Showing papers on "Nanofluidics published in 2021"

Fluids at the Nanoscale: From Continuum to Subcontinuum Transport

Abstract: Nanofluidics has firmly established itself as a new field in fluid mechanics, as novel properties have been shown to emerge in fluids at the nanometric scale. Thanks to recent developments in fabri...

Superfast Water Transport Zwitterionic Polymeric Nanofluidic Membrane Reinforced by Metal-Organic Frameworks.

Abstract: Nanofluidics derived from low-dimensional nanosheets and protein nanochannels are crucial for advanced catalysis, sensing, and separation. However, polymer nanofluidics is halted by complicated preparation and miniaturized sizes. This work reports the bottom-up synthesis of modular nanofluidics by confined growth of ultrathin metal-organic frameworks (MOFs) in a polymer membrane consisting of zwitterionic dopamine nanoparticles (ZNPs). The confined growth of the MOFs on the ZNPs reduces the chain entanglement between the ZNPs, leading to stiff interfacial channels enhancing the nanofluidic transport of water molecules through the membrane. As such, the water permeability and solute selectivity of MOF@ZNPM are one magnitude improved, leading to a record-high performance among all polymer nanofiltration membranes. Both the experimental work and the molecular dynamics simulations confirm that the water transport is shifted from high-friction-resistance conventional viscous flow to ultrafast nanofluidic flow as a result of rigid and continuous nanochannels in MOF@ZNPM.

Two-Dimensional Nanofluidic Membranes toward Harvesting Salinity Gradient Power

Abstract: ConspectusThe salinity gradient between seawater and river water has been identified as a promising, clean, renewable, and sustainable energy source that can be converted into electricity using ion-selective membranes in a reverse electrodialysis (RED) configuration. However, the major hindrance to current salinity gradient power (SGP) conversion is its poor energy efficiency due to the use of low-performance membrane processes, which affords power for neither miniaturized devices nor industrial-level applications. Nanofluidics, which combines strong confinement and surface charge effects at the nanoscale, contributes to novel transport properties, including excellent ion selectivity and high ion throughput; thus, nanofluidics may lead to technological breakthroughs and act as an emerging platform for harnessing SGP. Recently, two-dimensional (2D) materials have provided impressive energy extraction performance and further insight into fundamental transport mechanisms and theoretical feasibility. To reach the commercialization benchmark and real-world applications, an array of nanopores and channels that can be scaled up to industrial sizes is in high demand; additionally, it remains challenging to develop macroscale nanofluidic membranes that meet the "selectivity versus throughput" dual requirement. In the first section, we start with our understanding of the underlying mechanism of ion-channel interactions and transport characteristics in nanofluidic channel systems from the microscale to the macroscale. We review our recent efforts in this field by constructing a heterojunction with asymmetric ion transport behavior that generates rectification of the ion flux and creates an osmotic diode, which is composed of two nanofluidic layers with opposite polar charges and different chemical compositions. Another efficient way to improve the performance of the system is introducing charged functional materials intercalated into laminar 2D nanosheets. The intercalated nanofluidic material can be explained by two classical models to account for the synergistic effects that (i) improve the stability and mechanical properties of 2D materials with a fixed interlayer spacing and (ii) provide space charge for modulating ion diffusion; both of these effects contribute to its considerable energy conversion performance. Further, layer-by-layer membranes are superior to traditional membranes consisting of a simple stack because they retain their repulsion effect toward co-ions, largely strengthening the efficiency of ion separation and conversion. In particular, we highlight our views on the role of the 2D phase structure (e.g., semiconductor 2H phase and metallic 1T phase) in which the two phases differ from each other in physical and chemical properties, including ionic conductance, surface charge, and wetting, thereby presenting a state-of-the-art avenue for controlling ion transport. In view of the nature of 2D materials, we also report improved osmotic energy harvesting by exploiting the photoinduced heat gradient and electrons that increase ion mobility and surface charge, respectively. Finally, we point out specific research topics in which a combined project can certainly come into the limelight. For example, we discuss the combination of SGP with desalination systems and water splitting. We expect that this Account will stimulate further efforts toward functionalized 2D nanoporous materials and facilitate interdisciplinary efforts in chemistry, material engineering, environmental science, and nanotechnology.

