Flow In Nanochannels Research Articles

Scale and shear investigation of the viability and consequences of controlling viscous heating for Argon flows in nano-channels

Molecular dynamics (MD) is used to investigate the combined effects of channel size and wall shear stress on force-driven Argon flows in nanochannels. The adaptive wall thermostat (AWT) is used to automatically adjust wall temperature refraining the viscous heating upsurge with the increased body force from increasing mean fluid temperature. AWT is functional up to “wall shear stress limit” (WSSL), classifying wall shear stress range into controllable and uncontrollable viscous heating ranges (CVH and UVH) before and beyond WSSL, respectively. With increasing wall shear stress in CVH, liquid Argon at ∼119.9 K exhibits more non-uniform temperature profile in larger channels. Consequently, further shear dependence of density profile and fluid molecular layering which leads to controversial size-dependent slip length. Within UVH, supercritical Argon emerges from the significant increase of temperature and pressure exceeding critical state; consequently, density profile and molecular layering decrease noticeably drifting toward walls which results in minor and proximate values of slip length in all channels approaching no-slip condition. The apparent viscosity is higher in larger channels with decreasing and increasing trends within CVH and UVH, respectively, associated with changing fluid state and phase. Finally, the continuum solution suggests the possibility of estimating the mass flow rate with reasonable accuracy relative to MD if implemented with appropriate properties and slip boundary condition.

Insights into Waterflooding in Hydrocarbon-Bearing Nanochannels of Varying Cross Sections from Mesoscopic Multiphase Flow Simulations

Waterflooding is one of the geotechniques used to recover fuel sources from nanoporous geological formations. The scientific understanding of the process that involves the multiphase flow of nanoconfined fluids, however, has lagged, mainly due to the complex nanopore geometries and chemical compositions. To enable the benchmarked flow of nanoconfined fluids, architected geomaterials, such as synthesized mesoporous silica with tunable pore shapes and surface chemical properties, are used for designing and conducting experiments and simulations. This work uses a modified many-body dissipative particle dynamics (mDPD) model with accurately calibrated parameters to perform parametric flow simulations for studying the influences of waterflooding-driven power, pore shape, surface roughness, and surface wettability on the multiphase flow in heptane-saturated silica nanochannels. Remarkably, up to an 80% reduction in the effective permeability is found for water-driven heptane flow in a baseline 4.5-nm-wide slit channel when compared with the Hagen-Poiseuille equation. In the 4.5-nm-wide channels with architected surface roughness, the flow rate is found to be either higher or lower than the baseline case, depending on the shape and size of cross sections. High wettability of the solid surface by water is essential for achieving a high recovery of heptane, regardless of surface roughness. When the solid surface is less wetting or nonwetting to water, the existence of an optimal waterflooding-driven power is found to allow for the highest possible recovery. A detailed analysis of the evolution of the transient water-heptane interface in those nanochannels is presented to elucidate the underlying mechanisms that impact or dictate the multiphase flow behaviors.

Non-equilibrium flow of van der Waals fluids in nano-channels

The Enskog–Vlasov equation provides a consistent description of the microscopic molecular interactions for real fluids based on the kinetic and mean-field theories. The fluid flows in nano-channels are investigated by the Bhatnagar–Gross–Krook (BGK) type Enskog–Vlasov model, which simplifies the complicated Enskog–Vlasov collision operator and enables large-scale engineering design simulations. The density distributions of real fluids are found to exhibit inhomogeneities across the nano-channel, particularly at large densities, as a direct consequence of the inhomogeneous force distributions caused by the real fluid effects including the fluid molecules' volume exclusion and the long-range molecular attraction. In contrast to the Navier–Stokes equation with the slip boundary condition, which fails to describe nano-scale flows due to the coexistence of confinement, non-equilibrium, and real fluid effects, the Enskog–Vlasov–BGK model is found to capture these effects accurately as confirmed by the corresponding molecular dynamics simulations for low and moderate fluid densities.

