Flow In Nanochannels Research Articles

Viscous heating and temperature profiles of liquid water flows in copper nanochannel

Understanding nanoscale fluidic transport becomes increasingly important due to the rapid development of nanotechnology and nanofabrication. By using molecular dynamics (MD) simulations, we investigated the viscous heating of water flows in copper nanochannels. The two scenarios that were studied are Couette flows and Poiseuille flows. We observed the scale effects on the distribution of fluid density, streaming velocity, fluid viscosity, and temperature across the channel. The results revealed the significant effects of surface forces on causing a large deviation between simulation results and classical hypothesis. We found that the energy equation coupled with the thermal-slip boundary conditions still fails to predict the temperature distributions. Hereby, further scale effects are taken into account, which leads to better predictions. The model that we developed in this study shows the relative deviation to the simulation data within 5 %, which is small compared to the conventional continuum approach (i.e., up to 51 %).

Experimental verification of Poiseuille flow in nanochannels

The prediction of water transport in the nanochannels of a nanoporous membrane is crucial for the design and problem analysis of nanofluidic systems. In this study, we experimentally verified the no-slip boundary condition of the Poiseuille flow through the nanochannels by using a computational image processing method. Vertically-aligned nanochannel membranes (VNMs) were fabricated using the anodic aluminum oxidation process and chemical wet-etching process. Scanning electron microscope images of the VNMs were captured and processed computationally for determining the cross-sectional dimensions of the nanochannels. An experiment of water transport through the nanochannels of the VNMs was conducted, and the results were compared with theoretical calculations using image processing. The correspondence between the experimental results and theoretical calculations was greater than 97.5%.

Effects of ion size, ion valence and pH of electrolyte solutions on EOF velocity in single nanochannels

Electroosmotic flow (EOF) in nanochannels as small as 85 nm and in small microchannels is systematically investigated by using the current-slope method. The effects of ion size, ion valence and pH of electrolyte solutions on the velocity of electroosmotic flow are experimentally studied. The results show that the large size of hydrated ions leads to higher EOF velocity in large nanochannels and microchannels, and, however, lower EOF velocity in small nanochannels with an overlapped electric double layer (EDL). Furthermore, the EOF velocity is proportional to the pH of the electrolyte solution and contrariwise proportional to the valence of counter-ions. It is also observed that the EOF velocity depends on the channel size in nanochannels, even without overlapped EDL. Several models of electric double layer (EDL) are proposed to explain the experimental results and provide an improved understanding of the effects of ion size, ion valence and pH of electrolyte solutions on the EOF velocity in nanochannels in terms of EDL structure.

Density and viscosity profiles governing nanochannel flow

The density and viscosity profiles across the nanochannel height were respectively compared between the flow factor approach model and molecular dynamics simulation for the Poiseuille flow in a nanochannel, when the agreements between these two approaches were good in calculating both the flow velocity profile across the channel height and the volume flow rate through the channel. It was found that except close to the confining wall, the comparisons in the two regards showed satisfactory agreements; Because of the solidification of the thin layer close to the wall, the errors of the flow factor approach model in calculating both the local density and local viscosity close to the wall cannot considerably affect its calculation accuracy of both the flow velocity profile across the channel height and the volume flow rate through the channel, both of which were actually governed by the density and viscosity profiles of the flowing media in the middle section of the channel height.

Heat Transport of Electrokinetic Flow in Slit Soft Nanochannels.

Soft nanochannels are defined as nanochannels with a polyelectrolyte layer (PEL) on the rigid walls. In the present study, the thermal transport properties of the fluids through slit soft nanochannels are investigated under the combined influences of pressure-driven and streaming potential. Based on the analytical solutions of electric potential and velocity distributions, a dimensionless temperature of electrolyte solution in soft nanochannels is obtained by resolving the energy equation. Then, a finite difference method is used to compute the energy equation and test the validity of the analytical solution. Results show that the temperature increases with the decrease of dimensionless velocity and the heat transfer rate for rigid nanochannel are higher than that for the soft one. Moreover, we find the total entropy generation decreases with the increases of the ratio Kλ of the electrical double layer (EDL) thickness in PEL to the EDL thickness on the solid wall.

