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1.
Recent developments in superhydrophobic surfaces have enabled significant reduction in the frictional drag for liquid flow through microchannels. There is an apparent risk when using such surfaces, however, that under some conditions the liquid meniscus may destabilize and, consequently, the liquid will wet the entire patterned surface. This paper presents analytical and experimental results that compare the laminar flow dynamics through microchannels with superhydrophobic walls featuring ribs and cavities oriented both parallel and transverse to the direction of flow under both wetting and non-wetting conditions. The results show the reduction in the total frictional resistance is much greater in channels when the liquid phase does not enter the cavity regions. Further, it is demonstrated that the wetting and non-wetting cavity results represent limiting cases between which the experimental data lie. Generalized expressions enabling prediction of the classical friction factor-Reynolds number product as a function of the relevant governing dimensionless parameters are also presented for both the superhydrophobic and wetting states. Experimental results are presented for a range of parameters in the laminar flow regime. 相似文献
2.
The flow of a compressible, isothermal gas under slightly rarefied conditions in a 2D planar geometry is considered. The gas
is shear driven and is also subject to an applied pressure gradient, which is also known as Couette–Poiseuille (CP) flow.
In this paper, the full Navier-Stokes (NS) equations are solved using a perturbation expansion up to the first order. The
pressure profile is solved numerically. On the basis of the solutions, effects of rarefaction and compressibility on the flow
characteristics are investigated in detail. The results show the parallel flow assumption to be invalid for cases with slight
rarefaction. The axial and vertical velocity components are found to depend on the degree of rarefaction, applied pressure
gradient and wall velocity. The effects of rarefaction on the occurrence of back flow are also discussed. In addition, the
results for the Poiseuille and CP flow with and without rarefaction can be easily obtained from our results. 相似文献
3.
We study a model for Poiseuille flow instability in a porous medium of Brinkman type. In particular, we analyse the effect of slip boundary conditions on the onset of instability. Due to numerous applications in micro-electro-mechanical-systems and other microfluidic devices, such a study is essential. We accurately analyse as to when instability will commence and determine the critical Reynolds number as a function of the slip coefficient. 相似文献
4.
The surface of microchannels, especially polymer channels, often needs to be treated to acquire specific properties. This
study investigated the capillary flow and the interface behavior in several glass capillaries and fabricated microchannels
using a photographic technique and image analysis. The effect of air plasma treatment on the characteristics of capillary
flow in three types of microfluidic chips, and the longevity of the acquired surface properties were also studied. It was
observed that the dynamic contact angles in microchannels were significantly larger than those measured from a flat substrate
and the angle varied with channel size. This suggests that dynamic contact angle measured in situ must be used in the theoretical
calculation of capillary flow speed, especially for microfabricated microchannels since the surface properties are likely
to be different from the native material. This study also revealed that plasma treatment could induce different interface
patterns in the PDMS channels from those in the glass and PC channels. The PDMS channel walls could acquire different level
of hydrophilicity during the plasma treatment, and the recovery to hydrophobicity is also non-homogeneous. 相似文献
5.
Slip flow in non-circular microchannels 总被引:1,自引:2,他引:1
Microscale fluid dynamics has received intensive interest due to the emergence of Micro-Electro-Mechanical Systems (MEMS)
technology. When the mean free path of the gas is comparable to the channel’s characteristic dimension, the continuum assumption
is no longer valid and a velocity slip may occur at the duct walls. Non-circular cross sections are common channel shapes
that can be produced by microfabrication. The non-circular microchannels have extensive practical applications in MEMS. Slip
flow in non-circular microchannels has been examined and a simple model is proposed to predict the friction factor and Reynolds
product fRe for slip flow in most non-circular microchannels. Through the selection of a characteristic length scale, the square root
of cross-sectional area, the effect of duct shape has been minimized. The developed model has an accuracy of 10% for most
common duct shapes. The developed model may be used to predict mass flow rate and pressure distribution of slip flow in non-circular
microchannels. 相似文献
6.
This paper presents numerical results pertaining to the effects of interface curvature on the effective slip behavior of Poiseuille flow through microchannels and microtubes containing superhydrophobic surfaces with transverse ribs and grooves. The effects of interface curvature are systematically investigated for different normalized channel heights or tube diameters, shear-free fractions, and flow Reynolds numbers. The numerical results show that in the low Reynolds number Stokes flow regime, when the channel height or tube diameter (normalized using the groove–rib spacing) is sufficiently large, the critical interface protrusion angle at which the effective slip length becomes zero is θ c ≈ 62°–65°, which is independent of the shear-free fraction, flow geometry (channel and tube), and flow driving mechanism. As the normalized channel height or tube diameter is reduced, for a given shear-free fraction, the critical interface protrusion angle θ c decreases. As inertial effects become increasingly dominant corresponding to an increase in Reynolds number, the effective slip length decreases, with the tube flow exhibiting a more pronounced reduction than the channel flow. In addition, for the same corresponding values of shear-free fraction, normalized groove–rib spacing, and interface protrusion angle, longitudinal grooves are found to be consistently superior to transverse grooves in terms of effective slip performance. 相似文献
7.
