Nano Channels Research Articles

Thermal Lattice Boltzmann Simulation of Rarefied Gas Flows in Nanochannels for Wide Range of Knudsen Number

Using a modified Lattice Boltzmann Method (LBM), developing thermal flow through micro and nano channels has been modeled. Based on the improving of the dynamic viscosity and thermal conductivity, an effective relaxation time formulation is proposed which is able to simulate wide range of Knudsen numbers, Kn,. The results show that in spite of the standard LBM, the temperature distributions and the local Nusselt number obtained from this modified thermal LBM, agree well with the other numerical and empirical results in a wide range of Knudsen numbers.

Non-equilibrium molecular dynamics simulation of electrokinetic effects on heterogeneous ionic transport in nano-channel

The presence of the electric double layer (EDL) near the solid/liquid interface has a great impact on the liquid flowing through nano channels. In this paper, a non-equilibrium molecular dynamics method (NEMD) model is developed to investigate the transport characteristics of the heterogeneous ionic fluid flowing in a 4.672 nm-depth channel due to the electrokinetic effect induced by the EDL. An external perturbing velocity is introduced on the liquid instead of a constant force to study the electrokinetic effects on the flow. The effect of charges adsorbed on channel surfaces on flow resistance is especially considered. Two different charged surfaces with discrete or uniform charge distributions are compared, respectively. Besides Lennard-Jones (L-J) potential energy and Coulomb force, the ion–dipole interaction between the charged particle and the water molecule in the liquid is especially taken into account. According to the simulation results, the liquid density reaches the peak value in the EDL. However, unlike that predicted by the Poisson–Boltzmann theory, the liquid density profile is not exactly an anti-parabola curve, which has a significant fall before the density reaching the peak value in the EDL. It is shown that the charges absorbed on the channel surface have little influence on the liquid density distribution, but they play an important role in the fluid velocity distribution. Meanwhile, the liquid velocity slippage in the EDL emerges, which is well in agreement with the prior experimental data available. Moreover, though the non-dimensional velocities are nearly the same under different external velocity fields, the velocity slippage for the uniformly charged wall is much larger than that for the discretely charged wall. All of these results provide a probable reason why the flow behavior in a nano-channel is particularly different from that in a macro duct.

A novel modified lattice Boltzmann method for simulation of gas flows in wide range of Knudsen number

To cover a wide range of the flow regimes, a new relaxation time formulation by considering the rarefaction effect and the effective dynamic viscosity has been obtained. By using the modified lattice Boltzmann method (LBM), pressure driven flow through micro and nano channels has been modeled for wide range of Knudsen number, Kn, covering the slip, transition and to some extent the free molecular regimes. The results agree very well with existing empirical and numerical data. The velocity profile was predicted as well as the volumetric flow rate and for the first time, the well known Knudsen minimum effect has been captured about Kn = 1.

Experimental Study on the Filling of Nano Structures with Infrared Mold Surface Heating

Abstract In the fabrication of plastic components with micro or nano structures, injection molding is one of the simple and most cost-effective processes. In the injection molding process, a complete filling of the micro or nano structures is necessary to fabricate a successful product. However, it is difficult to fully fill nano structures with an aspect ratio close to one or higher by the conventional molding process. In this study, mold inserts with nano channels were constructed by a LIGA-like process. The effects of the processing parameters and infrared heating on the filling of the nano structures were explored experimentally. Increasing the mold temperature, pressure, or filling rate did not improve the filling distance in nano channels with the conventional molding process significantly. Thus, an infrared mold surface heating system was introduced to assist the molding of the nano structures. To enhance the heating efficiency and reduce the cycle time, a heat-resistant layer was inserted under the mold cavity. The heat resistant plate led the heating system to work more efficiently. It easily increased the surface temperature over the glass transition temperature of plastic, thus fully filling the nano structures.

