Ultrafast Water Transport Research Articles

Ultrafast water diffusion along the interface between oxidized and pristine regions in graphene oxide: Reactive molecular dynamics study

Graphene oxide (GO) is a promising membrane material due to its high water permeability. However, the exact physical mechanisms governing this process at the molecular level remain poorly understood, despite more than a decade of practical applications. In this article, we use classical molecular dynamics with the reactive potential ReaxFF to study the mobility of water molecules intercalated in GO and analyze the influence of its structure on diffusion processes. We highlight the previously unmentioned role of the interfacial area between oxidized and pristine graphene regions, which, according to our calculations, may be responsible for the ultrafast water transport observed in GO. This diffusion exhibits characteristics of a ballistic regime, suggesting another possible mechanism underlying GO’s high water permeability.

Ultrafast Water Transport of Reverse Osmosis Membrane Based on Quasi-Vertically Oriented 2D Interlayer.

Interlayered thin-film composite (i-TFC) membranes based on 2D materials have been widely studied due to their high efficiency in mass transfer. However, the randomly stacked 2D nanosheets usually increase the fluid path length to some extent. Herein, in situ-grown quasi-vertically oriented 2D ZIF-L was introduced as an interlayer for preparing high-performance reverse osmosis (RO) membranes. Through the optimization of the crystal growth based on the inert polyethylene substrate, the novel i-TFC RO membrane via interfacial polymerization shows an outstanding water permeance (5.50 L m-2 h-1 bar-1) and good NaCl rejection (96.3%). The membrane also shows promising potential in domestic water purification and organic solvent separation applications. Compared with the randomly stacked ZIF-L interlayer, the advantages of the vertically oriented one were ascribed to the excellent storage capacity of the amine monomers and the intensified gutter effect. This work will encourage more exploration on the interlayer architectures for high-performance i-TFC membranes.

Bioinspired Composite Fabrics with Nanoneedle Structures for High Wicking-Evaporation performance

Achieving ultrafast antigravity water transport and evaporation in high-performance moisture-wicking fabrics remains a significant challenge in textile engineering. In this study, we introduce two novel asymmetric composite fabrics, NN-TS/PET/BS and NS-TS/PET/BS, developed through a straightforward one-step method. These fabrics incorporate different hydrophilic nanostructures—nanoneedles (NN) and nanosheets (NS) of cobalt carbonate—to enhance wicking-evaporation capabilities. The NN-TS/PET/BS fabric exhibited superior unidirectional water transport and evaporation performance, achieving a one-way transport value (R) of up to 330 %, an outstanding overall moisture management capacity (OMMC) of 0.87, and a rapid water evaporation rate of 0.5 g h⁻¹, which is 2 and 2.3 times higher than that of NS-TS/PET/BS and Coolmax, respectively. Detailed analyses revealed that the curvature gradients in the nanoneedles significantly enhanced the wetting and evaporation stages, facilitating efficient antigravity water transport. Wear studies confirmed that the NN-TS/PET/BS fabric could swiftly remove sweat from the skin surface, enhancing comfort and performance. These findings offer new perspectives for developing high-performance unidirectional water transport fabrics with broad application potential, including smart textiles, fog harvesting, and wound dressings.

In-situ confined preparation of COF@GO nanofiltration membranes for high-efficiency dye removal