Entrance loss of capillary flow in narrow slit nanochannels

The entrance loss of capillary flow at the nanoscale is crucial but often overlooked. This study investigates the entrance loss of capillary flow in narrow slit nanochannels using molecular dynamics simulations. The results show that the early stage of capillary flow is determined by entrance loss. During this period, capillary length increases linearly, while the capillary velocity remains constant. The effect of length-dependent friction loss becomes more apparent in the subsequent stages, causing the capillary length to deviate from linear and the capillary velocity to decrease. Roscoe's equation, which describes the flow through an infinitely thin slit, is used to model the entrance loss. Finite element simulations of flow through slits of varying height and length demonstrate the validity of Roscoe's equation in the continuum theory framework. Based on this, a capillary flow model is proposed that can accurately depict the hydrodynamic behavior of a capillary flow. Additionally, an approximate model ignoring the friction loss is proposed that predicts the linear increase in capillary length at the early stage. Theoretical analysis shows that the effect of entrance loss on capillary velocity is limited to the early stage, while the effect on capillary length can be extended to a large scale. Overall, the results of this study and the proposed models provide important theoretical support for applications related to capillary flows in nanoslits. The study emphasizes the importance of considering entrance loss in the early stages of a capillary flow and demonstrates the applicability of Roscoe's equation in modeling capillary flows in nanochannels.

Molecular dynamics study of flow of Ar-Kr mixture across a smooth walled nano channel with and without obstacles

Nanofluidics has emerged as an interesting field involving science and engineering, in which a fluid is made to flow across the structures having diameter in the nanometers range such as nanochannels. Nanochannels are channels having any one or all dimensions in the range of nanometers. As a flow in the reduced-size dimensional system, the nanochannel flow manifests a series of special properties and has gained a large attention. The understanding of different properties and dynamic behavior of fluid flows across a nanochannel has great impact on the study of fluid dynamics and various applications in the field of physics, chemistry, medicine and electronics. In the current study, we have studied fluid flow in smooth walled nanochannel, with and without obstacles introduced in the path of fluid flow. For both the cases we studied the velocity profiles of the system and analyzed the effect of varying channel width, varying force field, and varying densities on the velocity profile of the system. It is inferred that there is variation in the mean velocity of system in all cases. The mean velocity of the system increases with increase in the channel width, and applied force and decrease in density of mixture. We have taken pure Ar, pure Kr and Ar-Kr binary mixture as a fluid for the study. The velocity profile of the Ar-Kr mixture lies between that of pure Ar and pure Kr. For Ar system, the value of mean velocity is highest and for Kr system the value is lowest. There is a decrease in the mean velocity of the system with introduction of obstacles in the path of flow of the nanochannel. The value of viscosity of the system remains constant even with the introduction of obstacles in the path of the flow of system.[Copyright information to be updated in production process]

Electro-osmotic flow in different phosphorus nanochannels

The electrokinetic transport in a neutral system consists of an aqueous NaCl solution confined in a nanochannel with two similar parallel phosphorene walls and is investigated for different black, blue, red, and green phosphorene allotropes in the presence of an external electric field in the directions x (parallel to the walls roughness axis) and y (perpendicular to the walls roughness axis). The results show that irrespective of the electric field direction, the thickness of the Stern layer increases with the increase in the magnitude of the negative electric surface charge density (ESCD) on the nanochannel walls, and it also increases with the increase in the roughness ratio for different allotropes. Moreover, three different regimes of Debye–Hückel (DH), intermediate, and flow reversal appear as the absolute value of the negative ESCD on the walls grows. With the increase in the absolute value of the negative ESCD, in the DH regime, the flow velocity grows, then in the intermediate regime, it decreases, and finally, at sufficiently high ESCD, the flow reversal occurs. When the external electric field is applied in the y direction, the dynamics of the system are slower than that of the x direction; therefore, the flow reversal occurs at the smaller absolute values of the negative ESCD.

Field-Induced Hydration Shell Reorganization Enables Electro-osmotic Flow in Nanochannels.

