High Aspect Ratio Channels Research Articles

Numerical Analysis of Three-Dimensional Flow of Supercritical Fluid in Cooling Channels

The knowledge of the flow behavior inside asymmetrically heated channels is of great importance to improve design and performance of regeneratively cooled rocket engines. The modeling of the coolant flow is a challenging task because of its particular features, such as the high wall temperature gradient, the high Reynolds number, the three-dimensional geometry of the passages, and the possible supercritical conditions of the fluid. In the present work, a numerical approach to study the turbulent flow of supercritical fluids is presented and validated by comparison with experimental data. Solutions of the supercritical nitrogen flowfield in an asymmetrically heated three-dimensional channel with a high-aspect ratio (channel height-to-width ratio) are presented and discussed. Emphasis is given to the analysis of the peculiar behavior and cooling performance of the supercritical fluid as compared with perfect gas. In particular, a long channel is considered, such that entrance effects are negligible, to analyze in detail wall heat-flux evolution throughout the channel.

Enhanced particle filtration in straight microchannels using shear-modulated inertial migration

In this work, we introduce a novel method for enhanced particle filtration using shear-modulated inertial migration in straight microchannels. Depending on their size, inertial lift causes particles to migrate toward microchannel walls. Using microchannels with high aspect ratio cross sections, the fluidic shear can be modulated, resulting in preferential equilibration of particles along the longer microchannel walls. Due to large lift forces generated in these high aspect ratio channels, complete particle filtration can be achieved in short distances even at low flow rates (Re<50). Based on this principle, we use a straight microfluidic channel with a rectangular cross section to passively and continuously filter 1.9 μm polystyrene particles. Overall, the proposed technique is versatile and can be easily integrated with on-chip microfluidic systems for filtration of a wide range of particle sizes, from micro- to nanoparticles.

High aspect ratio silicon nanomoulds for UV embossing fabricated by directional thermaloxidation using an oxidation mask

Nanomoulding is simple and economical but moulds with nanoscale features are usuallyprohibitively expensive to fabricate because nanolithographic techniques are mostly serialand time-consuming for large-area patterning. This paper describes a novel, simple andinexpensive parallel technique for fabricating nanoscale pattern moulds by silicon etchingfollowed by thermal oxidation. The mask pattern can be made by direct photolithographyor photolithography followed by metal overetching for submicron- and nanoscale features,respectively. To successfully make nanoscale channels having a post-oxidationcross-sectional shape similar to that of the original channel, an oxidation mask to promoteunidirectional (specifically horizontal) oxide growth is found to be essential. A siliconnitride or metal mask layer prevents vertical oxidation of the Si directly beneath it.Without this mask, rectangular channels become smaller but are V-shaped afteroxidation. By controlling the silicon etch depth and oxidation time, moulds with highaspect ratio channels having widths ranging from 500 to 50 nm and smaller canbe obtained. The nanomould, when passivated with a Teflon-like layer, can beused for first-generation replication using ultraviolet (UV) nanoembossing andsecond-generation replication in other materials, such as polydimethylsiloxane (PDMS).The PDMS stamp, which was subsequently coated with Au, was used for transferprinting of Au electrodes with a 600 nm gap which will find applications in plasticsnanoelectronics.

Acoustic limitations on the efficiency of machining by femtosecond laser-induced optical breakdown

The authors find an unexpected strong influence of acoustic phenomena on the efficiency of water-assisted femtosecond laser nanomachining. Analysis of acoustic interactions across a metastable structure of two phase flow in a nanocapillary, predicts acoustic nodes that strongly limit machining efficiency. Strategies for fabrication of high aspect ratio channels (>1000) are identified: increasing the speed of acoustic transmission delays formation of nodes, and can be accomplished by maximizing hydrogen in the gas phase or by varying pressure. These results reveal that laser machining can be strongly limited by acoustic phenomena not previously considered in the analysis of optical breakdown.

