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1.
Non-intrusive local temperature measurement in convective microchannel flows using infrared (IR) thermography is presented. This technique can be used to determine local temperatures of the visualized channel wall or liquid temperature near this wall in IR-transparent heat sinks. The technique is demonstrated on water flow through a silicon (Si) microchannel. A high value of a combined liquid emissivity and substrate overall transmittance coupled with a low uncertainty in estimating this factor is important for quantitative temperature measurement using IR thermography. The test section design, and experimental and data analysis procedures that provide increased sensitivity of the detected intensity to the desired temperature are discussed. Experiments are performed on a 13-mm long, 50 μm wide by 135 μm deep Si microchannel at a constant heat input to the heat sink surface for flow rates between 0.6 and 1.2 g min−1. Uncertainty in fluid temperature varies from a minimum of 0.60°C for a Reynolds number (Re) of 297 to a maximum of 1.33°C for a Re of 251. 相似文献
2.
Temperature gradients aroused from the Joule heating in a non-uniform electrical field can induce inhomogeneities of electric
conductivity and permittivity of the electrolyte, thus causing an electrothermal force that generates flow motion. A 2D numerical
investigation of a micromixer, utilizing electrothermal effect to enhance its mixing efficiency, is proposed in this paper.
Results for temperature and velocity distributions, as well as sample concentration distribution are obtained for an electrolyte
solution in a microchannel with different pairs of electrodes under AC potentials with various frequencies. Numerical solutions
were first carried out for one pair of electrodes, with a length of 10 μm separated by a gap of 10 μm, on one side wall of
a microchannel having a length of 200 μm and a height of 50 μm. It is found that the electrothermal flow effect, in the frequency
range for which Coulomb force is predominant, induces vortex motion near the electrodes, thus stirring the flow streams and
enhancing its mixing efficiency. If more than one pair of electrodes is located on the opposite walls of the microchannel,
the mixing efficiency depends on the AC potential applied pattern and the electrodes arrangement pattern. The distance between
two pairs of electrodes on two opposite walls is then optimized numerically. Sample mixing efficiencies, using KCl solutions
as the working fluid in microchannels with different number of electrodes pairs at optimal electrodes arrangement pattern,
are also investigated. If root mean squared voltages of 10 V in an AC frequency range of 0.1–10 MHz are imposed on 16 pairs
of electrodes separated at an optimal distance, the numerical results show that a mixing efficiency of 98% can be achieved
at the end of the microchannel having a length of 700 μm and a height of 50 μm at Re = 0.01 Pe
C = 100, and Pe
T = 0.07. However, the mixing efficiency decreases sharply at a frequency higher than 10 MHz owing to the drastically decrease
in the Coulomb force. 相似文献
3.
Dongming Qiu Laura Silva Anna Lee Tonkovich Ravi Arora 《Microfluidics and nanofluidics》2010,8(4):531-548
Micro-droplet formation from an aperture with a diameter of micrometers is numerically investigated under the cross-flow conditions
of an experimental microchannel emulsification process. The process involves dispersing an oil phase into continuous phase
fluid through a microchannel wall made of apertured substrate. Cross-flow in the microchannel is of non-Newtonian nature,
which is included in the simulations. Micro-droplets of diameter 0.76–30 μm are obtained from the simulations for the apertures
of diameter 0.1–10.0 μm. The simulation results show that rheology of the bulk liquid flow greatly affects the formation and
size of droplets and that dispersed micro-droplets are formed by two different breakup mechanisms: in dripping regime and
in jetting regime characterized by capillary number Ca. Relations between droplet size, aperture opening size, interfacial
tension, bulk flow rheology, and disperse phase flow rate are discussed based on the simulation and the experimental results.
Data and models from literature on membrane emulsification and T-junction droplet formation processes are discussed and compared
with the present results. Detailed force balance models are discussed. Scaling factor for predicting droplet size is suggested. 相似文献
4.
This paper describes the microfocusing in a microchannel using the thermal actuation of a pair of microbubbles. A microbubble
was produced from de-ionized (DI) water with an integrated microheater, and the volume was controlled by applying voltage.