Nanomorphing with Ultrafast Lasers and Biomedical Applications

ABSTRACTClassically, the limit for optical machining is on the order of the wavelength of the incident light. However, by taking advantage of precise, nonlinear damage mechanisms that occur for femtosecond laser pulses, damage can be achieved on a scale an order of magnitude lower, allowing precise removal of very small amounts of material to produce holes mere tens of nanometers wide. Femtosecond laser nanomachining can be carried out in a variety of dielectrics, and in transparent substrates machining can be sub-surface, in contrast to other nanomachining techniques such as using an electron beam or focused ion beam. We focus on the use of glass, as it is in many ways an ideal material for use in biological applications due to its chemical, optical, electrical and mechanical properties. By precisely placing laser pulses in glass, three dimensional nano and microfluidic channels and devices can be formed including nozzles, mixers, and separation columns. Recent advances in this technique allow the formation of high aspect ratio nanochannels from single pulses, thus helping address the fabrication speed limitations presented by serial processing. These nanochannels have a range of applications including the fabrication of nanoscale pores and nanowells that may serve as vias between fluidic channels, or from channels to a surface. These nanochannels have applications as a standalone technique for fabrication of nanopores and nanowells, but can also complement other fabrication techniques by allowing precisely placed jumpers that can connect channels that are out of plane. We discuss applications for diagnostic microfluidic devices, and basic cell biology research.

Confined growth of silicon nanowires as a possible process for third generation solar cells

AbstractA generic method for the growth of high density silicon nanowires in nano channels of alumina is presented. All fabrication steps are shown. Using nanoimprint lithography, fabricated porous alumina matrices present a perfect triangular array of vertical cylindrical pores. The gold catalyst was deposited at the bottom of the pores by an electrochemical way and catalytic growth of silicon nanowires was performed in a Chemical‐Vapour‐Deposition reactor. Scanning Electron Microscopy images are presented and detailed. We show that this system is the first step for the elaboration of a solar cell based on silicon core‐shell nanowires. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental and analytical study on filling of nano structures in micro injection molding

Nano technology is attracting more attention and has increasing applications in the recent years. Among the products with nano technology, many of them are made of polymer plastics. Micro injection molding is one of the critical processes for polymer plastics. In micro injection molding, the ability for the polymer melt to flow into the micro/nano structures is a crucial factor for successful molding. The flow behavior of polymer in nano structures needs to be explored further to facilitate the molding process. In this study, we investigated the effects of the processing parameters on the filling of nano structures analytically and experimentally. Firstly, mold inserts with nano channels were constructed by LIGA-Like process. Secondly, an analytical model was developed to model the filling of polymer melt in the nano channels. Molding experiments were performed to verify the analytical filling model. With this verified model, a theoretical penetration distance can be obtained, and the suitable processing parameters can be estimated for different product geometries.

Dynamic and Reversible Organization of Zeolite L Crystals Induced by Holographic Optical Tweezers

Organization and patterning of zeolite L crystals with their unique properties such as their one-dimensional nano channel system is of highest topical interest with various applications in many areas of science. We demonstrate full three-dimensional optical control of single zeolite L crystals and for the first time fully reversible, dynamic organization of a multitude of individually controlled zeolite L crystals.

Friction and heat transfer coefficient in micro and nano channels filled with porous media for wide range of Knudsen number

Invoking the velocity slip and temperature jump, numerical simulation of Darcy–Brinkman–Forchheimer flow model and forced convection in a circular micro/nano channel filled with porous media are presented. Relating the viscosity to the local Knudsen number, Kn, a generalized diffusion coefficient is obtained in such a way that it can model wide range of Kn regimes of flow. The effect of Kn and Darcy coefficient on velocity and temperature distribution is described. It is shown that despite of the fact that in most of previous researches it is assumed that Kn is constant along the channel, the variations of Kn due to the pressure variations, have considerable effects on heat transfer and temperature distribution across the channel cross section.

Formation of Tin Whiskers and Spheres on Anodic Aluminum Oxide Template

Nanosized tin whiskers and spheres 15 nm in diameter form on anodic aluminum oxide (AAO) templates via a two-step anodization. The driving force, residual stress released from the template at a high temperature, causes tin whiskers of uniform size to grow on AAO within a few minutes. Various thicknesses AAO templates of in nano channels were fabricated from high-purity aluminum metals. The formation of hexagonal pores on aluminum was elucidated with reference to the equilibrium surface tension at each anodic condition. During a faster heating and cooling rate, tin formed coarser whiskers on the AAO, compared with fine nanowhiskers, which prefer slower rates of tempering. A series of scanning electron microscopy (SEM) images under various conditions of fabrication are introduced. Hence, the transformation of tin whiskers to a spherical shape is presented in detail in this study as well.