Understanding flow enhancement in graphene-coated nanochannels.

In this work, we investigate pressure-driven water flows in graphene-coated copper nanochannels through molecular dynamics simulations. It is found that the flow rate in bare copper nanochannel can be significantly enhanced by a factor of 45 when the nanochannel is coated with monolayer graphene. The enhancement factor for the flow rate reaches about 90 when the nanochannel is modified with 3 or more graphene layers. The dipole relaxation time and the hydrogen bond lifetime of interfacial water molecules show that the graphene coating promotes the mobility of water molecules at the interface. The distribution of the potential of mean force and the free energy barriers also confirm that graphene coating reduces the flow resistance and 3 layers of graphene can fully screen the surface effects. The results in this work provide important information for the design of graphene-based nanofluidic systems for flow enhancement.

Surface effects on Couette gas flows in nanochannels

A three-dimensional non-equilibrium molecular dynamics method is adopted to investigate the influence of wall force field on the nanoscale gas flow in the transition regime. For the gas flow under nanoscale condition, the dominant effect of the wall force field on the flow field is more obvious, and the flow physical quantity is more sensitive to the change of the wall condition and system temperature. The motion of the wall atoms is governed by the Einstein theory, with using an elastic coefficient <i>k</i> to model the surface stiffness. The results indicate that the surface stiffness has little effect on the physical quantity distribution of the bulk flow region, but a certain influence on that of the near wall region. Increasing the value of the stiffness changes the velocity peak of the gas in the near-wall region and the tangential momentum adaptation coefficient (TMAC) towards lower values, thus demoting the momentum adaptability of the gas molecules to the surface. The wall roughness is simulated by a typical pyramidal model. It is found that the influence of wall roughness on the flow is very obvious, whether it is in the bulk flow region or in the near wall region. For the former case, the increase of roughness leads gas velocity and shear stress to increase, with density and normal stress remaining constant. The linear distribution of physical quantities is also affected to some extent. While for the latter case, as the roughness increases, the velocity of the fluid increases rapidly and approaches to the wall velocity. The peak of density increases, and the adsorption of gas molecules at the surface is obvious. The TMAC approaches to 1, suggesting that the gas and the surface achieve a complete momentum adaptation. Besides, the influence of system temperature on the gas flow in the nanochannel is also studied. The system temperature is controlled by the Nose-Hoover thermostat, making the flow field maintained at the target temperature through the damping coefficient. The results show that the effect of temperature is global in the whole flow region. The increase of temperature causes the flow velocity of the whole flow field to decrease, while the normal stress and shear stress to increase. A higher temperature leads to more frequent collisions between gas molecules, thus increasing the effective viscosity of the gas. At the same time, the degree of gas molecule adsorption in the near-wall region is reduced, contributing to a smaller TMAC value, and consequently a weaker gas-surface interaction.

Molecular dynamics simulation of water flow in nano channel

Molecular dynamics simulation of water flow in nano channel

Electro-osmotic flow in hydrophobic nanochannels.

Hydrophobic surfaces with large slip lengths have the potential to enhance electro-osmotic flows. Existing theories of electroosmosis in hydrophobic channels postulate immobile surface charges and/or make a number of simplifying assumptions by considering mostly weakly charged surfaces and thin diffuse layers compared to channel dimension. In this paper, we extend prior models by focusing on planar and cylindrical nanochannels. Our theory accounts for a hydrodynamic slip and a mobility of surface charges, and is valid not only on the scale of the nanochannel with thin diffuse layers, but also on the scale of the overlapping diffuse layer. The model is simple enough to allow us to derive analytical approximations for the electro-osmotic velocities even when the surface potential and charge density are quite large. We also present numerical solutions to validate the analysis and illustrate the variation of electro-osmotic velocities in response to changes in the channel size, potential, surface charge and its mobility, hydrodynamic slip length, and salt concentration. Our results are directly relevant for carbon nanotubes, graphene nanochannels, and conventional nanoporous membranes.