The microfocusing was demonstrated with a polydimethylsiloxane (PDMS) device consisting of two layers. The top layer included
a microchannel that was 300 μm wide and 50 μm high. It was flanked by a pair of reservoirs. The bottom layer provided a microheater
underneath the reservoir. Upon heating, DI water boiled immediately over the microheater and formed a microbubble that came
out of the reservoir in a perpendicular direction toward the fluid. The fluid was focused from 300 to 22 μm, as the distance
between the apexes of the arch-shaped microbubbles was shortened due to expansion, which was maintained at a flow velocity
up to approximately 17.8 mm s−1. The temperature of the water in the reservoir was estimated to reach the boiling point within 62 or 160 ms, depending on
the substrate. 相似文献
5.
An experimental study of pulsatile flow in microchannel is reported in this paper. Such a study is important because time-varying
flows are frequently encountered in microdevices. The hydraulic diameter of the microchannel is 144 μm and deionized water
is the working fluid. The pressure drop across the microchannel as a function of time is recorded, from which the average
and r.m.s. pressure drops are obtained. The experiments have been performed in the quasi-steady flow regime for a wide range
of flow rate, frequency of pulsations, and duty cycle. The results suggest that the pressure with pulsations lies between
the minimum and maximum steady state pressure values. The average pressure drop with pulsation is approximately linear with
respect to the flow rate. The theoretical expression for pressure has also been derived wherever possible and the experimental
data is found to lie below the corresponding theoretical values. The difference with respect to the theoretical value increases
with an increase in frequency and a decrease in flow rate, with a maximum difference of 32.7%. This is attributed to the small
size of the microchannel. An increase in frequency of square waveform leads to a larger reduction in pressure drop as compared
to rectangular waveform, irrespective of the duty cycle. The results can be interpreted with the help of a first-order model
proposed here; the model results are found to compare well against the experimental results. A correlation for friction factor
in terms of the other non-dimensional governing parameters is also proposed. Experimental study of mass-driven pulsatile flow
in microchannel is being conducted for the first time at these scales and the results are of both fundamental and practical
importance. 相似文献
6.
Junjie Zhu Robert Cameron Canter Gyunay Keten Pallavi Vedantam Tzuen-Rong J. Tzeng Xiangchun Xuan 《Microfluidics and nanofluidics》2011,11(6):743-752
Particle and cell separations are critical to chemical and biomedical analyses. This study demonstrates a continuous-flow
electrokinetic separation of particles and cells in a serpentine microchannel through curvature-induced dielectrophoresis.
The separation arises from the particle size-dependent cross-stream dielectrophoretic deflection that is generated by the
inherent electric field gradients within channel turns. Through the use of a sheath flow to focus the particle mixture, we
implement a continuous separation of 1 and 5 μm polystyrene particles in a serpentine microchannel under a 15 kV/m DC electric
field. The effects of the applied DC voltages and the serpentine length on the separation performance are examined. The same
channel is also demonstrated to separate yeast cells (range in diameter between 4 and 8 μm) from 3 μm particles under an electric
field as low as 10 kV/m. The observed focusing and separation processes for particles and cells in the serpentine microchannel
are reasonably predicted by a numerical model. 相似文献
7.
An experimental method for evaluating pressure fields in a microchannel flow was studied using μPIV measurement in conjunction with the pressure Poisson equation. The pressure error due to the influence of numbers of measurement planes, computational grids for solving pressure Poisson equation, and an experimental error in μPIV measurement was evaluated with respect to the exact solution of Navier–Stokes equation for straight microchannel flow. The mean velocity field in microchannel junction flows with bifurcation and confluence was measured by a μPIV system, which consists of a CCD camera and a microscope with an in-line illumination of white light from stroboscopes. The planar velocity fields at various cross-sections of the microchannel flow were measured by traversing the focal plane within a depth of focus of the microscope. The pressure contour in the microchannel flow was evaluated by solving the pressure Poisson equation with the experimental velocity data. The results indicate that the pressure field in the microchannel junction flow agrees closely with the numerical simulation of laminar channel flow, which suggests the validity of the present method. 相似文献
8.
