Evaluation of commercial nanofiltration and reverse osmosis membrane filtration to remove per-and polyfluoroalkyl substances (PFAS): Effects of transmembrane pressures and water matrices.

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are now widely found in aquatic ecosystems, including sources of drinking water and portable water, due to their increasing prevalence. Among different PFAS treatment or separation technologies, nanofiltration (NF) and reverse osmosis (RO) both yield high rejection efficiencies (>95%) of diverse PFAS in water; however, both technologies are affected by many intrinsic and extrinsic factors. This study evaluated the rejection of PFAS of different carbon chain length (e.g., PFOA and PFBA) by two commercial RO and NF membranes under different operational conditions (e.g., applied pressure and initial PFAS concentration) and feed solution matrixes, such as pH (4-10), salinity (0- to 1000-mM NaCl), and organic matters (0-10 mM). We further performed principal component analysis (PCA) to demonstrate the interrelationships of molecular weight (213-499 g·mol-1 ), membrane characteristics (RO or NF), feed water matrices, and operational conditions on PFAS rejection. Our results confirmed that size exclusion is a primary mechanism of PFAS rejection by RO and NF, as well as the fact that electrostatic interactions are important when PFAS molecules have sizes less than the NF membrane pores. PRACTITIONER POINTS: Two commercial RO and NF membranes were both evaluated to remove 10 different PFAS. High transmembrane pressures facilitated permeate recovery and PFAS rejection by RO. Electrostatic repulsion and pore size exclusion are dominant rejection mechanisms for PFAS removal. pH, ionic strength, and organic matters affected PFAS rejection. Mechanisms of PFAS rejection with RO/NF membranes were explained by PCA analysis.

Evaluating the Efficiency of Magnetic Treatment for Feed Water in Reverse Osmosis Processes.

The paper presents a new methodology for short-term (5-25 min) benchtop tests to evaluate the effectiveness of magnetic treatment of feed water for reducing mineral scaling on a reverse osmosis (RO) membrane. Scale deposition is measured at a controlled level of salt supersaturation in water flowing through an RO unit in once-through mode. A magnetic water conditioner is tested in a transient flow regime when variations of the permeate flux along the flow path are insignificant. Scale formation under these conditions is governed by salt crystallization on the membrane surface. The proposed method was implemented to investigate the influence of magnetic treatment on gypsum deposition on RO membranes in supersaturated aqueous CaSO4/NaCl solutions. The effects of magnetic water treatment on scale formation under our experimental conditions were found to be statistically insignificant with a confidence level of 95%. However, this outcome should not be considered to negate the potential efficiency of magnetic water treatment in specific applications. The proposed methodology of testing under a controlled level of salt supersaturation will also be useful for evaluating the efficiency of other water treatment technologies.

Effect of Current Density Ramping on the Growth Rate and Structure of AA2024-T3.

The presented study successfully demonstrated advantages of multistep anodization of AA2024-T3. Coating properties and morphology were studied in detail for five anodization processes: a conventional Base process with a constant applied current density and processes with current density applied in one (OS1 and OS2) and five (MS1 and MS2) steps at different magnitudes during the ramp period. Due to lower oxygen infusion, processes MS1 and MS2 produced a more intact coating with reduced porosity and enhanced abrasion resistance and hardness. The presented results clearly demonstrate that starting anodization at a low voltage and then slowly ramping current density will form coatings with a higher aluminum/oxygen ratio and enhanced properties over a shorter period of processing.

Role of diffusion in crystallization of hard-sphere colloids.

Vital for a variety of industries, colloids also serve as an excellent model to probe phase transitions at the individual particle level. Despite extensive studies, origins of the glass transition in hard-sphere colloids discovered about 30 y ago remain elusive. Results of our numerical simulations and asymptotic analysis suggest that cessation of long-time particle diffusivity does not suppress crystallization of a metastable liquid-phase hard-sphere colloid. Once a crystallite forms, its growth is then controlled by the particle diffusion in the depletion zone surrounding the crystallite. Using simulations, we evaluate the solid-liquid interface mobility from data on colloidal crystallization in terrestrial and microgravity experiments and demonstrate that there is no drastic difference between the respective mobility values. The insight into the effect of vanishing particle mobility and particle sedimentation on crystallization of colloids will help engineer colloidal materials with controllable structure.

Electro-Hydrodynamic Drop-on-Demand Printing of Aqueous Suspensions of Drug Nanoparticles.

