Water In Nanochannels Research Articles

Magnetic field modulation of confined water structure and transport in nanochannel

As a simple model of aquaporins, one-dimensional confined water in nanochannels, especially carbon nanotubes (CNTs), has attracted much attention due to its unique structure and rapid transport. The influence of external fields such as pressure, heat, electric field, and terahertz electromagnetic wave on confined water in CNTs has been widely studied. Magnetic field plays an important role in the regulation of water transport, but its research is limited at present. Therefore, in order to fill this gap, the effects of magnetic field with different strengths and directions on the structure and transport property of confined water in CNT were studied by molecular dynamics simulation. Simulation results showed that the magnetic field increased the number of hydrogen bonds by 1.10 % and accelerated the turnover frequency of the confined water (i.e. flipping interval decreased by 36.84 %), finally leading to a decrease in the water flow rate by 16.83 %. The effect of adjusting the magnetic field strength and direction on the hydrogen bond structure of confined water was more obvious. The results will help to understand the effect of magnetic field on confined water and provide guidance for the design of magnetic field regulation in water transport.

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. Specifically, under appropriate field frequency, the water flow presents a sharp increase with the increase in field strength, indicating an excellent gating behavior. These remarkable findings are attributed to the resonance effect between the terahertz electric field and inherent vibration of water hydrogen bonds, causing the water molecules to change from the frozen to super permeation states. The amount of confined water exhibits a sudden reduction, confirming the breakdown of the hydrogen bond network. The distributions of density profiles, hydrogen bond number and dipole orientation demonstrate more details of the water structural change. Furthermore, under a certain field strength, the water flow shows a peculiar maximum behavior with the increase in field frequency, implying frequency optimization for water transport. These findings not only enhance our understanding of the phase transition behavior of water molecules confined within DWCNTs under the influence of a terahertz electric field but also provide a promising avenue for designing innovative nanofluidic devices.

Molecular dynamics simulation of infrared absorption spectra of one-dimensional ordered single-file water

Compared with bulk water (BW), the water in nanochannels usually shows unique structural and dynamic properties, which is still unable to be effectively detected and characterized by existing experimental techniques. The spectrum is an effective technical means for studying and identifying the material composition and characteristics. In this study, the infrared absorption spectra of one-dimensional ordered single-file water (SW) confined in (6, 6) single-walled carbon nanotubes are calculated by molecular dynamics simulation. It is found that the ordered arrangement of SW results in an obvious blue shift and enhancement of the spectral peak in the 0–35 THz range relative to the bulk water. The analysis shows that this phenomenon is caused by the change of coupling weight of libration vibrations (including rock, twist and wag modes) of SW. The twist vibration mode and wag vibration mode with higher frequency are relatively easy to occur because the binding energy decreases under the single chain structure of water, which results in the blue shift and enhancement of the spectral peak. Meanwhile, the present study shows that the spectral component characteristics of SW can well predict and explain the structural and dynamic properties of SW. Further, terahertz simulation experiments show that the infrared absorption capacity of SW basically conforms with the spectral distribution characteristics.

Electropumping of water in nanochannels using non-uniform electric fields

Pumping fluids at the nanoscale is of fundamental importance to nanofluidic systems. We perform molecular dynamics simulations and provide the extended Navier-Stokes (ENS) equation to prove the electropumping possibility of using non-uniform electric fields to induce the continuous and unidirectional net flow of water confined in nanochannels. The net flow can be seen as a result of the combined effects of the unidirectional orientation of water molecules and the net force induced by the electric field intensity difference between O and H atoms in a water molecule. Higher net flow rates can be achieved by enhancing the electric field intensity gradients through the characteristics parameter adjustment of external electric fields. Importantly, the electropumping is more effective than the pressure-driven flow due to the ordered arrangements of water molecules in electric fields. The ENS equation proposed is more applicable at the flow field with high order degree of water molecules.

The first-principles phase diagram of monolayer nanoconfined water.