Optical monitoring of micro fluidic flow conditions, employing surface plasmon resonance sensing

Near surface flow conditions in micro / nano channels are difficult to assess by experiment. Associated flow and materials transport properties yet are poorly understood, and inconclusive. A novel optical tool for determination of the near wall transport parameters in a micro channel, utilizing the evanescent field of a surface plasmon resonance (SPR) set-up as the sensing probe, is presented. The method is based on the transient flow response due to convective diffusion, in absence of specific adsorption. An approximately step-function type temporal solute concentration variation serves as the input signal. The time varying optical response of a surface plasmon resonance sensor, acting as an integral part of a micro channel, has been taken as the output signal. It provides the flow dependent change of the solute concentration in the channel within the optical detection and near wall distance interval 0 < d < 0.5 μm. The temporal signal evolution and response time, until an initially plain aqueous solution is replaced by the solute, varies inversely with solute concentration and flow rate. In the asymptotic limits, the near wall forced convective and diffusive channel transit times, along with the associated velocities, can be extracted and separated. The validity of the scaling relation for Fickian diffusive transport has been confirmed by experiment. Furthermore, solute diffusion coefficients in the solution can be determined with high precision. Convective near wall flow reveals a distorted parabolic flow profile and indicates relaxation of the no-slip condition, and presence of slip flow. Comparison with Poiseuille flow conditions, calculated for the same channel geometry, reveals a threshold characteristic. The experimental data allow estimations of the slip length / slip velocity that varies non-linearly with the macroscopic flow rate in the channel.

An ammonia gas sensor based on non-catalytically synthesized carbon nanotubes on an anodic aluminum oxide template

In this study, an ammonia gas sensor structure for mass production is realized with an array of carbon nanotubes synthesized on an anodic aluminum oxide (AAO) template. The device is expected to yield a uniform performance due to the regular and uniform nano channels in the AAO. It is a vertical structure made with open-ended multi-wall carbon nanotubes through deposition on the inside wall of the AAO so that the gas would diffuse into the nano channels for adsorption. The carbon nanotubes are in the form of flaked carbons having many defect sites. The results show that the sensor has high sensitivity, fast response time of less than 1 min and good reproducible recovery behaviors in atmospheric pressure at room temperature.

MESOSCOPIC MODELLING OF FLUID FLOWS IN MICRO AND NANO CHANNEL

We review some recent results in the theory of Lattice Boltzmann Equation applied in micro and nano channel flows. With a suitable generalization of the Shan-Chan model, we are able to relate in a systematic and self consistent way, the model parameters with the contact angle. Comparison with Molecular Dynamics simulations show remarkable agreement.

Embedded nano channels fabricated by non-selective reverse contact UV nanoimprint lithography technique

In this work, a novel nanofabrication technique is presented, namely “Reverse contact Ultraviolet Nanoimprint Lithography” (RUVNIL). It is based on reverse nanoimprint lithography and ultraviolet contact lithography. It provides flexibility in building complex three-dimensional structures allowing selective imprint over pre-patterned surfaces with or without residual layer in opposition to what is often encountered in the normal NIL process. We have investigated and optimized the imprinting parameters that are required for three-dimensional nanofabrication and applied it to the fabrication of nano-fluidic channels. This lithography technique is a very promising process for three-dimensional nanofabrication.

A nanoscale probe for fluidic and ionic transport

Surface science and molecular biology are often concerned with systems governed by fluid dynamics at the nanoscale, where different physical behaviour is expected. With advances in nanofabrication techniques, the study of fluid dynamics around a nano-object or in a nano channel is now more accessible experimentally and has become an active field of research. However, developing nanoscale probes that can act as flow sensors and that can be easily integrated remains difficult. Many studies demonstrate that carbon nanotubes (CNTs) have outstanding potential for nanoscale sensing, acting as strain or charge sensors in chemical and biological environments. Although nanotube flow sensors composed of bulk nanotubes have been demonstrated, they are not readily miniaturized to nanoscale dimensions. Here we report that individual carbon nanotube transistors of ~2 nm diameter, incorporated into microfluidic channels, locally sense the change in electrostatic potential induced by the flow of an ionic solution. We demonstrate that the nanotube conductance changes in response to the flow rate, functioning as a nanoscale flow sensor.

Application of continuum mechanics to fluid flow in nanochannels

A fundamental understanding of the transport phenomena in nanofluidic channels is critical for systematic design and precise control of such miniaturized devices towards the integration and automation of Lab-on-a-chip devices. The goal of this study is to develop a theoretical model of electroosmotic flow in nano channels to gain a better understanding of transport phenomena in nanofluidic channels. Instead of using the Boltzmann distribution, the conservation condition of ion number and the Nernst equation are used in this new model to find the ionic concentration field of an electrolyte solution in nano channels. Correct boundary conditions for the potential field at the center of the nanochannel and the concentration field at the wall of the channel are developed and applied to this model. It is found that the traditional plug-like velocity profile is distorted in the center of the channel due to the presence of net charges in this region opposite to that in the electrical double layer region. The developed model predicted a trend similar to that observed in experiments reported in the literature for the area-average velocity versus the ratio of Debye length to the channel height.

