In-situ measurement of the internal compaction of a soft material caused by permeation flow.

HYPOTHESIS: The compaction of hydrogel films under permeation flow can be measured, in-situ, by tracking the internal displacements of their structure, thereby revealing the internal deformation profile. Additionally, monitoring the permeation flow rate and applied pressure over time enables determination of variations in the hydrogel's permeability due to flow-induced compaction. Hydrogels are soft porous materials capable of containing high amounts of water within their polymeric matrix. Flow-induced internal deformation can modify the hydrogel's permeability and selectivity, which are important attributes in separation processes, both industrial (e.g., membrane-based water purification) and natural (mucous filters in suspension feeders and intestinal lining) systems. Measuring the flow-induced compaction in thin hydrogels films can reveal the interplay between flow and permeability. However, the micro-scale internal compaction remains uncharted for due to experimental challenges. EXPERIMENTS: A technique is demonstrated for analyzing the compaction and stratification of permeable soft materials, in-situ, created by a pressure-driven permeation flow. To this end, the internal deformations within a soft material layer are calculated, based on tracking the positions of fluorescent micro-tracers that are embedded within the soft material. We showcase the capabilities of this technique by examining a hundred-micron-thick calcium-alginate cake deposited on a nanofiltration membrane, emphasizing the achieved micro-scale resolution of the local compaction measurements. FINDINGS: The results highlight the possibility to examine thin hydrogel films and their internal deformation produced by flow-induced stresses when varying the flow conditions. The method enables the simultaneous calculation of the soft material's permeance, as the pressure-driven flow conditions are continuously monitored. In summary, the proposed method provides a powerful tool for characterizing the behaviour of permeable soft materials under permeation conditions, with potential applications in engineering, biophysics and material science.

Pseudo-bottle-brush decorated thin-film composite desalination membranes with ultrahigh mineral scale resistance.

High water recovery is crucial to inland desalination but is impeded by mineral scaling of the membrane. This work presents a two-step modification approach for grafting high-density zwitterionic pseudo-bottle-brushes to polyamide reverse osmosis membranes to prevent scaling during high-recovery desalination of brackish water. Increasing brush density, induced by increasing reaction time, correlated with reduced scaling. High-density grafting eliminated gypsum scaling and almost completely prevented silica scaling during desalination of synthetic brackish water at a recovery ratio of 80%. Moreover, scaling was effectively mitigated during long-term desalination of real brackish water at a recovery ratio of 90% without pretreatment or antiscalants. Molecular dynamics simulations reveal the critical dependence of the membrane's silica antiscaling ability on the degree to which the coating screens the membrane surface from readily forming silica aggregates. This finding highlights the importance of maximizing grafting density for optimal performance and advanced antiscaling properties to allow high-recovery desalination of complex salt solutions.

Connecting the Non-Brownian Dots: Increasing Near-Neighbor Particle-Tracking Efficiency by Coordinate System Manipulation.

The nearest-neighbor algorithm (N-N) for single particle tracking (SPT) is widely employed for studying the deformation and mechanics of soft materials, or to detect flow in microfluidic systems. However, this algorithm may not perform well under certain conditions of oscillatory or directed motion of the studied tracers. Here, a method is presented with the goal of improving the performance of NN-SPT algorithms when studying directed and oscillatory motions. Specifically, the approach applies a change-of-basis matrix to the detected particles positions, prior to the calculations made by the NN-SPT algorithm. The presented results demonstrate the superior tracking efficiency when analyzing these systems, manifested via lower tracking mismatches and less spurious results than the original N-N algorithm.

Atomic Layer Deposition for Gradient Surface Modification and Controlled Hydrophilization of Ultrafiltration Polymer Membranes.

In recent years, atomic layer deposition (ALD) has emerged as a powerful technique for polymeric membrane surface modification. In this research, we study Al2O3 growth via ALD on two polymeric phase-inverted membranes: polyacrylonitrile (PAN) and polyetherimide (PEI). We demonstrate that Al2O3 can easily be grown on both membranes with as little as 10 ALD cycles. We investigate the formation of Al2O3 layer gradient through the depth of the membranes using high-resolution transmission electron microscopy and elemental analysis, showing that at short exposure times, Al2O3 accumulates at the top of the membrane, reducing pore size and creating a strong growth gradient, while at long exposure time, more homogeneous growth occurs. This detailed characterization creates the knowledge necessary for controlling the deposition gradient and achieving an efficient growth with minimum pore clogging. By tuning the Al2O3 exposure time and cycles, we demonstrate control over the Al2O3 depth gradient and membranes' pore size, hydrophilicity, and permeability. The oil antifouling performance of membranes is investigated using in situ confocal imaging during flow. This characterization technique reveals that Al2O3 surface modification reduces oil droplet surface coverage.

