Interfacial friction of ethanol–water mixtures in graphene pores

The interfacial friction of fluid within nanoscale pores is important to nanofluidic devices and processes. Herein, molecular dynamics simulations have been used to study the interfacial flow resistance of ethanol–water mixtures confined within graphene-based nanochannels. The friction coefficients of the mixtures were investigated by considering the effects of slit pore width and mixture composition. The simulated results show that the flow friction coefficient is sensitive to the graphene slit pore size for ethanol-containing solution systems. In particular, the mixture composition has a significant impact on the friction coefficients for the mixture in 7–10 Å nanoslits, while the composition dependence of friction coefficients becomes weak at larger pore widths. In addition, qualitative theoretical analysis has been carried out to reveal the molecular origin of mixture friction behavior. The ethanol–wall interaction accounts for the major role on the mixture friction coefficients. The changing behavior of mixture friction coefficient is caused by the joint effects from the interfacial ethanol density and the potential energy barrier felt by ethanol molecules.     

The end of nanochannels

Current theories of nanochannel flow impose no upper bound on flow rates, and predict friction through nanochannels can be vanishingly small. We reassess neglecting channel entry effects in extremely long channels and find violations at the nanoscale. Even in frictionless nanochannels, end effects provide a finite amount of friction. Hence, the speed at which nanochannels transport liquids is limited. Flow-rate and slip-length measurements are reevaluated using calculations which include end-effect friction. End effects are critical for the design of new technological devices and to understand biological transport.     

Effects of surface roughness and interface wettability on nanoscale flow in a nanochannel

Non-equilibrium molecular dynamics simulations have been carried out to investigate the effect of surface roughness and interface wettability on the nanorheology and slip boundary condition of simple fluids in a nanochannel of several atomic diameters width. The solid surfaces decorated with periodic nanostrips are considered as the rough surface in this study. The simulation results showed that the interface wettability and the surface roughness are important in determining the nanorheology of the nanochannel and fluid slip at solid–fluid interface. It is observed that the presence of surface roughness always suppresses the fluid slip for hydrophilic and hydrophobic surface nanochannels. For fluids over smooth and hydrophobic surfaces, the snapshots of fluid molecules show that an air gap or nanobubble exists at the fluid–solid interface, resulting in the apparent slip velocity. For a given surface with fixed interface wettability, the fluid velocities increase by increasing the driving force, while the driving force has no significant influence on the density structure of fluid molecules. The fluid slip and the flow rate are measured for hydrophilic and hydrophobic nanochannels. The flow rates in rough surface nanochannels are smaller than those of smooth surface walls due to the increase of drag resistance at the solid–fluid interface. The dependence between fluid slip and flow rate showed that the slip length increases approximately linearly with the flow rate for both the hydrophobic and hydrophilic surface nanochannels.     

The effects of wall roughness on the methane flow in nano-channels using non-equilibrium multiscale molecular dynamics simulation

This paper presents a non-equilibrium multiscale molecular dynamics simulation method to investigate the effects of periodic wall surface roughness on the structure and mass transfer of methane fluid through the silicon nano-channels. In order to accurately capture the trajectories and microstructure of methane nano-fluidics, the present modification of OPLS fully atomic model is employed. Meanwhile, we introduce the corresponding coarse-grained model to solve the problem of wall–fluid interaction for methane Poiseuille flow within silicon atomic walls using the classical Lorentz–Berthelot mixing rules. The geometries of the upper wall roughness are modeled by rectangular waves with different amplitudes and wavelengths. The three-dimensional number densities of C (H) atom and kinetic energy distribution plots give a clear observation of the impacts of surface roughness on the localization micro-information of methane fluid. Moreover, the slip length of fluid over rough surface decreases with the increase in amplitude. The diffusion coefficients appear anisotropic, and the radial distribution functions decrease with the increase in the amplitude. These properties should be taken into account in the design of energy-saving emission reduction nano-fluidic devices. All numerical results also indicate that the presented method not only can well solve the issue of wall–fluid interactions, but also could accurately predict the micro-information and dynamic properties of methane Poiseuille flow.     

Surface wettability effects on flow in rough wall nanochannels

The effect of rough-wall/fluid interaction on flow in nanochannels is investigated by NEMD. Hydrophobic and hydrophilic surfaces are studied for walls with nearly atomic-size rectangular protrusions and cavities. Our NEMD simulations reveal that the number of liquid atoms temporarily trapped in the cavities is affected by the strength of the potential energy inside the cavities. Regions of low potential energy are possible trapping locations. Fluid atom localization is also affected by the hydrophilicity/hydrophobicity of the surface. Potential energy is greater between two successive hydrophilic protrusions, compared to hydrophobic ones. Moreover, groove size and wall wettability are factors that control effective slip length. Surface roughness and wall wettability have to be taken into account in the design of nanofluidic devices.     

