Modified ceramic membrane with pH/ethanol induced switchable superwettability for antifouling separation of oil-in-acidic water emulsions

Recent advances in membrane technologies applied in oil-water separation

Effective treatment of oily wastewater, which is toxic and harmful and causes serious environmental pollution and health risks, has become an important research field. Membrane separation technology has emerged as a key area of investigation in oil-water separation research due to its high separation efficiency, low costs, and user-friendly operation. This review aims to report on the advances in the research of various types of separation membranes around emulsion permeance, separation efficiency, antifouling efficiency, and stimulus responsiveness. Meanwhile, the challenges encountered in oil-water separation membranes are examined, and potential research avenues are identified.

A comprehensive review on anticorrosive/antifouling superhydrophobic coatings: Fabrication, assessment, applications, challenges and future perspectives

Superhydrophobicity (SHP) is an incredible phenomenon of extreme water repellency of surfaces ubiquitous in nature (E.g. lotus leaves, butterfly wings, taro leaves, mosquito eyes, water-strider legs, etc). Historically, surface exhibiting water contact angle (WCA) > 150° and contact angle hysteresis <10° is considered as SHP. The SHP surfaces garnered considerable attention in recent years due to their applications in anti-corrosion, anti-fouling, self-cleaning, oil-water separation, viscous drag reduction, anti-icing, etc. As corrosion and marine biofouling are global problems, there has been focused efforts in combating these issues using innovative environmentally friendly coatings designs taking cues from natural SHP surfaces. Over the last two decades, though significant progress has been made on the fabrication of various SHP surfaces, the practical adaptation of these surfaces for various applications is hampered, mainly because of the high cost, non-scalability, lack of simplicity, non-adaptability for a wide range of substrates, poor mechanical robustness and chemical inertness. Despite the extensive research, the exact mechanism of corrosion/anti-fouling of such coatings also remains elusive. The current focus of research in recent years has been on the development of facile, eco-friendly, cost-effective, mechanically robust chemically inert, and scalable methods to prepare durable SHP coating on a variety of surfaces. Although there are some general reviews on SHP surfaces, there is no comprehensive review focusing on SHP on metallic and alloy surfaces with corrosion-resistant and antifouling properties. This review is aimed at filling this gap. This review provides a pedagogical description with the necessary background, key concepts, genesis, classical models of superhydrophobicity, rational design of SHP, coatings characterization, testing approaches, mechanisms, and novel fabrication approaches currently being explored for anticorrosion and antifouling, both from a fundamental and practical perspective. The review also provides a summary of important experimental studies with key findings, and detailed descriptions of the evaluation of surface morphologies, chemical properties, mechanical, chemical, corrosion, and antifouling properties. The recent developments in the fabrication of SHP -Cr-Mo steel, Ti, and Al are presented, along with the latest understanding of the mechanism of anticorrosion and antifouling properties of the coating also discussed. In addition, different promising applications of SHP surfaces in diverse disciplines are discussed. The last part of the review highlights the challenges and future directions. The review is an ideal material for researchers practicing in the field of coatings and also serves as an excellent reference for freshers who intend to begin research on this topic.

Flexible droplet transportation and coalescence via controllable thermal fields

Flexible droplet transportation and coalescence are significant for lots of applications such as material synthesis and analytical detection. Herein, we present an effective method for controllable droplet transportation and coalescence via thermal fields. The device used for droplet manipulation is composed of a glass substrate with indium tin oxide-made microheaers and a microchannel with two transport branches and a central chamber, and it's manipulated by sequentially powering the microheaters located at the bottom of microchannel. The fluid will be unevenly heated when the microheater is actuated, leading to the formation of thermal buoyancy convection and the decrease of interfacial tension of fluids. Subsequently, the microdroplets can be transported from the inlets of microchannel to the target position by the buoyancy flow-induced Stokes drag. And the droplet migration velocity can be flexibly adjusted by changing the voltage applied on the microheater. After being transported to the center of central chamber, the coalescence behaviors of microdroplets can be triggered if the microheater located at the bottom of central chamber is continuously actuated. The droplet coalescence is the combined effect of decreased fluid interfacial tension, the shortened droplet distance by buoyancy flow and the increased instability of droplet under the elevated temperature. The droplet coalescence efficiency is also related to the voltage of microheater, by increasing the voltage from 3.5 V to 7 V, the needed time for droplet coalescence dramatically decrease from 220s to 1.4 s. Finally, by the droplet coalescence-triggered calcium hydroxide precipitation reaction, we demonstrate the applicability of the droplet manipulation method on specific sample detection. Therefore, this approach used for droplet transportation and coalescence can be attractive for many droplet-based applications such as analytical detection.

