Mobile Liquid Gating Membrane System for Smart Piston and Valve Applications

Energy harvesting from water streaming at charged surface

Water flowing at a charged surface may produce electricity, known as streaming current/potentials, which may be traced back to the 19th century. However, due to the low gained power and efficiencies, the energy conversion from streaming current was far from usable. The emergence of micro/nanofluidic technology and nanomaterials significantly increases the power (density) and energy conversion efficiency. In this review, we conclude the fundamentals and recent progress in electrical double layers at the charged surface. We estimate the generated power by hydrodynamic energy dissipation in multi-scaling flows considering the viscous systems with slipping boundary and inertia systems. Then, we review the coupling of volume flow and current flow by the Onsager relation, as well as the figure of merits and efficiency. We summarize the state-of-the-art of electrokinetic energy conversions, including critical performance metrics such as efficiencies, power densities, and generated voltages in various systems. We discuss the advantages and possible constraints by the figure of merits, including single-phase flow and flying droplets.

Oil-gated isoporous membrane with micro-apertures for controllable pressure-induced passive flow regulator

The pressure-driven liquid flow controller is one of the key components in diverse applications including microfluidic systems, biomedical drug injection devices, and pressurized water supply systems. Electric feedback loop based flow controllers are fine-tunable but expensive and complex. The conventional safety valves based on spring force are simple and low cost, but their diverse application is limited due to their fixed pressure range, size, and shape. Herein, we propose a simple and controllable liquid-flowing system combining a closed liquid reservoir and an oil-gated isoporous membrane (OGIM). The ultra-thin and flexible OGIM acts as an immediately responsive and precisely controlled gas valve to maintain internal pneumatic pressure as designed to induce constant liquid flow. The oil filling apertures act as a gate for gas flow depending on the applied pressure and the threshold (gating) pressure of the gate is determined by the surface tension of the oil and the gate diameter. It is confirmed that the gating pressure is precisely controlled by varying the gate diameter, which agrees with the theoretically estimated pressures. Based on stably maintained pressure due to the function of OGIM, the constant liquid flow rate is achieved even with the high gas flow rate.

Self-Oscillating Liquid Gating Membranes with Periodic Gas Transport

Liquid gating membranes with molecular-level smooth liquid lining layers break through the limitations of traditional porous membrane materials in gas transport control. Owing to the stable, self-healing, and reconfigurable properties, liquid gating membranes have shown wide application prospects in microfluidics, intelligent valves, chemical reactions, and beyond. Here, we develop a periodic gas transport control system based on the self-oscillating liquid gating membrane. Under continuous gas injection, the gas–liquid interface is reversibly deformed, enabling self-oscillating behavior for discontinuous and periodic gas transport without the need for any complex external changes to the original system. Meanwhile, our experimental analysis reveals that the periodic time and periodic gas release in the system can be regulated. Based on the cycle stability of the system, we further demonstrate the controllability of the system for periodic droplet manipulation in microfluidics. Looking forward, it will offer new opportunities for various applications, such as pneumatic robots, gas-involved chemical reactions, droplet microfluidics, and beyond.

Carbon Dioxide Chemically Responsive Switchable Gas Valves with Protonation-Induced Liquid Gating Self-Adaptive Systems

Carbon dioxide (CO₂ ) capture and storage technologies are promising to limit CO₂ emission from anthropogenic activities, to achieve carbon neutrality goals. CO₂ capture requires one to separate CO₂ from other gases, and therefore a gas flow system that exhibits discernible gating behaviors for CO₂ would be very useful. Here we propose a self-adaptive CO₂ gas valve composed of chemically responsive liquid gating systems. The transmembrane critical pressures of the liquid gate vary upon the presence of CO₂ , due to the superamphiphiles assembled by poly(propylene glycol) bis(2-aminopropyl ether) and oleic acid in gating liquids that are protonated specifically by CO₂ . It is shown that the valve can perform self-adaptive regulation for specific gases and different concentrations of CO₂ . This protonation-induced liquid gating mechanism opens a potential platform for applications of CO₂ separators, detectors, sensors and beyond.

