Molecular dynamics simulation studies on some topics of water molecules on hydrophobic surfaces

———————————— 1 Introduction Molecular dynamics simulations have been rec- ognized as one of the most popular tools in the study of the behavior of systems at the nanoscale. Why wa- ter is so special and recognized as life matrix [1] has long been discussed. And it is well known that hydro- phobic force is one of the most important interactions in biological systems.[2] This article will focus on two of the most often discussed problems: the nanobubbles and the water molecules perme…     

Hydrodynamic dispersion of neutral solutes in nanochannels: the effect of streaming potential

An analytical model is developed to account for the effect of streaming potential on the hydrodynamic dispersion of neutral solutes in pressure-driven flow. The pressure-driven flow and the resulting electroosmotic backflow exhibit coupled dispersion effects in nanoscale channels where the hydraulic diameter is on the order of the electrical double layer thickness. An effective diffusion coefficient for this regime is derived. The influence of streaming potential on hydrodynamic dispersion is found to be mainly dependent on an electrokinetic parameter, previously termed the “figure of merit”. Results indicate that streaming potential decreases the effective diffusion coefficient of the solute, while increasing the dispersion coefficient as traditionally defined. This discrepancy arises from the additional effect of streaming potential on average solute velocity, and discussed herein.     

Simulation of low speed 3D nanochannel flow

This paper presents a new technique of simulating low speed nanochannel flows using molecular dynamics simulation (MDS) method. When using the molecular dynamics simulation method to study low speed nanoscale flow problems, a major difficulty is the extraction of the true flow velocity because of the highly nonlinear coupling of the low bulk flow velocity and the high velocity of molecules’ thermal motion. In all published papers the reported flow velocity is the average value of the sum of these two velocities over time. For high speed flow problems the conventional MDS method can give satisfactory result. However, when the flow velocity is much smaller than the thermal velocity, the conventional molecular dynamics simulation method cannot predict the true flow velocity. To overcome this difficulty, in this study, a new linearized algorithm is developed. The new algorithm separates the flow velocity increment caused by external forces from the thermal motion velocity at each time step. The detailed process of the new algorithm is derived in this paper and several cases of 3D nanochannel flows of liquid argon are simulated by using this method. The numerical results show that the new algorithm is valid for nanoscale flows.     

A molecular dynamics study of perturbed Poiseuille flow in a nanochannel

This paper reports the results from a nonequilibrium molecular dynamics (NEMD) simulation and analytical solution of Poiseuille flow through a nanochannel. Two kinds of external perturbing forces, sinusoidal and step pulse, have been applied on the flow passing through a nanochannel. A total number of 2,000 particles of simple fluid interacting with one another, according to the Week–Chandler–Anderson (WCA) potential model between two parallel plates, has been considered in this study. The flow is bounded by horizontal walls in one direction and periodic boundary conditions are imposed in the other two directions. The velocity profile predicted by molecular dynamics is a second-order polynomial and is in good agreement with the analytical solution based on the Navier–Stokes equations. The temperature profile obtained from the molecular dynamics simulation also conforms to the overall continuum predictions of a fourth-order polynomial energy equation. Moreover, in the vicinity of the boundaries, a jump in temperature profile has been observed.     

Electrodeposition of cobalt based ferro-magnetic metal nanowires in polycarbonate films with cylindrical nanochannels fabricated by heavy-ion-track etching

Polycarbonate films of thickness 30 μm were irradiated with heavy ions by applying a flux of 10⁸ ions cm⁻² to produce straight tracks perpendicular to the film surface. The tracks were preferentially etched in 6 M aqueous solution of sodium hydroxide to prepare cylindrical nanochannels. The channel diameters were tuned between 200 and 600 nm by varying the etching time. Co₈₁Cu₁₉ alloy nanowires were electrodeposited potentiostatically, while Co/Cu multilayered nanowires, consisting of alternating Co and Cu layers with thickness 10 nm, were synthesized by means of a pulse plating technique in channels of length 30 μm and diameter 200 nm. Co₈₁Cu₁₉ alloy nanowires showed an anisotropic magnetoresistance effect of 0.6%, and the giant magnetoresistance of Co/Cu multilayered nanowires reached up to 8.0%.     