Computational and Functional Evaluation of a Microfluidic Blood Flow Device

The development of microfluidic devices supporting physiological blood flow has the potential to yield biomedical technologies emulating human organ function. However, advances in this area have been constrained by the fact that artificial microchannels constructed for such devices need to achieve maximum chemical diffusion as well as hemocompatibility. To address this issue, we designed an elastomeric microfluidic flow device composed of poly (dimethylsiloxane) to emulate the geometry and flow properties of the pulmonary microcirculation. Our chip design is characterized by high aspect ratio (width > height) channels in an orthogonally interconnected configuration. Finite element simulations of blood flow through the network design chip demonstrated that the apparent pressure drop varied in a linear manner with flow rate. For simulated flow rates <250 mul min, the simulated pressure drop was <2000 Pa, the flow was laminar, and hemolysis was minimal. Hemolysis rate, assayed in terms of [total plasma hemoglobin (TPH) (sample - control)/(TPH control)] during 6 and 12 hour perfusions at 250 mul/min, was <5.0% through the entire period of device perfusion. There was no evidence of microscopic thrombus at any channel segment or junction under these perfusion conditions. We conclude that a microfluidic blood flow device possessing asymmetric and interconnected microchannels exhibits uniform flow properties and preliminary hemocompatibility. Such technology should foster the development of miniature oxygenators and similar biomedical devices requiring both a microscale reaction volume and physiological blood flow.

Mode Converters for Coupling to High Aspect Ratio Silicon-on-Insulator Channel Waveguides

The design, simulation, and experimental performance of mode converters for coupling from single-mode silicon-on-insulator ridge waveguides to high aspect ratio channel waveguides are described. The converters consist of a two-level adiabatic taper structure. The final channel waveguide is 1.5 mum high by 0.8 mum wide. Simulations predict that for total coupler lengths longer than 20 mum, the coupling loss from the fundamental ridge mode to the slit mode is better than -0.2 dB. The couplers and waveguides were fabricated using a two-step self-aligned process. The measured coupling loss for fabricated mode converters is -0.4 dB

High-performance microfluidic vanadium redox fuel cell

We demonstrate a new microfluidic fuel cell design with high-surface area porous carbon electrodes and high aspect ratio channel, using soluble vanadium redox species as fuel and oxidant. The device exhibits a peak power density of 70 mW cm −2 at room temperature. In addition, low flow rate operation is demonstrated and single pass fuel utilization levels up to 55% are achieved. The proposed design facilitates cost-effective and rapid fabrication, and would be applicable to most microfluidic fuel cell architectures.

Fabrication of FinFETs by Damage-Free Neutral-Beam Etching Technology

A high aspect ratio and damage-free vertical ultrathin channel for a vertical-type double-gate MOSFET was fabricated by using low-energy neutral-beam etching (NBE). NBE can completely eliminate the charge build-up and photon-radiation damages caused by the plasma. The fabricated FinFETs realize a higher device performance (i.e., higher electron mobility) than that obtained by using a conventional reactive-ion etching. The improved mobility is well explained by the NB-etched atomically flat surface. These results strongly support the effectiveness of the NB technology for nanoscale CMOS fabrication

Patterning of platinum microelectrodes in polymeric microfluidic chips

Miniaturized electrodes are one of the most important components of lab-on-a-chip devices for separation, pumping, sensing, and other bioanalyses. In microfluidic-based chemical and bioanalytical operations, platinum electrodes are preferred to minimize the interaction with chemicals or biomolecules due to their chemical inertness. Although microfabrication techniques for patterning integrated platinum microelectrodes on silicon, quartz, or glass substrates are available, no techniques have been reported so far for depositing platinum electrodes in soft polymeric microchannels. A novel fabrication scheme is described for forming integrated microelectrodes in a poly-di-methyl-siloxane (PDMS) microchip. The electrode fabrication technique consists of photolithography, thermal processing, sequential sputtering of titanium and platinum, and stripping off photoresist, while soft lithography is used to form the microfluidic channels on PDMS. This approach facilitates precise positioning of the electrodes with a micron-size gap between them, and it can be used for both low and high aspect ratio channels. Platinum electrodes, formed on the PDMS channel surface, demonstrate very good interfacial adhesion with the substrate due to the use of a very thin titanium layer between the platinum and PDMS. The sputtered electrodes have a surface roughness of 50 nm and are able to sense picoA level current through benzene.

