Deep bed filtration is widely applied in bioprocessing, virus filtration, and water purification to remove small impurities, such as particles or virus fragments. However, more needs to be understood about the parameters that influence particle capture and deposition. Detailed simulations and microfluidic filtration experiments with straight pores have been widely investigated, yet realistic membrane porosity features such as pore gradients and roughness have not been addressed in detail. The internal membrane morphology is assumed to have a strong effect on filtration performance. Therefore, this study introduces a new microfluidic membrane mimicking device (MMD) and methodology that analyzes spatio-temporal particle collections in a deep bed filtration MMD with gradually decreasing pore size toward the retentate side. The design allows us to systematically tailor an internal filter surface roughness to investigate its influence on differently sized soft particles and the consequences for filtration performance, in particular, particle capture. Optical investigation and quantitative evaluation show enhanced particle-pillar interactions for increased roughness. This results in a reduced penetration depth and reduced particle breakthrough. The modes of capture or filtration phenomena, such as pore blocking, bridging, or dendrite formation, differ significantly between smooth and rough membrane filter structures. In the case of rough inner filter surfaces, those phenomena lead to a less distinct decrease in volumetric flow, allowing a longer filtration process. The microfluidic study offers a detailed investigation of how microscale phenomena influence the filtration process. The results provide a fundamental understanding of microscale filtration effects based on roughness and, hence, motivate a tailoring of the internal membrane filter surface according to the filtration problem at hand.

Colloidal fouling is a notoriously limiting factor in many membrane processes. Although intensely studied in mostly macroscopic filtration systems, the underlying phenomena leading to generally decreased fouling remain unknown. This study systematically investigates fouling and backwashing of anionic polystyrene particles in microfluidic structures with engineered surface charge and hydrophilicity. We interpret the observed phenomena with the extended Derjaguin–Landau–Verwey–Overbeek (xDLVO) theory for interaction potentials. Polyelectrolyte coatings of the membrane-mimicking structure allow us to investigate particle–membrane interactions in (1) hydrophobic structures with negative zeta potential, (2) hydrophilic structures with positive zeta potential, and (3) hydrophilic structures with negative zeta potential. We assess the qualitative changes in membrane–particle interaction potentials regarding their hydrophilic and electrostatic double-layer interactions on the basis of the extensive study of particle deposition and resuspension during filtration and backwash. The results generally confirm the known trend of decreased fouling with increased hydrophilicity. However, we also show that hydrophilicity alone is an insufficient measure to estimate a membrane's particle deposition and removal behavior. More important to achieve a low-fouling membrane that can be cleaned with pure flow reversal during regular backwashing is the design of a hydrophilic membrane with an engineered surface charge.

Polyelectrolyte composite hollow fiber membranes show a high potential to become applied in industrial nanofiltration applications such as water purification or downstream processing in several industries. This material platform allows multiple routes to achieve tailored selectivities and high permeances. Hollow fiber membranes promise a higher process efficiency due to higher packing densities and their backwash stability. However, the fabrication of composite polyelectrolyte hollow fiber nanofiltration membranes is more challenging compared to state of the art spiral wound modules. While the Layer-by-Layer deposition has proven versatile opportunities and is recently entering the commercial market, the multi-step post-treatment is elaborate. In this work, we streamline the fabrication of polyelectrolyte composite hollow fiber membranes. We extend our recently presented "chemistry-in-a-spinneret" approach with a membrane modification in a subsequent coating bath. The additional modification can be incorporated in the composite hollow fiber membrane manufacturing process without additional process steps. Sulfonated polyethersulfone as a polyanionic additive is used in the polymer solution which complexates with the polycations polyethylenimine (PEI) or poly(diallyldimethylammonium chloride (PDADMAC) in the bore fluid during fiber spinning. The extruded fibers have a positively charged lumen surface which allows a subsequent modification with the polyanion poly(sodium 4-styrenesulfonate) (PSS) in the coating bath. The spun fibers reveal with 16-4 LMH/bar pure water permeance (PWP) and 2000-1100 Dalton (Da) molecular weight cut-off (MWCO) open nanofiltration characteristics, while the PEI+PSS modified fibers show dense nanofiltration characteristics (0.6 LMH/bar PWP and 360 Da MWCO). The fibers are backwash stable and the PDADMAC and PDADMAC+PSS fibers show a significantly higher sodium hypochlorite (NaOCl) stability compared to the PEI and PEI+PSS fibers. This new technique creates composite hollow fiber nanofiltration membranes with a polyelectrolyte bilayer without multi-step post-treatment. Therefore, it provides a promising alternative to the Layer-by-Layer post-modification, where two coating steps and one water washing post-treatment step are required to create a polyelectrolyte bilayer on a porous hollow fiber membrane support.