DSMC investigation of rarefied gas flow through diverging micro- and nanochannels

Direct simulation Monte Carlo (DSMC) method with simplified Bernoulli trials (SBT) collision scheme has been used to study the rarefied pressure-driven nitrogen flow through diverging micro- and nanochannels. The fluid behaviours flowing between two plates with different divergence angles ranging between 0° and 17° are described at different pressure ratios (1.5 ≤ Π ≤ 2.5) and Knudsen numbers (0.03 ≤ Kn ≤ 12.7). The primary flow field properties, including pressure, velocity, and temperature, are presented for divergent micro- and nanochannels and are compared with those of a micro- and nanochannel with a uniform cross section. The variations of the flow field properties in divergent micro- and nanochannels which are influenced by the area change, the channel pressure ratio, and the rarefication are discussed. The results show no flow separation in divergent micro- and nanochannels for all the range of simulation parameters studied in the present work. It has been found that a divergent channel can carry higher amounts of mass in comparison with an equivalent straight channel geometry. A correlation between the mass flow rate through micro- and nanochannels, the divergence angle, the pressure ratio, and the Knudsen number has been suggested. The present numerical findings prove the occurrence of Knudsen minimum phenomenon in micro- and nanochannels with non-uniform cross sections.     

A method to generate pressure gradients for molecular simulation of pressure-driven flows in nanochannels

One of the difficulties in molecular simulation of pressure-driven fluid flow in nanochannels is to find an appropriate pressure control method. When periodic boundary conditions (PBCs) are applied, a gravity-like field has been widely used to replace actual pressure gradients. The gravity-fed method is not only artificial, but not adequate for studying properties of fluid systems which are essentially inhomogeneous in the flow direction. In this paper, a method is proposed which can generate any desired pressure difference to drive the fluid flow by attaching a ??pump?? to the nanofluidic system, while the model is still compatible with PBCs. The molecular dynamics model based on the proposed method is applied to incompressible flows in smooth nanochannels, and the predicted velocity profiles are identical to those by the gravity-fed method, as expected. For compressible flows, the proposed model successfully predicts the changes of fluid density and velocity profile in the flow direction, while the gravity-fed method can only predict constant fluid properties. For fluid flows in nanochannels with a variable cross-sectional area, the proposed model predicts higher mass flow rates as compared to the gravity-fed method and possible reasons for the difference are discussed.     

Prediction and analysis of flow behavior of a polymer melt through nanochannels using artificial neural network and statistical methods

A new methodology, namely, artificial neural network (ANN) approach was proposed for modeling and predicting flow behavior of the polyethylene melt through nanochannels of nanoporous alumina templates. Wetting length of the nanochannels was determined to be a function of time, temperature, diameter of nanochannels, and surface properties of the inner wall of the nanochannels. An ANN was designed to forecast the relationship between the length of wetting as output parameter and other aforementioned parameters as input variables. It was demonstrated that the ANN method is capable of modeling this phenomenon with high accuracy. The designed ANN was then employed to obtain the wetting length of the nanochannels for those cases, which were not reported by the wetting experiments. The results were then analyzed statistically to identify the effect of each independent variable, namely, time, temperature, diameter of nanochannels, and surface properties of the inner wall of nanochannels as well as their combinations on the wetting length of the nanochannels. Interesting results were attained and discussed.     

AmphiBot I: an amphibious snake-like robot

This article presents a project that aims at constructing a biologically inspired amphibious snake-like robot. The robot is designed to be capable of anguilliform swimming like sea-snakes and lampreys in water and lateral undulatory locomotion like a snake on ground. Both the structure and the controller of the robot are inspired by elongate vertebrates. In particular, the locomotion of the robot is controlled by a central pattern generator (a system of coupled oscillators) that produces travelling waves of oscillations as limit cycle behavior. We present the design considerations behind the robot and its controller. Experiments are carried out to identify the types of travelling waves that optimize speed during lateral undulatory locomotion on ground. In particular, the optimal frequency, amplitude and wavelength are thus identified when the robot is crawling on a particular surface.     

Molecular dynamics simulation of nanoscale liquid flows

Molecular dynamics (MD) simulation is a powerful tool to investigate the nanoscale fluid flow. In this article, we review the methods and the applications of MD simulation in liquid flows in nanochannels. For pressure-driven flows, we focus on the fundamental research and the rationality of the model hypotheses. For electrokinetic-driven flows and the thermal-driven flows, we concentrate on the principle of generating liquid motion. The slip boundary condition is one of the marked differences between the macro- and micro-scale flows and the nanoscale flows. In this article, we review the parameters controlling the degree of boundary slip and the new findings. MD simulation is based on the Newton's second law to simulate the particles' interactions and consists of several important processing methods, such as the thermal wall model, the cut-off radius, and the initial condition. Therefore, we also reviewed the recent improvement in these key methods to make the MD simulation more rational and efficient. Finally, we summarized the important discoveries in this research field and proposed some worthwhile future research directions.     