Water in nanoscale cavities is ubiquitous and of central importance to everyday phenomena in geology and biology. However, the properties of nanoscale water can be substantially different fromthose of bulk water, as shown, for example, by the anomalously low dielectric constant of water in nanochannels1, near frictionless water flow2 or the possible existence of a square ice phase3. Such properties suggest that nanoconfined water could be engineered for technological applications in nanofluidics4, electrolyte materials5 and water desalination6. Unfortunately, challenges in experimentally characterizing water at the nanoscale and the high cost of first-principles simulations have prevented the molecular-level understanding required to control the behaviour of water. Here we combine a range of computational approaches to enable a first-principles-level investigation of a single layer of water within a graphene-like channel. We find that monolayer water exhibits surprisinglyrich and diverse phase behaviour that is highly sensitive to temperature and the van der Waals pressure acting within the nanochannel. In addition to multiple molecular phases with melting temperatures varying non-monotonically by more than 400kelvins with pressure, we predict a hexatic phase, which is an intermediate between a solid and a liquid, and a superionic phase with a high electrical conductivity exceeding that of battery materials. Notably, this suggests that nanoconfinement could be a promising route towards superionic behaviour under easily accessible conditions.

From normal to anomalous diffusion of water molecules in narrow carbon nanotubes with defects, gases, and salts

In this letter, we study the diffusion of water molecules inside narrow carbon nanotubes in the presence of nanotube defects, gas molecules and salt ions by using molecular dynamics simulations. Mean square displacement (MSD) with a power-law fitting parameter , and the probability distribution function (PDF) with an distribution parameter a, of water molecules are calculated. It is found that within the diffusion time scale, water molecules exhibit a superdiffusion behavior with inside a perfect nanotube and the superdiffusion is weakened in the presence of gas molecules and salt ions. Meanwhile, a normal diffusion behavior with is observed inside a defective nanotube, and it is replaced by a subdiffusion behavior with when gas molecules and salt ions are involved. PDF of water molecules is found to be Gaussian-like with subject to superdiffusion and normal diffusion, while it becomes a center-peaked and long-tailed Lévy distribution with a < 2 subject to subdiffusion. Furthermore, it is found that the diffusion behavior of water molecules approaches normal diffusion as the radius size of the nanotube increases. Our result implies anomalous transport behavior of water in nanochannels due to the common presence of defects, gases and salts in nanochannels.

Water structure in 100 nm nanochannels revealed by nano X-ray diffractometry and Raman spectroscopy

Microscopic water structure confined in 100 nm nanofluidic channels was investigated with nano X-ray diffractometry and Raman spectroscopy. A new nano X-ray diffractometry device was fabricated to protect the dry-up of water in the nanochannels during a long-term (greater than 30 h) measurement. Bragg peaks from the substrate material, which affected the analysis of a radial distribution function (RDF), were completely removed by changing the materials. As a result, the small change of the RDF by the confinement effect of water was successfully revealed. Raman spectra suggested the strong hydrogen bonding of water in nanochannels. Based on both X-ray and Raman results, a stronger hydrogen-bonding network and higher density than bulk water were suggested for water in nanochannels. This information is significant to discuss the liquid property changes and the water structure in 10–100 nm spaces, which are seen in many chemical and biological systems.

Effects of channel size, wall wettability, and electric field strength on ion removal from water in nanochannels

Molecular dynamics simulations are employed to estimate the effect of nanopore size, wall wettability, and the external field strength on successful ion removal from water solutions. It is demonstrated that the presence of ions, along with the additive effect of an external electric field, constitute a multivariate environment that affect fluidic interactions and facilitate, or block, ion drift to the walls. The potential energy is calculated across every channel case investigated, indicating possible ion localization, while electric field lines are presented, to reveal ion routing throughout the channel. The electric field strength is the dominant ion separation factor, while wall wettability strength, which characterizes if the walls are hydrophobic or hydrophilic has not been found to affect ion movement significantly at the scale studied here. Moreover, the diffusion coefficient values along the three dimensions are reported. Diffusion coefficients have shown a decreasing tendency as the external electric field increases, and do not seem to be affected by the degree of wall wettability at the scale investigated here.

Disjoining Pressure of Water in Nanochannels.

The disjoining pressure of water was estimated from wicking experiments in 1D silicon dioxide nanochannels of heights of 59, 87, 124, and 1015 nm. The disjoining pressure was found to be as high as ∼1.5 MPa while exponentially decreasing with increasing channel height. Such a relation resulting from the curve fitting of experimentally derived data was implemented and validated in computational fluid dynamics. The implementation was then used to simulate bubble nucleation in a water-filled 59 nm nanochannel to determine the nucleation temperature. Simultaneously, experiments were conducted by nucleating a bubble in a similar 58 nm nanochannel by laser heating. The measured nucleation temperature was found to be in excellent agreement with the simulation, thus independently validating the disjoining pressure relation developed in this work. The methodology implemented here integrates experimental nanoscale physics into continuum simulations thus enabling numerical study of various phenomena where disjoining pressure plays an important role.