Utilizing carbon dioxide (CO2 ) as a resource for carbon monoxide (CO) production using renewable energy requires electrochemical reactors with gas diffusion electrodes that maintain a stable and highly reactive gas/liquid/solid interface. Very little is known about the reasons why gas diffusion electrodes suffer from unstable long-term operation. Often, this is associated with flooding of the gas diffusion electrode (GDE) within a few hours of operation. A better understanding of parameters influencing the phase behavior at the electrolyte/electrode/gas interface is necessary to increase the durability of GDEs. In this work, a microfluidic structure with multi-scale porosity featuring heterogeneous surface wettability to realistically represent the behavior of conventional GDEs is presented. A gas/liquid/solid phase boundary was established within a conductive, highly porous structure comprising a silver catalyst and Nafion binder. Inoperando visualization of wetting phenomena was performed using confocal laser scanning microscopy (CLSM). Non-reversible wetting, wetting of hierarchically porous structures and electrowetting were observed and analyzed. Fluorescence lifetime imaging microscopy (FLIM) enabled the observation of reactions on the model electrode surface. The presented methodology enables the systematic evaluation of spatio-temporally evolving wetting phenomena as well as species characterization for novel catalyst materials under realistic GDE configurations and process parameters.

Efficiency in fluid-fluid displacement is drastically reduced by viscous fingering, limiting the overall effectiveness in enhanced oil recovery, membrane science, and lateral flow devices used in biomedical applications. Local instabilities at the fluid-fluid interface lead to finger-like patterns when a less viscous fluid displaces an immiscible fluid of higher viscosity. This widely observed phenomenon in multiphase flow inside porous media is infamously intricate to control, especially for given geometry and viscosity ratio. The presented study uses a highly controlled microfluidic porous network structure with tailored ionic surface strength. The direct correlation of viscous fingering evolution on the porous structure's zeta potential at a pore-scale level is demonstrated via polyelectrolyte coatings using a layer-by-layer technique. Displacement patterns are tuned from vigorous viscous fingering over stable displacement to corner flow events across a broad range of capillary numbers depending on the applied coatings. The experimental data show an increasing trend of oil recovery with increasing surface wettability, consistent with several previous findings. Furthermore, the results reveal that surface zeta potential correlates positively with recovery rate but negatively with the displacement stability quantified by the fractal dimension. These insights enable a more targeted porous media design to obtain optimal multiphase flow control.

During the first wave of Covid-19 infections in Germany in April 2020, clinics reported a shortage of filtering face masks with aerosol retention> 94% (FFP2 & 3, KN95, N95). Companies all over the world increased their production capacities, but quality control of once-certified materials and masks came up short. To help identify falsely labeled masks and ensure safe protection equipment, we tested 101 different batches of masks in 993 measurements with a self-made setup based on DIN standards. An aerosol generator provided a NaCl test aerosol which was applied to the mask. A laser aerosol spectrometer measured the aerosol concentration in a range from 90 to 500 nm to quantify the masks' retention. Of 101 tested mask batches, only 31 batches kept what their label promised. Especially in the initial phase of the pandemic in Germany, we observed fluctuating mask qualities. Many batches show very high variability in aerosol retention. In addition, by measuring with a laser aerosol spectrometer, we were able to show that not all masks filter small and large particles equally well. In this study we demonstrate how important internal and independent quality controls are, especially in times of need and shortage of personal protection equipment.

Wetting of the membrane to displace air or conditioning liquids is important to exploit the complex porosity of a filtration membrane. This study reveals the details of wetting in multibore membrane based fi?ltration modules. Using magnetic resonance imaging (MRI), we quantify the uid distribution patterns during initial membrane wetting in dead-end permeation mode. The spatio-temporal evolution of aqueous copper sulfate solution wetting the membrane fi?bers was investigated as a function of the applied flux, packing density, and position along the membrane module length.Three initial wetting conditions were examined: delivery-state membranes, ethanol-washed and dried (air-fillled) membranes, and ethanol-?lled membranes. Signi?cant changes in wetting patterns were observed due to interfacial and polymer swelling effects. This in-situ investigation reveals a slow wetting progression over six hours and more to obtain complete wetting, even at high fluxes of 200 LMH. However, an increased flux leads to faster wetting kinetics as the evolving wetting patterns are flux dependent. Packing density of the multibore ?bers additionally impacts the wetting kinetics by shifting the prevalent pressure conditions. Although in dead-end mode, the wetting progression is non-uniform along the membrane module length.In addition to this parameter study, different pre-wetting agents' effect on the displacement behavior was investigated in depth. This study helps to understand (a) complex wetting phenomena inside multibore membranes in dead-end ?ltration, (b) the membranes' interaction with their surroundings due to neighboring membranes, and (c) the effect of the used fluid system for displacement on the resulting wetting patterns.