Dissolutive flow in nanochannels: transition between plug-like and Poiseuille-like

Dynamic properties of dissolutive flow in nanochannels were investigated by molecular dynamics simulations. It turned out that the liquid flow pattern changes greatly after the dissolution effect taken into consideration. Liquid inside the channel has a plug-like velocity profile when the dissolubility is low, whereas a Poiseuille-like flow was observed as the dissolubility increases. By introducing the dissolution term to molecular kinetic theory, we explained the physical mechanisms of velocity transition. During which a modified dimensionless Galilei number was proposed to describe the effect of main forces. The results showed that in pressure-driven flow, when the dissolubility is low, the dominant dissipation is the viscous dissipation and the theoretical model of insolubility is acceptable. However, as the dissolubility increases, the dissolving dissipation takes priority, which results in the velocity profiles becoming Poiseuille-like. In addition, we analyzed the evolution of fluid density, number of dissolved solid particles and concentration distribution of solute. The liquid density varying from layered oscillation to uniform distribution was obtained, which can be described by a critical number. The analysis of solute concentration helps to establish the scaling relation among the dissolution rate, convection velocity, and diffusion coefficient. These findings not only help to understand the physical mechanisms of dissolutive flow but also help to control and optimize the flow patterns in dissoluble channels.

Scaling law for slip flow of gases in nanoporous media from nanofluidics, rocks, and pore-scale simulations

In unconventional reservoirs, as the effective pore size becomes close to the mean free path of gas molecules, gas transport in porous media begins to deviate from Darcy’s law. The objective of this study is to explore the similarities of gas flows in nanochannels and core samples as well as those simulated by direct simulation BGK (DSBGK), a particle-based method that solves the Bhatnagar-Gross-Krook (BGK) equation.Due to difficulties in fabrication and experimentation, previous study on gas flow experiments in nanochannels is very limited. In this work, steady-state gas flow was measured in reactive-ion etched nanochannels with a controlled channel size on a sillicon wafer. A core-based permeability measurement apparatus was used to perform steady-state gas flow measurements on carbonate and shale samples. Klinkenberg permeability was obtained under varying pore pressures but constant temperature and effective stress. Methane was used in nanofluidic and rock experiments, making them directly comparable. Results from both experiments were then compared to gas flow simulations by DSBGK method carried out on several independently constructed geometry models. DSBGK uses hundreds of millions of simulated molecules to approximate gas flow inside the pore space. The intermolecular collisions are handled by directly integrating the BGK equation along each molecules trajectory, rather than through a sampling scheme like that in the direct simulation Monte-Carlo (DSMC) method. Consequently, the stochastic noise is significantly reduced, and simulation of nano-scale gas flows in complex geometries becomes computationally affordable.The slippage factors obtained from these independent studies varied across three orders of magnitude, yet they all appear to collapse on a single scaling relation where the slippage factor in the slip flow regime is inversely proportional to the square root of intrinsic permeability over porosity. Our correlation could also fit the data in the literature, which were often obtained using nitrogen, after correcting for temperature and gas properties. This study contributes to rock characterization, well testing analysis as well as the understanding of rarefied gas transport in porous media.