Effect of Modification on the Fluid Diffusion Coefficient in Silica Nanochannels.

The diffusion behavior of fluid water in nanochannels with hydroxylation of silica gel and silanization of different modified chain lengths was simulated by the equilibrium molecular dynamics method. The diffusion coefficient of fluid water was calculated by the Einstein method and the Green–Kubo method, so as to analyze the change rule between the modification degree of nanochannels and the diffusion coefficient of fluid water. The results showed that the diffusion coefficient of fluid water increased with the length of the modified chain. The average diffusion coefficient of fluid water in the hydroxylated nanochannels was 8.01% of the bulk water diffusion coefficient, and the diffusion coefficients of fluid water in the –(CH2)3CH3, –(CH2)7CH3, and –(CH2)11CH3 nanochannels were 44.10%, 49.72%, and 53.80% of the diffusion coefficients of bulk water, respectively. In the above four wall characteristic models, the diffusion coefficients in the z direction were smaller than those in the other directions. However, with an increase in the silylation degree, the increased self-diffusion coefficient due to the surface effect could basically offset the decreased self-diffusion coefficient owing to the scale effect. In the four nanochannels, when the local diffusion coefficient of fluid water was in the range of 8 Å close to the wall, Dz was greater than Dxy, and beyond the range of 8 Å of the wall, the Dz was smaller than Dxy.

Temperature effect on water dynamics in tetramer phosphofructokinase matrix and the super-arrhenius respiration rate

Advances in understanding the temperature effect on water dynamics in cellular respiration are important for the modeling of integrated energy processes and metabolic rates. For more than half a century, experimental studies have contributed to the understanding of the catalytic role of water in respiration combustion, yet the detailed water dynamics remains elusive. We combine a super-Arrhenius model that links the temperature-dependent exponential growth rate of a population of plant cells to respiration, and an experiment on isotope labeled 18O2 uptake to H218O transport role and to a rate-limiting step of cellular respiration. We use Phosphofructokinase (PFK-1) as a prototype because this enzyme is known to be a pacemaker (a rate-limiting enzyme) in the glycolysis process of respiration. The characterization shows that PFK-1 water matrix dynamics are crucial for examining how respiration (PFK-1 tetramer complex breathing) rates respond to temperature change through a water and nano-channel network created by the enzyme folding surfaces, at both short and long (evolutionary) timescales. We not only reveal the nano-channel water network of PFK-1 tetramer hydration topography but also clarify how temperature drives the underlying respiration rates by mapping the channels of water diffusion with distinct dynamics in space and time. The results show that the PFK-1 assembly tetramer possesses a sustainable capacity in the regulation of the water network toward metabolic rates. The implications and limitations of the reciprocal-activation–reciprocal-temperature relationship for interpreting PFK-1 tetramer mechanisms are briefly discussed.

Molecular Dynamics Simulations of Ion Drift in Nanochannel Water Flow.

The present paper employs Molecular Dynamics (MD) simulations to reveal nanoscale ion separation from water/ion flows under an external electric field in Poiseuille-like nanochannels. Ions are drifted to the sidewalls due to the effect of wall-normal applied electric fields while flowing inside the channel. Fresh water is obtained from the channel centerline, while ions are rejected near the walls, similar to the Capacitive DeIonization (CDI) principles. Parameters affecting the separation process, i.e., simulation duration, percentage of the removal, volumetric flow rate, and the length of the nanochannel incorporated, are affected by the electric field magnitude, ion correlations, and channel height. For the range of channels investigated here, an ion removal percentage near 100% is achieved in most cases in less than 20 ns for an electric field magnitude of E = 2.0 V/Å. In the nutshell, the ion drift is found satisfactory in the proposed nanoscale method, and it is exploited in a practical, small-scale system. Theoretical investigation from this work can be projected for systems at larger scales to perform fundamental yet elusive studies on water/ion separation issues at the nanoscale and, one step further, for designing real devices as well. The advantages over existing methods refer to the ease of implementation, low cost, and energy consumption, without the need to confront membrane fouling problems and complex electrode material fabrication employed in CDI.

