Fluid In Nanochannel Research Articles

Electro-osmotic flow instability of viscoelastic fluids in a nanochannel

The study of the complex rheological properties of viscoelastic fluids in nanochannels will facilitate the application of nanofluidics in biomedical and other fields. However, the flow of viscoelastic fluids in nanochannels has significant instabilities, and numerical simulation failures are prone to occur at high Weissenberg numbers (Wi). In this study, the simplified Phan-Thien-Tanner viscoelastic fluid model is solved using the log-conformation tensor approach, and the effects of rheological parameters of the viscoelastic fluid, such as the Weissenberg number (Wi), extensibility parameter (ε), and viscosity ratio (β), on the flow characteristics and flow instability within the nanochannel are investigated. The results indicate that the variation of rheological parameters of viscoelastic fluids has a significant effect on the flow state and flow instability of fluids in nanochannels. When the rheological parameters are in a specific range, the flow velocity and outlet current in the nanochannel exhibit relatively regular periodic fluctuations. As the flow transitions from an up-and-down moving single-vortex state to a symmetric double-vortex state, the average velocity of the central axis in the nanochannel is increased by about 15%. Furthermore, when Wi increases from 150 to 400, the length and height of the vortex increase by 50% and 100%, respectively.

Analysis of Electroviscous Effect for Flow of Micropolar Fluids in a Nanochannel with Overlapping Electric Double Layers at High Zeta Potential.

Flows in nanofluidic channels under the influence of pressure gradient often lead to the overlapping of the electrical double layers (EDLs) augmenting the electroviscous effect. In this work, we analyze the electroviscous effect for the flow of a micropolar fluid in a parallel plate nanochannel, considering EDL overlapping and interfacial slip. Closed-form expressions of EDL potential, velocity, microrotation, yielded streaming potential, and volumetric flow rate are derived semi-analytically for high zeta potential without accounting for the Boltzmann distribution. We observe a significant change in streaming potential with inverse ionic Peclet number (R) for its lower values, and the range of streaming potential is more for thicker EDL. The micropolarity parameter does not have any influence on the streaming potential for weaker EDL overlapping at a lower R value. For thicker EDL, velocity decreases with the micropolarity parameter, while it increases drastically with interfacial slip. However, velocity shows a non-monotonic behavior with interfacial slip and micropolarity parameter for thinner EDL. The microrotation remains invariant with interfacial slip for thicker EDL, whereas its magnitude decreases with slip length for thinner EDL. Although the sensitivity of the flow rate on slip (Qs*) increases with R for thicker EDL, the behavior is non-monotonic for thinner EDL. Furthermore, Qs* varies non-monotonically with the micropolarity parameter at higher couple stress parameter values and vice versa. In fact, the volumetric flow rate is highly sensitive to slip for thicker EDL overlapping.

Molecular dynamics simulation of effect of non-condensable gases on heat transfer of water molecule flow in nanochannels

With the development of higher performance and miniaturization of electronic components, the flow heat transfer of working fluids in nanochannels has received more attention. To elucidate this phenomenon, molecular dynamics simulations are used to simulate the behaviors of fluids within nanochannels at temperatures of 300 K, 325 K, and 350 K. Water serves as a flow medium, with argon substituted for any non-condensable gases. In the flow process, argon atoms aggregate into clusters that are characterized by high potential energy. As the temperature rises, the concomitant increases in the fluid’s potential energy, which leads to the gradual diminution or complete dissipation of these clusters. A minor presence of gas atoms can facilitate fluid movement; however, an excess of argon promotes the formation of larger gaseous clusters in the central region of the channel, thereby impeding fluid flow. Concurrently, the application of heat to the fluid appreciably diminishes the coefficient of flow resistance. The temperature of the fluid in the near-wall region exceeds that of the central area. In the clusters, the atoms exhibit heightened activity, leading to an increase in the average molecular kinetic energy and a concomitant rise in temperature. The inherent hydrogen-bonding structure of water enhances heat transfer within the nanochannels. Argon atoms exert an influence on the number of hydrogen bonds, and rising temperatures disrupts the hydrogen-bond network established by water molecules, ultimately leading to a decrease of the Nusselt number. This investigation offers insights into the heat transfer dynamics of water molecular flow within microchannels under the perturbation of non-condensable gases, thereby furnishing theoretical guidance for enhancing heat transfer within electronic devices.

