The present work’s originality lies in the evidence of a non negligible effect of the fluid ions’ and co-ions’ interaction with the wall surface in a microtube. This study is based on the EDL theory (Electrical Double Layer) which is developed here for a circular geometry. High electrical surface potentials are taken into account for the present study; they induce the nonlinearity of the problem’s main equation (Poisson-Boltzmann equation). The electrical field is determined, then the velocity profile and finally the Poiseuille number. We show that even with the EDL effect taken into account, the Poiseuille number does not depend on the mean velocity. Our model agrees with the experimental results for high surface potentials (> 25 mV). This is found by comparing with experiments previously carried out with microtubes ranging from 530 to 50 μm.
`Electrokinetics has become a popular method of both particulate and fluidic control in microdevices. In an effort to address the problems associated with conventional electrokinetic control, we use silver-silver chloride electrodes for low-voltage, spatially localized electrokinetic control in microfluidic devices. This work presents an analysis of the electric fields generated by silver-silver chloride electrodes and establishes a nondimensional design parameter that governs the device performance. In addition, an optimal parameter space for maximum electrode longevity is presented.
An experimental study of a submerged 125-mu m circular microscale jet impingement is presented. These jets flows are associated with low exit Reynolds number and a correspondingly high, subsonic Mach number. Detailed distributions of heated and adiabatic wall temperature, and local and average Nusselt number variations are presented for five laminar exit Reynolds numbers in the range of 690 to 1770 at three nozzle-to-surface spacings of 2, 4, and 6 times the nozzle diameter. The corresponding jet exit Mach numbers range between 0.26 and 0.63. An infrared radiometer is used in conjunction with a heated-thin-foil technique to measure detailed surface temperatures. Results indicate that the adiabatic surface temperature distribution is relatively insensitive to nozzle-to-surface spacing within the parameter range studied. With an increase in Reynolds number, the adiabatic surface temperature decreases significantly near the stagnation point. The average Nusselt numbers are higher compared to Martin's correlation [1] for large Reynolds numbers. A similar observation has been reported previously in a numerical study on microscale jet impingement [5] and attributed to compressibility of the flow and the possible existence of a slip flow.
Abstract By solving Maxwell's equations, we observed photon localization and strong electromagnetic field enhancement in laser-irradiated, one-dimensional random multilayer and two-dimensional random media, with particle size around the laser wavelength. Higher refractive index contrast and degree of randomness result in stronger localization. The probability density and the location of enhanced fields are investigated as functions of refractive index and particle size distribution. For weakly dissipative media, local absorption enhancement is observed, and the expectation intensity distribution is obtained by averaging among many realizations. For comparison, the equation of radiative transfer (ERT) is also solved for random porous media, using the two-flux model. Since no interference effects are allowed, the classical diffusion, rather than localization, is predicted. Consequently, ERT can represent a good statistical average of the intensities only as absorption is dominant over localization. As expected, the particle treatment of ERT does not allow for predicting photon localization and field enhancement. 236pt
The thickness of the interfacial region dividing bulk liquid and vapor regions is typically on the order of a few molecular diameters. Furthermore, in systems where the characteristic length scale is the same order of magnitude as the thickness of the interface such as a thin liquid film on a solid surface, behavior may be different than that for larger systems. The small thickness of such films leads to difficult experimental observation of phenomena within various regions of the film: the wall-affected region, the bulk liquid, and the liquid-vapor interfacial region. A hybrid simulation methodology is applied that combines a deterministic molecular dynamics simulation of the liquid regions with a stochastic treatment of the far-field vapor region boundary. In this simulation scheme, the imposed far-field pressure may be held fired or iterated as the simulation is advanced in time until the mass in the system stabilizes at the specified temperature, which establishes the equilibrium saturation vapor pressure for the specified temperature as dictated by the intermolecular force interaction models for the fluid and molecules near the solid surface. Simulation results are presented for an argon liquid film on a solid metallic surface.