Novel interconnected nanochannel hydroxyapatite ceramics: synthesis,microstructure, and permeability

Uniform interconnected micro/nanoporous ceramics with good mechanical properties hold universal applications in biomedical and engineering fields. Herein, using hydroxyapatite (HAP) microtubes as the raw material instead of traditional particles, a novel interconnected nanochannel hydroxyapatite ceramic was fabricated successfully through one-step microwave sintering method without the addition of pore generators. The tubular structure of the HAP microtubes remains even after microwave sintering, which endows the ceramic with uniform interconnected nanochannels and 3-D porous structure. The HAP microtube nanochannel ceramic has a narrow pore size distribution from 400 nm to 600 nm, and exhibits well permeability, high adsorption/desorption ability. The porosity is about 30%, the HAP microtube nanochannel ceramic can be totally dyed by methylene blue within several minutes, and the blue dye can be desorbed completely in 45 min by ultrasonic vibration. In addition, due to the one-dimensional structure of the HAP microtubes, the HAP microtube nanochannel ceramic has smaller shrinkage, bigger porosity, and better toughness than the control sample fabricated by nanoparticles. Base on the uniform interconnected nanochannel structure, well permeability, high adsorption/desorption ability, the HAP microtube nanochannel ceramic fabricated here may be a promising candidate for many applications in biomedical engineering, environmental engineering, and energy engineering.     

Fabrication of nanochannels with ladder nanostructure at the bottom using AFM nanoscratching method

This letter presents a novel atomic force microscopy (AFM)-based nanomanufacturing method combining the tip scanning with the high-precision stage movement to fabricate nanochannels with ladder nanostructure at the bottom by continuous scanning with a fixed scan size. Different structures can be obtained according to the matching relation of the tip feeding velocity and the precision stage moving velocity. This relationship was first studied in detail to achieve nanochannels with different ladder nanostructures at the bottom. Machining experiments were then performed to fabricate nanochannels on an aluminum alloy surface to demonstrate the capability of this AFM-based fabrication method presented in this study. Results show that the feed value and the tip orientation in the removing action play important roles in this method which has a significant effect on the machined surfaces. Finally, the capacity of this method to fabricate a large-scale nanochannel was also demonstrated. This method has the potential to advance the existing AFM tip-based nanomanufacturing technique of the formation these complex structures by increasing the removal speed, simplifying the processing procedure and achieving the large-scale nanofabrication.     

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.     

Slip behavior of liquid flow in rough nanochannels

The molecular dynamics simulation is applied to investigate the liquid flow in rough nanochannels with a focus on interfacial velocity slip via three-dimensional Couette flow system. The typical liquid spatial distribution, velocity profile and slip length for liquid flow in rough nanochannels are evaluated and compared with smooth nanochannel. The effects of liquid–solid interaction, surface roughness and shear flow orientation on slip behavior of liquid flow in rough nanochannels are all investigated and discussed. The results indicate that, regardless of whether the liquid flow in transverse or longitudinal flow configuration, the rough surface induces extra energy losses and contributes to the reduction of interfacial velocity in nanochannel when compared with smooth surface. A larger roughness size introduces a more irregular near-wall flow, which results in a smaller interfacial velocity slip. In addition, irrespective of surface condition, increases in liquid–solid interaction strength lead to small interfacial velocity slip and expand the extent of velocity nonlinearity in wall-neighboring region. In particular, the slip behavior of liquid flow in rough nanochannels is also influenced by the shear flow orientation. Interestingly, we find that interfacial velocity slip at the rough solid surface in transverse flow configuration is smaller than that in longitudinal flow configuration.     

A novel technique for fabrication of micro- and nanofluidic device with embedded single carbon nanotube

This paper describes a novel technique for fabrication of micro- and nanofluidic device that consists of a carbon nanotube (CNT) and a polydimethylsiloxane (PDMS) microchannel. Single CNT was placed at desired locations using dielectrophoresis (DEP) and PDMS microchannel was constructed on the aligned CNT via photolithography and soft lithography techniques. This technique enables a CNT to be seamlessly embedded in a PDMS microchannel. Moreover, controlling the PDMS curing condition enables the construction of the device with or without a CNT (the device without CNT has a trace nanochannel in PDMS). Preliminary flow tests such as capillary effect and pressure-driven flow were performed with the fabricated devices. In the capillary effect tests, the flow stopped at the nanochannel in both devices. In the pressure-driven flow lower flow resistance was observed in the device with a CNT.