Nanoscale Flow Research Articles

GS34 格子ボルツマン法によるナノ流路内の熱流動解析

The present study discusses thermal nano-scale flows by the lattice Boltzmann method(LBM). Although recent LBMs discuss nano-flows, reports on nano-thermal flows are rare. Our group showed that the regularization procedure was needed for complex nano-scale flows. However our preliminary study also showed that the common thermal model did not capture thermal jumps on a wall when the regularization was coupled with. In this study, the model is improved to capture thermal jumps with the regularization. To verify the present model, Fourie, Poiseuille and square cylinder flows are simulated. It is confirmed that the results of the present model correspond well with the reference data.

Molecular dynamics simulation on fluid flow and heat transfer in rough nanochannels

Fluid flow and heat transfer in a microstructure may depart from the traditional behavior due to the scale effect, and its velocity slip and temperature jump will occur at the fluid-solid interface. A molecular dynamics model of coupled fluid flow and heat transfer in rough nanochannels is developed to investigate the effect of surface roughness on nanoscale fluid flow and heat transfer, as well as velocity slip and temperature jump at the fluid-solid interface. The fluid microscopic structure, velocity and temperature distributions, interfacial velocity slip and temperature jump in a rough nanochannel are evaluated and compared with the corresponding smooth nanochannel. Effects of solid-liquid interaction and wall stiffness on the velocity slip and temperature jump are analyzed. Results indicate that the velocity of the fluid flow under an external force in a nanochannel in a bulk region is of a parabolic distribution, and the viscous dissipation due to shear flow induces the fourth-order temperature profile in the nanochannel. And the velocity slip and temperature jump will occur at the fluid-solid interface. The presence of roughness may introduce an extra viscous dissipation in shear flow, leading to a reduction of overall velocity and an increase in temperature in the nanochannel when compared with the smooth nanochannel. In addition, the degree of velocity slip and temperature jump at a rough liquid-solid interface is smaller than that at a smooth interface. In particular, the increase in fluid-solid interaction strength and reduction in wall stiffness will lead to a small velocity slip and temperature jump.

Multiscale Fluid Mechanics and Modeling

In recent years, there has been a tremendous growth of activity on multiscale modeling and computation. In particular, the multiscale hybrid numerical methods are those that combine multiple models defined at fundamentally different length and time scales within the same overall spatial and temporal domain. For examples, a framework of hybrid continuum and molecular dynamics multiscale method has been developed to simulate micro- and nanoscale fluid flows, which combines the continuum computational fluid dynamics (CFD) or the mesoscopic lattice Boltzmann method for the bulk flow region and the atomistic molecular dynamics for the interface region. The similar idea of constrained variation has also been used in developing multiscale fluid turbulent models for constrained dynamic subgrid-scale stress model, Reynolds stress constrained large eddy simulation (RSC-LES) for wall-bounded turbulent flows with massive separation and heat flux constrained large eddy simulation. For RSC-LES, our model is able to solve the traditional log-layer mismatch problem in RANS/LES approaches and can predict mean velocity, turbulent stress and skin friction coefficients more accurate than pure dynamic large eddy models and traditional detached eddy simulation using the same grid resolution. Our results demonstrate the capability of multiscale simulation methods for complex fluid systems and the necessity of physical constraints on the multiscale methods.

Breaking through tradeoff of Polysulfone ultrafiltration membranes by zeolite 4A

Pressure-driven ultrafiltration by porous membrane often encountered a tradeoff between permeability and selectivity due to the transport mechanism based on pore dimension. This paper aims to break through the tradeoff of Polysulfone (PSf) ultrafiltration membranes by incorporating specific zeolite 4A. The influences of particle size, micro-channel dimension, zeta potential and content of zeolite 4A on tradeoff were discussed. The morphological evolution investigated by Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) showed that zeolites 4A embedded in the selective layer within the depth of less than 1μm increased the surface roughness and improved the hydrophilicity. Thermal and mechanical stability were also evaluated by thermal gravimetric analysis (TGA) and tensile testing respectively. The filtration experiments showed that both pure water flux and rejection were simultaneously improved due to the morphology regulation, abundant nano-scale water flow channels, negatively charged membrane surfaces and precise embedding position of zeolite 4A in the membrane selective layer.