Hydrodynamic-Colloidal Interactions of an Oil Droplet and a Membrane Surface.

Membranes have been shown to be exceptionally successful in the challenging separation of stable oil/water emulsions but suffer from severe fouling that limits their performance. Understanding the mechanisms leading to oil deposition on the membrane surface, as influenced by hydrodynamics and colloidal surface interactions, is imperative for informing better engineered membrane surfaces and process conditions. Here, we study the interactions between an oil droplet and a membrane surface. Hydrodynamics within the water film, confined between the droplet and the membrane, are captured within the framework of the lubrication approximation, coupled with the van der Waals (vdW) and electrostatic interactions through the droplet shape, which is governed by an augmented Young-Laplace equation. The model is used to calculate possible equilibrium positions, where the droplet is held at a finite distance from the membrane by a balance of the forces present. An equilibrium phase diagram is constructed as a function of various process parameters and is shown in terms of the scaled permeation rate through the membrane. The phase diagram identifies the range of conditions leading to deposition, characterized by a "critical" permeation rate, beyond which no equilibrium exists. When equilibrium positions are permitted, we find that these may be classified as stable/unstable, in the kinetic sense. Further, our results demonstrate the link between the deformation of the droplet and the stability of equilibria. An upward deflection of the droplet surface, owing to a dominant, long-range repulsion, has a stabilizing effect, as it maintains the separation between the droplet and membrane. Conversely, a downward deflection is destabilizing because of the self-amplifying effect of strongly increasing attractive forces with separation distance-as the surfaces are pulled together because of deformation, the attractive force increases, causing further deformation. This is also manifested by a dependence of the bistable region on the deformability of the droplet, which is represented by a capillary number, modified so as to account for the effect of the permeable boundary. As the droplet becomes more easy to deform, the transition from an unconditionally stable region of the phase diagram to a point beyond which there is no equilibrium (interpreted as deposition) becomes abrupt. These results provide valuable physical insights into the mechanisms that govern oil fouling of membrane surfaces.

Mineral Scale Prevention on Electrically Conducting Membrane Distillation Membranes Using Induced Electrophoretic Mixing.

The growth of mineral crystals on surfaces is a challenge across multiple industrial processes. Membrane-based desalination processes, in particular, are plagued by crystal growth (known as scaling), which restricts the flow of water through the membrane, can cause membrane wetting in membrane distillation, and can lead to the physical destruction of the membrane material. Scaling occurs when supersaturated conditions develop along the membrane surface due to the passage of water through the membrane, a process known as concentration polarization. To reduce scaling, concentration polarization is minimized by encouraging turbulent conditions and by reducing the amount of water recovered from the saline feed. In addition, antiscaling chemicals can be used to reduce the availability of cations. Here, we report on an energy-efficient electrophoretic mixing method capable of nearly eliminating CaSO4 and silicate scaling on electrically conducting membrane distillation (ECMD) membranes. The ECMD membrane material is composed of a percolating layer of carbon nanotubes deposited on porous polypropylene support and cross-linked by poly(vinyl alcohol). The application of low alternating potentials (2 Vpp,1Hz) had a dramatic impact on scale formation, with the impact highly dependent on the frequency of the applied signal, and in the case of silicate, on the pH of the solution.

Oil Deposition on Polymer Brush-Coated NF Membranes.