Graphene oxide membrane (GOM) possesses ultrafast and tunable two-dimensional water transport pathways, making it an ideal candidate for nanofiltration separation of dye wastewater. However, GOM also faces great challenges such as poor aqueous stability, collapse of non-oxygenated regions under high pressure, and limitations imposed by trade-off effects. In this study, covalent organic frameworks (COFs) were in-situ formed within GOM interlayers and on membrane surface to enlarge interlayer spacings and enhance hydrophilicity of GOM simultaneously via a two-step method including monomer retention in GOM and in-situ confined interfacial polymerization of COFs. Fourier transformation infrared spectroscopy (FT-IR) and X-ray photoelectron spectrometry (XPS) results confirmed the successful synthesis of COF on GOM surface, which enhanced membrane surface hydrophilicity and bridged the split defects of GOM. According to X-ray diffraction measurements (XRD) and scanning electron microscope (SEM) results, COF@GO membranes possessed enlarged interlayer spacings and increased thickness of selective layer, which indicated the successful intercalation of COF into GO interlayer. COF@GO-50 membrane exhibited an ultrahigh pure water permeability (PWP) of 148.1 L/(m2·h·bar), with 99.0 % rejection for direct red 80 (DR80) aqueous solution and 9.6 % NaCl rejection. The ultrahigh water permeance might be ascribed to the enlarged interlayer pathways, the shortened vertical pathways provided by in-plane pores of COF nanosheets, and enhanced water adsorption capacity due to enhanced membrane hydrophilicity. Moreover, COF@GO nanofiltration membrane demonstrated excellent anti-fouling properties and long-term stability in 30h continuous operation, maintaining considerable separation efficiency under harsh conditions. This work provides a straightforward and applicable strategy to simultaneously enlarge interlayer spacings and enhance surface hydrophilicity of GOM via in-situ confined preparation of COFs, achieving ultrahigh water permeance and overcoming the trade-off effect between permeability and selectivity of GOM.

Built-in Electric Fields in Heterostructured Lamellar Membranes Enable Highly Efficient Rejection of Charged Mass.

Separation membranes with homogeneous charge channels are the mainstream to reject charged mass by forming electrical double layer (EDL). However, the EDL often compresses effective solvent transport space and weakens channel-ion interaction. Here, built-in electric fields (BIEFs) are constructed in lamellar membranes by assembling the heterostructured nanosheets, which contain alternate positively-charged nanodomains and negatively-charged nanodomains. We demonstrate that the BIEFs are perpendicular to horizontal channel and the direction switches alternately, significantly weakening the EDL effect and forces ions to repeatedly collide with channel walls. Thus, highly efficient rejection for charged mass (salts, dyes, and organic acids/bases) and ultrafast water transport are achieved. Moreover, for desalination on four-stage filtration option, salt rejection reaches 99.9 % and water permeance reaches 19.2 L m-2 h-1 bar-1. Such mass transport behavior is quite different from that in homogeneous charge channels. Furthermore, the ion transport behavior in nanochannels is elucidated by validating horizontal projectile motion model.

Built‐in Electric Fields in Heterostructured Lamellar Membranes Enable Highly Efficient Rejection of Charged Mass

Separation membranes with homogeneous charge channels are the mainstream to reject charged mass by forming electrical double layer (EDL). However, the EDL often compresses effective solvent transport space and weakens channel‐ion interaction. Here, built‐in electric fields (BIEFs) are constructed in lamellar membranes by assembling the heterostructured nanosheets, which contain alternate positively‐charged nanodomains and negatively‐charged nanodomains. We demonstrate that the BIEFs are perpendicular to horizontal channel and the direction switches alternately, significantly weakening the EDL effect and forces ions to repeatedly collide with channel walls. Thus, highly efficient rejection for charged mass (salts, dyes, and organic acids/bases) and ultrafast water transport are achieved. Moreover, for desalination on four‐stage filtration option, salt rejection reaches 99.9% and water permeance reaches 19.2 L m‐2 h‐1 bar‐1. Such mass transport behavior is quite different from that in homogeneous charge channels. Furthermore, the ion transport behavior in nanochannels is elucidated by validating horizontal projectile motion model.

Ultrafast Water Transport in Two-dimensional Vermiculite–Ionic Liquid Composite Membrane to Enhance Rapid Humidity Sensing

Recent advances in humidity sensors have emphasized their critical roles in various fields, from food processing to healthcare. Vermiculite (V), as a two-dimensional (2D) material, can be exploited in humidity sensors with numerous advantages such as low cost, thermal stability, and ease of functionalization for large-scale manufacturing. Here we demonstrated that the 2D characteristics of V, combined with ultrafast transport of confined water in its nanocapillaries, significantly enhance the rapid adsorption and desorption of water, thereby endowing the humidity sensor with rapid sensing capabilities. Furthermore, we employed the ionic liquid (IL), [EMIM][BF4] as an intercalating agent to modify V utilizing the electrostatic and hydrogen bonding interactions between them. The ultrafast transport of water in the V-[EMIM][BF4] membrane was not only improved, but the confined water in nanocapillaries was also transformed from a “constrained” to a comparatively “relaxed” state. This “relaxed” water allowed it to rotate into suitable orientation for efficient proton transfer. Consequently, the V-[EMIM][BF4] membrane-based sensor exhibited the improved transient response of 5 s and 34 s in the range of 30%–80% relative humidity. This study leveraged the benefits of IL-modified V membranes to pave the way for cost-effective humidity sensing devices with rapid responses.