Electro-osmotic flow is the motion of fluid driven by an applied electric field, for which an electric double layer near a charged surface is deemed essential. Here, we find that electro-osmotic flow can occur in electrically neutral nanochannels in the absence of definable electric double layers through extensive molecular dynamics simulations. An applied electric field is shown to cause an intrinsic channel selectivity between cations and anions, by reorienting the hydration shells of these confined ions. The ion selectivity then results in a net charge density in the channel that induces the unconventional electro-osmotic flow. The flow direction is amenable to manipulation by the field strength and the channel size, which will inform ongoing efforts to develop highly integrated nanofluidic systems capable of complex flow control.

Electrokinetic energy conversion of high pressure-driven flow with pressure-viscosity effect at high zeta potential

In this study, the electrokinetic energy conversion efficiency is investigated theoretically in high pressure-driven flow with the pressure-viscosity effect. Calculations of the electrokinetic flow in a straight nanochannel are based on the Poisson-Boltzmann equation and modified fluid momentum equation. Once the viscosity of fluid is dependent on the pressure, the strongly non-linear governing equation should be solved simultaneously due to the unknown fluid viscosity. Thus, the domain perturbation method is used to solve the distributions of flow velocity and pressure. Based on obtained velocity and pressure, the maximum conversion efficiency is calculated by thermodynamic analysis. Results show the maximum efficiency increases with the decrease of salt concentration and the peak efficiency can be obtained at sufficiently low salt concentration. For a given salt concentration, the maximum efficiency decreases monotonously with the increase of pressure-viscosity coefficient. Besides, the maximum energy conversion efficiency of ignoring the pressure-viscosity effect is larger than that of considering the pressure-viscosity effect.

Hydrovoltaic Nanogenerators for Self-Powered Sweat Electrolyte Analysis.

Human sweat comprises various electrolytes that are health status indicators. Conventional potentiometric electrolyte sensors require an electrical power source, which is expensive, bulky, and requires a complex architecture. Herein, this work demonstrates an electric nanogenerator fabricated using silicon nanowire (SiNW) arrays comprising modified carbon nanoparticles. The SiNWarrays platform is demonstrated as an effective self-powered sensor for sweat electrolyte analysis. It has been shown that an evaporation-induced water flow in nanochannels can yield an open-circuit voltage (Voc ) of 0.45V and a short-circuit current of 10.2µA at room temperature as a result of overlapped electric double layers. The electrolyte in the water flow results in a Voc decrease due to the charge shielding effect. The Voc is inversely proportional to the electrolyte concentration. The fabricated hydrovoltaic device shows the capability for sensing electrolytes in human sweat, which is useful in evaluating the hydration status of volunteers following intense physical exercise. The device depicts a novel response mechanism compared to conventional electrochemical sensors. Furthermore, the hydrovoltaic device shows a maximum output power of 1.42 µW, and as such has been successfully shown to drive various electronic devices including light-emitting diodes, a calculator, and an electronic timer.

Water flow in graphene nanochannels driven by imposed thermal gradients: the role of flexural phonons.

Accurate control of fluid transport in nanoscale structures is key to enable the design of foreseeable nanofluidic devices with applications in many fields such as chip cooling, energy conversion, drug delivery and medical diagnosis. Here, inspired by the experimental observation of intrinsic thermal ripples in graphene and by recent advances in the manipulation of 2D nanomaterials, we introduce a graphene-based thermal nanopump which produces controlled and continuous liquid flow in nanoslit channels. We investigate the performance of this thermal nanopump employing large scale molecular dynamics simulations. Upon systematically imposing thermal gradients, a net water flow towards the low-temperature zone is observed, achieving flow velocities up to 4 m s-1. We observe that water flow rates increase monotonically due to larger ripple fluctuations on the graphene layers as higher thermal gradients are applied. Moreover, we find that the out-of-plane flexural phonons in graphene are responsible for flow generation wherein lower frequency phonon branches are activated with higher imposed thermal gradients. Furthermore, by modifying the wettability of the channel walls, an increase of 50% in the water flow rates is observed, showing that the efficiency of the proposed thermal pump can be enhanced by tuning the channel wall hydrophobicity. Our results indicate that thermal gradients can be employed to drive continuous water flow in graphene nanoslit channels with potential applications in nanofluidic devices.