We demonstrate the ability to fabricate dosage forms of a poorly water-soluble drug by using wet stirred media milling of a drug powder to produce an aqueous suspension of nanoparticles and then print it onto a porous biocompatible film. Contrary to conventional printing technologies, a deposited material is pulled out from the nozzle. This feature enables printing highly viscous materials with a precise control over the printed volume. Drug (griseofulvin) nanosuspensions prepared by wet media milling were printed onto porous hydroxypropyl methylcellulose films prepared by freeze-drying. The drug particles retained crystallinity and polymorphic form in the course of milling and printing. The versatility of this technique was demonstrated by printing the same amount of nanoparticles onto a film with droplets of different sizes. The mean drug content (0.19-3.80 mg) in the printed films was predicted by the number of droplets (5-100) and droplet volume (0.2-1.0 µL) (R2 = 0.9994, p-value < 10-4). Our results also suggest that for any targeted drug content, the number-volume of droplets could be modulated to achieve acceptable drug content uniformity. Analysis of the model-independent difference and similarity factors showed consistency of drug release profiles from films with a printed suspension. Zero-order kinetics described the griseofulvin release rate from 1.8% up to 82%. Overall, this study has successfully demonstrated that the electro-hydrodynamic drop-on-demand printing of an aqueous drug nanosuspension enables accurate and controllable drug dosing in porous polymer films, which exhibited acceptable content uniformity and reproducible drug release.

Single-bubble water boiling on small heater under Earth's and low gravity.

Today's trends for enhancing boiling heat transfer in terrestrial and space applications focus on removal of bubbles to prevent formation of a vapor layer over the surface at high overheat. In contrast, this paper presents a new boiling regime that employs a vapor-air bubble residing on a small heater for minutes and driving cold water over the surface to provide high heat flux. Single-bubble boiling of water was investigated under normal gravity and low gravity in parabolic flights. Experiments demonstrated a negligible effect of gravity level on the rate of heat transfer from the heater. Due to self-adjustment of the bubble size, the heat flux provided by boiling rose linearly up with increasing heater temperature and was not affected by a gradually rising water temperature. The fast response and stable operation of single-bubble boiling over a broad range of temperatures pave the way for development of new devices to control heat transfer by forming surface domains with distinct thermal properties and wettability. The bubble lifetime can be adjusted by changing the water temperature. The ability of heating water on millimeter scales far above 100 °C without an autoclave or a powerful laser provides a new approach for processing of biomaterials and chemical reactions.

Three-dimensional piezoelectric fibrous scaffolds selectively promote mesenchymal stem cell differentiation.

The discovery of electric fields in biological tissues has led to efforts in developing technologies utilizing electrical stimulation for therapeutic applications. Native tissues, such as cartilage and bone, exhibit piezoelectric behavior, wherein electrical activity can be generated due to mechanical deformation. Yet, the use of piezoelectric materials have largely been unexplored as a potential strategy in tissue engineering, wherein a piezoelectric biomaterial acts as a scaffold to promote cell behavior and the formation of large tissues. Here we show, for the first time, that piezoelectric materials can be fabricated into flexible, three-dimensional fibrous scaffolds and can be used to stimulate human mesenchymal stem cell differentiation and corresponding extracellular matrix/tissue formation in physiological loading conditions. Piezoelectric scaffolds that exhibit low voltage output, or streaming potential, promoted chondrogenic differentiation and piezoelectric scaffolds with a high voltage output promoted osteogenic differentiation. Electromechanical stimulus promoted greater differentiation than mechanical loading alone. Results demonstrate the additive effect of electromechanical stimulus on stem cell differentiation, which is an important design consideration for tissue engineering scaffolds. Piezoelectric, smart materials are attractive as scaffolds for regenerative medicine strategies due to their inherent electrical properties without the need for external power sources for electrical stimulation.

Detection of a dynamic cone-shaped meniscus on the surface of fluids in electric fields.

A cone-shaped meniscus of electrified fluids, often called a Taylor cone, is observed in rain drops and lightning and employed in various physical instruments and experimental techniques, but the way it evolves from a rounded shape to a cone is a long-standing puzzle. Earth's gravity and microgravity measurements on the meniscus whose height is just shy of droplet ejection reveal that field-driven cusp evolution exhibits a universal self-similarity insensitive to the forcing field and scaled by the fluid surface tension and density. Our work paves the way for dynamic control of field-driven phenomena in fluids.

Fast drying of biocompatible polymer films loaded with poorly water-soluble drug nano-particles via low temperature forced convection.