Shear viscosity calculation of water in nanochannel: molecular dynamics simulation

Shear viscosity is one of the important transport properties which affects different phenomena in nanoconfined water. This study aims to investigate the effect of sub-Angstrom variations of nanochannel size on the shear viscosity of water confined in a silicon wall by employing equilibrium molecular dynamics (EMD) simulations. Simulation results demonstrate that water molecules confined in the slits are layered and for channels width less than 21 A, the number of layers varies from one to six. We show that if the capillary size becomes less than 18.5 A, the sub-Angstrom variations significantly affect the layered structure of the confined water. This causes the anomalous behavior of water viscosity and therefore, the flow resistance of nanoconfined water. According to the previous studies, the shear viscosity is greatly enhanced for subnanometer capillaries so that the shear viscosity increases dramatically by decreasing the channel size; however, we found that shear viscosity obeys an oscillatory behavior and has a complicated behavior which originates from the consistency between the channel size and the space required to embed one layer of water molecules. Results show that five minima and four maxima values for the viscosity are observed for channels width less than 18.5 A. Such unfamiliar behavior of viscosity and, consequently, the flow resistance, friction coefficient and slip length should be taken into account in investigation and design of such nanoconfined water.

3D Interconnected Gyroid Au-CuS Materials for Efficient Solar Steam Generation.

Surface plasmon resonance (SPR), a promising technology, is beneficial for various applications, such as photothermal conversion, solar cells, photocatalysts, and sensing. However, the SPR performance may be restricted by the 1D- or 2D-distributed hotspots. The bicontinuous interconnected gyroid-structured materials have emerged in light energy conversion due to a high density of 3D-distributed hotspots, ultrahigh light-matter interactions and large scattering cross-section. Here, a series of bioinspired Au-CuS gyroid-structured materials are fabricated by precisely controlling the deposition time of CuS nanoparticles (NPs) and then adopted for solar steam generation. Specifically, Au-CuS/GMs-80 present the highest evaporation efficiency of 88.8% under normal 1 sun, with a suitable filling rate (57%) and a large inner surface area (∼2.72 × 105 nm2 per unit cell), which simultaneously achieves a dynamic balance between water absorption and evaporation as well as efficient heat conduction with water in nanochannels. Compared with other state-of-the-art devices, Au-CuS/GMs-80 steam generator requires a much lower photothermal component loading (<1 mg cm-2) and still guarantees outstanding evaporation performance. This superior evaporation performance is attributed to broadband light absorption, continuous water supply, excellent heat generation and thermal insulation, and good light-heat-water interaction. The combination of 3D interconnected nanostructures with controllable metal-semiconductor deposition could provide a new method for the future design of high-performance plasmonic devices.

Molecular dynamics simulations of ion separation in nano-channel water flows using an electric field

ABSTRACTRemoval of undesired substances from water is a field of investigation recently focused at the nanoscale. Towards this direction, molecular dynamics simulations are conducted in this paper to investigate unwanted ion removal in nanochannel flows. The simulation method incorporates a Poiseuille-like water/ion flow system at the nanoscale where an electric field, of various magnitudes in the range of E = 0.25–1.5 V/Å, is applied perpendicular to the flow, leading anions and cations close to the wall regions, similar to the Capacitive De-Ionization method. The time needed for ions to reach equilibrium, i.e. to flow in the region near the walls while pure water flows in the channel interior, is t = 1.3 ns when E = 1.5 V/Å and t = 4.0 ns when E = 0.25 V/Å, showing a dependency on the value of the electric field. Calculations on density, velocity, and temperature values report on fluid properties to be used in the proposed desalination configuration and could act as a basis to guide novel technological applications and extend to higher scales.

Experimental study on capillary filling in nanochannels

We investigated the capillary filling kinetics of deionized water in nanochannels with heights of 50–120nm. The measured position of the moving meniscus was proportional to the square root of time, as predicted by the LW equation. However, the extracted slopes were significantly smaller than the predictions based on the bulk properties. This unusual behavior was found to be mainly caused by the electro-viscous effect and dynamic contact angle, which was significantly larger than the static angle. In addition, when the filling distance reached about 600μm, bubbles tended to be formed, leading to the main meniscus was almost immobile.

Structural characteristics of hydrated protons in the conductive channels: effects of confinement and fluorination studied by molecular dynamics simulation.

