of the flowable pores in the skin region get blocked by aerosol dendrites that bridge across captured aerosols and fibers. These aerosol bridges subsequently build-up above the filter surface in the “surface filtration” regime. As more aerosol bridges stack-up on existing ones and interact with each other, they form ultimately a continuous cake layer on the filter surface. The pressure drop due to the skin effect, which is a combination of both pore filling in the skin region in the filter and the aerosol bridges, can be relatively low (i.e. low-skin effect) for low nanofiber packing density and a thin nanofiber filter. However, the establishment of the skin region during depth filtration and the aerosol bridging effect above the filter surface are still two essential processes prior to formation of the cake on the filter surface. Two experiments and related analysis on loading of thin nanofiber filters using nano-aerosols are used to investigate this scenario. It has been found that in the course of aerosol bridging, the pressure drop behaves predominantly concave upward with increasing specific aerosol deposit. This contrast with previous study with high-skin effect for a thick, less-porous nanofiber filter for which aerosol bridging predominantly yields a concave downward behavior with specific aerosol deposit during pressure drop excursion. Most importantly, the bridging model that predicts successfully the high-skin effect can also predict the low-skin effect where pressure drop has a concave upward behavior with increasing specific aerosol deposit. Finally, we have demonstrated the aerosol holding capacity for a thin, highly porous, nanofiber filter can still be significant, with a small fraction of the total aerosols captured by depth filtration in the filter, and majority of deposited aerosols above the filter in the surface filtration regime, partly in the aerosol bridges and mostly in the cake.

Highlights • Cyclic loading/cleaning nanofiber filter with nano-aerosols. • Thin skin layer upstream of filter controls loading behavior, permeability drops to 10–20% of initial value. • Capillary model for aerosol deposit, pore bridging, cake model match 6-cycle loading tests. • Transit from depth-to-surface-filtration – tests and modelling., Nano-aerosols from viruses to virgin pollutant particulates from combustion, 100 nm or smaller, are harmful to our health as they penetrate readily into our body causing various diseases. Nanofiber filter can capture effectively these nano-aerosols. However, over time the pressure drop increases dramatically and cleaning of the filter by backpulse/backblow is essential for filter reuse. The cyclic loading-and-cleaning of a nanofiber filter has been investigated for the first time experimentally and theoretically. The “skin” layer, a thin region upstream of the nanofiber filter, plays a pivoting role in controlling the pressure drop excursion of the filter. We model the skin layer to be made up of numerous fine capillaries and examine how continuous aerosols deposited in the capillaries affect rapid rise in pressure drop followed by bridging of aerosols across the capillary openings leading to more bridging and ultimately formation of cake on top of the bridges and filter surface. We have been able to describe the deposition of aerosols in the capillary pores for depth filtration, the deposition of aerosols in the cake (surface filtration), and the intermediate bridging regime between these two. We can depict the complete pressure drop excursion including the S-shaped curve behavior from depth filtration transiting to surface filtration for a filter with severe skin effect. Our prediction matches extremely well with the 6 cycles of loading/cleaning on a 280-nm nanofiber filter subject to challenging nano-aerosols, 50–400 nm. During cyclic loading and cleaning, the porosity and permeability in the skin layer for our experiment drop to 68% and 11–21% of their original values, respectively, and the effective pore diameter also drops from 1.2 to 0.6 μm.

Solution-processable organic–inorganic hybrid perovskites have been successfully used as light-harvesting materials in efficient photovoltaic cells. Here, an efficient and simple approach is reported for using dimethylsulphoxide (DMSO) to increase crystal size and hydrochloric acid (HCl) to further retard crystallization thereby improving uniformity and crystallinity of the perovskite film. With this synergistic approach, large uniform perovskite crystals over one micron in size with superior quality perovskite film without pinholes resulted as confirmed by both XRD and SEM images. The fabricated planar heterojunction solar cell has reached efficiency of 17.8%. Both the best and average solar cells fabricated with this approach using DMSO reveal that a 10%-11% increase in efficiency as a result of conditioning with HCl which controls crystallization preventing undesirable non-uniform crystal growth, which leads to pinholes and hole-electron recombination sites. Using this method, a uniform perovskite layer that maximizes light harvesting can be readily obtained. Further, the incorporation of chloride using the HCl formulation has demonstrated to have better stability against degradation from moisture, strong solar irradiation, and high temperature.

