Adhesion of emulsified oil droplets to hydrophilic and hydrophobic surfaces - effect of surfactant charge, surfactant concentration and ionic strength

Intertwisted superhydrophilic and superhydrophobic collagen fibers enabled anti-fouling high-performance separation of emulsion wastewater

Oil-contaminated wastewater has been one of the most concerned environmental issues. Superwetting materials-enabled remediation of oil contamination in wastewater faces the critical challenge of fouling problems due to the formation of intercepted phase. Herein, high-performance separation of emulsions wastewater was accomplished by developing collagen fibers (CFs)-derived water-oil dual-channels that were comprised of intertwisted superhydrophilic and superhydrophobic CFs. The dual-channels relied on the superhydrophilic CFs to accomplish efficient demulsifying, which played the role as water-channel to enable fast transportation of water, while the superhydrophobic CFs served as the oil-transport channel to permit oil transportation. The mutual repellency between water-channel and oil-channel was essential to guarantee the stability of established dual-channels. The unique dual-channel separation mechanism fundamentally resolved the intercepted phase-caused fouling problem frequently engaged by the superwetting materials that provided single-channel separation capability. Long-lasting (1440 min) anti-fouling separations were achieved by the superwetting CFs-derived dual-channels with separation efficiency high up to 99.99%, and more than 4-fold of stable separation flux as compared with that of superhydrophilic CFs with single-channel separation capability. Our investigations demonstrated a novel strategy by using superwetting CFs to develop water-oil dual-channels for achieving high-performance anti-fouling separation of emulsions wastewater. ENVIRONMENTAL IMPLICATION: Industrial processes discard a large amount of emulsion wastewater, which seriously imperils the aquatic ecosystem. This work demonstrated a conceptual-new strategy to achieve effective remediation of emulsion wastewater via the water-oil dual-channels established by the intertwisted superhydrophilic and superhydrophobic collagen fibers (CFs). The superhydrophilic CFs enabled efficient demulsification of emulsions and played the role of water-channel for the rapid transportation of water, while the superhydrophobic CFs worked as oil-channel to permit the efficient transportation of oil pollutants. Consequently, the long-term (1440 min) anti-fouling high-performance separation of emulsion wastewater was achieved.

A comprehensive review on the behavior and evolution of oil droplets during oil/water separation by membranes

Membrane separation technology has significant advantages for treating oil-in-water emulsions. Understanding the evolution of oil droplets could reveal the interfacial and colloidal interactions, facilitate the design of advanced membranes, and improve the separation performances. This review on the characteristic behavior and evolution of oil droplets focuses on the advanced analytical techniques, and the subsequent fouling as well as demulsification effects during membrane separation. A detailed introduction is provided on microscopic observations and numerical simulations of the dynamic evolution of oil droplets, featuring real-time in-situ visualization and accurate reconstruction, respectively. Characteristic behaviors of these oil droplets include attachment, pinning, wetting, spreading, blockage, intrusion, coalescence, and detachment, which have been quantified by specific proposed parameters and criteria. The fouling process can be evaluated using Hermia and resistance models. The related adhesion force and intrusion pressure as well as droplet-droplet/membrane interfacial interactions can be accurately quantified using various force analysis methods and advanced force measurement techniques. It is encouraging to note that oil coalescence has been achieved through various effects such as electrostatic interactions, mechanical actions, Laplace pressure/surface free energy gradients, and synergistic effects on functional membranes. When oil droplets become destabilized and coalesce into larger ones, the functional membranes can overcome the limitations of size-sieving effect to attain higher separation efficiency. This not only bypasses the trade-off between permeability and rejection, but also significantly reduces membrane fouling. Finally, the challenges and potential research directions in membrane separation are proposed. We hope this review will support the engineering of advanced materials for oil/water separation and research on interface science in general.

Pickering Emulsions Stabilized by Tea Water-Insoluble Protein Nanoparticles From Tea Residues: Responsiveness to Ionic Strength