The relationship between the proton conductive channel and the hydrated proton structure is of significant importance for understanding the deformed hydrogen bonding network of the confined protons which matches the nanochannel. In general, the structure of hydrated protons in the nanochannel of the proton exchange membrane is affected by several factors. To investigate the independent effect of each factor, it is necessary to eliminate the interference of other factors. In this paper, a one-dimensional carbon nanotube decorated with fluorine was built to investigate the independent effects of nanoscale confinement and fluorination on the structural properties of hydrated protons in the nanochannel using classical molecular dynamics simulation. In order to characterize the structure of hydrated protons confined in the channel, the hydrogen bonding interaction between water and the hydrated protons has been studied according to suitable hydrogen bond criteria. The hydrogen bond criteria were proposed based on the radial distribution function, angle distribution and pair-potential energy distribution. It was found that fluorination leads to an ordered hydrogen bonding structure of the hydrated protons near the channel surface, and confinement weakens the formation of the bifurcated hydrogen bonds in the radial direction. Besides, fluorination lowers the free energy barrier of hydronium along the nanochannel, but slightly increases the barrier for water. This leads to disintegration of the sequential hydrogen bond network in the fluorinated CNTs with small size. In the fluorinated CNTs with large diameter, the lower degree of confinement produces a spiral-like sequential hydrogen bond network with few bifurcated hydrogen bonds in the central region. This structure might promote unidirectional proton transfer along the channel without random movement. This study provides the cooperative effect of confinement dimension and fluorination on the structure and hydrogen bonding of the slightly acidic water in the nanoscale channel.

Nonstraight nanochannels transfer water faster than straight nanochannels.

Understanding the flow of liquids and particularly water in nanochannels is important for scientific and technological applications, such as for filtration and drug delivery. Here we perform molecular dynamics simulations to investigate the transfer of single-file water molecules across straight or nonstraight single-walled carbon nanotubes (SWCNTs). In contrast with the macroscopic scenario, the nonstraight nanostructure can increase the water permeation. Remarkably, compared with the straight SWCNT, the nonstraight SWCNT with the minimal bending angle of 35° in the simulations can enhance the water transport up to 3.5 times. This enhancement mainly originates from the Lennard-Jones interaction between water molecules and nonstraight nanostructures. Our work offers an additional freedom to design high-flux nanochannels by choosing nonstraight nanostructures and provides an insight into water flow across biological water nanochannels, which are often nonstraight since they are composed of integral membrane proteins.

Pressure dependence of Kapitza resistance at gold/water and silicon/water interfaces

We conducted non-equilibrium molecular dynamics simulations to investigate Kapitza length at solid/liquid interfaces under the effects of bulk liquid pressures. Gold and silicon were utilized as hydrophilic and hydrophobic solid walls with different wetting surface behaviors, while the number of confined liquid water molecules was adjusted to obtain different pressures inside the channels. The interactions of solid/liquid couples were reparameterized accurately by measuring the water contact angle of solid substrates. In this paper, we present a thorough analysis of the structure, normal stress, and temperature distribution of liquid water to elucidate thermal energy transport across interfaces. Our results demonstrate excellent agreement between the pressures of liquid water in nano-channels and published thermodynamics data. The pressures measured as normal stress components were characterized using a long cut-off distance reinforced by a long-range van der Waals tail correction term. To clarify the effects of bulk liquid pressures on water structure at hydrophilic and hydrophobic solid surfaces, we defined solid/liquid interface spacing as the distance between the surface and the peak value of the first water density layer. Near the gold surface, we found that interface spacing and peak value of first water density layer were constant and did not depend on bulk liquid pressure; near the silicon surface, those values depended directly upon bulk liquid. Our results reveal that the pressure dependence of Kapitza length strongly depends on the wettability of the solid surface. In the case of the hydrophilic gold surface, Kapitza length was stable despite increasing bulk liquid pressure, while it varied significantly at the hydrophobic silicon surface.

MD Simulations of the Water Transportation in Nanochannels under the Environments of Electric Fields

The transportation properties of water in nanochannels have been widely studied by molecular dynamics (MD) simulations. In the environments of electric fields, the behaviors of water, such as molecular dipole orientations, flux, diffusion rate, water filling/empty equilibriums and phase-transition processes, etc., are much influenced. This review surveys the methods of introducing electric fields in MD simulations, including assigning charges near tubes, adding ions or charged amino acids to the water phases on both sides of nanotubes, and directly applying electric fields through the whole nanotubes. Besides, the relevant applications using electric fields, such as flow switch, signal transmission, water pump, stable storage, etc., are also included. Finally, some issues in the relevant MD studies are presented.