DPD simulation of non-Newtonian electroosmotic fluid flow in nanochannel

ABSTRACTWe use the dissipative particle dynamics (DPD) method to simulate the non-Newtonian electroosmotic flow (EOF) through nanochannels. Contrary to a large amount of past computational efforts dedicated to the study of EOF profile, this work pays attention to the EOF of non-Newtonian fluids, which has been rarely touched in past publications. Practically, there are many MEMS/NEMS devices, in which the EOF behaviour should be treated assuming both non-continuum and non-Newtonian conditions. Therefore, our concern in this work is to simulate the EOF through nanochannels considering both non-Newtonian fluid properties and non-continuum flow conditions. We have chosen DPD as our working tool because it provides several important advantages comparing with the classical time consuming molecular dynamics method. Using the DPD method, we explore the effect of a few important fluid properties and nanochannel parameters on the EOF behaviour and the resulting flow rate magnitudes. Our investigation will result in a number of findings, which have not been reported in past research works.

Gas concentration polarization and transport mechanism transition near thin polymeric membranes

Advanced material engineering gave rise to cutting-edge characteristics of sustainable separation by fabrication of high permeable and gas selective membranes. A developed fabrication process of ultrathin polymers with controlled microporosity helps understanding pore size dependent transport and performance of thin membranes with constant pore number from continuum to free-molecular to solution-diffusion regimes. Fabrication of polymer layers with controlled pore size was realized by stage-wise physical vapor deposition of dimethylsiloxane oligomers onto porous alumina support. Single process step forms high permeable porous films of 13 nm in thickness. Further deposition allows fabrication of up to 120 nm nanopore-free uniform layers with defined gas selectivity. Permeances as high as 1300 GPU through gas-selective, uniform dense films results in gas concentration polarization and apparent selectivity loss, so far only observed for porous materials. Our fabrication process enabled observation of two unseen separation regimes in the transport physics transition from (nano) channel flow to solution diffusion supported by mathematical model derived for transport through ultra-thin membranes.

A design method for nanofluidic circuits

A design method is proposed for nanofluidic circuits, based on the flow equation for a nanoscale fluid flow. This method incorporates the use of the concepts of the flow resistance, the flow rate, the pressure drop and the power loss, as like in electric circuits. The equations for calculating the flow resistance and the power loss in exemplary nanofluidic circuits including in a nanotube tree are presented. It was found that the nanotube size and the fluid-tube wall interaction both have great influences on the flow resistance and the power loss in nanochannel flow. Exemplary design analysis is given for some nanofluidic circuits, based on the proposed method.

A discrete phase hybrid continuum-atomistic model for electrokinetics in nanofluidics

The ever-growing field of micro- and nanotechnology has a great deal of interest in simulating dynamic phenomena of multiscale systems. Hybrid approaches that produce a trade-off between accuracy and computational costs play a key role in this area. In this study, an improved hybrid continuum-atomistic model is proposed for the simulation of electroosmotic flows in nanochannels. The aqueous solvent phase is modeled by the continuum four-way coupled Navier-Stokes equations, while a Lagrangian approach is used for the ion transport. Different forces, including the drag, buoyancy, Brownian, electrostatic, and ion-ion/wall-ion collision, and torques, including the drag and collision, govern the motion of ion particles. The ion-ion/wall-ion collision is taken into account by a discrete phase model, and the electric field is derived by the Poisson-Boltzmann closure. Results of the model, such as the change in bulk velocity with surface electric charge density, are validated by several molecular dynamics simulations and experimental observations available in the literature. It is shown that the present hybrid model is capable of predicting the main features of the problem. Moreover, the significance of different forces and the other alternative for modeling the external electric field, i.e., the discrete Coulomb’s approach with the modified particle mesh Ewald boundary treatment, are also examined. The proposed model would be extremely useful for future studies on the electrokinetics in nanochannels, especially in more complex geometries where the molecular dynamics approaches are limited due to the computational costs.