Lin Gui Bo Yang Yu Carolyn L. Ren Jan P. Huissoon 《Microfluidics and nanofluidics》2011,10(2):435-445
A phase change (PC) microvalve with an integrated two-level cooling/heating system is developed for microfluidic applications
in this article. This PC microvalve utilizes the liquid–solid PC of a small portion of the working medium in a microchannel
to switch on/off the flow in the microchannel. The size of the working medium for the PC microvalve is 5-mm long, 50-μm high,
and 80-μm wide (50 μm × 80 μm is the cross-sectional area of the channel) in this study. The switch is actuated by using a
two-level cooling/heating system integrated on the chip. The first-level cooling/heating unit keeps the working medium in
the valve area in the temperature range of supercooling state. Based on the supercooling state, the second-level cooling/heating
unit either heats up or cools down the medium in the valve area to trigger its PC between liquid and solid for valving purposes.
The proposed microfluidic PC microvalve is characterized experimentally in microfluidic chips. The thermal impact of one PC
microvalve in one particular microchannel on its adjacent channels is discussed by establishing a preliminary analytical model
and a numerical model. In addition to no leakage and no moving element, this PC microvalve with a two-level cooling/heating
system can achieve a very short cooling time (i.e., 2.72 s). 相似文献
9.
Taotao Fu Denis Funfschilling Youguang Ma Huai Z. Li 《Microfluidics and nanofluidics》2010,8(4):467-475
The present study aims at scaling the formation of slug bubbles in flow-focusing microfluidic devices using a high-speed digital
camera and a micro particle image velocimetry (μ-PIV) system. Experiments were conducted in two different polymethyl methacrylate
square microchannels of respectively 600 × 600 and 400 × 400 μm. N2 bubbles were generated in glycerol–water mixtures with several concentrations of surfactant sodium dodecyl sulfate. The influence
of gas and liquid flow rates, the viscosity of the liquid phase and the width of the microchannel on the bubble size were
explored. The bubble size was correlated as a function of the width of the microchannel W
c, the ratio of the gas/liquid flow rates Q
g/Q
l and the liquid Reynolds number. During the pinch-off stage, the variation of the minimum width of the gaseous thread W
m with the remaining time could be scaled as
_boxclose_boxclose ()^ - 0.15 (T - t)^1/3 . W_{\text{m}} \propto ({\frac{{Q_{\text{g}} }}{{Q_{\text{l}} }}})^{ - 0.15} (T - t)^{1/3} . The velocity fields in the liquid phase around the thread, determined by μ-PIV measurements, were obtained around a forming
bubble to reveal the role of the liquid phase. 相似文献
10.
C. F. Kung C. F. Chiu C. F. Chen C. C. Chang C. C. Chu 《Microfluidics and nanofluidics》2009,6(5):693-697
This study aims to identify distinct blood flow characteristics in a microchannel at different sloping angles. The channel
is determined by a bottom hydrophilic stripe on a glass substrate and a fully covered hydrophobic glass substrate. The channel
has a height of 3 μm, and a width of 100 μm. It is observed that increasing the sloping angle from −90° (downward flow) to
90° (upward flow) increases the blood flow rate monotonically. These peculiar behaviors on the micro scale are explained by
a dynamic model that establishes the balance among the inertial, surface tension, gravitational, and frictional forces. The
frictional force is further related to the effective hematocrit. The model is used to calculate the frictional force, and
thus the corresponding hematocrit, which is smaller when the blood flows upward, reducing the frictional force. 相似文献
11.
Scalable attoliter monodisperse droplet formation using multiphase nano-microfluidics 总被引:1,自引:1,他引:0
We demonstrate a robust method to produce monodisperse femtoliter to attoliter droplets by using a nano-microfluidic device.