Modeling and optimization of bulk SiGe thin-film coolers are described. Thin-film coolers can provide large cooling power densities compared to commercial thermoelectrics. Thin-film SiGe coolers have been demonstrated with maximum cooling of 4degreesC at room temperature and with cooling power density exceeding 500 W/cm(2). Important parameters in the design of such coolers are investigated theoretically and are compared with experimental data. Thermoelectric cooling, joule heating, and heat conduction are included in the model as well as non-ideal effects such as contact resistance, geometrical effects, and three-dimensional thermal and electrical spreading resistance of the substrate. Simulations exhibit good agreement with experimental results for bulk Si and SiGe thin-film coolers. It turned out that in many spot cooling applications using two n- and p-elements electrically in series and thermally in parallel does not give significant improvement over single leg elements. This is in contrast to conventional thermoelectric modules and is due to the aspect ratio and special geometry of thin film coolers. With optimization of SiGe thin-film cooler, simulations predict it can provide over 16degreesC with cooling power density of over 2000 W/cm(2).
In microchannel flow, gas-liquid interface behavior will be important for developing a wide range of microfluidic applications, especially in micro reactors. In this article, we discuss some topics related to capillary action and two-phase fluid behavior in a microchannel, One of the topics is interface motion in the flow driven only by capillary action. We examined circular and rectangular microchannels with diameters of 50 mu m 85 mu m x 68 mu m, and 75 mu m x 48 mu m, respectively. For the circular channel, experiments well agreed with the previous theory in the case of ethyl alcohol as the test liquid. The effects of inner surface condition are found to be critical for interface motion on a microscopic scale. We have extended our theory to a rectangular microchannel. We obtained the same formula of relation between non-dimensional time and interface position as that of the circular channel. We compared predictions with experimental results of a PDMS microchannel. They agreed qualitatively, but not quantitatively. The difference was considered to be caused by contact angle estimation.
One of the inter-connected factors that can lead to failures in the flow plates of PEM fuel cells is the pressure differences that exist between adjacent flow channels. These pressure differences lead to stresses in the channel supports, i.e., the ribs, which can be important in the presence of stresses arising due to other factors such as temperature gradients in the flow plates. In order to investigate the magnitudes of the pressure differences across the supports and the places where the maximum pressure differences occur, the flow and pressure variations in various forms of serpentine channels, these channels having a rectangular cross-sectional shape, have been numerically calculated. The presence of the diffusion layer has been ignored and the flow has been calculated using a commercial finite-element software package using the governing equations written in dimensionless form. Solutions have been obtained for various values of the Reynolds number for each of the flow geometries considered for two channel height-to-width ratios (one and three). Except for the flow in the vicinity of the bends in channels, the pressure has been found, as is to be expected, not vary significantly across the channel cross-section. The difference between the center point dimensionless pressure in a given channel with those at the same longitudinal position in the adjacent channels has been determined. The dependence of the highest dimensionless pressure difference between channel on the input parameters has been examined.
Compressive SiO2 films on Si substrates irradiated with sub-nanosecond laser pulses at fluences above 0.65 J/cm2 delaminate from the substrate. The volume of the blister created during the laser processing, determined through volume measurements of AFM and cross-sectional SEM data, is ∼0.1 μ3. Ion bombardment of the film prior to laser processing enhances the delamination, permitting the creation of blisters with volumes on the order of 0.3 μm3 for ion doses from 7.5 × 1012 to 3.0 × 1013 ions/cm2.
Molecular dynamics simulation has been performed on an ultra-thin lubrication liquid film, where the liquid film of diatomic molecules having a thickness of molecular scale (several nanometers) is sheared by two parallel solid walls moving at different speeds. The Couette-like flow is generated and energy conversion from the macroscopic flow energy to thermal energy, which is the viscous heating in a macroscopic sense, occurs in the liquid film. It was observed in the present simulations that the thermal phenomena in the liquid film are far from the macroscopically expected ones; thermal energy is not distributed evenly to each degree of freedom of molecular motion, and energy transfer in the liquid adjacent to the solid surface is contributed by molecular motion in a manner different from those in a bulk liquid.