Molecular dynamics study of interface thermal resistance in asymmetric nanochannel

Heat transfer in a micro-scale system has less thermal inertia and faster thermal response, which has unique advantages in controlling heat transfer. Interface thermal resistance is an important physical quantity that reflects the heat transfer performance of the interface on a micro-scale. In this paper, the interface thermal resistance os static fluid and flowing fluid in nanochannel, which are different in the wall temperature and wettability, are studied by the molecular dynamics method. In the static fluid, the results show that the wall wettability has a significant influence on the interface thermal resistance, and the stronger the wall wettability, the smaller the values of interface thermal resistance is. For the walls with different temperatures, it can be observed that the interface thermal resistance on high temperature wall is higher than that on low temperature, when the wall wettability is weaker. On the contrary, when the wall wettability is stronger, the effect of wall temperatures on the interface thermal resistance is negligible. An external force applied to the fluid domain makes the fluid flow. In the flowing fluid, the results show that the variation of wall wettability and external force can lead to the slip to different degrees at the interface, and the slip-induced frictional viscous heat is generated at the solid-liquid interface, and thus increasing the fluid temperature and the heat flux of the system. The effect of external force on the thermal resistance is limited by the wall wettability. When the wall wettability is weaker, the increase of the external force will make the interface slip more easily and the thermal resistance decrease. With the stronger wall wettability, it is difficult to make the interface slip obviously with the increase of external force, and the influence of external force on interface thermal resistance decreases. The heat transfer performance at the solid-liquid interface is related to the number of fluid molecules adsorbed on the wall surface. The results show that in the static fluid, the increase of wall wettability will make more fluid molecules adsorbed on the wall, and the arrangement becomes more and more regular, which causes the interface thermal resistance to decrease and is beneficial to the interface heat transfer. In the flowing fluid, the change of external force has less influence on the number of adsorbed molecules, and the wall wettability is the main factor affecting the adsorption of fluid molecules on the wall.

Hydrodynamic and thermal flow in nanochannel to study effects of roughness by estimation the atoms positions via MD method

PurposeThis paper aims to investigate atoms type and channel roughness effects on fluid behavior in nanochannel.Design/methodology/approachThe results of mechanical properties of these structures are reported in this work by using molecular dynamics method.FindingsThe results show that nanochannel roughness is a limiting factor in flowing fluid in nanochannel. Moreover, fluids with less atomic weight have more free movement in ideal and non-ideal nanochannels.Originality/valueFor the study of mechanical properties of fluid/nanochannel system, the authors calculated parameters such as potential energy, density, temperature and velocity profiles of simulated fluids.

Thermal, caloric and transport properties of the Lennard–Jones truncated and shifted fluid in the adsorbed layers at dispersive solid walls

Fluid properties change when the fluid is adsorbed at a wall. The effect of the adsorption on the fluid properties was studied here by molecular simulation. There is much previous work in this field on fluids in nanochannels that are so small that the adsorbed layers on both walls interfere. In this work, the channel width was so large that the adsorbed layers did not interfere, such that information on the adsorbed layer on single walls was obtained and average values of thermodynamic properties of the fluid in that layer were determined. The studied fluid properties are: pressure p, density ρ, internal energy u, enthalpy h, isobaric heat capacity , thermal expansion coefficient , thermal conductivity λ, shear viscosity η and self-diffusion coefficient D. For the study, non-equilibrium molecular dynamics simulations were carried out. The fluid and the solid were modelled with the Lennard–Jones potential truncated and shifted at . The overall density of the fluid was . The overall temperature and the solid–fluid interaction were varied. The corresponding bulk states are liquid or supercritical. The results for the fluid properties in the adsorbed layer were compared to the corresponding bulk values and the deviations are generally below .

Pressure-driven flow of oligomeric fluid in nano-channel with complex structure. A dissipative particle dynamics study

We develop a simulational methodology allowing for simulation of the pressure-driven flow in the pore with flat and polymer-modified walls. Our approach is based on dissipative particle dynamics and we combine earlier ideas of fluid-like walls and reverse flow. As a test case we consider the oligomer flow through the pore with flat walls and demonstrate good thermostatting qualities of the proposed method. We found the inhomogeneities in both oligomer shape and alignment across the pore leading to a non-parabolic velocity profiles. The method is subsequently applied to a nano-channel decorated with a polymer brush stripes arranged perpendicularly to the flow direction. At certain threshold value of a flow force we find a pillar-to-lamellar morphological transition, which leads to the brush enveloping the pore wall by a relatively smooth layer. At higher flow rates, the flow of oligomer has similar properties as in the case of flat walls, but for the narrower effective pore size. We observe stretching and aligning of the polymer molecules along the flow near the pore walls.