Lian Zhang Jae-Mo Koo Linan Jiang Asheghi M. Goodson K.E. Santiago J.G. Kenny T.W. 《Journal of microelectromechanical systems》2002,11(1):12-19
Two-phase forced convective flow in microchannels is promising for the cooling of integrated circuits. There has been limited research on boiling flow in channels with dimensions below 100 μm, in which bubble formation and flow regimes can differ from those in larger channels. This work develops single and multi-channel experimental structures using plasma-etched silicon with pyrex glass cover, which allow uniform heating and spatially-resolved thermometry and provide optical access for visualization of boiling regimes. Boiling was observed with less than 5°C of super-heating in rectangular channels with hydraulic diameters between 25 and 60 μm. The channel wall widths are below 350 μm, which minimizes solid conduction and reduces variations in the heat flux boundary condition. Pressure drop and wall temperature distribution data are consistent with predictions accounting for solid conduction and homogeneous two-phase convection 相似文献
8.
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10.
E. E. Licon Bernal V. I. Kovalchuk E. K. Zholkovskiy A. Yaroshchuk 《Microfluidics and nanofluidics》2016,20(4):58
This work describes the steady-state transport of an electrolyte due to a stationary concentration difference in straight long channels under conditions of electroosmotic circulation. The electroosmotic flow is induced due to the slip produced at the charged channel walls. This flow is assumed to be compensated by a pressure-driven counterflow so that the net volume flow through the channel is exactly zero. Owing to the concentration dependence of electroosmotic slip, there is an involved coupling between the solute transfer, hydrodynamic flow and charge conservation. Nevertheless, for such a system the Taylor–Aris dispersion (TAD) theory is shown to be approximately applicable locally within an inner part of the channel for a wide range of Péclet numbers (Pe) irrespective of the concentration difference. Numerical simulations reveal only small deviations from analytical solutions for the inner part of the channel. The breakdown of TAD theory occurs within boundary regions near the channel ends and is related to the variation of the dispersion mechanism from the purely molecular diffusion at the channel ends to the hydrodynamic dispersion within the inner part of the channel. This boundary region is larger at the lower-concentration channel edge and its size increases nearly linearly with Pe number. It is possible to derive a simple analytical approximation for the inner profile of cross-section-averaged electrolyte concentration in terms of only few parameters, determined numerically. Such analytical approximations can be useful for experimental studies of concentration polarization phenomena in long microchannels. 相似文献
11.
Electroosmotic flow in microchannels with prismatic elements 总被引:1,自引:0,他引:1
Yandong Hu Xiangchun Xuan Carsten Werner Dongqing Li 《Microfluidics and nanofluidics》2007,3(2):151-160
Fundamental understanding of liquid flow through microchannels with 3D prismatic elements is important to the design and operation
of lab-on-a-chip devices. In this paper, we studied experimentally and theoretically the electroosmotic flow (EOF) in slit
microchannels with rectangular 3D prismatic elements fabricated on the bottom channel wall. The average electroosmotic velocity
measured by the current-monitoring technique was found lower than that in a smooth microchannel. This velocity reduction becomes
larger in microchannel with larger but less number of the prisms even though the space taken by the prisms are identical.
The velocity distribution and streamlines on two typical horizontal planes in the microchannel are measured and visualized
by a particle-based technique. These experimental observations are in good agreement with the numerical simulation. The comparison
of streamlines near the prisms in the pressure-driven flow with that in the EOF showed that the EOF was more sensitive to
the local geometry. 相似文献
12.
13.
Poiseuille number of rarefied gas flow in channels with designed roughness is studied and a multiplicative decomposition of Poiseuille number on the effects of rarefaction and roughness is proposed. The numerical methodology is based on the mesoscopic lattice Boltzmann method. In order to eliminate the effect of compressibility, the incompressible lattice Boltzmann model is used and the periodic boundary is imposed on the inlet and outlet of the channel. The combined bounced condition is applied to simulate the velocity slip on the wall boundary. Numerical results reveal the two opposite effects that velocity gradient and friction factor near the wall increase as roughness effect increases; meanwhile, the increments of the rarefaction effect and velocity slip lead to a corresponding decrement of friction factor. An empirical relation of Poiseuille number which contains the two opposite effects and has a better physical meaning is proposed in the form of multiplicative decomposition, and then is validated by available experimental and numerical results. 相似文献
14.