Diffusion of a chemically reactive species of a power-law fluid past a stretching surface

A numerical solution for the steady magnetohydrodynamic (MHD) non-Newtonian power-law fluid flow over a continuously moving surface with species concentration and chemical reaction has been obtained. The viscous flow is driven solely by the linearly stretching sheet, and the reactive species emitted from this sheet undergoes an isothermal and homogeneous one-stage reaction as it diffuses into the surrounding fluid. Using a similarity transformation, the governing non-linear partial differential equations are transformed into coupled nonlinear ordinary differential equations. The governing equations of the mathematical model show that the flow and mass transfer characteristics depend on six parameters, namely, the power-law index, the magnetic parameter, the local Grashof number with respect to species diffusion, the modified Schmidt number, the reaction rate parameter, and the wall concentration parameter. Numerical solutions for these coupled equations are obtained by the Keller-Box method, and the solutions obtained are presented through graphs and tables. The numerical results obtained reveal that the magnetic field significantly increases the magnitude of the skin friction, but slightly reduces the mass transfer rate. However, the surface mass transfer strongly depends on the modified Schmidt number and the reaction rate parameter; it increases with increasing values of these parameters. The results obtained reveal many interesting behaviors that warrant further study of the equations related to non-Newtonian fluid phenomena, especially shear-thinning phenomena. Shear thinning reduces the wall shear stress.     

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.

     

Gated transport in nanofluidic devices

The surface property of the nanochannel plays an important role in controlling the ion transport through the nanochannel. Embedding electrodes outside the nanochannel (referred to as gated nanochannels) is a simple way to control the surface charge density of the nanochannel. Based on the numerical simulations using coupled Poisson–Nernst–Planck and Stokes equations, we show that a relative difference between the applied voltage and the gate voltage would alter the space charge density along the nanochannel. Thus, the gate voltage can tune the nanochannel into a p- or n-type field effect transistor, enabling the control of fluid flow in the nanochannel. The ionic currents reveal that the ionic flux can be controlled by the gate voltage. Analytical expressions are derived to estimate the effective space charge density and the fluid flow in the nanochannels for a fixed gate voltage. We also suggest potential applications of the gated nanochannels.     

An Enskog based Monte Carlo method for high Knudsen number non-ideal gas flows

A Monte Carlo method based on the Enskog equation for dense gas is developed by considering high density effect on collision rates and both repulsive and attractive molecular interactions for a Lennard-Jones fluid. The appropriate internal energy exchange model is introduced with consistency with the collision model. The equation of state for a non-ideal gas is therefore derived involving the finite density effect and the van der Waals intermolecular force, changing from the Clapeyron equation to the van der Waals equation. In contrast to previous Monte Carlo approaches, the present predictions agree better with experimental data for the gas transport properties at high densities and in a wide temperature region. The numerical modeling of non-ideal gas flow in micro and nanochannels show that the high gas density affects greatly flow behavior and heat transfer characteristics. The high density of gas leads to a lower skin friction coefficient on the wall surfaces than the predictions by the perfect gas assumption.     

Numerical analysis on electroosmotic flows in a microchannel with rectangle-waved surface roughness using the Poisson–Nernst–Planck model

The present study has numerically investigated two-dimensional electroosmotic flows in a microchannel with dielectric walls of rectangle-waved surface roughness to understand the roughness effect. For the study, numerical simulations are performed by employing the Nernst–Planck equation for the ionic species and the Poisson equation for the electric potential, together with the traditional Navier–Stokes equation. Results show that the steady electroosmotic flow and ionic-species transport in a microscale channel are well predicted by the Poisson–Nernst–Planck model and depend significantly on the shape of surface roughness such as the amplitude and periodic length of wall wave. It is found that the fluid flows along the surface of waved wall without involving any flow separation because of the very strong normal component of EDL (electric double layer) electric field. The flow rate decreases exponentially with the amplitude of wall wave, whereas it increases linearly with the periodic length. It is mainly due to the fact that the external electric-potential distribution plays a crucial role in driving the electroosmotic flow through a microscale channel with surface roughness. Finally, the present results using the Poisson–Nernst–Planck model are compared with those using the traditional Poisson–Boltzmann model which may be valid in these scales.     

