Length-dependent water permeation through a graphene channel.

Water confined in two-dimensional channels exhibits unique properties, such as rich morphology, specific phase transition and a low dielectric constant. In this work, molecular dynamics simulations have been used to study the water transport in two-dimensional graphene channels. The structures and dynamics of water under confinement show strong dependence on the channel length and thickness of the channels.

Double-Walled Carbon Nanotubes Enable Breakdown of the Trade-off between Ion Selectivity and Water Permeability.

Although evidence has been presented for desalination potentials in single-walled carbon nanotubes (SWCNTs), it is still very challenging to overcome the trade-off between ion selectivity and water permeability by simply tuning the carbon nanotube (CNT) size. In this work, we prove that double-walled carbon nanotubes (DWCNTs) can make it. Employing a series of molecular dynamics simulations, we find a striking phenomenon that tuning the combination architecture of DWCNTs can significantly improve the desalination performance, with the salt rejection rate even reaching 100% in some cases while maintaining high levels of water flux.

Terahertz electric field induced melting and transport of monolayer water confined in double-walled carbon nanotubes.

Understanding the structures and dynamics of confined water in nanochannels holds great promise for various applications, ranging from membrane separation to blue energy collection. A setting of particular interest is the confined monolayer water within double-walled carbon nanotubes (DWCNTs), which demonstrates rich ice morphologies; however, the dynamics of this peculiar system are still unexplored. In this work, a series of molecular dynamics (MD) simulations reveal that the two-dimensional ice in DWCNTs can be effectively melted by terahertz electric fields but not by static electric fields, exhibiting an interesting ice to vapor-like transition along with extraordinary dynamical behaviors.

Enhanced Rectification Performance in Bipolar Janus Graphene Oxide Channels by Lateral Electric Fields.

Improving the ionic rectification in nanochannels enables versatile applications such as biosensors, energy harvesting, and fluidic diodes. While previous work mostly focused on the effect of channel geometry and surface charge, in this work via a series of molecular dynamics simulations, we find a striking phenomenon that the ionic current rectification (ICR) ratio in Janus graphene oxide (GO) channels can be tremendously promoted by lateral electric fields. First, under a given axial electric field, an additional lateral electric field can improve the ICR ratio by several times to an order, depending on the channel symmetry.

Desalination Performance in Janus Graphene Oxide Channels: Geometric Asymmetry vs Charge Polarity.

Improving the desalination performance of membranes is always in the spotlight of scientific research; however, Janus channels with polarized surface charge as nanofiltration membranes are still unexplored. In this work, using molecular dynamics simulations, we demonstrate that Janus graphene oxide (GO) channels with appropriate geometry and surface charge can serve as highly efficient nanofiltration membranes. We observe that the water permeability of symmetric Janus GO channels is significantly superior to that of asymmetric channels without sacrificing much ion rejection, owing to weakened ion blockage and electrostatic effects.

Ionic transport through a bilayer nanoporous graphene with cationic and anionic functionalization.

Understanding the ionic transport through multilayer nanoporous graphene (NPG) holds great promise for the design of novel nanofluidic devices. Bilayer NPG with different structures, such as nanopore offset and interlayer space, should be the most simple but representative multilayer NPG. In this work, we use molecular dynamics simulations to systematically investigate the ionic transport through a functionalized bilayer NPG, focusing on the effect of pore functionalization, offset, applied pressure and interlayer distance.

Effect of terahertz electromagnetic field on single-file water transport through a carbon nanotube.

With the advancement in terahertz technology, the terahertz electromagnetic field has been proven to be an effective strategy to tune the nanofluidic transport. In this study, we utilize molecular dynamics simulations to systematically analyze the transport of single-file water through a carbon nanotube (CNT) under terahertz electromagnetic fields, focusing on the CNT length, field strength, polarization direction and frequency. Strikingly, with the increase in field strength, the water flow exhibits a transition from normal to super permeation states because of the resonance effect, and the threshold field shifts to low values for long CNTs.