Thermal and Flow Analysis of Fully Developed Electroosmotic Flow in Parallel-Plate Micro- and Nanochannels with Surface Charge-Dependent Slip.

This study analytically investigates the coupled effects of surface charge and boundary slip on the fully developed electroosmotic flow and thermal transfer in parallel plate micro and nanochannels under the high zeta potential. The electric potential, velocity, temperature, flow rate, and Nusselt number are obtained analytically. The main results are that the velocity of bulk flow is significantly reduced in the presence of the surface charge-dependent slip. Moreover, the maximum velocity at ζ = -125 mV is approximately twice as large as that at ζ = -25 mV. The velocity and dimensionless temperature increase as the zeta potential increases. The dimensionless temperature of the surface charge-dependent slip flow is larger than that of the surface charge-independent slip flow. For the surface charge-dependent slip flow, the maximum temperature at ζ = -125 mV is approximately four times larger than that at ζ = -25 mV. The Nusselt number decreases with Joule heating and increases with a positive heat transfer coefficient. The Nusselt number decreases as the electric field and the magnitude of the zeta potential increase. In the surface charge-dependent slip flows, the Nusselt number is smaller than that in the surface charge-independent slip flows.

Imbibition dynamics and steady flows in graphene nanochannels with sparse geometric and chemical defects

Geometric and chemical defects are frequently found or created on smooth graphene for applications of nanofluidics. In this work, imbibition dynamics and steady flows of water in graphene nanochannels with sparse defects are explored by molecular dynamics. The water contact angle is raised slightly by geometric defects (hole and protrusion) but lowered significantly by chemical defects (hydroxyl and epoxide groups). In steady flows, the mean velocity and slip length are always reduced by sparse defects and the effect of chemical defects is more significant than that of geometric defects. Moreover, it is interesting to find that the velocity profile is plug-like for geometric defects but becomes parabolic for chemical defects, regardless of the slip length. Sparse defects on graphene nanoslits also affect the imbibition dynamics remarkably, which generally follows Washburn's equation with the slip length. For chemical defects, surface friction (slip length) dominates over the driving force associated with surface wettability (contact angle). Nonetheless, for protrusion defects, the stick-slip behavior caused by contact line pinning and thermal fluctuations can be observed. Our new and novel findings indicate that the defect nature is crucial in nanoscale flows and imbibition processes, which the conventional hydrodynamic theory fails to depict.

Molecular Insights into the Wall Slip Behavior of Pseudoplastic Polymer Melt in Nanochannels during Micro Injection Molding

Wall slip directly affects the molding quality of plastic parts by influencing the stability of the filling flow field during micro injection molding. The accurate modeling of wall slip in nanochannels has been a great challenge for pseudoplastic polymer melts. Here, an effective modeling method for polymer melt flow in nanochannels based on united-atom molecular dynamics simulations is presented. The effects of driving forces and wall–fluid interactions on the behavior of polyethylene melt under Poiseuille flow conditions were investigated by characterizing the slip velocity, dynamics information of the flow process, and spatial configuration parameters of molecular chains. The results indicated that the united-atom molecular dynamics model could better describe the pseudoplastic behavior in nanochannels than the commonly used finitely extensible nonlinear elastic (FENE) model. It was found that the slip velocity could be increased with increasing driving force and show completely opposite variation trends under different orders of magnitude of the wall–fluid interactions. The influence mechanism was interpreted by the density distribution and molecular chain structure parameters, including disentanglement and orientation, which also coincides with the change in the radius of gyration.