Two-dimensional compressible momentum and energy equations are solved to obtain the heat transfer characteristics of gaseous flows in parallel-plate microchannels. The numerical methodology is based on the arbitrary-Lagrangian-Eulerian (ALE) method. The computations were performed for channels with adiabatic walls to obtain the adiabatic wall temperature. The channel height ranges from 10 to 100 μm and the channel length is fixed at 30 mm. The stagnation pressure varies from 1.1 × 105 to 4 × 106 Pa. The outlet pressure is fixed at the atmosphere. The computations were also performed for channels with isothermal walls. The aspect ratio of the channel length and height is 100 or 200. The channel height also ranges from 10 to 100 μm. The bulk and total temperatures are compared with that of the incompressible flow in the conventional-sized parallel plate channel.
In recent years, micro-technologies have become very important to cutting edge industries such as aerospace and biotechnology. In the fluidic research area, two-phase flow study in microchannels has been an emerging topic in the past few years. Characteristics of two-phase flow in microchannels such as flow regimes, pressure drop, void fraction, and heat transfer are now being extensively studied by numerous groups. The ultimate goal of this research ranges from compact heat exchangers to small-sized refrigeration systems. One of the major drawbacks in this area to date is the lack of a universal flow regime map enabling the prediction of flow regimes in microchannels.In the present study, a new test rig was designed and constructed to extend the range of the existing data on flow regimes. Several flow regime maps, for a hydraulic diameter of less than 1.0 mm obtained from a comprehensive literature review, are tested and compared. A total of approximately 1475 experimental data points from present and previous studies performed in channels with D-h≤ 1.0 mm were used for comparison. Two universal flow regime maps were created, one each for horizontal and vertical channels with a hydraulic diameter ranging between 0.1 mm and 1.0 mm. The resulting universal maps presented here are based on flow regimes observed in all the studies on two-phase flow in microchannels. We suggest that the use of these flow regimes could help diminish the amount of discord present among research groups pertaining to the definition of flow regimes. Based on the comparisons between the universal maps and different experimental flow regime maps, appropriate conclusions on the effects of channel orientation and geometry are suggested.
This is the first investigation on the possibility of the isothermal crystallization induced by an external force field using molecular dynamics simulation. External cyclic forces with a DC bias are superimposed on the intermolecular forces, which govern the global behavior of molecules. It is discovered that field-enhanced movements of susceptor molecules can induce crystallization effectively without heating problem, the crystallization process becomes more efficient when the external cyclic force is shifted by a DC bias, a radial distribution function is a sufficient tool to observe the progress of the crystallization, and the optimal values for the external forces are close to the averaged intermolecular forces.
The use of boiling propagation as the actuation mechanism of micropumps is proposed. The process of boiling propagation along a film heater surface, the propagation velocity, propagating bubble size, maximum repetition frequency, and the wall superheat conditions under which propagation occurs are investigated using ink for a thermal inkjet printer under pool boiling conditions. A prototype micropump that uses boiling propagation is developed. A film heater placed facing the microchannel is powered stepwise over a short duration and unidirectional boiling propagation over the entire heater length is triggered at a high wall superheat by generating a vapor bubble at the end of the heater. A continuous pumping action in a U-shaped microchannel via boiling propagation repeated at a prescribed frequency of up to 20 Hz is confirmed based on the head difference generated between liquid columns in the vertical sections.
A novel concept of microthermophotovoltaics (MTPV) systems is proposed for powering MEMS devices. The system uses, hydrogen or hydrocarbon as fuel and does not involve any moving pails. Its fabrication and assembly are relatively simple. In this article, energy conversion efficiencies of a GaSb MTPV system incorporating broadband SiC and selective emitter material were first analyzed. Numerical and experimental studies on microcombustion processes in the MTPV system were carried out. The results show that uniform temperature distribution above 1000 K along the wall of microcylindrical combustors with a unique backward-facing step can be achieved. Finally, a prototype MTPV power system using SiC as the material for combustor and emitter, and a hexagonal GaSb cell array for energy conversion, was fabricated and tested. Electric power output ranging from 0.07 W to 0.74 W was measured. The potentials and further approaches of MTPV system were discussed. It is believed that MTPV would be a very attractive and competitive system among other power MEMS developments.