Impact of technology scaling on leakage power in nano-scale bulk CMOS digital standard cells

Leakage estimation is an important step in nano-scale technology digital design flows. While reliable data exist on leakage trends with bulk CMOS technology scaling in stand-alone devices and circuits, there is a lack of public domain results on the effect of scaling on leakage power consumption for a complete standard cell set. We present an analysis on a standard cell library applying a logic-level estimation model, supported by SPICE BSIM4 comparison. The logic-level model speedup over SPICE is >103 with average accuracy below 1% error. We therefore explore the effects of scaling on the whole standard cell set with respect to different leakage mechanisms (sub-threshold, body, gate) and to input pattern dependence. While body leakage appears to be dominant, sub-threshold leakage is expected to increase more than other components with scaling. Detailed data of the whole analysis are reported for use in further research on leakage aware digital design.

Thermo-flow and temperature sensing behaviour of graphene based on surface heat convection

This letter studies the surface heat convection of thin graphene sheets and the application of graphene wires as nanoscale flow and temperature sensors. Graphene wires with relatively large length-to-width ratios were designed and fabricated using bi- and few-layer graphene sheets. Prior to testing, the devices were packaged in a microfluidic chamber with capillary tubes as upstream and downstream connections to minimise environmental interference. The thermal inertia of the graphene wire was studied at 70°C and the flow sensing behaviour of the device was characterised by monitoring normalised resistance changes at different flow rates. The authors experimental results demonstrated the negative temperature coefficients of the bi- and few-layer graphene films. Moreover, the flow sensing resolutions of ~ 0.07 l/min and 0.1 l/min were achieved from the bi- and few-layer graphene hot wires, respectively. The temperature sensing behaviour of the graphene thermistor was studied in a small temperature range from room temperature to 80°C. The larger negative temperature coefficient of the bi-layer graphene resulted in a higher sensing response than the few-layer one.

Conformational Manipulation of DNA in Nanochannels Using Hydrodynamics

The control over DNA elongation in nanofluidic devices holds great potential for large-scale genomic analysis. So far, the manipulation of DNA in nanochannels has been mostly carried out with electrophoresis and seldom with hydrodynamics, although the physics of soft matter in nanoscale flows has raised considerable interest over the past decade. In this report the migration of DNA is studied in nanochannels of lateral dimension spanning 100 to 500 nm using both actuation principles. We show that the relaxation kinetics are 3-fold slowed down and the extension increases up to 3-fold using hydrodynamics. We propose a model to account for the onset in elongation with the flow, which assumes that DNA response is determined by the shear-driven lift forces mediated by the proximity of the channels’ walls. Overall, we suggest that hydrodynamic actuation allows for an improved manipulation of DNA in nanochannels.

Using nanoscale thermocapillary flows to create arrays of purely semiconducting single-walled carbon nanotubes

Among the remarkable variety of semiconducting nanomaterials that have been discovered over the past two decades, single-walled carbon nanotubes remain uniquely well suited for applications in high-performance electronics, sensors and other technologies. The most advanced opportunities demand the ability to form perfectly aligned, horizontal arrays of purely semiconducting, chemically pristine carbon nanotubes. Here, we present strategies that offer this capability. Nanoscale thermocapillary flows in thin-film organic coatings followed by reactive ion etching serve as highly efficient means for selectively removing metallic carbon nanotubes from electronically heterogeneous aligned arrays grown on quartz substrates. The low temperatures and unusual physics associated with this process enable robust, scalable operation, with clear potential for practical use. We carry out detailed experimental and theoretical studies to reveal all of the essential attributes of the underlying thermophysical phenomena. We demonstrate use of the purified arrays in transistors that achieve mobilities exceeding 1,000 cm(2) V(-1) s(-1) and on/off switching ratios of ∼10,000 with current outputs in the milliamp range. Simple logic gates built using such devices represent the first steps toward integration into more complex circuits.

Influence of streaming potential on pulsatile pressure‐gradient driven flow through an annulus

We study pulsatile pressure-gradient driven flow through an annular confinement under the influence of streaming potential. Our study considers zeta potentials beyond the traditional Debye-Hückel limits, and takes care against aphysical predictions stemming from traditional Boltzmann statistics through modifications involving the finite size of the ionic species (steric effects). With these considerations, we demonstrate, rather non-intuitively, that an intrinsic asymmetry may be manifested in the velocity profile of the flow through the annulus, as a consequence of the streaming potential effects in presence of Steric interactions. Our findings are likely to provide a new design basis for micro- and nano-scale flow devices involving annular geometries.