Novel technologies have been on demand to develop improved photocatalyst for gas purification. Graphene has been used to improve the performance of photonic devices based on its high charge conductivity as well as other unique properties. Traditional approach uses discrete graphene sheets with sparse, sporadic deposition of semiconductor crystals (TiO 2 ) as photocatalyst, which results in poor light harvesting and electron-hole recombination at the sheet edges. In our novel configuration, the edge effect has been eliminated by having the graphene sheets being rolled up into “spiral rolls” inserted in the 80 nm TiO 2 /ZnO/Bi 2 O 3 (TZB) nanofiber, and free electrons can only travel unidirectional in the graphene rolls along the axis of the nanofiber. The nanofiber is fabricated with its surface packed with 10-nm size TZB nanocrystallites that increases the surface area, thereby improving light harvesting. Further, the addition of ZnO and Bi 2 O 3 reduce the band-gap energy of the composite facilitating harvesting of the visible light spectrum. Other than fast transport of electrons to sites where photocatalysis is needed, the graphene roll (exposed in between the pores of the TZB nanocrystallites) has proven to harvest more energy from the entire UV–vis spectrum and almost doubled the already large surface area of the nanofibers when maximum amount of graphene is added into the nanofibers. Note the larger surface area of the photocatalyst facilitates pollutant gas molecule adsorption, which is the first and an important step prior to photocatalysis. The TZB-Gr photocatalyst, after optimized with as much as 26.5% by mass of graphene in the nanofibers, has superior photoactivity in degradation of NO under solar irradiation. It is 35% higher than nanofibers with just TZB alone, at least 17% higher than traditional approach with discrete graphene sheets (with edge effect) with TZB particles deposited on the sheets, and 10 times better than 25-nm TiO 2 nanoparticles (P25). Other photonic devices, such as solar cells, and non-photonic devices, such as bio-chemical sensors and lithium batteries, can also benefit from the innovative configuration of semiconductor nanofibers with inserted graphene for the low band-gap energy, fast charge transit, long electron life time, reduced recombination rate, and large surface area.

This chapter describes process objectives, feed properties, bench-scale testing, and pilot-scale testing. Material balance is used to back-calculate the variables that are difficult to measure accurately, but are required as a part of the measuring indices or metrics. Laboratory spintube testing can be made to determine the separability, and the G's and t's to make separation. It can also be used to determine whether chemical additives, coagulants, and flocculants are required to enhance separation. A special designed spintube centrifuge system, equipped with optical transmission measurement, provides sedimentation behavior of the suspension (monodispersed versus polydispersed particles, and unflocculated versus flocculated suspension) over time as well as space. Pilot testing is a necessary program once a bench-scale test demonstrates the viability of the separation process. Various parameters in controlling and monitoring pilot centrifuge testing are also discussed.

The need for discharging a concentrated underflow is investigated in terms of recovery of extracellular expressed protein, otherwise the valuable protein is lost with the liquid in the underflow or concentrate stream. The underlying physics involved in cake compaction and percolation is described in this chapter. Once the G-forces of a given machine are selected, the process determining the kinetics of dewatering, dictates whether there is enough time in between shots to drain the liquid trapped in the pores between cake solids. Some designed cake compaction and percolation experiments can be carried out to determine the kinetics of dewatering. Under steady state, cake compaction and percolation depend on the cake solid stress, which in turn depends on the G-force and the concentrate thickness. There is general increase in solids concentration with increasing solids stress initially, fairly linearly, and subsequently at a rate of diminishing return. The underflow should not be too compacted, otherwise it might have discharge difficulty, and that might increase the centrate turbidity or solids.

This chapter deals with the selection and the sizing of centrifuges for biotech applications. All centrifuges, such as spintube, disk stack, tubular, chamber bowl, and decanter can be scaled by the appropriate dimensionless Le number. However, the Le number depends on the specific set of geometric and operating parameters for each type of centrifuge. Based on this, one can determine the separated size of cell particles—the cut size. The cut size, which depends on the operation and the geometry of the machine, should be smaller than the size of cell that needs to be separated. If there are any changes in viscosity (from temperature change) or solids concentration, it affects the Le number from which the feed rate and the G-force need to be modified; otherwise, the cut size would be different and that affects the separation outcome. Sizings are made in centrifuges for use in small organelles (0.1-1 micron) and relatively larger biological cells (1-20 microns and bigger).