Liquid-Based Adaptive Structural Materials

Structural materials are used to provide stable mechanical architectures and transmit or support forces, and they play an important role in materials science and technology. During the long process of the exploitation of structural materials, the functionality of structural materials has gained prominence. Adaptive structures responding to external stimuli have come to the fore with significant advantages in structural materials. However, many solid adaptive structural materials still suffer from their single function and the lack of dynamic performance, such as issue around fouling and energy consumption, defects present everywhere in materials at the microscale, etc. To meet the increasing demands, more and more researchers have started turning their attention to liquid-based materials owing to their intrinsic spontaneous, dynamic, and functional properties. Liquid-based adaptive structural materials (LASMs) have been proposed and developed. Building upon both dynamic liquids and fixed solids, LASMs have been demonstrated to possess both dynamic adaptivity (from the active liquid part) and stable mechanical structure (from the fixed solid part), which are desired in many applications such as 3D printing, droplet manipulation, omniphobic surfaces, microfluidics, mass separation, etc. A unifying view of the recent progress of LASMs is presented, including liquid with particles, liquid with surfaces, as well as liquid with membranes. In addition, the discussion of the prospects and challenges are provided for promoting the development of LASMs.

Integrated Iontronic Circuits Based on Single Nanochannels

This article presents an invention of iontronic circuits to achieve effective electronic signal manipulation base on paralleled single polydimethylsiloxane (PDMS) nanochannels. The regulation of ion transport through the designed device is achieved by the asymmetrical decoration of charged polyelectrolytes on nanochannels. This circuit can serve as either a bipolar junction transistor or an ionic rectifier. The functionalities of the circuit are demonstrated and confirmed in different operation modes. The fabrication of this device is relatively simple. High accuracy in signal rectification can be obtained through the circuit because of the significant consistency of integrated ionic diodes. The iontronic integrated circuits presented in this paper provide new possibilities in the fabrication of iontronic devices with various functionalities on PDMS chips, toward improved biological computing and sensing.

Liquid gating technology

Recent years have witnessed the emergence of liquid gating technologies that employ liquids as structural materials to provide dynamic gating control. Such technologies have attracted considerable attention globally owing their antifouling, energy-saving, reversible, and reconfigurable characteristics. This study considers a new perspective to discuss advancements in liquid gating technologies, including the concept, mechanisms, development, designs, and emerging applications. Moreover, recommendations are provided for the selection of the gating liquid and porous matrix, preparation processes, technical parameters, and theoretical modelling to guide related research. Emerging applications of liquid gating technologies, such as microscale flow control, multiphase separation, chemical detection, and biomedical catheters, are reported. Finally, the challenges currently faced by these technologies are discussed and potential directions for further research are explored to promote the use of these technologies in future applications.

Light-responsive and corrosion-resistant gas valve with non-thermal effective liquid-gating positional flow control

Safe and precise control of gas flow is one of the key factors to many physical and chemical processes, such as degassing, natural gas transportation, and gas sensor. In practical application, it is essential for the gas-involved physicochemical process to keep everything under control and safe, which significantly relies on the controllability, safety, and stability of their valves. Here we show a light-responsive and corrosion-resistant gas valve with non-thermal effective liquid-gating positional flow control under a constant pressure by incorporating dynamic gating liquid with light responsiveness of solid porous substrate. Our experimental and theoretical analysis reveal that the photoisomerization of azobenzene-based molecular photoswitches on the porous substrate enabled the gas valve to possess a light-responsive and reversible variation of substantial critical pressure of non-thermal effective gas flow switch. Moreover, the chemically inert gating liquid prevented the solid substrate from corrosion and, by combining with the high spatiotemporal resolution of light, the gas valve realizes a precisely positional open and close under a steady-state pressure. The application demonstrations in our results show the potentials of the new gas valve for bringing opportunities to many applications, such as gas-involved reaction control in microfluidics, soft actuators, and beyond.