Effect of charge density distribution of polyelectrolyte layer on electroosmotic flow and ion selectivity in a conical soft nanochannel

Electroosmotic flow (EOF) and ion transport through a conical nanochannel coated with polyelectrolyte layer (PEL) are numerically investigated. The diffuse character is defined by an ununiform distribution of the polymer segment density. Soft step (Type I), exponential (Type II), and sigmoidal (Type III) functions were considered to describe the charge density distribution of PEL. A nonlinear model based on the Poisson-Nernst-Planck and the Navier–Stokes equations is used. The equations were solved using the finite element method. At applied voltages of ±1V, the electroosmotic velocity within diffuse soft nanochannels follows the order of uTypeI>uTypeII>uTypeIII. The soft step function leads to more ion selectivity. Considering a charge density of 100 mol/m3 and a salt concentration of 100 mM, we demonstrate that the rectification factors for the conical nanochannels, from 3 by considering the uniform PEL, can reach the value of 18 by considering the soft step function for PEL.

Evaluation of different particle-actuation modes in molecular dynamics and their impact on nanoscale flow behaviors

An accurate understanding of nanoscale gas transport mechanism is a fundamental research concern in many engineering applications, which remains as a research challenge currently. Two particle-actuation modes, namely, force-driven and pressure-driven methods, are evaluated and compared by molecular dynamics simulations of flows in nano-channels focusing on the characteristics of gas adsorption and slip velocity behaviors. The force-driven method may lead to unphysical properties when fluid inhomogeneities are significant since all fluid molecules are subjected to a same external force. By contrast, fluid molecules move forwards through the central part of the flow domain as a predominate pathway in a pressure-driven method. Results show that there is a significant difference between the two methods at smooth or small rough wall conditions, while the results tend to be consistent as roughness increases. The density distribution is uniform along the flow direction in force-driven cases, while adsorbed gas density increases in pressure-driven cases, leading to a smaller slip velocity near the outlet region. The effects of fluid wettability strength on solid surfaces and system temperature on gas adsorption/flow behaviors are also investigated and analyzed. This study is helpful for better understanding nanoscale gas dynamics and has many practical implications, such as the shale gas production.

Molecular dynamics study of spontaneous capillary flow and heat transfer in nanochannels

Passive cooling technology has become increasingly used in microelectronic devices, with capillary force being the most commonly used driving method. In this study, the capillary flow and heat transfer mechanisms and characteristics in nanochannels are studied based on molecular dynamics (MD), which have rarely been explored in previous studies. The capillary speed increases as the solid-liquid interaction intensity (εwl) increased, but when the wall is completely wetted, the capillary speed slows slightly an increasing εwl. By introducing wall slip, the Lucas-Washburn (L-W) equation is modified and a better fit is obtained. Furthermore, the effects of εwl, the temperature difference of the solid-liquid and surface nanostructure on capillary flow and heat transfer are analyzed. The increase in εwl and wall temperature enhances the capillary speed and heat transfer, but a continuous increase in the wall temperature will accelerate liquid evaporation and cause climb stagnation. The introduction of nanostructures slows the capillary speed, but can enhance the solid-liquid heat transfer. Finally, the microscopic mechanism of the above findings is analyzed by potential energy and density analyses. The reduction of the liquid potential energy at the solid-liquid interface can accelerate the capillary speed to a certain extent, but when the potential energy is too low, the liquid atoms will gather near the wall surface, blocking the capillary climbing. The expansion of the low potential energy region and the high mass density region enhances the solid-liquid heat transfer.

Numerical Investigation of Diffusioosmotic Flow in a Tapered Nanochannel.

Diffusioosmosis concerns ionic flow driven by a concentration difference in a charged nano-confinement and has significant applications in micro/nano-fluidics because of its nonlinear current-voltage response, thereby acting as an active electric gating. We carry out a comprehensive computation fluid dynamics simulation to investigate diffusioosmotic flow in a charged nanochannel of linearly varying height under an electrolyte concentration gradient. We analyze the effects of cone angle (), nanochannel length (l) and tip diameter (), concentration difference ( = 0–1 mM), and external flow on the diffusioosmotic velocity in a tapered nanochannel with a constant surface charge density (). External flow velocity (varied over five orders of magnitude) shows a negligible influence on the diffusioosmotic flow inside the tapered nanochannel. We observed that a cone angle causes diffusioosmotic flow to move towards the direction of increasing gap thickness because of stronger local electric field caused by the overlapping of electric double layers near the smaller orifice. Moreover, the magnitude of average nanoflow velocity increases with increasing . Flow velocity at the nanochannel tip increases when is smaller or when l is greater. In addition, the magnitude of diffusioosmotic velocity increases with increasing . Our numerical results demonstrate the nonlinear dependence of tapered, diffusioosmotic flow on various crucial control parameters, e.g., concentration difference, cone angle, tip diameter, and nanochannel length, whereas an insignificant relationship on flow rate in the low Peclet number regime is observed.