Slip in nanoscale shear flow: mechanisms of interfacial friction

The atomistic mechanism of fluid–solid interfacial friction as the basis of slip is still not fully understood. This study explores the interfacial friction mechanisms and their interplay with the nanoscale slip behavior using non-equilibrium molecular dynamics simulations. Our results show that there is an abrupt jump of slip length at a critical shear rate, corresponding to the transition from “defect slip” at low shear rates to “collective slip” at high shear rates. Here, we identified two mechanisms of interfacial friction: surface potential and collision mechanisms. Their impacts on slip are elaborated through a quantitative scaling estimation and our results show that both mechanisms contribute to the defect slip at low shear rates, while the collision mechanism dominates the collective slip at high shear rates. We also verify the importance of the bulk viscous heating via a comparison among different thermostat strategies.

Surface–gas interaction effects on nanoscale gas flows

Molecular dynamics (MD) method is used to simulate shear driven argon gas flows in the early transition and free molecular flow regimes to investigate surface effects as a function of the surface–gas potential strength ratio (ewf/eff). Results show a bulk flow region and a near wall region that extends three molecular diameters away from the surfaces. Within the near wall region the velocity, density, and shear stress distributions exhibit deviations from the kinetic theory predictions. Increased ewf/eff results in increased gas density, leading toward monolayer adsorption on surfaces. The near wall velocity profile shows reduced gas slip, and eventually velocity stick with increased ewf/eff. Using MD predicted shear stress values and kinetic theory, tangential momentum accommodation coefficients (TMAC) are calculated as a function of ewf/eff, and TMAC values are shown to be independent of the Knudsen number. Presence of this near wall region breaks down the dynamic similarity between rarefied and nanoscale gas flows.

Bulk velocity extraction for nano-scale Newtonian flows

The conventional velocity extraction algorithm in MDS method has difficulty to determine the small flow velocity. This study proposes a new method to calculate the bulk velocity in nano-flows. Based on the Newtonʼs law of viscosity, according to the calculated viscosities and shear stresses, the flow velocity can be obtained by numerical integration. This new method can overcome the difficulty existed in the conventional MDS method and improve the stability of the computational process. Numerical results show that this method is effective for the extraction of bulk velocity, no matter the bulk velocity is large or small.

Molecular dynamics study of slip behaviors: Role of a soft attractive tail

The role of soft attractive tails in nano-scale cylindrical Couette flows is investigated by using molecular dynamics computer simulations. The cylindrical Couette flow is generated by rotating an inner wall with a fixed outer wall. By varying the cutoff distance of the Lennard-Jones potential, we can easily control the inclusion or exclusion of an attractive component. We found that the number density and the tangential velocity profiles along the radial direction show rather strikingly different behaviors that can be classified into four distinctive flow types of slip behaviors due to the soft attractive tail.

Micro- and nanoscale fluid flow on chemical channels

We study the time evolution and driven motion of thin liquid films lying on top of chemical patterns on a substrate. Lattice-Boltzmann and molecular dynamics methods are used for simulations of the flow of microscopic and nanoscopic films, respectively. Minimization of the fluid surface area is used to examine the corresponding equilibrium free energy landscapes. The focus is on motion across patterns containing diverging and converging flow junctions, with an eye towards applications to lab-on-a-chip devices. Both open liquid–vapor systems driven by body forces and confined liquid–liquid systems driven by boundary motion are considered. As in earlier studies of flow on a linear chemical channel, we observe continuous motion of a connected liquid film across repeated copies of the pattern, despite the appearance of pearling instabilities of the interface. Provided that the strength of the driving force and the volume of the liquid are not too large, the liquid is confined to the chemical channels and its motion can be directed by small variations in the geometry of the pattern.

Nanoscale Poiseuille Flows of Liquid Argon

In nanoscale flow systems, the flow motion is affected by many parameters, some of which may play different roles under different conditions. In this work, we investigate the flux of liquid argon in nanoscale Poiseuille flows through molecular dynamics simulations. By illustrating the flux as a function of a dimensionless number, which represents the effective surface effect on the fluid, we show that the fluid motion in nanochannels under small external forces can be categorized into two regimes based on the role of the temperature. For large external forces, a bimodal behavior in the flux is observed as the fluid-wall interaction is varied. The underlying mechanisms that govern the flow fashions are discussed.