Electrochemically mediated gating membrane with dynamically controllable gas transport

The regulation of mass transfer across membranes is central to a wide spectrum of applications. Despite numerous examples of stimuli-responsive membranes for liquid-phase species, this goal remains elusive for gaseous molecules. We describe a previously unexplored gas gating mechanism driven by reversible electrochemical metal deposition/dissolution on a conductive membrane, which can continuously modulate the interfacial gas permeability over two orders of magnitude with high efficiency and short response time. The gating mechanism involves neither moving parts nor dead volume and can therefore enable various engineering processes. An electrochemically mediated carbon dioxide concentrator demonstrates proof of concept by integrating the gating membranes with redox-active sorbents, where gating effectively prevented the cross-talk between feed and product gas streams for high-efficiency, directional carbon dioxide pumping. We anticipate our concept of dynamically regulating transport at gas-liquid interfaces to broadly inspire systems in fields of gas separation, miniaturized devices, multiphase reactors, and beyond.

Bioinspired liquid gating membrane-based catheter with anticoagulation and positionally drug release properties

Catheters are indispensable medical devices that are extensively used in daily medical treatment. However, existing catheter materials continue to encounter many problems, such as thrombosis, single functionality, and inadaptability to environmental changes. Inspired by blood vessels, we develop a self-adaptive liquid gating membrane-based catheter with anticoagulation and positionally drug release properties. Our multifunctional liquid gating membrane-based catheter significantly attenuates blood clot formation and can be used as a general catheter design strategy to offer various drugs positionally releasing applications to comprehensively enhance the safety, functionality, and performance of medical catheters' materials.

Highly stretchable and reliable graphene oxide-reinforced liquid gating membranes for tunable gas/liquid transport

The ability of membrane technologies to dynamically tune the transport behavior for gases and liquids is critical for their applications. Although various methods have been developed to improve membrane success, tradeoffs still exist among their properties, such as permeability, selectivity, fouling resistance, and stability, which can greatly affect the performance of membranes. Existing elastomeric membrane designs can provide antifracture properties and flexibility; however, these designs still face certain challenges, such as low tensile strength and reliability. Additionally, researchers have not yet thoroughly developed membranes that can avoid fouling issues while realizing precise dynamic control over the transport substances. In this study, we show a versatile strategy for preparing graphene oxide-reinforced elastomeric liquid gating membranes that can finely modulate and dynamically tune the sorting of a wide range of gases and liquids under constant applied pressures. Moreover, the produced membranes exhibit antifouling properties and are adaptable to different length scales, pressures, and environments. The filling of graphene oxide in the thermoplastic polyurethane matrix enhances the composites through hydrogen bonds. Experiments and theoretical calculations are carried out to demonstrate the stability of our system. Our membrane exhibits good stretchability, recovery, and durability due to the elastic nature of the solid matrix and dynamic nature of the gating liquid. Dynamic control over the transport of gases and liquids is achieved through our optimized interfacial design and controllable pore deformation, which is induced by mechanical stimuli. Our strategy will create new opportunities for many applications, such as gas-involved chemical reactions, multiphase separation, microfluidics, multiphase microreactors, and particulate material synthesis.

Soft interface design for electrokinetic energy conversion

The exploitation and utilization of renewable clean energy is of great significance to the sustainable development of society. Electrokinetic energy conversion (EKEC) based on micro/nanochannels is expected to provide immense potential for ocean energy harvesting, self-powered micro/nanodevices, and small portable power supplies through converting environmental energy into electrical energy. Herein, aiming to get a deeper understanding of EKEC based on micro/nanochannels, several classic theoretical models and corresponding calculation equations are introduced briefly. For high efficiency energy conversion, it is essential to clearly discuss the interface properties between the inner surface of the channel and the bulk electrolyte solution. Therefore, we put forward soft interface designs of solid-liquid and liquid-liquid interfaces, and summarize their recent progress. In addition, the different applications of EKEC, harvesting from environmental energy, are further discussed. We hope that this review will attract more scientists' attention to transform the experimental results of EKEC systems in the lab into available products on shelves.