Highly Permeable Sulfonated Polydopamine Integrated MXene Membranes for Efficient Surfactant-Stabilized Oil-in-Water Separation

MXene is an incredibly promising two-dimensional material with immense potential to serve as a high-performing separating or barrier layer to develop advanced membranes. Despite the significant progress made in MXene membranes, two major challenges still exist: (i) effectively stacking MXene nanosheets into defect-free membranes and (ii) the high fouling tendency of MXene-based membranes. To address these issues, we employed sulfonated polydopamine (SPD), which simultaneously serves as a binding agent to promote the compact assembling of Ti3C2Tx MXenes (MX) nanosheets and improves the antifouling properties of the resulting sulfonated polydopamine-functionalized MX (SPDMX) membranes. The SPDMX membrane was tested for challenging surfactant-stabilized oil-in-water separation with an impressive efficiency of 98%. Moreover, an ultrahigh permeability of 1620 LMH/bar was also achieved. The sulfonation of PD helps in improving the antifouling characteristics of SPDMX by developing a strong hydration layer and enhancing the oleophobicity of the membrane. The underwater SPDMX membrane appeared superoleophobic with an oil contact angle of 153°, whereas the ceramic membrane exhibited an oil contact angle of 137°. The SPDMX membranes showed an improved flux recovery (31%) compared to the nonsulfonated counterpart. This work highlights the appropriate functionalization of MXene as a promising approach to developing MXene membranes with high permeation flux and better antifouling characteristics for oily wastewater treatment.

The Preparation of Superhydrophobic Polylactic Acid Membrane with Adjustable Pore Size by Freeze Solidification Phase Separation Method for Oil-Water Separation

An environmentally friendly pore size-controlled, superhydrophobic polylactic acid (PLA) membrane was successfully prepared by a simpler freeze solidification phase separation method (FSPS) and solution impregnation, which has application prospects in the field of oil-water separation. The pore size and structure of the membrane were adjusted by different solvent ratios and solution impregnation ratios. The PLA-FSPS membrane after solution impregnation (S-PLA-FSPS) had the characteristics of uniform pore size, superhydrophobicity and super lipophilicity, its surface roughness Ra was 338 nm, and the contact angle to water was 151°. The S-PLA-FSPS membrane was used for the oil-water separation. The membrane oil flux reached 16,084 L·m^-2·h^-1, and the water separation efficiency was 99.7%, which was much higher than that of other oil-water separation materials. In addition, the S-PLA-FSPS membrane could also be applied for the adsorption and removal of oil slicks and underwater heavy oil. The S-PLA-FSPS membrane has great application potential in the field of oil-water separation.

Novel Strategy of Polymers in Combination with Silica Particles for Reversible Control of Oil-Water Interface

Hybrid smart emulsification systems are highly applicable in manipulating oil-in-water (O/W) droplets. Herein, novel switchable block polymers containing both zwitterionic and tertiary amine pendent groups were designed and synthesized to combine with charged silica particles to stabilize the O/W emulsion responsive to pH. This study was carried out in O/W emulsions stabilized with the polymer and silica particles under different pH conditions. The emulsion system was also simulated using molecular dynamics simulation to reveal the mechanism at molecular levels, thus gaining insight into the relationships between the emulsifying properties and the molecular interaction of the mixed system. Upon acidification of the continuous aqueous phase, protonated polymers with excellent hydrophilicity were induced by charged silica particles to cause rapid emulsion coalescence. In alkaline media, the mixed system conversely stabilized the O/W emulsions, cutting polymer consumption by over three-quarters. The emulsification and demulsification can be switched alternately by tuning the pH conditions. The applications exhibited excellent efficiency in separating heavy oil/water emulsions and proved the high conversion rate in emulsion polymerization. Overall, with this novel strategy to relieve tedious modifications on particle surfaces and massive consumption of polymers, the designed responsive emulsification systems can impart intelligent and controllable chemical reactivity to emulsions on demand in a more affordable and sustainable way.