Synergy of light and acid–base reaction in energy conversion based on cellulose nanofiber intercalated titanium carbide composite nanofluidics

Abstract: Employing chemical-potential with reverse electrodialysis is a sustainable way to generate electricity. Based on this observation, efforts have been made to utilize this energy, such as the development of nanomaterials and the introduction of external fields. However, the reported systems mainly focus on a single type of physical field on the enhancement of energy conversion but ignore the synergy between cross-factor types. Here, we report a light and acid–base reaction enhanced energy conversion system based on 1D/2D composite nanofluidics. The contributions of light (physical) and acid–base reaction (chemical) in a chemical-potential-driven ionic generation system are studied separately and jointly. With the synergistic effect of the joint system, the output power density increases to 87.23 W m−2, exceeding the output power density of the state-of-the-art membrane-based systems. Both experimental and theoretical results indicate that the physical–chemical synergistic effect can significantly improve the magnitude of ionic migration. Hence, our work demonstrates the vast potential of a physical–chemical synergistic assistant factor in the enhancement of chemical-potential-driven energy conversion.

Water friction in nanofluidic channels made from two-dimensional crystals.

Abstract: Membrane-based applications such as osmotic power generation, desalination and molecular separation would benefit from decreasing water friction in nanoscale channels. However, mechanisms that allow fast water flows are not fully understood yet. Here we report angstrom-scale capillaries made from atomically flat crystals and study the effect of confining walls’ material on water friction. A massive difference is observed between channels made from isostructural graphite and hexagonal boron nitride, which is attributed to different electrostatic and chemical interactions at the solid-liquid interface. Using precision microgravimetry and ion streaming measurements, we evaluate the slip length, a measure of water friction, and investigate its possible links with electrical conductivity, wettability, surface charge and polarity of the confining walls. We also show that water friction can be controlled using hybrid capillaries with different slip lengths at opposing walls. The reported advances extend nanofluidics’ toolkit for designing smart membranes and mimicking manifold machinery of biological channels. Flow through nanometer scale channels facilitates an unmasked study of water-surface molecular interactions. Here, Keerthi et al. show with conduits made from graphite and hexagonal boron nitride that strong hydrophobicity does not rule out enhanced stickiness and friction.

Biomimetic Nanocomposite Membranes with Ultrahigh Ion Selectivity for Osmotic Power Conversion.

Abstract: Ion transport in nanoconfinement exhibits significant features such as ionic rectification, ionic selectivity, and ionic gating properties, leading to the potential applications in desalination, water treatment, and energy conversion. Two-dimensional nanofluidics provide platforms to utilize this phenomenon for capturing osmotic energy. However, it is challenging to further improve the power output with inadequate charge density. Here we demonstrate a feasible strategy by employing Kevlar nanofiber as space charge donor and cross-linker to fabricate graphene oxide composite membranes. The coupling of space charge and surface charge, enabled by the stabilization of interlayer spacing, plays a key role in realizing high ion selectivity and the derived high-performance osmotic power conversion up to 5.06 W/m2. Furthermore, the output voltage of an ensemble of the membranes in series could reach 1.61 V, which can power electronic devices. The system contributes a further step toward the application of energy conversion.

Electroosmotic flow: From microfluidics to nanofluidics

Abstract: Electroosmotic flow (EOF), a consequence of an imposed electric field onto an electrolyte solution in the tangential direction of a charged surface, has emerged as an important phenomenon in electrokinetic transport at the micro/nano-scale. Because of their ability to efficiently pump liquids in miniaturized systems without incorporating any mechanical parts, electroosmotic methods for fluid pumping have been adopted in versatile applications-from biotechnology to environmental science. To understand the electrokinetic pumping mechanism, it is crucial to identify the role of an ionically polarized layer, the so-called electrical double layer (EDL), which forms in the vicinity of a charged solid-liquid interface, as well as the characteristic length scale of the conducting media. Therefore, in this tutorial review, we summarize the development of EDL models from a historical point of view to elucidate the interplay and configuration of water molecules and ions in the vicinity of a solid-liquid interface. Moreover, we discuss the physico-chemical phenomena owing to the interaction of EDL when the characteristic length of the conducting media is decreased from the micro-scale to the nano-scale. Finally, we highlight the pioneering studies and the most recent works on EOF devoted to both theoretical and experimental aspects. This article is protected by copyright. All rights reserved.