Scalable Superhydrophilic Solar Evaporators for Long-Term Stable Desalination, Fresh Water Collection and Salt Collection by Vertical Salt Deposition.

Solar-driven interfacial evaporation (SIE) is very promising to solve the issue of fresh water shortage, however, poor salt resistance severely hinders long-term stable SIE and fresh water collection. Here, we report design of superhydrophilic solar evaporators for long-term stable desalination, fresh water collection and salt collection by vertical salt deposition. The evaporators are prepared by sequentially deposition of silicone nanofilaments, polypyrrole and Au nanoparticles on a polyester fabric composed of microfibers. The evaporators feature excellent photothermal effect and ultrafast water transport, due to their unique micro-/nanostructure and superhydrophilicity. As a result, during SIE the salt gradually deposits vertically rather than occupies larger area on the evaporators. Consequently, long-term stable SIE with high evaporation rates of 2.4-2.1 kg m-2 h-1 for 3.5-20 wt % brine in continuous 10 h is achieved under 1 sun illumination. Meanwhile, the loosely deposited salt can be easily collected, realizing zero brine discharge. Moreover, scalable preparation of the evaporator is achieved, which exhibits efficient collection of high quality fresh water (10.08 kg m-2 in 8 h) via SIE desalination under weak natural sunlight (0.46~0.66 sun). This strategy sheds a new light on the design of high-performance solar evaporators and their real-world fresh water collection.

Bioinspired structural hydrogels with highly ordered hierarchical orientations by flow-induced alignment of nanofibrils

Natural structural materials often possess unique combinations of strength and toughness resulting from their complex hierarchical assembly across multiple length scales. However, engineering such well-ordered structures in synthetic materials via a universal and scalable manner still poses a grand challenge. Herein, a simple yet versatile approach is proposed to design hierarchically structured hydrogels by flow-induced alignment of nanofibrils, without high time/energy consumption or cumbersome postprocessing. Highly aligned fibrous configuration and structural densification are successfully achieved in anisotropic hydrogels under ambient conditions, resulting in desired mechanical properties and damage-tolerant architectures, for example, strength of 14 ± 1 MPa, toughness of 154 ± 13 MJ m−3, and fracture energy of 153 ± 8 kJ m−2. Moreover, a hydrogel mesoporous framework can deliver ultra-fast and unidirectional water transport (maximum speed at 65.75 mm s−1), highlighting its potential for water purification. This scalable fabrication explores a promising strategy for developing bioinspired structural hydrogels, facilitating their practical applications in biomedical and engineering fields.

From Sheep Track to Motorway: Supramolecular-Mediated 2D Nanofluidic Channels for Ultrafast Water Transport.

Atomic thick 2D materials hold great potential as building blocks to construct highly permeable membranes, yet the permeability of laminar 2D material membranes is still limited by their irregularity sheep track-like interlayer channels. Herein, a supramolecular-mediated strategy to induce the regular assembly of high-throughput 2D nanofluidic channels based on host-guest interactions is proposed. Inspired by the characteristics of motorways, supramolecular-mediated ultrathin 2D membranes with broad and continuous regular water transport channels are successfully constructed using graphene oxide (GO) as an example. The prepared membrane achieves an ultrahigh water permeability (369.94LMHbar-1) more than six times higher than that of the original membranes while maintaining dye rejection above 98.5%, which outperforms the reported 2D membranes. Characterization and simulation results show that the introduction of hyaluronate-grafted β-cyclodextrin not only expands the interlayer channels of GO membranes but also enables the membranes to operate stably under harsh conditions with the help of host-guest interactions. This universal supramolecular assembly strategy provides new opportunities for the preparation of 2D membranes with high separation performance and reliable and stable nanofluidic channels.