The electroviscous effect in nanochannels with overlapping electric double layers considering the height size effect on surface charge

The electroviscous effect has been extensively investigated in micro/nano scale ions transport, fluidic flow and heat transfer, etc. Previous researches often used constant surface charge in theoretic calculation and numerical simulations. However, for thick overlapping electric double layer (EDL) nanochannels, the channel height size effect on surface charge cannot be neglected. In this work, we consider the channel height size effect on surface charge to investigate the related ions transport, fluidic flow and heat transfer in nanochannels. The corresponding results show that the deviation considering the height effect or not is significant, especially in the situation of boundary slip. The relative errors of local ionic concentration and potential can reach 50% and 8%, respectively. The maximum relative errors of induced electrical field strength and the ratio of apparent viscosity to fluidic viscosity reach 10% and 9%, respectively, indicating strong height size effect on the fluidic flow. The deviation of Nu is relatively small and almost not affected by the surface charge. These deviations will decrease to zero when the channel height is large enough or the solution concentration is high enough to reach a non-overlapping EDL situation. The results of this work are meaningful to further understand the electroviscous effect in nanochannels, which may provide useful guidelines for the design and performance optimization of nanofluidic devices.

Time periodic electroosmotic flow in a pH-regulated parallel-plate nanochannel

In this paper, the separation of variables method is applied to investigate the effects of solution pH, background salt concentration and AC electric field frequency on time periodic electroosmotic flow in a pH-regulated parallel-plate nanochannel. The surface charge is generated by the protonation and deprotonation of the functional group SiOH. The background salt is KCl. The pH value of the solution is adjusted by HCl and KOH. Analytical and semi-analytical solutions for electric potential and velocity distributions are obtained. The results show that the electric potential caused by the electric double layer depends greatly on the solution pH and background salt concentration. The amplitudes of the velocity and flow rate of the time periodic electroosmotic flow decrease with the background salt concentration and increase with the deviation of the solution pH from the isoelectric point. In a nanochannel having a height less than 100 nm, the electroosmotic velocity amplitude is not affected by the AC electric field frequency because the oscillating Reynolds number is much less than unity.

Effect of copper nanoparticles on thermal behavior of two-phase argon-copper nanofluid flow in rough nanochannels with focusing on the interface properties and heat transfer using molecular dynamics simulation

A comparison between the efficacy of surface boundary structure and presence of nanoparticles on the condensation two-phase flow inside rough nanochannels has been accomplished by applying molecular dynamics procedure to evaluate the thermal conductivity and flow characteristics. Simulation is performed in a computational region with two copper walls containing rectangular rough elements under different saturated temperatures. The main properties of liquid-vapor interface including density and the number of liquid atoms, are obtained. It is observed that the density profile is more affected by nanoparticles than the roughness. Also, compared to the condensation of nanofluid in a smooth nanochannel, the rough wall causes a greater drop in the temperature at the early time steps and by development of liquid films, effects of the wall roughness reduce. At the first of the condensation process, adding nanoparticle causes that transferring argon particles to the liquid phase increases with a steeper slope. Furthermore, HCACF for nanofluid condensation flow over considered correlation time is analyzed and following that the thermal conductivity for different saturated conditions is calculated. It has been represented that at lower temperatures the roughness makes more significant influence on the heat transfer of two-phase flow, while at higher temperatures the importance of nanoparticles prevails.