Fabrication of smart biointerfaces in nanochannels with a thiolated temperature-responsive polymer

Event Abstract Back to Event Fabrication of smart biointerfaces in nanochannels with a thiolated temperature-responsive polymer Yan Xu1, Misato Shinomiya1, 2 and Atsushi Harada2 1 Osaka Prefecture University, Nanoscience and Nanotechnology Research Center, Japan 2 Osaka Prefecture University, Department of Applied Chemistry, Graduated School of Engineering, Japan Introduction: Recently, nanofluidic devices have attracted much attention in the fields of chemistry, biology, and medicine. For example, nanofluidic devices are very promising in the development of early diagnostic tools for diseases because nanochannels in the nanofluidic devices allow confining biomolecules such as proteins, DNAs at the single molecular level[1]. However, these applications rely on the precise control of fluids in nanochannels, which is still a challenge in nanofluidics due to the extreme difficulty to construct fluid control units in nanochannels. Therefore, we proposed a concept of smart nanofluidic control (SNC). In SNC, the ‘open’ and ‘closed’ states of nanochannels are smartly controlled by the change of the external stimuli such as temperature, pH, light, through self-assembled stimuli-responsive interfaces which are locally constructed in nanochannels (Figure 1). Towards the realization of SNC, we have first achieved the fabrication of gold nanopatterns in glass nanochannels[2]-[4]. The gold nanopatterns in nanochannels provide robust surfaces to form self-assembled monolayers (SAM) when reacting with thiolated molecules. In this study, we focused on temperature-responsive SNC. A thiolated temperature-responsive polymer, poly(N-isopropylacrylamide) (referred to as PNIPAAm-SH, Figure 2), was specially designed and synthesized to fabricate smart biointerfaces in nanochannels for SNC. Materials and Methods: The SNC chips (Figure 3) were fabricated on fused-silica glass substrates using the methods previously reported by us[2]-[4]. PNIPAAm-SH was synthesized by a reversible addition fragmentation chain transfer (RAFT) polymerization. While SAMs of PNIPAAm-SH prepared on the gold-deposited glass substrates were used to characterized the bulk interfacial properties, SAMs of PNIPAAm-SH formed on the gold pattern surfaces in nanochannels were used to evaluate their capability to control fluids in nanochannels using an experimental setup previously reported by us[5]. Results and Discussion: The well-controlled structures of synthesized PNIPAAm-SH were identified by H1 NMR. Independent on polymerization time, PNIPAAm-SH with small molecular weight lower than 10,000 and narrow polydispersity lower than 1.2 were obtained, which is considered to be appropriate to inject into nanochannels. The SAMs formed on the gold surface using PNIPAAm-SH of different concentration were elementally confirmed by XPS and exhibited smooth morphology having extremely small roughness lower than that of glass substrate. Further, the synthesized PNIPAAm-SH exhibited sharp sol-gel phase transition at 30.8 °C in solution, suggesting that it would possess a quick temperature responsiveness in nanochannel, which is very ideal for SNC. Finally, nanofluidic experiments conducted below and above the sol-gel phase transition temperature revealed that the smart biointerfaces consisting of SAMs of PNIPAAm-SH on the gold pattern surfaces in the nanochannels allowed the smart control of fluids in the nanochannels. Conclusion: By fabricating a smart PNIPAAm-SH biointerfaces in nanochannels, SNC was achieved for the first time. This work was partially supported by JSPS KAKENHI Grant Numbers 26706010,26630403, and MEXT KAKENHI Grant Number 26107714, and the Asahi Glass Foundation.

Stabilized lattice Boltzmann-Enskog method for compressible flows and its application to one- and two-component fluids in nanochannels

A numerically stable method to solve the discretized Boltzmann-Enskog equation describing the behavior of nonideal fluids under inhomogeneous conditions is presented. The algorithm employed uses a Lagrangian finite-difference scheme for the treatment of the convective term and a forcing term to account for the molecular repulsion together with a Bhatnagar-Gross-Krook relaxation term. In order to eliminate the spurious currents induced by the numerical discretization procedure, we use a trapezoidal rule for the time integration together with a version of the two-distribution method of He et al. [J. Comput. Phys. 152, 642 (1999)]. Numerical tests show that, in the case of a one-component fluid in the presence of a spherical potential well, the proposed method reduces the numerical error by several orders of magnitude. We conduct another test by considering the flow of a two-component fluid in a channel with a bottleneck and provide information about the density and velocity field in this structured geometry.