Biomimetic Murray nanofiber membranes with pore/wetting double gradient for ultrafast directional water transport and evaporative textiles

Directional water transport (DWT) textiles, possessing moisture-wicking and evaporative fast-drying capabilities, help in creating a comfortable microenvironment for the human body. However, fabricating synthetic materials that follow Murray’s law and replicate the pore gradient of vascular plants remains challenging, thereby impeding the achievement of a good combination of moisture conduction, fast drying, and osmosis resistance. In this study, DWT membranes comprising three layers of pore/wetting gradients were constructed using a straightforward electrospinning/netting technique. The inner and intermediate layers, comprising hydrophobic polyurethane (PU) and hydrophilic PU-hydrolyzed polyacrylonitrile (PU-HPAN) nanofibers with average diameters of 1.83 µm and 255 nm, respectively, were prepared via electrospinning. Furthermore, the superhydrophilic outer layer (HPAM) comprised HPAN and a blend of acrylic acid/acrylamide with an average diameter of 76 nm. This layer was prepared via the electro-netting of dilute solution with high electrical conductivity, resulting in a spontaneous and continuous water transport, coupled with rapid drying. The DWT membranes exhibited an ultrahigh one-way transport capability (R) of 1270%, achieving an evaporation rate of 0.86 g h−1. Additionally, they demonstrated rapid drying within 16 min, effectively preventing reverse osmosis under pressure. Therefore, these membranes can be applied for moisture wicking, water extraction, and micro fluidic control.

Construction of robust one-dimensional nanowire-regulated graphene oxide membranes for efficient dye/salt separation

Loose nanofiltration (NF) for graphene oxide (GO) membranes is an emerging efficient strategy for separating dye/salt mixtures in textile wastewater to realize sustainable resource recovery. This study proposes a novel approach for constructing a laminated GO membrane with interlayer water-transporting nanochannels by intercalating tannic acid nanowires (TANs). The interlayer spacing of the GO membrane can be expanded by one-dimensional TANs while cross-linking between hydrophilic TANs and GO nanosheets enhance the mechanical stability of this sheet-wire structure and interlayer channels. The membrane is able to achieve excellent water permeability without sacrificing selectivity due to the dual driving force resulting from the ultrafast water transport ability of primitive graphene planes and interlayer hydrophilic nanowires. The modified GO membrane exhibits a high water permeability of 52.1 L m−2 h−1 bar−1, which is twice that of the pure GO membrane and excellent selectivity of 85.5 to the methyl blue/NaCl mixed solution. Overall, this work provides a feasible strategy for constructing a mixed-dimensional GO-based membrane with a robust structure and multi-pathways to achieve excellent perm-selectivity for dye/salt separation.

Thermal resistance analysis of micro heat pipes incorporated with carbon nanotubes nanocapillaries

The remarkable performance improvement and thermal optimization of micro heat pipes (MHPs) incorporated with carbon nanotubes (CNTs) of different density are analysed using a thermal resistance model. The ultrafast water transport characteristic of CNT-nanocapillaries induces large-surface-area water film where the evaporation, condensation, and condensate circulation processes, which govern the MHP's overall performance, are simultaneously enhanced. By employing a thermal resistance model, the pertinent parameters inside an MHP, which are not measurable due to the miniature size of the microchannels, are estimated. By benchmarking with the uncoated MHP, the condenser and evaporator heat transfer coefficients of the optimal 5 mg/CNTs MHPs are enhanced remarkably by the up to 249% and 264%, respectively. Furthermore, the circulation rate attains a maximum increase of 212%. Consequently, an enhancement up to 250% in the overall performance of MHP gauged by the effective thermal conductivity and a significant temperature drop of 20.8 °C on the heat source as compared to the uncoated MHP case are achieved. By minimizing the total thermal resistance of MHP, the optimized length ratio of the evaporator and condenser, which is useful in the optimal design of the MHP, can be obtained. The thermal resistance model serves as a useful analytical tool to provide insightful thermal analysis and optimized design of an MHP.

Construction of 2D/0D Graphene Oxide/Copper(I) Oxide-Incorporated Titanium Dioxide Mixed-Dimensional Membranes with Ultrafast Water Transport and Enhanced Antifouling Properties.