Fast drying of nano-drug particle laden strip-films formed using water-soluble biocompatible polymers via forced convection is investigated in order to form films having uniform drug distribution and fast dissolution. Films were produced by casting and drying a mixture of poorly water soluble griseofulvin (GF) nanosuspensions produced via media milling with aqueous hydroxypropyl methylcellulose (HPMC E15LV) solutions containing glycerin as a plasticizer. The effects of convective drying parameters, temperature and air velocity, and film-precursor viscosity on film properties were investigated. Two major drying regimes, a constant rate period as a function of the drying conditions, followed by a single slower falling rate period, were observed. Films dried in an hour or less without any irreversible aggregation of GF nanoparticles with low residual water content. Near-infrared chemical imaging (NIR-CI) and the content uniformity analysis indicated a better drug particle distribution when higher viscosity film-precursors were used. Powder X-ray diffraction showed that the GF in the films retained crystallinity and the polymorphic form. USP IV dissolution tests showed immediate release (~20 min) of GF. Overall, the films fabricated from polymer-based suspensions at higher viscosity dried at different conditions exhibited similar mechanical properties, improved drug content uniformity, and achieved fast drug dissolution.

Directed motion of colloidal particles in a galvanic microreactor.

The mechanisms leading to the deposition of colloidal particles in a copper-gold galvanic microreactor are investigated. Using in situ current density measurements and particle velocimetry, we establish correlations between the spatial arrangement and the geometry of the electrodes, current density distribution, and particle aggregation behavior. Ionic transport phenomena are responsible for the occurrence of strongly localized high current density at the edges and corners of the copper electrodes at large electrode separation, leading to a preferential aggregation of colloidal particles at the electrode edges. Preferential aggregation appears to be the result of a combination of electrophoretic effects and changes in bulk electrolyte flow patterns. We demonstrate that electrolyte flow is most likely driven by electrochemical potential gradients of reaction products formed during the inhomogeneous copper dissolution.

Electrodeless electrohydrodynamic drop-on-demand encapsulation of drugs into porous polymer films for fabrication of personalized dosage units.

Noncontact drop-on-demand (DOD) dosing is a promising strategy for manufacturing of personalized dosage units. However, current DOD methods developed for printing chemically and thermally stable, low-viscosity inks are of limited use for pharmaceuticals due to fundamentally different functional requirements. To overcome their deficiency, we developed a novel electrohydrodynamic (EHD) DOD (Appl, Phys, Lett. 97, 233501, 2010) that operates on fluids of up to 30 Pa·s in viscosity over a wide range of droplet sizes and provides a precise control over the droplet volume. We now evaluate the EHD DOD as a method for fabrication of dosage units by printing drug solutions on porous polymer films prepared by freeze-drying. Experiments were carried out on ibuprofen and griseofulvin, as model poorly water-soluble drugs, polyethylene glycol 400, as a drug carrier, and hydroxypropyl methylcellulose films. The similarities between drug release profiles from different dosage units were assessed by model-independent difference, f(1) , and similarity, f(2) , factors. The results presented show that EHD DOD offers a powerful tool for the evolving field of small-scale pharmaceutical technologies for tailoring medicines to individual patient's needs by printing a vast array of predefined amounts of therapeutics arranged in a specific pattern on a porous film.

A novel concept of dielectrophoretic engine oil filter.

A novel concept of an alternating current (AC) dielectrophoretic filter with a three-dimensional electrode array is presented. A filter is constructed by winding into layers around the core tube two sheets of woven metal wire-mesh with several sheets of woven insulating wire-mesh sandwiched in between. Contrary to conventional dielectrophoretic devices, the proposed design of electrodes generates a high-gradient field over a large working volume by applying several hundred volts at a standard frequency of 60 Hz. The operating principle of filtration is based on our recently developed method of AC dielectrophoretic gating for microfluidics. The filtration efficiency is expressed in terms of two non-dimensional parameters, which describe the combined influence of the particle polarizability and size, the oil viscosity and flow rate, and the field gradient on the particle captivity. The proof-of-concept is tested by measuring the single-pass performance of two filters on positively polarized particles dispersed in engine oil: spherical glass beads, fused aluminum oxide powder, and silicon metal powder, all smaller than the mesh opening. The results obtained are used to consider the potential of using AC dielectrophoretic filtration and provide critical design guidelines for the development of a filter based on the retention capability of challenge particles.

Field-induced alignment of flexible polyelectrolytes in solution.

Molecular dynamics simulations are used to study dynamical coupling between conformational fluctuations of a highly charged flexible macromolecule and its surrounding ionic cloud. We find that the basic model of a polyelectrolyte as a chain of charged monomers captures the main experimental findings on the field-induced alignment of polyelectrolytes despite its simplicity. Contrary to current theories, the correlated local charge and field fluctuations in the vicinity of the macromolecule are found to dominate alignment along an external field. We suggest that short-lived monomer-counterion clustering can be probed by measuring the field-induced anisotropy of x-ray and neutron scattering from polymer solutions.

Inhibition and promotion of copper corrosion by CTAB in a microreactor system.