Turbine Blade Internal Cooling Passages with Rib Turbulators

Gas turbines are extensively used for aircraft propulsion, land-based power generation, and industrial applications. Developments in turbine cooling technology play a critical role in increasing the thermal efficiency and power output of advanced gas turbines. Gas turbine blades are cooled internally by passing the coolant through several rib-enhanced serpentine passages to remove heat conducted from the outside surface. For internal cooling, focus is placed on the effect of rotation on rotor blade coolant passage heat transfer with rib turbulators. In particular, the most recent publications are covered that deal with the rotational effects on internal cooling passage heat transfer with low and high aspect ratio channels with various high-performance rib geometries. To better understand the complex three-dimensional flow physics in the complicated blade internal coolant passage geometry, the computational flow and heat transfer results are presented and compared using the Reynolds averaged Navier-Stokes method with various turbulence models such as k-e, and second-moment closure models.

Efficient Vectorized Finite-Difference Method to Solve the Incompressible Navier–Stokes Equations for 3-D Mixed-Convection Flows in High-Aspect-Ratio Channels

A very efficient vectorized code is tailored to solve 3-D incompressible Navier–Stokes equations for mixed-convection flows in high streamwise aspect ratio channels. It is based on Goda's algorithm, second-order finite differences, an incremental factorization method of alternating direction implicit (ADI) type, spectral decomposition of the 1-D Laplace operators, and the tridiagonal matrix algorithm (TDMA). It is shown to be of second order in both space and time by a general method of determining code convergence orders and to have good performance on a NEC-SX5 supercomputer. It is validated through experiments of various Poiseuille-Rayleigh-Bénard flows with steady longitudinal, unsteady transverse, and convectively unstable wavy rolls.

Potassium chloride nanowire formation inside a microchannel glass array

The synthesis of KCl nanowires has been achieved by atomic layer deposition inside high aspect ratio channels of microchannel glass. The average diameter of the KCl nanowires is 250 nm, with a minimum observed diameter of 50 nm, and lengths up to 5μm. The Cl precursor was TaCl5, while the source of K was determined to be impurities in the microchannel glass substrate. The process for KC1 nanowire formation is a three-step chemical process that simultaneously etches K from the substrate concomitant with the formation of chlorine gas. It is postulated that the curvature of the channels may influence the diameters of the KCl nanowires.

Micropowder blasting with nanoparticles dispersed polymer mask for rapid prototyping of glass chip

We present a new rapid prototyping technique without a photolithography step to produce a glass chip with high aspect ratio channel for a micro total analysis system (µTAS). This technique consists of a powder blasting technique and direct laser patterning of Au nanoparticles dispersed polymer mask technique, and is useful in the development stage of a glass chip. The mask thickness and powder blasting condition were optimized for the fabrication of a glass chip with a higher aspect ratio channel. Under the optimized processing condition, the microchannel with a maximum aspect ratio of 2.1 in a glass substrate was successfully realized. The proposed technique was applied to a glass chip for electrophoresis and its performance for DNA separation analysis was confirmed.

Thermal Management Using Synthetic Jet Ejectors

The utility of a synthetic jet ejector for thermal management at low flow rates is discussed. A synthetic jet ejector typically consists of a primary zero-mass-flux unsteady jet driving a secondary airflow through a low profile, high aspect ratio channel. A simple configuration of a nominally two-dimensional jet ejector in a rectangular channel is used to investigate the effects of channel width on the induced flow rate, power dissipated, heat transfer coefficient and thermal efficiency. An active heat sink for high power microprocessors is developed using the jet ejector concept and its performance is discussed.

Electroless Nickel-Plated UV-Embossed Microstructured Surface with Very High Aspect Ratio Channels

The very high aspect ratio (14) and deep (132 μm) channels of an UV-embossed polymeric microstructured surface were successfully metallized by electroless nickel plating. A submicron conformal and highly adherent nickel coating was uniformly deposited on all the hydrophobic silicone-enriched surfaces of the channels. The nickel surface layer acts as a diffusion barrier and also allows for easy subsequent chemical modification by chemical grafting, electroplating, or electropolymerization for diverse applications in micromolding, microfluidics, sensors, microreactors, biomedical devices, and so forth. Hence, electroless nickel plating combined with UV embossing allows easy fabrication of high aspect ratio metallized polymeric microstructures with tailored surfaces.