Tea water-insoluble protein nanoparticles (TWIPNs) can be applied to stabilize Pickering emulsions. However, the effect of ionic strength (0–400 mmol/L) on the characteristics of Pickering emulsions stabilized by TWIPNs (TWIPNPEs) including volume-averaged particle size (d_4,3), zeta potential, microstructure and rheological properties is still unclear. Therefore, this work researched the effect of ionic strength on the characteristics of TWIPNPEs. The d_4,3 of TWIPNPEs in the aquatic phase increased with the increase in ionic strength (0–400 mmol/L), which was higher than that in the SDS phase. Furthermore, the flocculation index of TWIPNPEs significantly (P < 0.05) increased from 24.48 to 152.92% with the increase in ionic strength. This could be verified from the microstructure observation. These results indicated that ionic strength could promote the flocculation of TWIPNPEs. Besides, the absolute values of zeta potential under different ionic strengths were above 40 mV in favor of the stabilization of TWIPNPEs. The viscosity of TWIPNPEs as a pseudoplastic fluid became thin when shear rate increased from 0.1 to 100 s⁻¹. The viscoelasticity of TWIPNPEs increased with increasing ionic strength to make TWIPNPEs form a gel-like Pickering emulsion. the possible mechanism of flocculation stability of TWIPNPEs under different ionic strengths was propose. TWIPNs adsorbed to the oil-water interface would prompt flocculation between different emulsion droplets under the high ionic strength to form gel-like behavior verified by CLSM. These results on the characteristics of TWIPNPEs in a wide ionic strength range would provide the theoretical basis for applying Pickering emulsions stabilized by plant proteins in the food industry.

Polyelectrolyte Complex Hollow Fiber Membranes Prepared via Aqueous Phase Separation

Hollow fiber (HF) membrane geometry is the preferred choice for most commercial membrane operations. Such fibers are conventionally prepared via the non-solvent-induced phase separation technique, which heavily relies on hazardous and reprotoxic organic solvents such as N-methyl pyrrolidone. A more sustainable alternative, i.e., aqueous phase separation (APS), was introduced recently that utilizes water as a solvent and non-solvent for the production of polymeric membranes. Herein, for the first time, we demonstrate the preparation of sustainable and functional HF membranes via the APS technique in a dry-jet wet spinning process. The dope solution comprising poly(sodium 4-styrenesulfonate) (PSS) and polyethyleneimine (PEI) at high pH along with an aqueous bore liquid is pushed through a single orifice spinneret into a low pH acetate buffer coagulation bath. Here, PEI becomes charged resulting in the formation of a polyelectrolyte complex with PSS. The compositions of the bore liquid and coagulation bath were systematically varied to study their effect on the structure and performance of the HF membranes. The microfiltration-type membranes (permeability ∼500 to 800 L·m^-2·h^-1·bar^-1) with complete retention of emulsion droplets were obtained when the precipitation rate was slow. Increasing the concentration of the acetate buffer in the bath led to the increase in precipitation rate resulting in ultrafiltration-type membranes (permeability ∼12 to 15 L·m^-2·h^-1·bar^-1) having molecular weight cut-offs in the range of ∼7.8-11.6 kDa. The research presented in this work confirms the versatility of APS and moves it another step closer to large-scale use.

Recent Mitigation Strategies on Membrane Fouling for Oily Wastewater Treatment

The discharge of massive amounts of oily wastewater has become one of the major concerns among the scientific community. Membrane filtration has been one of the most used methods of treating oily wastewater due to its stability, convenience handling, and durability. However, the continuous occurrence of membrane fouling aggravates the membrane's performance efficiency. Membrane fouling can be defined as the accumulation of various materials in the pores or surface of the membrane that affect the permeate's quantity and quality. Many aspects of fouling have been reviewed, but recent methods for fouling reduction in oily wastewater have not been explored and discussed sufficiently. This review highlights the mitigation strategies to reduce membrane fouling from oily wastewater. We first review the membrane technology principle for oily wastewater treatment, followed by a discussion on different fouling mechanisms of inorganic fouling, organic fouling, biological fouling, and colloidal fouling for better understanding and prevention of membrane fouling. Recent mitigation strategies to reduce fouling caused by oily wastewater treatment are also discussed.

Hydrophobized metallic meshes can ease water droplet rolling

Rolling liquid droplets are of great interest for various applications including self-cleaning of surfaces. Interfacial resistance, in terms of pinning and shear rate, has a critical role in droplet rolling dynamics on hydrophobic surfaces. Lowering the interfacial resistance requires reducing the droplet wetting length and droplet fluid contact area on hydrophobic surfaces. The present study examines droplet rolling behavior on inclined hydrophobized metallic meshes, which facilitate reduced wetting length and contact area of droplets. Experiments are carried out using a high-speed recording facility to evaluate droplet translational and rolling velocities over various sizes of hydrophobized meshes. The flow field inside the droplet fluid is simulated in 3-dimensional space mimicking the conditions of experiments. The findings reveal that droplet translational velocity attains significantly higher values for hydrophobized meshes than plain hydrophobized metallic surfaces. Increasing the mesh size enhances the droplet velocity and reduces the droplet kinetic energy dissipation created by interfacial surface tension and shear forces. Increasing the droplet volume enhances the droplet velocity despite the fact that pinning and frictional forces increase at the liquid-mesh interface. Hence, for rolling droplets on the mesh surface, the increase in the gravitational force component becomes larger than the increase in interfacial pinning and frictional forces.