We report on an optical microscopy technique for the analysis of corrosion kinetics of metal thin films in microreactor systems and use it to study the role of cetyltrimethylammonium bromide surfactant as a corrosion inhibitor in a copper-gold galvanic coplanar microsystem. A minimum in the dissolution rate of copper is observed when the surfactant concentration is approximately 0.8 mM. To explain why the inhibitory role of the surfactant does not extend to higher concentrations, we use zero resistance ammetry with separated half cells and show that while the surfactant inhibits cathodic reactions on gold, it also promotes the corrosion of copper because of the catalytic action of bromide counterions. These two competing processes lead to the observed minimum in the dissolution rate.

Dynamical clustering of counterions on flexible polyelectrolytes.

Molecular dynamics simulations are used to study the spatiotemporal dynamics of charge fluctuations around a polyelectrolyte molecule at charge densities above and below the classic counterion condensation threshold. Surprisingly, the counterions form weakly interacting clusters which exhibit slowly decaying short range orientational order. Local charge fluctuations create energy fluctuations at the order of k_(B)T that is sufficient to affect the polyelectrolyte interaction with an approaching ligand molecule. The predictions of the classical theory appear to be appropriate only over much longer time scales.

Dielectric spectroscopy of liquid crystalline dispersions.

We describe dielectric spectroscopy measurements on dispersions of two thermotropic liquid crystals (5CB and 8CB) in a poly(dimethylsiloxane) matrix. 5CB exhibits nematic and isotropic phases, while 8CB exhibits smectic, nematic, and isotropic phases. The spectra of the dispersions exhibit a temperature-dependent dielectric relaxation in the interval from 100 to 1000 Hz, with relaxation times that depend strongly on whether the dispersed phase is isotropic, nematic, or smectic. The dielectric relaxation times also depend on the viscosity of the matrix fluid. These results suggest a coupling between the electric field and the mechanics of the interface that affects the spectrum of the dispersed phase and shifts the Maxwell-Wagner interfacial polarization peak.

New electric-field-driven mesoscale phase transitions in polarized suspensions.

We report the discovery of a new class of an electric field-driven bulk phase transition due solely to dipolar interactions in a suspension under the action of a uniform ac field where the effects of other competing forces are suppressed. This transition appears after the well-known chain-column formation and causes the uniform suspension of columns to rearrange into a cellular pattern consisting of particle-free domains surrounded by particle-rich walls. Interestingly, the characteristic size of these domains scales linearly with the interelectrode spacing and remains insensitive to the size of the particles.

Hysteresis, force oscillations, and nonequilibrium effects in the adhesion of spherical nanoparticles to atomically smooth surfaces.

Equilibrium and nonequilibrium aspects of particle adsorption on the walls of fluid-filled nanochannels are examined via molecular dynamics simulations. The force on the particle and the free energy of the system are found to depend on the particle's history (hysteresis), in addition to its radial position and the wetting properties of the fluid, even when the particle moves quasistatically. The hysteresis is associated with changes in the fluid density in the gap between the particle and the wall, which persist over surprisingly long times. The force and free energy exhibit large oscillations with distance when the lattice of the structured nanoparticle is held in register with that of the tube wall, but not if the particle is allowed to rotate freely. Adsorbed particles are trapped in free-energy minima in equilibrium but can desorb if forced along the channel.

Combined negative dielectrophoresis and phase separation in nondilute suspensions subject to a high-gradient ac electric field.

Experiments were conducted on concentrated suspensions of neutrally buoyant particles which exhibit negative dielectrophoresis. We found that, due to interparticle electrical interactions, such suspensions undergo a phase separation when subjected to a high-gradient ac field (approximately kV/mm) and form a propagating distinct front between the regions enriched with and depleted of particles. A generalization of our theory for the thermodynamics of the field-induced phase transitions in suspensions of polarized particles [Phys. Rev. E 52, 1669 (1995); 54, 5428 (1996); 60, 3015 (1999)] is proposed for the front propagation, and its predictions are shown to be consistent with the experiments even though the model contains no fitting parameters. The combination of field-induced dielectrophoresis and phase transition provides a method for strongly concentrating particles in prespecified regions of dielectrophoretic devices.

Adsorption phenomena in the transport of a colloidal particle through a nanochannel containing a partially wetting fluid.

Using molecular dynamics simulations, we study the motion of a closely fitting nanometer-size solid sphere in a fluid-filled cylindrical nanochannel at low Reynolds numbers. At early times, when the particle is close to the middle of the tube, its velocity is in agreement with continuum calculations, despite large thermal fluctuations. At later times, partially wetting fluids exhibit novel adsorption phenomena: the sphere meanders away from the center of the tube and adsorbs onto the wall, and subsequently either sticks to the wall and remains motionless on average, or separates slightly from the tube wall and then either slips parallel to the mean flow or executes an intermittent stick-slip motion.