Vibration and Orientation of Diatomic Molecules Flowing Through Small Carbon Nanotubes

With their unique long cylindrical shape, carbon nanotubes may one-day form nozzles for nano-scale printing or flow into a chamber. Since the scale of the flowing molecules is similar to the diameter of the nanotubes, molecular vibration, orientation and density become influenced by the confinement during flow. We have studied the flow of diatomic molecules through carbon nanotube nozzles using non-equilibrium molecular dynamics simulations, in an effort to gain a greater understanding about the fundamental properties of such molecules in such a setting. The frequency of vibration of the molecules is shown to be dependent on the density inside the nanotubes and follow the same relation as an experimental micro-scale density-frequency study suggests, although only for nanotubes above a certain diameter. Meanwhile no relation is found between the frequency of vibration and the flow rate. The effect of nanotube diameter on the orientation of the molecules is also examined in detail, showing the transition between axial and radial orientation, with "pull" and "push" effects determining the orientation.

Multiscale simulation strategies and mesoscale modelling of gas and liquid flows

This paper presents a review of multiscale simulation strategies for the modelling of micro- and nanoscale flows. These have been developed in the last two decades in an attempt to bridge the application gap between molecular and continuum simulation methods preventing the simulation of many micro- and nanofluidic devices. The paper focuses on hybrid molecular-continuum methods and reviews different coupling strategies, including geometrical decomposition in conjunction material state and flux coupling. point wise coupling, the heterogeneous multiscale method and the equation-free approach. Different applications of these methods are briefly discussed.

DSMC simulation of subsonic flow through nanochannels and micro/nano backward-facing steps

In this study, we use direct simulation Monte Carlo method to simulate subsonic flow in nanochannels and micro/nanoscale backward-facing (BF) step considering a wide range of Knudsen number regimes. The nanochannel flow simulation indicates that the nanoscale flow through the nanochannel resembles unique features such as encountering negative pressure deviation behavior and observing flat velocity profiles at higher Knudsen number regimes. On the other hand, the micro/nano BF step flow simulations demonstrate that the length of separation region considerably decreases as the flow becomes more rarefied and approaches the transition regime. Meanwhile, the variations in the flow properties are much slower in the mid-transition and free-molecular regimes compared with the slip and early transition regime cases.

The equivalence between volume averaging and method of planes definitions of the pressure tensor at a plane

It is shown analytically that the method of planes (MOP) [Todd, Evans, and Daivis, Phys. Rev. E 52, 1627 (1995)] and volume averaging (VA) [Cormier, Rickman, and Delph, J. Appl. Phys. 89, 99 (2001)] formulas for the local pressure tensor, P(α, y)(y), where α ≡ x, y, or z, are mathematically identical. In the case of VA, the sampling volume is taken to be an infinitely thin parallelepiped, with an infinite lateral extent. This limit is shown to yield the MOP expression. The treatment is extended to include the condition of mechanical equilibrium resulting from an imposed force field. This analytical development is followed by numerical simulations. The equivalence of these two methods is demonstrated in the context of non-equilibrium molecular dynamics (NEMD) simulations of boundary-driven shear flow. A wall of tethered atoms is constrained to impose a normal load and a velocity profile on the entrained central layer. The VA formula can be used to compute all components of P(αβ)(y), which offers an advantage in calculating, for example, P(xx)(y) for nano-scale pressure-driven flows in the x-direction, where deviations from the classical Poiseuille flow solution can occur.

Molecular Dynamics Simulation of Nanoflow Inside Non-Smooth Pipe

This paper presents simulations of 3D nanoscale flow inside non-smooth pipe with molecular dynamics simulation method. For nanoscale flow problems the influence of non-smooth surface is very important. This work constructs a nanoscale pipe with non-smooth inside surface and analyzes the steady liquid flow in the non-smooth pipe. The distribution of density, temperature and flow velocity is studied. Lennard-Jones potential is used in the simulations. Differences are obvious between smooth and non-smooth solutions. The numerical results show that the non-smooth surface should be considered for many nanoscale flow problems. The results of this paper are helpful to analyze wetting and non-wetting phenomenon at a solid-fluid interface. Keywords: Computer simulations, Molecular dynamics, Nanoflow, nonlinear distribution, non-smooth surface, wetting