Surface charge density governs the ionic current rectification direction in asymmetric graphene oxide channels.

Charged asymmetric channels are extensively investigated for the design of artificial biological channels, ionic diodes, artificial separation films, These applications are attributed to the unique ionic current rectification phenomenon, where the surface charge density of the channel has a deep influence. In this work, we use molecular dynamics simulations to study the rectification phenomenon in asymmetric graphene oxide channels. A fascinating finding is that the ionic current rectification direction reverses from the negative to positive electric field direction with an increase in surface charge density.

Enhanced Ion Rejection in Carbon Nanotubes by a Lateral Electric Field.

Reverse osmosis membranes hold great promise for dealing with global water scarcity. However, the trade-off between ion selectivity and water permeability is a serious obstacle to desalination. Herein, we introduce an effective strategy to enhance the desalination performance of the membrane.

Asymmetric transport and desalination in graphene channels.

Inspired by the high water permeability and excellent salt rejection of hourglass-shape aquaporin channels, asymmetric channels have attracted increasing attention in recent years. In this work, we use molecular dynamics simulations to explore the transport of an ionic solution through asymmetric graphene channels under a pressure difference. We observe an interesting asymmetric desalination phenomenon when changing the channel geometry.

Promoting Electroosmotic Water Flow through a Carbon Nanotube by Weakening the Competition between Cations and Anions in a Lateral Electric Field.

Understanding the electroosmotic flow through a nanochannel is essential to the design of novel nanofluidic devices, ranging from desalination to nanometer water pumps. Nonetheless, the competition between cation and anion in electric fields inevitably leads to a limited pumping of water, and thus weakening their competition could be a new avenue for the fundamental control of water transport. In this work, through a series of molecular dynamics simulations, we find a surprising phenomenon in which under the drive of a traditional longitudinal electric field, an additional lateral electric field can significantly weaken the competitive transport of a cation and anion through a carbon nanotube, which spontaneously leads to a massive increase in electroosmotic water flux.

Electropumping Phenomenon in Modified Carbon Nanotubes.

Controlling the water transport in a given direction is essential to the design of novel nanofluidic devices, which is still a challenge because of thermal fluctuations on the nanoscale. In this work, we find an interesting electropumping phenomenon for charge-modified carbon nanotubes (CNTs) through a series of molecular dynamics simulations. In electric fields, the flowing counterions on the CNT inner surface provide a direct driving force for water conduction.

Rectification Correlation between Water and Ions through Asymmetric Graphene Channels.

Rectification phenomena occurring in asymmetric channels are essential for the design of novel nanofluidic devices such as nanodiodes. Previous studies mostly focus on ion current rectification, while its correlations with water dynamics are rarely explored. In this work, we analyze the transport of water and ions through asymmetric graphene channels under the drive of electric fields using molecular dynamics simulations.

Effect of temperature on the coupling transport of water and ions through a carbon nanotube in an electric field.

Temperature governs the motion of molecules at the nanoscale and thus should play an essential role in determining the transport of water and ions through a nanochannel, which is still poorly understood. This work devotes to revealing the temperature effect on the coupling transport of water and ions through a carbon nanotube by molecular dynamics simulations. A fascinating finding is that the ion flux order changes from cation > anion to anion > cation with the increase in field strength, leading to the same direction change of water flux.

Osmotic Water Permeation through a Carbon Nanotube.

Osmosis are essential for not only the application of nanofluidic devices but also the understanding of working principles of biological transmembrane proteins. Despite considerable experimental interests, comprehensive simulation work is still lacking, possibly because of the periodic boundary condition that inevitably leads to the spontaneous exchange of two side reservoirs. To eliminate this disadvantage, herein we design a simple model system by introducing a dipalmitoylphosphatidylcholine bilayer into a common carbon-nanotube (CNT)-based setup, which allows long-time osmotic simulations.

The Role of Interface Ions in the Control of Water Transport through a Carbon Nanotube.