Molecular dynamics-based prediction of boundary slip of fluids in nanochannels

Computational modeling and simulation can provide an effective predictive capability for flow properties of the confined fluids in micro/nanoscales. In this paper, considering the boundary slip at the fluid–solid interface, the motion property of fluids confined in parallel-plate nanochannels are investigated to couple the atomistic regime to continuum. The corrected second-order slip boundary condition is used to solve the Navier–Stokes equations for confined fluids. Molecular dynamics simulations for Poiseuille flows are performed to study the influences of the strength of the solid–fluid coupling, the fluid temperature, and the density of the solid wall on the velocity slip at the fluid boundary. For weak solid–fluid coupling strength, high temperature of the confined fluid and high density of the solid wall, the large velocity slip at the fluid boundary can be obviously observed. The effectiveness of the corrected second-order slip boundary condition is demonstrated by comparing the velocity profiles of Poiseuille flows from MD simulations with that from continuum.

EFFECT OF MASS ON SHEAR VISCOSITY OF BINARY FLUID MIXTURE CONFINED TO NANOCHANNELS

Flow of fluid in nanochannel has been studied by evaluating transverse stress auto-correlation function and shear viscosity of the Ar–Kr and isotopic fluid mixture interacting via Lennard Jones potential. The enhancement in viscosity due to confinement has been investigated for different mass ratios and concentration of one of the fluids in a binary fluid mixture. It has been found that at a given number density and temperature, mixing fluid of lower molar weight reduces the enhancement in viscosity and also lower the chances of freezing of liquid near the walls. We have also studied the effect of concentration of two species on the viscosity. It is found that the enhancement in viscosity is less when the concentration of particles with lighter mass increases.

Hydrokinetic simulations of nanoscopic precursor films in rough channels

We report on simulations of capillary filling of highly wetting fluids in nanochannels withand without obstacles. We use atomistic (molecular dynamics) and hydrokinetic (latticeBoltzmann; LB) approaches which indicate clear evidence of the formation of thinprecursor films, moving ahead of the main capillary front. The dynamics of the precursorfilms is found to obey a square-root law like that obeyed by the main capillary front,, although with a larger prefactor, which we find to take the same value for the differentgeometries (2D–3D) under inspection. The two methods show a quantitative agreementwhich indicates that the formation and propagation of thin precursors can be handled at amesoscopic/hydrokinetic level. This can be considered as a validation of the LB methodand opens the possibility of using hydrokinetic methods to explore space–time scales andcomplex geometries of direct experimental relevance. Then, the LB approach is used tostudy the fluid behaviour in a nanochannel when the precursor film encounters a squareobstacle. A complete parametric analysis is performed which suggests that thin-filmprecursors may have an important influence on the efficiency of nanochannel-coatingstrategies.

Molecular dynamics of fluids and droplets in patterned nanochannels

Molecular dynamics studies of chain molecule fluids in nanochannels reveal new kinds of stationary flows when the walls of the channel are patterned chemically. In the single-fluid case, the patterning is shown to give rise to spatial switching between Poiseuille-like and plug-flow behaviours. In the two-fluid case, such that the two immiscible fluids wet different patches of a chemically patterned channel, the velocity field of the individual components is modulated in a way that is phase shifted relative to the single-fluid case. When the channel is homogeneous and wetting to one fluid component but non-wetting to the other then the resulting coating of the channel walls provides dissipation and thus stabilises flow of the other component. Wettability and geometrical patterning of substrates gives rise to a lotus-like effect in a rolling motion of nanodroplets along the substrate.

Molecular dynamics of immiscible fluids in chemically patterned nanochannels

Molecular dynamics simulations of chain molecules are used to elucidate physical phenomena involved in flows of dense immiscible fluids in nanochannels. We first consider a force driven flow in which the channel walls are homogeneous and wetting to one fluid and nonwetting to the other fluid. The coating of the walls by the wetting fluid provides a fluctuating surface that confines the flow of the nonwetting fluid. The resulting dissipation yields stationary Poiseuille-like flows in contrast to the accelerating nature of flow in the absence of the coating. We then consider walls consisting of patches whose wetting preferences to a fluid alternate along the walls. In the resulting flow, the immiscible components exhibit periodic structures in their velocity fields such that the crests are located at the wettability steps in contrast to the behavior of a single fluid for which the crest occurs in the wetting region. We demonstrate that for a single fluid, the modulated velocity field scales with the size of the chain molecules.