Jean Berthier Kenneth A. Brakke David Gosselin Fabrice Navarro Naceur Belgacem Didier Chaussy 《Microfluidics and nanofluidics》2016,20(7):100
Capillary flows are increasingly used in biotechnology, biology, chemistry, energy and space applications. Motivated by these new developments, designs of capillary channels have become more sophisticated. In particular, capillary microsystems often use winding channels for reasons such as compactness, or mixing. The behavior of capillary microflows in curved channels is still underdeveloped. In this work, we investigate this type of behavior. In the case of suspended capillary flows, is shown that the flow profile in the curved section is approximately analogous to that in a rectilinear section. In the case of open U-grooves where inner corners are present, the importance of the turn sharpness and of the presence of capillary filaments is pointed out. For sharp turns, and/or in the presence of precursor capillary filaments, we found the phenomenon that the inner filament precedes the outer filament in the channel. Analysis of the capillary flow in curved channels is performed experimentally using rectangular U-grooves and suspended channels. The experimental observations are compared to Surface Evolver numerical software results. 相似文献
15.
The Schmidt number is a fundamental parameter characterizing the properties of quantum states, and local projections are fundamental operations in quantum physics. We investigate the relation between the Schmidt numbers of bipartite states and their projected states. We show that there exist bipartite positive partial transpose entangled states of any given Schmidt number. We further construct the notion of joint Schmidt number for multipartite states and explore its relation with the Schmidt number of bipartite reduced density operators. 相似文献
16.
We consider pipe Poiseuille flow subjected to a disturbance which is highly localized in space. Experiments by Peixinho and Mullin have shown this disturbance to be efficient in triggering turbulence, yielding a threshold dependence on the required amplitude as R-1.5 on the Reynolds number, R. The experiments also indicate an initial formation of hairpin vortices, with each hairpin having a length of approximately one pipe radius, independent of the Reynolds number in the range of R=2000-3000. We perform direct numerical simulations for R=5000. The results show a packet of hairpin vortices traveling downstream, each having a length of approximately one pipe radius. The perturbation remains highly localized in space while being advected downstream for approximately 10 pipe diameters. Beyond that distance from the disturbance origin, the flow becomes severely disordered. 相似文献
17.
Debashis Dutta 《Microfluidics and nanofluidics》2011,10(3):691-696
In this article, we investigate the effect of channel sidewalls on the transport of neutral samples through rectangular conduits under pressure-driven flow and small zeta potential conditions. Our analyses show that while these structures can significantly reduce the streaming potential in small aspect ratio rectangular channels, they introduce a very minor variation in the sample velocity with the extent of Debye layer overlap in the system. Moreover, the increase in sample dispersion due to the channel side-regions has been shown to be nearly independent of the Debye layer thickness and very comparable to that reported under simple pressure-driven flow conditions. Interestingly however, a simple one-dimensional (1D) model that decouples band broadening arising due to diffusional limitations across the depth and width of the rectangular conduit has been shown to capture the predicted dependence of the Taylor–Aris dispersion coefficient on the channel aspect ratio under all operating conditions with less than 3% error. 相似文献
18.
A new non-intrusive measurement technique for two-phase flow in microchannels is presented. The development of an evanescent field-based optical fiber Bragg grating (FBG) sensor is described, and experiments coupled with flow visualization demonstrating the performance of this sensor are presented. Two adjacent 1-mm FBGs in etched D-shaped fiber are embedded into the surface of a PDMS microchannel. Experiments are conducted in both droplet and slug flow regimes and high-speed digital video is captured synchronously with the sensor data. The FBGs exhibit an on?Coff type response to the passage droplets which is shown to correlate precisely with the passage of the liquid phase. This correlation enables the measurement of droplet average velocity and size using only the sensor data. In addition to the use of both FBG signals for the purpose of measuring droplet speed and size, it is shown that for droplets larger than the FBG length, a single FBG can be used to estimate the convection velocity and size of fast moving droplets. This sensing method is potentially useful for monitoring two-phase flow in fuel cells and microfluidic applications such as micro-heat exchangers and lab-on-a-chip systems. 相似文献
19.
Jens Ducrée Stefan Haeberle Thilo Brenner Thomas Glatzel Roland Zengerle 《Microfluidics and nanofluidics》2006,2(2):97-105
We demonstrate how the speed of mixing under laminar conditions can be appreciably enhanced in concurrent centrifugal flows
through straight, low-aspect-ratio microchannels pointing in radial direction in the plane of rotation. The convective mixing
is driven by the inhomogeneous distribution of the velocity-dependent Coriolis pseudo force and the interaction of the so-induced
transverse currents with the side walls. By investigating the key impact parameters, which are the geometry of the channels
and the speed of rotation, it is shown that the contact surface between two laminar flows can be folded to shorten mixing
times by up to two orders of magnitude! 相似文献
20.
We demonstrate the capability of a molecular tracer based laser induced fluorescence photobleaching anemometer for measuring
fluid velocity profile in microfluidics. To validate the feasibility and accuracy of this measurement system, the velocity
profiles of the cylindrical and rectangular microchannels are measured, respectively. We compare our experimental results
with theoretical prediction. The theoretical prediction shows a good consistency with practical measurement results. The spatial
resolutions are analyzed and validated. Finally, the error source of the presented system is analyzed, which makes it possible
to compensate some error quantitatively and guides the application of this system. 相似文献