Membrane-based processes are attractive for treating oily wastewaters. However, membrane fouling due to the deposition of oil droplets on the membrane surface compromises performance. Here, real-time observation of the deposition of oil droplets by direct confocal microscopy was conducted. Experiments were conducted in dead-end and crossflow modes. Base NF 270 nanofiltration membranes as well as membranes modified by grafting poly(N-isopropylacrylamide) chains from the membrane surface using atom transfer radical polymerization were investigated. By using feed streams containing low and high NaCl concentrations, the grafted polymer chains could be induced to switch conformation from a hydrated to a dehydrated state, as the lower critical solution temperature for the grafted polymer chains moved above and below the room temperature, respectively. For the modified membrane, it was shown that switching conformation of the grafted polymer chains led to the partial release of adsorbed oil. The results also indicate that, unlike particles such as polystyrene beads, adsorption of oil droplets can lead to coalescence of the adsorbed oil droplets on the membrane surface. The results provide further evidence of the importance of membrane properties, feed solution characteristics, and operating mode and conditions on membrane fouling.

Field-Induced Redistribution of Surfactants at the Oil/Water Interface Reduces Membrane Fouling on Electrically Conducting Carbon Nanotube UF Membranes.

Membrane-based treatment of oily wastewater remains a significant challenge, particularly under high salinity conditions. The main difficulty associated with this separation process is membrane fouling, mostly caused by wetting and coalescence of emulsified oil droplets on the membrane surface. In this study, electrically conducting carbon nanotube-based ultrafiltration membranes were used to treat an emulsified oil suspension at ionic strengths as high as 100 mM. By tuning the electrical potential applied to the membrane surface, we demonstrate how fouling can be dramatically reduced, even under high salinity conditions. Permeate water quality is shown to improve upon application of a negative potential. Using optical microscopy, we observed dramatic changes in the shape of oil droplets at the membrane/water interface in response to the applied electric potential; this change is associated with a redistribution of charged surfactant molecules at the oil/water interface in response to the external electric field. Specifically, using the membrane as a cathode repels surfactant molecules away from the oil/membrane interface, while anodic conditions lead to increased surfactant concentrations. We speculate that this change in surfactant molecule distribution is responsible for changes in the surface tension of oil droplets at the membrane/water interface, which results in a decrease in oil coalescence and subsequent fouling. The membranes used in this study offer an attractive treatment option when separating emulsified oil from water under high salinity conditions.

Periodic energy conversion in an electric-double-layer capacitor.

Electrostatic conversion devices operate through periodic modulation of capacitance. Such devices have a wide range of configurations, involving either changes in permittivity, electrode-plate spacing or wetting area. The presented study examines, theoretically, a potential configuration of an electric-double-layer capacitor (EDLC)-based transducer, as it converts concentration and temperature oscillations into an electric alternating current. A constant voltage applied at the EDLC electrodes results in the formation of two opposite-sign EDLs, and an electric current is generated when ionic charges pass from one EDL to the other. In the examined configuration, this ionic charge transfer is induced by boundary modulation of temperature and/or concentration. To capture the oscillating dynamics of the ion distribution and ion flux, we solve the full set of Poisson-Nernst-Planck (PNP) equations coupled with the energy equation. We find that the transducer's optimal conditions for conversion, for which the device's frequency response is maximized, are governed by three main factors: low irreversible Joule heating, confined geometry, where the capacitor thickness is a close as possible to the EDL's characteristic screening length, and, most importantly, 'tuning' the system to a resonance frequency dictated by the interplay between geometry and characteristic time scales for mass and heat diffusion.

Microscale Dynamics of Oil Droplets at a Membrane Surface: Deformation, Reversibility, and Implications for Fouling.

Despite their excellent capabilities, wide implementation of membranes for oil/water emulsion separation is limited due to severe fouling. To date, microscale dynamics of the oil-water-membrane system are poorly understood. The present study uses confocal microscopy at unprecedented resolution for direct observation of oil droplet deposition, deformation, and detachment during separation and cleaning, respectively. The 3D shape of the droplets was imaged as a function of the permeation rate, J, droplet radius, R, membrane permeance, k, water viscosity, µ, and the water/oil interfacial tension coefficient, σ. These parameters yield a modified capillary number, [Formula: see text] = µVR1/2/σk1/2, which accounts for the extra viscous "suction" at close proximity to the membrane surface. A clear correlation was observed between the degree of droplet deformation and an increasing [Formula: see text]. Furthermore, the reversibility of droplet deposition and membrane performance were assessed through microscopic surface coverage and flux recovery analysis. In general, operation at a low flux (3.9 µm/s) yields spherical droplets that are easily removed by crossflow cleaning, whereas a high flux (85 µm/s) leads to significant deformation and mostly irreversible deposition. These results shed important new insight on the influence of hydrodynamic conditions on fouling reversibility during emulsion separation, and may guide better design of surface-modified membranes.