The transient thermoreflectance (TTR) technique coupled with a pump-probe experimental setup enables the observation of thermal transport phenomena on a sub-picosecond time scale. The reflectance from non-noble transition metals (at least one unoccupied d-orbital in the conduction band) can be shown to have a linear dependence when compared to small changes in the electron and lattice temperatures. This thermal dependence can be combined with the parabolic two step (PTS) model to enable measurement of the electron-phonon coupling factor and thermal conductivity of thin film materials. Experimental results are presented for thin film samples of the non-noble transition metals platinum and nickel. Results are presented using laser wavelengths ranging from 740 nm to 805 nm and using a range of laser fluences (ranging from ∼0.35 to 2 J/m2). Over this range of wavelengths and fluences the material properties are shown to be independent of the measurement conditions.
This article examines the role of slip conditions within surface-embedded microchannels for reducing entropy production of external flows with convective heat transfer. Viscous dissipation of mechanical energy into internal energy within the boundary layer leads to pressure losses and other irreversible losses of energy availability. These exergy losses entail additional input power needed to deliver a fixed mass flow across the surface, subject to a specified rate of heat transfer to/from the wall. By selectively altering geometrical and surface parameters which minimize the net entropy production, the benefits of drag reduction due to the slip-flow conditions can outweigh the higher irreversibility arising from added microchannel area. Predicted results illustrate the changes of optimal Reynolds number and entropy generation number with varying surface parameters for embedded parallel and diverging microchannels. Based on these results, it is viewed that surface micro-profiling offers a useful new technique of taking advantage of slip-flow microfluidic conditions for reducing drag and simultaneously increasing heat transfer effectiveness in external flows.
We present an extensive comparison between direct Monte Carlo simulations and the predictions of the Navier-Stokes description coupled to a recently proposed second-order slip model for hard sphere gases. Two one-dimensional, time-dependent channel flows are considered. In both cases excellent agreement is found between molecular simulation and the proposed model well into the transition regime for both the velocity and stress fields. The excellent quantitative agreement extends, approximately, to a Knudsen number based on the channel width of 0.4. The transient nature of the flows suggests that the slip model, despite its (quasi-)steady origins, remains reliable as long as the evolution timescale is long compared to the molecular collision time. Our discussion elucidates the effect of the Knudsen layer, a kinetic "boundary layer" in the vicinity of walls that needs to be superposed to the Navier-Stokes component of the flow in order to obtain the complete flow solution. The existence of this kinetic boundary layer, whose importance grows with increasing Knudsen number and which ultimately renders the Navier-Stokes component insufficient to describe the flow, implies that special care is needed in interpreting slip-flow flowfields.
The methodology used to simulate a monatomic film on a solid metallic surface is extended to polyatomic nonpolar fluids and water. In this extension, the wall potential is calculated in the same manner as for a monatomic fluid since the intermolecular distance is generally much larger titan the molecular size. Nonpolar films are simulated in a manner very similar to the monatomic system since no long-ranged Coulombic potentials are present. The water simulation methodology uses a corrected 3-D Ewald summation technique, but this technique limits the number of vapor molecules in the simulation domain such that the system must be equilibrated to a value far from the critical point. As a result, the lack of vapor molecules in the water system does not allow for calculation of the equilibrium saturation pressure via external pressure adjustment, but rather the simulation is run with a fixed external pressure. In this study, a diatomic nitrogen film at T/T-c = 0.55 and three water films (T = 100, 127, and 140 degrees C) were simulated. The resultant calculated bulk liquid density and surface tension compare well with ASHRAE recommended values, and these results are consistent with previous calculations for a monatomic system.
This work investigated the interface influence on the thermal resistance across double-layered thin films by non-equilibrium molecular dynamics (NEMD) with Lennard-Jones potential. Layer A is a solid argon with a face-centered cubic structure and Layer B is obtained by changing atomic mass only. A flat interface is formed when each of the contacting atomic planes from the two layers has the same kind of atoms. A staggered interface is obtained by mixing atoms A and B around the interface region. The temperature profile, vibration amplitude, and structure factor are studied to observe the interface effects. It is found that the thickness of the staggered atomic layer has significant influences on the normal thermal conductivity. With a staggered interface thickness of two atomic planes, the normal thermal conductivity is sharply increased. Further increasing the staggered thickness will gradually decrease the normal conductivity. This result suggests a possibility to control the thermal conductivity of the double-layered structure by engineering its interface condition.