Characterization of orientation state of carbon nanotubes in shear flow

To characterize and understand carbon nanotube (CNT) orientation state in thermoset composite processing, we surface treated multi-walled carbon nanotubes (MWNT) and suspended them in vinyl ester resin. The surface treatment prevented clustering of the nanotubes and maintained homogeneously separate nanotubes in the suspension ensuring good dispersion quality. The suspension was subjected to shear flow in a micro channel, a nano channel and across a micro cylinder. transmission electron microscope (TEM) was employed to characterize the position of the nanotubes at various locations in the suspension. Second order orientation tensor description was used to describe the MWNT orientations in these flow fields in the 2-D plane. The degree of MWNT alignment was found to increase with increasing shear. However, we did not observe near perfect alignment, as one would expect in case of short fiber suspensions. Although MWNTs diameter is of the order of nanometers, we show by an order of magnitude analysis that Brownian motion is not significant. The MWNT orientations are maintained even after the shear force is discontinued.

Study of ELOG GaN for Application in the Fabrication of Micro-channels for Optoelectronic Devices

AbstractGallium Nitride (GaN) is a promising wide band gap semiconductor material for many optoelectronic applications, especially in the near UV range. Over the past several years, an extensive technical effort has been focused on improving the quality of GaN films through various overgrowth techniques such as epitaxial lateral overgrowth (ELOG), facet controlled epitaxial lateral overgrowth (FACELO), and Pendeoepitaxy. ELOG has been shown to reduce the density of threading dislocations by up to five orders of magnitude [1], however a complete physical model describing lateral overgrowth is needed in order to take full advantage of the process. A lateral overgrowth model will allow for the design and fabrication of three dimensional structures that can lead to novel devices and also to efficient biosensors by integrating micro and nano channels on the same chip as the optoelectronic components.A two-step process has been used to successfully control the geometry of overgrown GaN. Conditions have been identified which give a reduced lateral growth rate, in order to allow expansion of the {112n} plane to form vertical sidewalls and for the design of channel width. These geometries are being examined for possible application in laser diode and micro-channel fabrication for integrating bio-agent detection modules.

Analysis of magnetic interaction in Ni nanowire array grown using electrodeposition process

Simulation and comparison of magnetic properties were performed on three types of Ni nanowire arrays grown by electrodeposition using commercial nano channel alumina (NCA) templates with nominal pore sizes of 200, 100 and 20nm. Experimental observations on the structural behaviour of the nanowires grown inside the pores of NCA templates show that the magnetic interaction depends on the effective pore diameter and the interpore distance. A numerical simulation model based on micromagnetic calculations using object-oriented micromagnetic computing framework (OOMMF) has been developed and reported experimental results on coercivity of Ni nanowires generally agreed with the simulated ones.

Characterisation of electrodeposited nickel nanowires using NCA template

This paper reports on the growth of nanowires of nickel by electrodeposition in nano channel alumina (NCA) membranes. To make ohmic contact, one side of the NCA was sputtered with aluminium and nickel was electrodeposited from the other side. Nickel ions in solution migrate into the pores by the action of electrical potential, filling them approximately to 80% of solid-metal density. These ions form wires imbedded in the NCA matrix. Scanning electron microscopy (SEM) images were used to examine the growth mechanism of the nanowires. The images revealed that the wires started to grow from the aluminium backing plate, although a small number of wires grew from the middle of the pores. One possible reason of this partial pore fill-up could be the trapped air pocket inside the pores when the NCA was placed into the electrolyte. The mechanism of growth of the wires was studied by varying the electrochemical cell parameters temperature, pH and time. Purer nickel wire growth was observed with less nickel hydroxide species contamination at higher temperatures and at lower pH. X-ray diffraction spectra of FCC nickel nanowires were found to be polycrystalline and had a preferred orientation of (200) plane at higher bath temperature (60°C). Bulk magnetic properties of nickel filled nanoporous arrays were investigated by the vibrating sample magnetometer (VSM). The magnetization hysteresis at room temperature showed enhancement of coercivity for magnetic field applied parallel to the nanowires compared to that for perpendicular applied field. Magneto-impedance was measured as a function of applied magnetic field and wire diameter.