Graphene oxide (GO) membranes are emerging for water treatment. Meanwhile, challenges remain due to membrane fouling and their instability in aqueous solutions. Herein, a novel GO-based mixed-dimensional membrane with superior antifouling and nonswelling properties was prepared by assembling two-dimensional (2D) GO nanosheets and zero-dimensional (0D) copper(I) oxide-incorporated titanium dioxide photocatalyst (CT). The decoration of CT in GO nanosheets tuned the microstructure and surface hydrophilicity while creating more transport channels in CT/GO membranes. This resulted in a high water permeance of 171.5 L m-2 h-1 bar-1 and improved selectivity to various dye molecules (96.2-98.6%). Due to the significantly enhanced antibacterial properties of the CT nanoparticles, the growth of bacteria on the surface of the CT/GO membrane was suppressed (threefold less than that on the GO membrane). Moreover, the embedding of photocatalysts also allowed CT/GO membranes to exhibit ∼9-fold improvement in antibacterial activity and organic dye degradation performance under visible-light irradiation. This study offers a powerful solution to enhance the nanofiltration performance and antibacterial properties of GO membranes toward practical applications.

Highly Salt-Resistant interfacial solar evaporators based on Melamine@Silicone nanoparticles for stable Long-Term desalination and water harvesting

Interfacial solar-driven evaporation (ISE) is one of the most promising solutions for collecting fresh water, however, poor salt-resistance severely limits the long-term stability of solar evaporators. Here, highly salt-resistant solar evaporators for stable long-term desalination and water harvesting were fabricated by depositing silicone nanoparticles onto melamine sponge, and then modifying the hybrid sponge sequentially with polypyrrole and Au nanoparticles. The solar evaporators have a superhydrophilic hull for water transport and solar desalination, and a superhydrophobic nucleus for reducing heat loss. Spontaneous rapid salt exchange and reduction in salt concentration gradient were achieved due to ultrafast water transport and replenishment in the superhydrophilic hull with a hierachical micro-/nanostructure, which effectively prevents salt deposition during ISE. Consequently, the solar evaporators have long-term stable evaporation performance of 1.65 kg m-2h−1 for 3.5 wt% NaCl solution under 1 sun illumination. Moreover, 12.87 kg m−2 fresh water was collected during consecutive 10 h ISE of 20 wt% brine under 1 sun without any salt precipitation. We believe that this strategy will shed a new light on the design of long-term stable solar evaporators for fresh water harvesting.

Moisture wicking textiles with hydrophilic oriented polyacrylonitrile layer: Enabling ultrafast directional water transport

Functional textiles with high-performance directional water transport for regulating human sweat are in high demand because of growing concerns about the role of comfort in the performance of wearer. However, the fabrication of such materials remains a critical job. Here, we report a facile strategy to develop hydrophilic oriented polyacrylonitrile (HOPAN)/hydrophilic polylactic acid @polyvinylidene fluoride (HPLA@PVDF) composite membrane with surface energy gradient for enhanced directional water transport. Three step fabrication strategy involves electrospinning of oriented polyacrylonitrile (OPAN fibers) on polylactic acid (PLA) nonwoven surface followed by dip-coating in hydrophilic agent, and single-side electrospray of PVDF dilute solution on HOPAN/HPLA. Combination of highly oriented fiber structure, differential pore size and asymmetric wettability between two layers enabled instant water transport. The resultant fabricated composite membranes offer superior properties with one-way transport capacity (R) of 1117%, overall moisture management capacity (OMMC) of 0.91, and excellent water vapor transmission rate of 11.6 kg m−2 d-1. The successful preparation of these fascinating directional water transport materials offers new insight into the role of fiber alignment along with differential apertures and asymmetric chemical structure for realizing membranes for quick-drying applications.

Sulfonated graphene oxide-based pervaporation membranes inspired by a tortuous brick and mortar structure for enhanced resilience against silica scaling and organic fouling