Two immiscible liquids are forced through a nanochannel where a steady nanoscopic liquid filament forms, thinning close to
the nanochannel exit to a microchannel due to the capillary focusing. When the nanoscopic filament enters the microchannel,
monodisperse droplets are formed by capillary instability. In a certain range of physical parameters and geometrical configurations,
the droplet size is only determined by the nanochannel height and independent of liquid flow rates and ratios, surfactants,
and continuous phase viscosity. By using nanochannels with a height of 100–900 nm, 0.4–3.5 μm diameter droplets (volume down
to 30 aL) have been produced. The generated droplets are stable for at least weeks. 相似文献
12.
In this study, a poly-methyl-methacrylate (PMMA) microfluidic chip with a 45° cross-junction microchannel is fabricated using
a CO2 laser machine to generate chitosan microfibers. Chitosan solution and sodium tripolyphosphate (STPP) solution were injected
into the cross-junction microchannel of the microfluidic chip. The laminar flow of the chitosan solution was generated by
hydrodynamic focusing. The diameter of laminar flow, which ranged from 30 to 50 μm, was controlled by changing the ratio between
chitosan solution and STPP solution flow rates in the PMMA microfluidic chip. The laminar flow of the chitosan solution was
converted into chitosan microfibers with STPP solution via the cross-linking reaction; the diameter of chitosan microfibers
was in the range of 50–200 μm. The chitosan microfibers were then coated with collagen for cell cultivation. The results show
that the chitosan microfibers provide good growth conditions for cells. They could be used as a scaffold for cell cultures
in tissue engineering applications. This novel method has advantages of ease of fabrication, simple and low-cost process. 相似文献
13.
Masoomeh Tehranirokh Burhanuddin Yeop Majlis Badariah Bais 《Microsystem Technologies》2009,15(5):753-762
In drug delivery systems microvalves are the key components that have been developed for active control of drugs. In this
research a normally closed microvalve with a glucose sensitive hydrogel actuating system is designed and simulated. Swelling
of the hydrogel forces a silicone rubber membrane to deflect and causes the valve to be opened. The component of the valve
that can be opened because of the hydrogel pressure is a silicon nitride cantilever beam which is sealed with a parylene layer.
Simulations have been done by FEM analysis and the results show that membrane deflection is large enough to enable the valve
to be opened and the fluid to flow through the microchannel. For both rectangular and trapezoidal microchannels with various
hydraulic diameters, output flow rates less than 50 μl/min to several hundreds of μl/min can be achieved. Final design has
been optimized for the opening point of microvalve at glucose concentration of 15 mM. Overall investigation has been done
for a microvalve with specific dimensions and with 4 kPa input pressure the output flow rate of 100 μl/min has been generated
which is in the desired range. 相似文献
14.
This study presents a sheathless and portable microfluidic chip that is capable of high-throughput focusing bioparticles based
on 3D travelling-wave dielectrophoresis (twDEP). High-throughput focusing is achieved by sustaining a centralized twDEP field
normal to the continuous through-flow direction. Two twDEP electrode arrays are formed from upper and lower walls of the microchannel
and extend to its center, which induce twDEP forces to provide the lateral displacements in two directions for focusing the
bioparticles. Bioparticles can be focused to the center of the microchannel effectively by twDEP conveyance when the characteristic
time due to twDEP conveying in the y direction is shorter than the residence time of the particles within twDEP electrode
array. Red blood cells can be effectively focused into a narrow particle stream (~10 μm) below a critical flow rate of 10 μl/min
(linear flow velocity ~5 mm/s), when under a voltage of 14 Vp–p at a frequency of 500 kHz is applied. Approximately 90% focusing efficiency for red blood cells can be achieved within two
6-mm-long electrode arrays when the flow rate is below 12 μl/min. Blood cells and Candida cells were also focused and sorted successfully based on their different twDEP mobilities. Compared to conventional 3D-paired
DEP focusing, velocity is enhanced nearly four folds of magnitude. 3D twDEP provides the lateral displacements of particles
and long residence time for migrating particles in a high-speed continuous flow, which breaks through the limitation of many
electrokinetic cell manipulation techniques. 相似文献
15.