Cellulose-based special wetting materials for oil/water separation: A review

Oil spill accidents and oily wastewater discharged by petrochemical industries have severely wasted water resources and damaged the environment. The use of special wetting materials to separate oil and water is efficient and environment-friendly. Cellulose is the most abundant renewable resource and has natural advantages in removing pollutants from oily wastewater. The application and modification of cellulose as special wetting materials have attracted considerable research attention. Therefore, we summarized cellulose-based superlipophilic/superhydrophobic and superhydrophilic/superoleophobic materials exhibiting special wetting properties for oil/water separation. The treatment mechanism, preparation technology, treatment effect, and representative projects of oil-bearing wastewater are discussed. Moreover, cellulose-based intelligent-responsive materials for application to oil/water separation and the removal of other pollutants from oily wastewater have also been summarized. The prospects and potential challenges of all the materials have been highlighted.

Unraveling driving regimes for destabilizing concentrated emulsions within microchannels

Coalescence is the most widely demonstrated mechanism for destabilizing emulsion droplets in microfluidic chambers. However, we find that depending on the channel wall surface functionalization, surface zeta potential, type of surfactant, characteristics of the oil as a dispersed phase, or even the presence of externally-induced stress, other different destabilization mechanisms can occur in subtle ways. In general, we observe four regimes leading to destabilization of concentrated emulsions: (i) coalescence, (ii) emulsion bursts, (iii) a combination of the two first mechanisms, attributed to the simultaneous occurrence of coalescence and emulsion bursts; and (iv) compaction of the droplet network that eventually destabilizes to fracture-like behavior. We correlate various physico-chemical properties (zeta potential, contact angle, interfacial tension) to understand their respective influence on the destabilization mechanisms. This work provides insights into possible ways to control or inflict emulsion droplet destabilization for different applications.

An overview of nanoemulsion characterization via atomic force microscopy

Nanoemulsion-based systems are widely applied in food industries for protecting active ingredients against oxidation and degradation and controlling the release rate of active core ingredients under particular conditions. Visualizing the interface morphology and measuring the interfacial interaction forces of nanoemulsion droplets are essential to tailor and design intelligent nanoemulsion-based systems. Atomic force microscopy (AFM) is being established as an important technique for interface characterization, due to its unique advantages over traditional imaging and surface force-determining approaches. However, there is a gap in knowledge about the applicability of AFM in characterizing the droplet interface properties of nanoemulsions. This review aims to describe the fundamentals of the AFM technique and nanoemulsions, mainly focusing on the recent use of AFM to investigate nanoemulsion properties. In addition, by reviewing interfacial studies on emulsions in general, perspectives for the further development of AFM to study nanoemulsions are also discussed.

Effects of feed composition on the fouling on cation-exchange membranes desalinating polymer-flooding produced water

HYPOTHESIS

Cation exchange membranes (CEMs) are subject to fouling when utilized to desalinate wastewater from the oil and gas industry, hampering their performance. The kind and extent of the fouling are most likely dependent on the composition of the stream, which in practical applications can vary significantly.

EXPERIMENTS

Fouling experiments were performed on commercial cation exchange membranes, which were used in electrodialysis runs to desalinate solutions of varying composition. The variations included ionic strength, type of ions, amount of viscosifying polyelectrolyte (partially hydrolyzed polyacrylamide), presence of crude oil, and surfactants. Performance parameters, like electric potential and pH, were monitored during the runs, after which the membranes were recovered and analyzed.

FINDINGS

Fouling was detected on most CEMs and occurred mainly in the presence of the viscosifying polyelectrolyte. Under normal pH conditions (pH ~ 8), the polyelectrolyte fouled the concentrate side of the CEMs, as expected due to electrophoresis. However, by applying a current in the opposite direction, the polyelectrolyte layer could be removed. Precipitation occurred mostly on the opposite side of the membrane, with different morphology depending on the feed composition.