How Does Chemistry Influence Liquid Wettability on Liquid-Infused Porous Surface?

Design of Nepenthes pitcher-inspired slippery liquid-infused porous surface (SLIPS) appeared as an important avenue for various potential and practically relevant applications. In general, hydrophobic base layers were infused with selected liquid lubricants for developing chemically inert SLIPS. Here, in this current study, an inherently hydrophilic (soaked beaded water droplet with ∼20° within a couple of minutes), porous and thick (above 200 μm) polymeric coating, loaded with readily chemically reactive acrylate moieties yielded a chemically reactive SLIPS, where residual acrylate groups in the synthesized hydrophilic and porous interface rendered stability to the infused lubricants. The chemically reactive SLIPS is capable of reacting with the solution of primary amine-containing nucleophiles in organic solvent through 1,4-conjugate addition reaction, both in the presence (referred as "in situ" modification) and absence (denoted as pre-modification) of lubricated phase in the porous polymeric coating. Such amine reactive SLIPS was further extended to (1) examining the impact of different chemical modifications on the performance of SLIPS and (2) developing a spatially selective and "in situ" postmodification with primary amine-containing nucleophiles through 1,4-conjugate addition reaction. Moreover, the chemically reactive SLIPS was capable of sustaining various physical abrasions and prolonged (minimum 10 days) exposure to complex and harsh aqueous phases, where infused lubricants protect the residual acrylate groups from harsh aqueous exposures. Such, principle will be certainly useful for spatially selective covalent immobilization of water-insoluble functional molecules/polymers directly from organic solvents, which would be of potential interest for various applied and fundamental contexts.

Metallic Liquid Gating Membranes

The development of liquid gating membrane (LGM) systems with tunable multiphase selectivity and antifouling properties is limited by the mechanical stability of the membrane materials. The mechanical integrity of most polymeric membranes can be compromised by deformation under harsh operating conditions (elevated temperatures, corrosive environments, foulants, etc.), ultimately leading to their failure. Here, a facile electrochemical approach to the fabrication of multifunctional metal-based liquid gating membrane systems is presented. The membrane porosity, pore size, and membrane surface roughness can be tuned from micro- to nanometer scale, enabling function under a variety of operating conditions. The prepared LGMs demonstrate controllable gas-liquid selectivity, superior resistance to corrosive conditions and fouling chemicals, and significant reduction of the transmembrane pressure required for the separation process, resulting in lower energy consumption. The stability of the gating liquid is confirmed experimentally through sustained fouling resistance and further supported by the interfacial energy calculations. The mechanically robust metal-based membrane systems reported in this study significantly extend the operating range of LGMs, prompting their applications in water treatment processes such as wastewater treatment, degassing, and multiphase separation.

Liquid-based porous membranes

The confluence of available membrane materials and the explorations into fluid behaviors have revolutionized liquid-based porous membranes, which deserve more attention.

A Focus on Soft Actuation

The present editorial paper analyzes the hundred recent research works on soft actuation to understand the current main research focus in the light of the grand challenges in the field. Two characteristic paper types were obtained: one focuses on soft actuator design, manufacturing and demonstration, while another includes in addition the development of functional materials. Although vast majority of the works showcased soft actuation, evaluation of its robustness by multi-cyclic actuation was reported in less than 50% of the works, while only 10% described successful actuation for more than 1000 cycles. It is suggested that broadening the research focus to include investigation of mechanisms underlying the degradation of soft functional material performance in real cyclic actuation conditions, along with application of artificial intelligence methods for prediction of muscle behavior, may allow overcoming the reliability issues and developing robust soft-material actuators. The outcomes of the present work might be applicable to the entire soft robotics domain.