Ionic conductance oscillations in sub-nanometer pores probed by optoelectronic control

Abstract: Summary Ionic Coulomb blockade is one of the ion transport phenomena revealing the quantized nature of ionic charges, which is of crucial importance to our understanding of the sub-continuum transport in nanofluidics and the mechanism of biological ion channels. Here, we report an experimental observation and plausible theoretical reasoning of ionic conduction oscillations. Our experiment is performed under confinement in sub-nanometer (sub-nm) MoS2 pores with optoelectronic control enabled for active tuning of pore surface charges. Under this charge control, we measure the ionic current at fixed voltages and observe multiple current peaks. Our analytical discussions and molecular dynamics simulations further reveal that the conductance oscillations may originate from the multi-ion interaction at the pore entry, particularly the electrostatic repulsion of ions external to the pore by ions bound inside the pore. Our work adds a further understanding of ionic Coulomb blockade effect and paves the way for developing advanced ionic machineries.

Diameter Dependence of Water Filling in Lithographically Segmented Isolated Carbon Nanotubes.

Abstract: Although the structure and properties of water under conditions of extreme confinement are fundamentally important for a variety of applications, they remain poorly understood, especially for dimen...

Realization of robust mesoscale ionic diodes for ultrahigh osmotic energy generation at mild neutral pH

Abstract: Ionic diodes, referring to fluidic devices with ion rectifying properties, have recently received considerable attention from the nanofluidics community due to their promising applications particularly in promoting clean energy. The past ionic diodes were realized with nanometer-scale pores, but those nanofluidic devices suffer from high resistance and high fabrication cost, thus limiting their practical applications. Here we report a simple strategy to fabricate a robust mesoscale ionic diode by electrokinetic functionalization of single conical mesoscale pores (700–800 nm tip diameter) with high molecular weight poly-L-lysine. Experiments show that the mesoscale ionic diode exploited can still function in mild neutral and 10 mM salt solution, under conditions at which the pore size is over 200-fold larger than the Debye length. The findings are supported by our simulation that attributes the mesoscale rectification to the effect of ion concentration polarization induced by the coating of highly space-charged polyelectrolytes. The application of the exploited mesoscale ionic diode in osmotic energy harvesting is also demonstrated, which achieves a power up to 254 pW at “neutral” pH and a 1000-fold salinity gradient, 5 times more than that of all the state-of-the-art single nanopore-based devices reported under the same testing conditions.

Fast and efficient nanoparticle trapping using plasmonic connected nanoring apertures.

Abstract: The manipulation of microparticles using optical forces has led to many applications in the life and physical sciences. To extend optical trapping towards the nano-regime, in this work we demonstrate trapping of single nanoparticles in arrays of plasmonic coaxial nano-apertures with various inner disk sizes and theoretically estimate the associated forces. A high normalized experimental trap stiffness of 3.50 fN nm-1 mW-1 μm-2 for 20 nm polystyrene particles is observed for an optimum design of 149 nm for the nanodisk diameter at a trapping wavelength of 980 nm. Theoretical simulations are used to interpret the enhancement of the observed trap stiffness. A quick particle trapping time of less than 8 s is obtained at a concentration of 14 × 1011 particles ml-1 with low incident laser intensity of 0.59 mW μm-2. This good trapping performance with fast delivery of nanoparticles to multiple trapping sites emerges from a combination of the enhanced electromagnetic near-field and spatial temperature increase. This work has applications in nanoparticle delivery and trapping with high accuracy, and bridges the gap between optical manipulation and nanofluidics.

Adjustable diffusion enhancement of water molecules in a nanoscale water bridge

Abstract: The emergence of nanofluidics in the last few decades has led to the development of various applications such as water desalination, ultrafiltration, osmotic energy conversion, etc. In particular, understanding water molecule transport in nanotubes is of importance for designing novel ultrafiltration and filtering devices. In this paper, we use an electric field to form a nanoscale water bridge as an artificial water channel to connect two separate disjoint nanotubes by molecular dynamics simulations. The extended length of the water bridge under different electric field strengths could adjust the diffusion process of the water molecules crossing the two disjoint nanotubes and the diffusion coefficients could be remarkably enhanced up to 4 times larger than the value in bulk water. By analyzing the structure of the water bridge, it is found that the diffusion enhancement originates from the strengthened interactions and the increase of hydrogen bonds between the water molecules due to the restrained reorientation from the external electric field. Our result provides a promising insight for realizing an efficient mass transport between various disjoint nanochannels.