Adsorption-Mediated Mass Streaming in a Standing Acoustic Wave.

Oscillating flows can generate nonzero, time-averaged fluxes despite the velocity averaging zero over an oscillation cycle. Here, we report such a flux, a nonlinear resultant of the interaction between oscillating velocity and concentration fields. Specifically, we study a gas mixture sustaining a standing acoustic wave, where an adsorbent coats the solid boundary in contact with the gas mixture. It is found that the sound wave produces a significant, time-averaged preferential flux of a "reactive" component that undergoes a reversible sorption process. This effect is measured experimentally for an air-water vapor mixture. An approximate model is shown to be in good agreement with the experimental observations, and further reveals the interplay between the sound-wave characteristics and the properties of the gas-solid sorbate-sorbent pair. The preferential flux generated by this mechanism may have potential in separation processes.

Elastic Relaxation of Fluid-Driven Cracks and the Resulting Backflow.

Cracks filled with fluid propagation when the pressurized fluid is injected into the crack. Subsequently, when the fluid inlet is exposed to a lower pressure, the fluid flows backwards (backflow) and the crack closes due to the elastic relaxation of the solid. Here we study the dynamics of the crack closure during the backflow. We find that the crack radius remains constant and the fluid volume in the crack decreases with time in a power-law manner at late times. The balance between the viscous stresses in the fluid and elastic stresses in the fluid and the elastic stresses in the solid yields a scaling law that agrees with the experimental results for different fluid viscosities, Young's moduli of the solid, and initial radii of the cracks. Furthermore, we visualize the time-dependent crack shapes, and the convergence to a universal dimensionless shape demonstrates the self-similarity of the crack shapes during the backflow process.

Thermodynamic analysis of osmotic energy recovery at a reverse osmosis desalination plant.

Recent years have seen a substantial reduction of the specific energy consumption (SEC) in seawater reverse osmosis (RO) desalination due to improvements made in hydraulic energy recovery (HER) as well as RO membranes and related process technologies. Theoretically, significant potential for further reduction in energy consumption may lie in harvesting the high chemical potential contained in RO concentrate using salinity gradient power technologies. Herein, "osmotic energy recovery" (OER) is evaluated in a seawater RO plant that includes state-of-the-art RO membranes, plant designs, operating conditions, and HER technology. Here we assume the use of treated wastewater effluent as the OER dilute feed, which may not be available in suitable quality or quantity to allow operation of the coupled process. A two-stage OER configuration could reduce the SEC of seawater RO plants to well below the theoretical minimum work of separation for state-of-the-art RO-HER configurations with a breakeven OER CAPEX equivalent to 42% of typical RO-HER plant cost suggesting significant cost savings may also be realized. At present, there is no commercially viable OER technology; hence, the feasibility of using OER at seawater RO plants remains speculative, however attractive.

Impacts of Operating Conditions and Solution Chemistry on Osmotic Membrane Structure and Performance.

Herein, we report on changes in the performance of a commercial cellulose triacetate (CTA) membrane, imparted by varied operating conditions and solution chemistries. Changes to feed and draw solution flow rate did not significantly alter the CTA membrane's water permeability, salt permeability, or membrane structural parameter when operated with the membrane skin layer facing the draw solution (PRO-mode). However, water and salt permeability increased with increasing feed or draw solution temperature, while the membrane structural parameter decreased with increasing draw solution, possibly due to changes in polymer intermolecular interactions. High ionic strength draw solutions may de-swell the CTA membrane via charge neutralization, which resulted in lower water permeability, higher salt permeability, and lower structural parameter. This observed trend was further exacerbated by the presence of divalent cations which tends to swell the polymer to a greater extent. Finally, the calculated CTA membrane's structural parameter was lower and less sensitive to external factors when operated in PRO-mode, but highly sensitive to the same factors when the skin layer faced the feed solution (FO-mode), presumably due to swelling/de-swelling of the saturated porous substructure by the draw solution. This is a first attempt aimed at systematically evaluating the changes in performance of the CTA membrane due to operating conditions and solution chemistry, shedding new insight into the possible advantages and disadvantages of this material in certain applications.