During the first wave of Covid-19 infections in Germany in April 2020, clinics reported a shortage of filtering face masks with aerosol retention > 94% (FFP2 & 3, KN95, N95). Companies all over the world increased their production capacities, but the quality control of once-certified materials and masks came up short. To help identify falsely labeled masks and ensure safe protection equipment, we tested 101 different batches of masks in 993 measurements with a self-made setup based on DIN standards. An aerosol generator provided a NaCl test aerosol which was applied to the mask. A laser aerosol spectrometer measured the aerosol concentration in a range from 90 to 500 nm to quantify the masks' retention. Of 101 tested mask batches, only 31 batches kept what their label promised. Especially in the initial phase of the pandemic in Germany, we observed fluctuating mask qualities. Many batches show very high variability in aerosol retention. In addition, by measuring with a laser aerosol spectrometer, we were able to show that not all masks filter small and large particles equally well. In this study, we demonstrate how important internal and independent quality controls are, especially in times of need and shortage of personal protection equipment.

Active layers of ion separation membranes often consist of charged layers that retain ions based on electrostatic repulsion. Conventional fabrication of these layers, such as polyelectrolyte deposition, can in some cases lead to excess coating to prevent defects in the active layer. This excess deposition increases the overall membrane transport resistance. The study at hand presents a manufacturing procedure for controlled polyelectrolyte complexation in and on porous supports by support wetting control. Pre-wetting of the microfiltration membrane support, or even supports with larger pore sizes, leads to ternary phase boundaries of the support, the coating solution, and the pre-wetting agent. At these phase boundaries, polyelectrolytes can be complexated to form partially freestanding selective structures bridging the pores. This polyelectrolyte complex formation control allows the production of membranes with evenly distributed polyelectrolyte layers, providing (1) fewer coating steps needed for defect-free active layers, (2) larger support diameters that can be bridged, and (3) a precise position control of the formed polyelectrolyte multilayers. We further analyze the formed structures regarding their position, composition, and diffusion dialysis performance.

Flow-electrode capacitive deionization (FCDI) is an electrically driven water desalination technology promising for many applications, such as industrial wastewater treatment. FCDI exploits the pumpability of carbon slurries, enabling a continuous process suitable for a wide range of feed salinities. Previously, we demonstrated the applicability of FCDI processes with incorporated ion-exchange membranes for the desalination and concentration of saline brines containing 60-120 g/L NaCl. At such elevated salinities, steep concentration gradients occur across the membranes of an FCDI cell. Hence, the characteristics of the membranes become crucial for the overall process performance. It is not yet fully understood, which physical phenomena dominantly influence the ion transport. In this article, we present the first FCDI process model focusing on brine treatment. While our previously published FCDI model (Rommerskirchen et al., 2018) was suitable for simulating the treatment of low salinity solutions, the model at hand includes more non-idealities and focuses on the ion transport through the ion-exchange membranes. We introduce a constriction factor (sigma-factor) for the diffusion coefficients to model the electrical double layer behavior within the membranes at steep concentration gradients. Hence, the model is now also suitable for the simulation of continuous FCDI processes for brine treatment and shows good correlation with experimental results.

We report the design and analysis of a salt metathesis process using Flow-Electrode Capacitive Deionization (FCDI) for the generation of a concentrated valuable magnesium sulfate solution from dilute MgCl2 and Na2SO4 (or K2SO4) solutions. First, a batch mode decomposition and recombination of the MgCl2, Na2SO4, and K2SO4 solutions was studied with varying initial concentrations. Current efficiencies of ∼100% were observed for each cycle. In a so-called decomposition step, two different salt solutions are decomposed into electrically charged slurries having the counterions adsorbed. Swapping the slurries with the stored counterions during the recombination step results into new salt solutions upon discharge including the desired product. Both, purity of products and overall conversion of ions into the products, depend on operational parameters, while maximal achieved MgSO4 purity was as high as 93% with a concentration factor of 6.3 and a discharge current efficiency of ∼85%. Finally, a semicontinuous FCDI metathesis system was investigated. Performing the recombination step at appropriate process conditions also allows the concentration of the resulting product solutions by a factor as high as 81.5 with MgSO4 purity of ∼80% and current efficiencies of 96%. Future improvements in process configurations and membrane ion selectivity will render the process even more selective.