FLOW DEVELOPMENT OF CO-FLOWING STREAMS IN RECTANGULAR MICRO-CHANNELS

The diffusion and flow development characteristics of two co-flowing, laminar streams in a high aspect ratio rectangular micro-channel have been examined. A long, thin splitter plate initially separates the two streams such that fully developed flow in each of the two channels is established prior to merging. The co-flowing micro-channel has an aspect ratio of 16 with a width of 1006 μm and a height of 63 μm. Micro-Particle Image Velocimetry (μPIV) was utilized to observe the interaction between the streams for a range of flow rate ratios ranging from one to nine, for Reynolds numbers of one and ten. For flow rate ratios greater than one, a cross-stream pressure gradient exists immediately downstream of the splitter plate, which results in a strong lateral flow of the faster moving fluid into the slower moving fluid. Despite this rapid expansion, the fluids in the two streams do not mix. The two streams eventually recover a fully developed velocity profile across the entire channel. A model is presented to predict this development length based on the pressure imbalance between the two streams. The model is expressed in terms of the flow rate ratio between the streams, which is shown to be a function of channel aspect ratio. An asymptotic condition for the development length is found for high flow rate ratios and high aspect ratio channels. It is shown that existing entrance length relationships greatly underpredict this development length.

Fabrication of on-chip microcapillary using photosensitive glass

The on-chip microcapillary applying for electrophoresis analysis device has been fabricated on photosensitive glass. This process can make a high aspect-ratio channel structure. The channel-depth can be controlled by varying wet-etching time without widening the channel width because the substrate material has vertical high etch rate selectivity. This high aspect-ratio structure obtains a long optical path and appropriate sample volume resulting in high-sensitivity for various analyses. The fabrication process is simpler than a usual glass micromachining process because of photo-resistless lithography process. In order to smooth the channel surface and to attach a cover glass onto the substrate, a polysilazane coating process is proposed and revealed to be effective.

Dimple Enhanced Heat Transfer in High Aspect Ratio Channels

Dimple Enhanced Heat Transfer in High Aspect Ratio Channels

Application of Microchip Fabricated of Photosensitive Glass for Thermal Lens Microscopy

Conventional wet chemical processes have not been successful in achieving a high aspect ratio channel on glass wafers because of isotropic etching of glass by hydrogen fluoride (HF). We fabricated a microchip of photosensitive glass in order to obtain a high aspect ratio channel (width:depth:aspect ratio = 150 µm:250 µm:1.67). The water glass bonding technique was applied to bond glass wafers at low temperature (40–80°C). The results of studying the characteristics of the water glass bonding technique for the micro total analysis system (µ-TAS) use show that dissolution of the material (sodium) from water glass layer was avoided by placing it in deionized water at 60°C for 4 h, and the bond strength was weakened by keeping it at high pH for a day. The microchip was available to analyze the amount of nitrite acid present by thermal lens microscopy as one method of optical chemical analysis. Although the etched glass surface had a roughness of about 1 µm, the calibration curve of thermal lens microscopy was similar to that of spectrophotometry.

Interfacing a polymer-based micromachined device to a nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometer.

Here we report the design, fabrication, and operation of a polymer-based microchip device interfaced to a nanoelectrospray ionization source and a Fourier transform ion cyclotron resonance mass spectrometer. The poly(methyl methacrylate) micromachined device was fabricated using X-ray lithography to produce a network of channels with high aspect ratios. Fabrication of high aspect ratio channels allows for zero dead volume interfaces between the microchip platform and the nanoelectrospray capillary interface. The performance of this device was evaluated with standard peptide and protein samples. High-quality mass spectral data from peptide and proteins (and mixtures thereof) were obtained without any interfering chemical noise from the polymer or the developers and plasticizers used in the fabrication process. Sample cross-contamination is not a problem using this polymer-based microchip device as demonstrated by the sequential analysis of several proteins. The nanoelectrospray source was operated at flow rates from 20 to 100 nL/min using pressure-driven flow, and uninterrupted operation for several hours is demonstrated without any noticeable signal degradation. The ability to fabricate multiple devices using injection molding or hot-embossing techniques of polymers provides a lower cost alternative to silica-based devices currently utilized with mass spectrometry.