This chapter describes several non-intuitive phenomena, such as nonlinear increase in pressure with linear change in radial distance as well as the Coriolis acceleration and associated force. In addition, other more intuitive phenomena, such as discrete particle settling versus aggregate of particles settling, are discussed. These cover the entire spectrum of real life possibilities. The chapter also presents Stokes' free settling of single particles and settling of a cluster of discrete particles at higher solids concentration at which particles settle under hydrodynamic influence of each other. Finally, various process functions of centrifugation, coveting separation, clarification, classification, degritting, thickening, and impurities removal/valuable recovery by separation as well as repulping are discussed.

In this chapter, the batch spintube and ultracentrifuge have been presented. The ultracentrifuge, capable of a million times gravity, has different operating modes that facilitate the measurement of sedimentation velocity and molecular weight and the separation of small cells and cellular organelles. Also, the versatile centrifugal filter that combines sedimentation, filtration, and chromatography carried out with different functional modules has been presented. Each process can be enhanced by the centrifugal field, including the chromatography process. In addition, a semibatch tubular centrifuge, capable of 20,000g, is discussed with either manual or automatic cake discharge. The traditional tubular uses a high length-to-diameter ratio, which requires manual cake removal, whereas the more modern tubular, which takes advantage of the solid-basket centrifuge design and has a length-to-diameter ratio of order up to 3, is designed with automatic cake discharge and more space for solid storage, and consequently it can take on feed with higher solids concentration.

Centrifugation accompanied by flocculation of fine biosolids is often utilized in wastewater treatment. Unfortunately, commonly used laboratory jar tests often over-estimate the size of the flocculated solids (flocs) that can be realized in-situ in the centrifuge as the fragile flocs can be easily broken by shear and turbulence during feed acceleration in the centrifuge. Currently, there is no satisfactory method to predict in-situ floc size in the centrifuge, despite the floc size is critical to separation and solids recovery. The difficulty in making predictions also leads to inaccuracy in predicting solids recovery by decanter centrifuge and scale-up between centrifuges of different sizes, designs, and operating conditions, which is an even more serious issue. This study attempts to address these hurdles. In this study, first the flow pattern in a decanter centrifuge in form of moving layer at the pool surface is demonstrated by two different experiments. Second, a model on separation of suspended flocs in the moving layer in a centrifuge is developed. Further, a two-parameter model is proposed to represent the floc size distribution wherein the first parameter represents the minimum floc size (primary particle without coagulation or flocculation) and the second parameter represents the median floc size. A closed form analytical solution for the model is obtained with results expressed by the ratio of minimum-to-median floc size and a dimensionless Leung number which measures feed rate to the clarification rate of the centrifuge. Third, a Buckingham-π analysis has been conducted on separation in a moving layer under centrifugal field confirming these two governing dimensionless parameters can also be derived independently from the more basic dimensionless π groups. Fourth, by matching the solids recovery obtained from field tests with the model prediction, the median floc size can be inferred. In this matching process, the minimum floc size is assumed to be equal to the size of the primary, unflocculated solids. Fifth, four sets of tests (over 20 runs) have been carried out using two decanter centrifuges of different sizes and designs operating, respectively, under different pool depths, feed rates, polymer dosages and rotation speeds in a wastewater treatment plant processing mixed sludge with 48% primary sludge and 52% waste activated sludge. The median floc size inferred from the present method is in the range of 4–9 mm from the three series of tests (14 tests). The improved feed acceleration design with speed matching closely that of the rotating pool results in less pool turbulence, larger flocs, and lower polymer dosage (7 kg polymer/ton sludge); while the poorer feed acceleration with more pool disturbance from the under-accelerated feed results in smaller flocs, and higher polymer dosage (9 kg/t). Sixth, an important application of the model is to predict solids recovery using an estimated median floc size and this approach has predicted the solids recovery reasonably well on the fourth set of tests (9 tests). Seventh, the scale-up for predicting solids recovery of flocculated suspension from decanter centrifuges of different sizes, designs, and operating conditions has been demonstrated using the dimensionless Leung number and the minimum-to-median floc size ratio.