Surfactant-dependent critical interfacial tension in silicon carbide membranes for produced water treatment

During fossil oil extraction, a complex water stream known as produced water (PW), is co-extracted. Membrane treatment makes PW re-use possible, but fouling and oil permeation remain major challenges. In this work, membrane fouling and oil retention of Synthetic PW stabilized with a cationic, anionic, zwitterionic or nonionic surfactant, were studied at various surfactant and salt concentrations. We discuss our results in the framework of the Young-Laplace (YL) equation, which predicts for a given membrane, pressure and oil-membrane contact angle, a critical interfacial tension (IFT) below which oil permeation should occur. We observe such a transition from high to low oil retention with decreasing IFT for the anionic (SDS), cationic (CTAB) and non-ionic (TX) surfactant, but at significantly higher critical IFTs than predicted by YL. On the other side, for the zwitterionic DDAPS we do not observe a drop in oil retention, even at the lowest IFT. The discrepancy between our findings and the critical IFT predicted by YL can be explained by the difference between the measured contact angle and the effective contact angle at the wall of the membrane pores. This leads to a surfactant-dependent critical IFT. Additionally, our results point out that zwitterionic surfactants even at the lowest IFT did not present a critical IFT and exhibited low fouling and low oil permeation.

Aqueous Phase Separation of Responsive Copolymers for Sustainable and Mechanically Stable Membranes

Membranes are often used in environmentally friendly applications and as a sustainable alternative to conventional processes. Unfortunately, the vast majority of polymeric membranes are produced via an unsustainable and environmentally unfriendly process that requires large amounts of harsh reprotoxic chemicals such as N-methyl-2-pyrrolidinone and dimethylformamide. In this work, we investigate an aqueous phase separation (APS) system that uses weak polyelectrolytes, whose charge is dependent on the pH (weak polyelectrolytes), to produce membranes. Specifically the copolymer polystyrene-alt-maleic acid (PSaMA) is used. PSaMA contains responsive monomers, required for APS, and also unresponsive hydrophobic monomers that provide mechanical stability to the resultant membranes. This work demonstrates that by controlling the precipitation of PSaMA, it is possible to prepare a wide range of membranes; from microfiltration membranes capable of treating oily waste water to dense nanofiltration-type membranes with excellent micropollutant retentions and high mechanical stability. While similar materials in prior work could only withstand 4 bar, the membranes presented here demonstrate stable operation up to 20 bar. The only solvents used in this APS system are water and the green solvent acetic acid, thus making our APS process significantly more sustainable and environmentally friendly as compared to the conventional membrane fabrication methods.

Stimuli-Responsive Membranes through Sustainable Aqueous Phase Separation

Polymeric membranes are used on huge scales for kidney dialysis, wastewater treatment, and drinking water production. However, almost all polymeric membranes are fabricated by a process reliant on the use of unsustainable, expensive, and reprotoxic dipolar aprotic solvents. In this work, we propose an aqueous phase separation approach for preparing porous membrane films. Poly(4-vinylpyridine) (P4VP), a pH-responsive polymer, is first dissolved at low pH where the polymer is charged and subsequently cast as a thin film. Switching to a high pH where the polymer is uncharged and insoluble results in controlled phase separation and solidification of the polymer into porous membrane structures. This approach gives a large degree of control over membrane structure, leading to symmetric porous microfiltration membranes and asymmetric dense nanofiltration membranes. Moreover, the use of a pH-responsive polymer leads directly to a pH-responsive membrane, where the degree of responsive behavior can be tuned by the degree of cross-linking. Such responsive behavior allows effective cleaning of the membrane, without the use of harsh chemicals. This work outlines an approach toward preparing membranes in a more sustainable fashion-an approach that allows control over the membrane structure and one that naturally leads to advanced membranes with responsive properties.

Large scale and integrated platform for digital mass culture of anchorage dependent cells

Industrial applications of anchorage-dependent cells require large-scale cell culture with multifunctional monitoring of culture conditions and control of cell behaviour. Here, we introduce a large-scale, integrated, and smart cell-culture platform (LISCCP) that facilitates digital mass culture of anchorage-dependent cells. LISCCP is devised through large-scale integration of ultrathin sensors and stimulator arrays in multiple layers. LISCCP provides real-time, 3D, and multimodal monitoring and localized control of the cultured cells, which thereby allows minimizing operation labour and maximizing cell culture performance. Wireless integration of multiple LISCCPs across multiple incubators further amplifies the culture scale and enables digital monitoring and local control of numerous culture layers, making the large-scale culture more efficient. Thus, LISCCP can transform conventional labour-intensive and high-cost cell cultures into efficient digital mass cell cultures. This platform could be useful for industrial applications of cell cultures such as in vitro toxicity testing of drugs and cosmetics and clinical scale production of cells for cell therapy.

Large scale culture of adherent cells would benefit from a platform for continuous monitoring and control of cell growth and culture conditions. Here the authors develop an integrated, smart cell culture platform where cells are grown on multiple layers of thin sensors that can be wirelessly integrated across several incubators.