A novel tortuous brick-and-mortar structure utilizing intercalation of polyvinyl alcohol (PVA) on sulfonated graphene oxide (SGO) membranes was specifically tailored for brine treatment by pervaporation to ensure excessive resistance to silica scaling and organic fouling, as well as ultrafast water transport without compromising salt rejection. The synthesized SGO membrane showed a smoother surface morphology, improved zeta potential, and a higher hydration capacity than the graphene oxide (GO) membrane. Further intercalation of PVA through glutaraldehyde (GA) crosslinking, confirmed by Fourier transform infrared spectroscopy and X-ray diffraction analysis, conferred increased cohesiveness, and the SGO-PVA-GA membrane was therefore able to withstand ultrasonication tests without any erosion of the coating layer. According to a pervaporative desalination test, the SGO-PVA-GA membrane exhibited 62 kg m−2 h−1 of permeate flux, with an extraordinary salt rejection of 99.99% for a 10 wt% NaCl feed solution at 65 °C. The 72 h organic fouling, silica scaling, and combined fouling and scaling tests proved that the SGO-PVA-GA membrane sustains a stable flux with less scaling and fouling than the GO-PVA-GA membrane, attributable to dense surface negative charges and great hydration capacities caused by sulfonic acid. Thus, the SGO-PVA-GA membrane offers superlative advantages for long-term brine treatment by pervaporation, related to its ability to withstand silica scaling and organic fouling.

Electrochemical Opening of Impermeable Nanochannels in Laminar Graphene Membranes for Ultrafast Nanofiltration.

Reduced graphene oxide (rGO) could be theoretically used to construct highly permeable laminar membranes with nearly frictionless nanochannels for water treatment. However, their pristine (sp2 C-C) regions usually restack into impermeable channels as a result of van der Waals interactions, resulting in a much low permeance. In this study, we demonstrate that the restacked regions could be electrochemically expanded to form ultrafast water transport nanochannels by providing a low positive potential (e.g., +1.00 V vs SCE) to the rGO membrane. Experimental investigations indicate that the structural expansion is attributed to the intercalation of water molecules into the restacked regions, driven by hydrogen bond interactions between water molecules and hydroxyl groups that are electrochemically produced on edges of rGO nanosheets. The structural expansion could be promoted by weakening the graphene-OH- interactions through intermittent application of the potential. As a result of more ultrafast water transport nanochannels available, the electrochemically treated rGO membranes could have a permeance 2 orders of magnitude higher than that of the pristine one and ∼3 times higher than that of graphene oxide membranes. Because of their smaller average pore size, the rGO membranes also have a higher ionic/molecular rejection performance than graphene oxide membranes.

Improving water transport through sub‐nanometer channels by alternating hydrophilic–hydrophobic patterns

AbstractHydrophilicity is key factor to sub‐nanometer channels for water transport. However, how to effectively integrate the respective advantages of hydrophilicity and hydrophobicity to realize ultrafast water transport at the sub‐nanometer scale is still confusing. By using sub‐nanometer channels formed by two‐dimensional covalent organic frameworks as typical models, we herein demonstrate that alternating hydrophilic–hydrophobic patterns in channels are capable of significantly improving water transport. Alternatingly stacking hydrophilic and hydrophobic monolayers is testified to be energetically favorable to directly piling their multilayers. Meanwhile, alternatingly‐stacked hydrophilic–hydrophobic multilayers are able to achieve much higher water permeability than individual hydrophilic or hydrophobic multilayers. The effect of framework flexibility is also investigated. The exceptional performances benefit from the perfect balance between excellent wettability of hydrophilicity and low friction of hydrophobicity. In order to give full play to the synergistic advantages, the thickness of hydrophobic patterns spaced by hydrophilic ones should be less than 1 nm.

Micro/nanofluidic transport by special fibers deposited by electrohydrodynamic direct-writing

Recently micro/nanofluidic transportation is important in many natural and engineering processes, such as water collection, fluid patterning and microfluidic chip. But the natural microstructures for fluid transportation are usually difficult to manufacture and adjust. Here we find that some fibers (the diameters < 10 μm) deposited by electrohydrodynamic direct-writing(EDW) can be directly used for micro/nanofluidic transportation due to the geometries of fibers. The liquid paraffin (its minimum width < 100 nm) can transport along the circular fiber because of the wedge microcavities formed by circular fiber and substrate. Ultrafast directional water transport (about 22 mm/s) has been achieved by oblique elliptic fiber array which has been placed at a distance of 300 μm above the substrate. Moreover, we found that the contact angles of the test liquids on the substrates and on the fiber materials also play a crucial role in this structure. So electrowetting has been used to change the contact angles of water on the substrate to fast switch (response time < 0.4 s) direction of water transport. We think electrohydrodynamic direct-writing the fibers with various geometries opens up new low-cost, high efficiency, fast and accurate pathways to realize fabric-based wearable microfluidic device.