Controlling the water transport toward a given direction is still challenging, particularly due to thermal fluctuations of water motion at the nanoscale. While most of the previous works focus on the symmetric hydrophobic membrane systems, the role of the membrane in affecting the water transport remains largely unexplored. In this work, by using extensive molecular dynamics simulations, we find an interesting electropumping phenomenon, that is, the flowing counterions on an asymmetric hydrophobic-hydrophilic membrane can significantly drive the single-file water transport through a carbon nanotube, suggesting a nanometer water pump in a highly controllable fashion.

How ions block the single-file water transport through a carbon nanotube.

Understanding the blockage of ions for water transport through nanochannels is crucial for the design of desalination nanofluidic devices. In this work, we systematically clarify how ions block the single-file water transport through a (6,6) carbon nanotube (CNT) by using molecular dynamics simulations. We consider various pressure differences and salt concentrations.

A Nanometer Water Pump Induced by the Brownian and Non-Brownian Motion of a Graphene Sheet on a Membrane Surface.

Energy-saving water pump and efficient semipermeable membranes are the cores of reverse osmosis technology. Applying nanotechnology to improve the performance is a fashion in recent years. Based on the competitive effect of water's spontaneous infiltration of two sides of a carbon nanotube, we design a water pump that makes use of the natural permeability by weakening one side's competitiveness based on a small graphite sheet laying on the membrane.

Understanding the role of pore size homogeneity in the water transport through graphene layers.

Graphene is a versatile 2D material and attracts an increasing amount of attention from a broad scientific community, including novel nanofluidic devices. In this work, we use molecular dynamics simulations to study the pressure driven water transport through graphene layers, focusing on the pore size homogeneity, realized by the arrangement of two pore sizes. For a given layer number, we find that water flux exhibits an excellent linear behavior with pressure, in agreement with the prediction of the Hagen-Poiseuille equation.

Interface nanoparticle control of a nanometer water pump.

Nanoparticles are highly versatile and exhibit broad applications in tuning material properties. Herein, we show through molecular dynamics simulations the possibility of a nanometer water pump, driven by the motion of nanoparticles (NPs) on a membrane surface. Surprisingly, considerable net water flux can be induced through a carbon nanotube (CNT) that is perpendicular to the NP motion.

Asymmetric osmotic water permeation through a vesicle membrane.

Understanding the water permeation through a cell membrane is of primary importance for biological activities and a key step to capture its shape transformation in salt solution. In this work, we reveal the dynamical behaviors of osmotically driven transport of water molecules across a vesicle membrane by molecular dynamics simulations. Of particular interest is that the water transport in and out of vesicles is highly distinguishable given the osmotic force are the same, suggesting an asymmetric osmotic transportation.

Ultra-fast single-file transport of a simple liquid beyond the collective behavior zone.

We use molecular dynamics simulations to analyze the single-file transport behavior of a simple liquid through a narrow membrane channel. With the decrease of the liquid-channel interaction, the liquid flow exhibits a remarkable maximum behavior owing to the competition of liquid-liquid and liquid-channel interactions. Surprisingly, this maximum flow is coupled to a sudden reduce of the liquid occupancy, where the liquid particle is moving through the channel alone at nearly constant velocity, rather than in a collective motion mode.

A current-driven nanometer water pump.

The design of a water pump, which has huge potential for applications in nanotechnology and daily life, is the dream of many scientists. In this paper, we successfully design a nanometer water pump by using molecular dynamics simulations. Ions of either sodium or chlorine in a narrow channel will generate electric current under electric fields, which then drives the water through a wider channel, similar to recent experimental setups.

Vesicle Geometries Enabled by Dynamically Trapped States.

Understanding and controlling vesicle shapes is a fundamental challenge in biophysics and materials design. In this paper, we design dynamic protocols for enlarging the shape space of both fluid and crystalline vesicles beyond the equilibrium zone. By removing water from within the vesicle at different rates, we numerically produced a series of dynamically trapped stable vesicle shapes for both fluid and crystalline vesicles in a highly controllable fashion.