Neuromorphic van der Waals crystals for substantial energy generation

Abstract: Controlling ion transport in nanofluidics is fundamental to water purification, bio-sensing, energy storage, energy conversion, and numerous other applications. For any of these, it is essential to design nanofluidic channels that are stable in the liquid phase and enable specific ions to pass. A human neuron is one such system, where electrical signals are transmitted by cation transport for high-speed communication related to neuromorphic computing. Here, we present a concept of neuro-inspired energy harvesting that uses confined van der Waals crystal and demonstrate a method to maximise the ion diffusion flux to generate an electromotive force. The confined nanochannel is robust in liquids as in neuron cells, enabling steady-state ion diffusion for hundred of hours and exhibiting ion selectivity of 95.8%, energy conversion efficiency of 41.4%, and power density of 5.26 W/m2. This fundamental understanding and rational design strategy can enable previously unrealisable applications of passive-type large-scale power generation. Controlling ion transport in nanofluidics is fundamental to numerous material applications but designing a material for ion selection is challenging. Here the authors report a confined van der Waals graphene oxide membrane as cation selective channel for energy generation inspired by neuron electromotive force.

Competition between electroosmotic and chemiosmotic flow in charged nanofluidics

Abstract: In electrolyte solutions, charged nanoscale pores or channels with overlapping electrical double layers are charge selective, thereby benefiting a wide range of applications such as desalination, bio-sensing, membrane technology, and renewable energy. As an important forcing mechanism, a gradient of electrolyte concentration along a charged nano-confinement can drive flow without an external electrical field or applied pressure difference. In this paper, we numerically investigate such a diffusioosmotic nanoflow, particularly for dilute electrolyte concentrations (0.01 mM–1 mM), and calculate the corresponding electrical and concentration fields in a charged nanochannel connecting two reservoirs of different salt concentrations—a typical fluidic configuration for a variety of experimental applications. Under a wide range of parameters, the simulation results show that the flow speed inside the nanochannel is linearly dependent on the concentration difference between the two reservoir solutions, Δc, whereas the flow direction is primarily influenced by three key parameters: nanochannel length (l), height (h), and surface charge density (σ). Through a comparison of the chemiosmotic (due to ion-concentration difference) and electroosmotic (as a result of the induced electric field) components of this diffusioosmotic flow, a non-dimensional number ( C = h / l λ G C) has been identified to delineate different nanoscale flow directions in the charged nanochannel, where λGC is a characteristic (so-called Gouy–Chapman) length associated with surface charge and inversely proportional to σ. This critical dimensionless parameter, dependent on the above three key nanochannel parameters, can help in providing a feasible strategy for flow control in a charged nanochannel.

Microscale Ionic Diodes: An Overview

Abstract: Ionic rectifier membranes or devices generate uni-directional ion transport to convert an alternating current (AC) ion current input into stored energy or direct current (DC) in the form of ion/salt gradients. Electrochemical experiments 80 years ago were conducted on biological membrane rectifier systems, but today a plethora of artificial ionic rectifier types has been developed and electroanalytical tools are employed to explore mechanisms and performance. This overview focuses on microscale ionic rectifiers with a comparison to nanoand macroscale ionic rectifiers. The potential is surveyed for applications in electrochemical analysis, desalination, energy harvesting, electrochemical synthesis, and in selective ion extraction.

Light-Controlled Ionic Transport through Molybdenum Disulfide Membranes.