Size effect on nanochannel flow explored by the flow factor approach model

The flow factor approach model (FFAM) was newly compared with molecular dynamics simulation (MDS) when simulating the Poiseuille flow in a nano slit pore where the effect of the solidified layer on the confining wall may be considerable. For direct comparisons, the MDS results were for no fluid-wall interfacial slippage under a relatively strong fluid-wall interaction; In this case, certain solidified layers may occur on both the confining walls. The comparisons show that these two approaches match well not only in the flow velocity profiles across the channel height but also in the volume flow rates through the channel. The study shows that when the total thickness of the solidified layers on the walls is comparable to the channel height, the effective channel height which is the nominal channel height minus the total thickness of the solidified layers on the walls, may need to be incorporated by FFAM for simulating a nanochannel flow; Otherwise, the effect of the solidified layer on the wall is negligible.

Hybrid finite-difference/lattice Boltzmann simulations of microchannel and nanochannel acoustic streaming driven by surface acoustic waves

The acoustic streaming flow in microchannels and nanochannels caused by complex fluid-structural coupling due to surface acoustic waves is studied using a hybrid continuum and mesoscale numerical model.

Thermal transport of combined electroosmotically and pressure driven nanofluid flow in soft nanochannels

In this paper, the heat transfer characteristics of the nanofluid through a parallel plate soft nanochannel are investigated under the fully developed condition. The flow is actuated by the combined effects of pressure gradient and implied electric field. Based on the ion partitioning effect and the Debye–Huckel linearization, the analytical solutions for electrokinetic flow in such nanochannel are obtained. Meanwhile, the uniform wall heat flux is utilized in the analysis, and the influences of viscous dissipation and the Joule heating are taken into account. The results for pertinent dimensionless parameters are presented graphically and discussed in brief. The relevant result reveals that the ion partitioning effect can greatly impact the electrostatic potential, velocity and temperature distribution. Furthermore, this ion partitioning effect can exert an influence on heat transfer of the nanofluid. The present study also indicates the possibility of alteration in the nanofluid heat transfer by the use of nanoparticle volume.

Driving mechanisms and streamwise homogeneity in molecular dynamics simulations of nanochannel flows

In molecular dynamics simulations, nanochannel flows are usually driven by a constant force, that aims to represent a pressure difference between inlet and outlet, and periodic boundary conditions are applied in the streamwise direction resulting in an homogeneous flow. The homogeneity hypothesis can be eliminated adding reservoirs at the inlet and outlet of the channel which permits to predict streamwise variation of flow properties. It also opens the door to drive the flow by applying pressure gradient instead of a constant force. We analyze the impact of these modeling modifications in the prediction of the flow properties and we show when they make a difference with respect to the standard approach. It turns out that both assumptions are irrelevant when low pressure differences are considered, but important differences are observed at high pressure differences. They include the density and velocity variation along the channel (the mass flow rate is constant) but, more importantly, the temperature increase and slip length decrease. Because viscous heating is important at high shear rates, these modeling issues are also linked to the use of thermostating procedures. Specifically, selecting the region where the thermostat is applied has a critical influence on the results. Whereas in the traditional homogeneous model the choices are limited to the fluid and/or the wall, in the inhomogeneous cases the reservoirs are also available, which permits to leave the region of interest, the channel, unperturbed.

Supercritical CO2-induced atomistic lubrication for water flow in a rough hydrophilic nanochannel.

A fluid flow in a nanochannel highly depends on the wettability of the channel surface to the fluid. The permeability of the nanochannel is usually very low, largely due to the adhesion of fluid at the solid interfaces. Using molecular dynamics (MD) simulations, we demonstrate that the flow of water in a nanochannel with rough hydrophilic surfaces can be significantly enhanced by the presence of a thin layer of supercritical carbon dioxide (scCO2) at the water-solid interfaces. The thin scCO2 layer acts like an atomistic lubricant that transforms a hydrophilic interface into a super-hydrophobic one and triggers a transition from a stick- to- a slip boundary condition for a nanoscale flow. This work provides an atomistic insight into multicomponent interactions in nanochannels and illustrates that such interactions can be manipulated, if needed, to increase the throughput and energy efficiency of nanofluidic systems.