基础激励作用下悬臂输流管的振动实验研究

通过实验的方法对基础激励作用下悬臂输流管的动力学行为进行振动测试分析.设计并搭建了悬臂输流管振动测试的实验平台,深入研究了悬臂输流管在不同流速及不同参数情况下的振动行为,分析了在基础激励作用下,悬臂输流管振幅的变化规律.结果表明,悬臂输流管的物理参数对悬臂输流管发生颤振失稳的临界流速有着极其显著的影响,在基础激励作用下,悬臂输流管的振幅明显增大,并且当激振力频率接近悬臂输流管的固有频率时,输流管的振幅达到最大.     

Mesoscopic simulations of electroosmotic flow and electrophoresis in nanochannels

We review recent dissipative particle dynamics (DPD) simulations of electrolyte flow in nanochannels. A method is presented by which the slip length δB at the channel boundaries can be tuned systematically from negative to infinity by introducing suitably adjusted wall-fluid friction forces. Using this method, we study electroosmotic flow (EOF) in nanochannels for varying surface slip conditions and fluids of different ionic strength. Analytic expressions for the flow profiles are derived from the Stokes equation, which are in good agreement with the numerical results. Finally, we investigate the influence of EOF on the effective mobility of polyelectrolytes in nanochannels. The relevant quantity characterizing the effect of slippage is found to be the dimensionless quantity κδB, where 1/κ is an effective electrostatic screening length at the channel boundaries.     

Flow-rate-insensitive deterministic particle sorting using a combination of travelling and standing surface acoustic waves

Manipulation of cells by acoustic forces in a continuous flow offers a means to sort on the basis of physical properties in a contactless, label-free and biocompatible manner. Many acoustic sorting systems rely on either standing waves or travelling waves alone and require specific exposure times to the acoustic field, fine-tuned by manipulating the bulk flow rate. In this work, we demonstrate a flow-rate-insensitive device for continuous particle sorting by employing a pressure field that utilises both travelling and standing acoustic wave components, whose non-uniform spatial distribution arises from the attenuation of a leaky surface acoustic wave. We show that in parts of the pressure field in which the travelling wave component dominates, particles migrate across multiple wavelengths. In doing so, they drift into areas of standing wave dominance, whereby particles are confined within their respective nodal positions. It is demonstrated that this final confinement location is dependent on the particle size and independent of the force field exposure time and thus the flow rate, permitting the continuous separation of 5.1-, 6.1- and 7.0-µm particles. Omitting the need to precisely control the bulk flow rate potentially enables sorting in systems in which flow is not driven by external pumps.     

Simulation of media behaviour in vibration assisted abrasive flow machining

Abrasive flow machining (AFM) is an abrasive-based precision finishing process used for achieving surface finish in micro and nano-level. The AFM polishes surfaces by extruding a visco-elastic media in contact with the workpiece. The media, also called a ‘flexible tool’, plays a key role in the performance of the process. Ultrasonic assisted abrasive flow machining (UAAFM) is a new variant of the AFM process in which the workpiece is subjected to mechanical vibration orthogonal to the media flow direction. In this process a high frequency, in the range of about 5–20 kHz, is given to the workpiece with the help of a piezo actuator and a specially designed fixture. The present work highlights on the possible behaviour of the tool (media) during UAAFM and its effect on the machining process through a computation based approach. Commercially available simulation tool was used to study the effect of the media in response to different set of machining conditions. The responses were evaluated in terms of changes in the fluid pressure, velocity profile of the fluid, temperature distribution in the working fluid and the possible wall shear on the work surface. A three-dimensional model was constructed for simulating the UAAFM process. The simulation shows that the abrasive particles tend to hit the target surface at an angle ‘θ’ which significantly affects the basic mechanisms involved and enhances the effectiveness of the process. The computed wall shear explains that the process will have higher finishing rate and hence the performance. The enhanced interaction of abrasive media in UAAFM while compared to simple AFM could be explained by the resultant pressure–velocity phenomena. Results show that while changes in the amplitude of applied vibration (10 μm and 50 μm) significantly affect the wall shear, the media velocity and pressure profiles are only marginally sensitive to this parameter. The simulation results also confirm that the rise in temperature during the process will not affect the media stability. Results have been discussed vis-a-vis the basic mechanism of the process through suitable illustrations.     

带强迫项的变系数KdV方程的多孤立波解及其应用

本文利用改进的齐次平衡法,首先得到了带强迫项的变系数KdV方程的多孤立波解,然后借助此解得到了强迫KdV方程的多孤立波解.最后作为应用例子,利用图形分析方法分析了Rossby孤立波的相互作用,指出了影响Rossby孤立波相对幅度、相位、传播方向及平衡位置的主要原因.