Abstract: In recent years, two-dimensional (2D) nanomaterials have been extensively explored in the field of nanofluidics due to their interconnected and well-controlled nanochannels. In particular, the investigation of 2D nanomaterials using their intrinsic properties for smart nanofluidics is receiving increased interest. Here, we report that MoS2 membranes can be used for light-controlled nanofluidic applications based on their photoelectrical properties. We show that the MoS2 membranes exhibit surface charge-governed ionic transport in NaCl and KCl solution without light illumination, while the ionic conductivity of the MoS2 membranes is up to 2 orders of magnitude higher at low concentration solution than that in bulk solution. We also show that the ionic conductivity of the membranes is enhanced under light illumination at 405 and 635 nm and reversible and stable switching of ionic current upon light illumination is observed. In addition, ionic current through membranes is enhanced by increasing light intensity. Therefore, our findings demonstrate that MoS2 membranes can be a potential platform for light-controlled nanofluidic applications.

Water mobility in MoS2 nanopores: effects of the dipole-dipole interaction on the physics of fluid transport.

Abstract: Use of nanoscale materials is a promising desalination technology. While fast water flow in nanotubes is well understood, this is not the case for water permeability in single-layer membranes. The advances in nanofluidics have opened up the possibility to shift the permeability–selectivity tradeoff. The physical–chemical balance between nanopore size, shape, and charge might be the answer. In this work, we investigate the role of the MoS2 nanopore charge distribution in water mobility by tuning its strength. We shed light on the competition between charge and nanopore size. The strong dipole interaction between water and the MoS2 nanopore is responsible for adding a constraint to the water angular orientation possibilities to travel through the nanopore, but this effect also depends on the nanopore size.

Laser-excited elastic guided waves reveal the complex mechanics of nanoporous silicon.

Abstract: Nanoporosity in silicon leads to completely new functionalities of this mainstream semiconductor. A difficult to assess mechanics has however significantly limited its application in fields ranging from nanofluidics and biosensorics to drug delivery, energy storage and photonics. Here, we present a study on laser-excited elastic guided waves detected contactless and non-destructively in dry and liquid-infused single-crystalline porous silicon. These experiments reveal that the self-organised formation of 100 billions of parallel nanopores per square centimetre cross section results in a nearly isotropic elasticity perpendicular to the pore axes and an 80% effective stiffness reduction, altogether leading to significant deviations from the cubic anisotropy observed in bulk silicon. Our thorough assessment of the wafer-scale mechanics of nanoporous silicon provides the base for predictive applications in robust on-chip devices and evidences that recent breakthroughs in laser ultrasonics open up entirely new frontiers for in-situ, non-destructive mechanical characterisation of dry and liquid-functionalised porous materials.

Clay-Based Nanofluidic Membrane Derived from Vermiculite Nanoflakes for Pressure-Responsive Power Generation

Abstract: The intrinsic thermal and chemical stability of vermiculite clay is exploited here to fabricate a robust and responsive energy device that consumes atmospheric water molecules as the cathode reagen...

Metal-Free Fabrication of Fused Silica Extended Nanofluidic Channel to Remove Artifacts in Chemical Analysis.

Abstract: In microfluidics, especially in nanofluidics, nanochannels with functionalized surfaces have recently attracted attention for use as a new tool for the investigation of chemical reaction fields. Molecules handled in the reaction field can reach the single–molecule level due to the small size of the nanochannel. In such surroundings, contamination of the channel surface should be removed at the single–molecule level. In this study, it was assumed that metal materials could contaminate the nanochannels during the fabrication processes; therefore, we aimed to develop metal-free fabrication processes. Fused silica channels 1000 nm-deep were conventionally fabricated using a chromium mask. Instead of chromium, electron beam resists more than 1000 nm thick were used and the lithography conditions were optimized. From the results of optimization, channels with 1000 nm scale width and depth were fabricated on fused silica substrates without the use of a chromium mask. In nanofluidic experiments, an oxidation reaction was observed in a device fabricated by conventional fabrication processes using a chromium mask. It was found that Cr6+ remained on the channel surfaces and reacted with chemicals in the liquid phase in the extended nanochannels; this effect occurred at least to the micromolar level. In contrast, the device fabricated with metal-free processes was free of artifacts induced by the presence of chromium. The developed fabrication processes and results of this study will be a significant contribution to the fundamental technologies employed in the fields of microfluidics and nanofluidics.

Time- and Pressure-Dependent Gas Diffusion in a Nanoconfined Liquid Phase

Abstract: Fundamentally understanding the gas–liquid interaction in a nanoenvironment is important in nanofluidics-based systems. Here, a systematically experimental study of the gas diffusion in a liquid ph...