One of the most important components in a microfluidic system is the microchannel which involves complicated flow and transport
process. This work presents microscale thermal fluid transport process inside a microchannel with a height of 37 μm. The channel
can be heated on the bottom wall and is integrated with arrays of pressure and temperature sensors which can be used to measure
and determine the local heat transfer and pressure drop. A more simplified model with modification of Young’s Modulus from
the experimental test is used to design and fabricate the arrays of pressure sensors. Both the pressure sensors and the channel
wall use polymer materials which greatly simplifies the fabrication process. In addition, the polymer materials have a very
low thermal conductivity which significantly reduces the heat loss from the channel to the ambient that the local heat transfer
can be accurately measured. The airflow in the microchannel can readily become compressible even at a very low Reynolds number
condition. Therefore, simultaneous measurement of both the local pressure drop and the temperature on the heated wall are
required to determine the local heat transfer. Comparison of the local heat transfer for a compressible airflow in microchannel
is made with the theoretical prediction based on incompressible airflow in large scale channel. The comparison has clarified
many of the conflicting results among different works. 相似文献
16.
This study presents a particle manipulation and separation technique based on dielectrophoresis principle by employing an
array of isosceles triangular microelectrodes on the bottom plate and a continuous electrode on the top plate. These electrodes
generate non-uniform electric fields transversely across the microchannel. The particles within the flowing fluid experience
a dielectrophoretic force perpendicular to the fluid flow direction due to the non-uniform electric fields. The isosceles
triangular microelectrodes were designed to continuously exert a small dielectrophoretic force on the particles. Particles
experiencing a larger dielectrophoretic force would move further in the perpendicular direction to the fluid flow as they
traveled past each microelectrode. Polystyrene microspheres were used as the model particles, with particles of ∅20 μm employed
for studying the basic characteristics of this technique. Particle separation was subsequently demonstrated on ∅10 and ∅15 μm
microspheres. Using an applied sinusoidal voltage of 20 Vpp and frequency of 1 MHz, a mean separation distance of 0.765 mm between them was achieved at a flow rate of 3 μl/min (~1 mm/s),
an important consideration for high throughput separation capability in a micro-scale technology device. This unique isosceles
triangular microelectrodes design allows heterogeneous particle populations to be separated into multiple streams in a single
continuous operation. 相似文献
17.
Keisuke Horiuchi Prashanta Dutta Cecilia D. Richards 《Microfluidics and nanofluidics》2007,3(3):347-358
This paper presents experimental and numerical results of mixed electroosmotic and pressure driven flows in a trapezoidal
shaped microchannel. A micro particle image velocimetry (μPIV) technique is utilized to acquire velocity profiles across the
microchannel for pressure, electroosmotic and mixed electroosmotic-pressure driven flows. In mixed flow studies, both favorable
and adverse pressure gradient cases are considered. Flow results obtained from the μPIV technique are compared with 3D numerical
predictions, and an excellent agreement is obtained between them. In the numerical technique, the electric double layer is
not resolved to avoid expensive computation, rather a slip velocity is assigned at the channel surface based on the electric
field and electroosmotic mobility. This study shows that a trapezoidal microchannel provides a tapered-cosine velocity profile
if there is any pressure gradient in the flow direction. This result is significantly different from that observed in rectangular
microchannels. Our experimental results verify that velocity distribution in mixed flow can be decomposed into pressure and
electroosmotic driven components. 相似文献
18.
S. C. H. Thian J. Y. H. Fuh Y. S. Wong H. T. Loh P. W. Gian Y. Tang 《Microsystem Technologies》2008,14(8):1125-1135
Microfluidic patterns of 100 μm in width and 50 μm height were replicated from a master using vacuum casting with silicone
rubber. These silicone copies are subjected to thorough analysis for dimensional accuracy against the master pattern. Analysis
of experimental results shows repeatability of the silicone rubber molds. To test the limits of vacuum casting with silicone
rubber, an attempt was undertaken to replicate 8 μm microchannel and submicron features. The results show that casting of
microfluidic channel via vacuum casting has high repeatability. 相似文献
19.
Phase separation of parallel laminar flow for aqueous two phase systems in branched microchannel 总被引:2,自引:1,他引:1
Aqueous two phase systems (ATPSs) have good biocompatibility and special selectivity. Their phase equilibrium and applications
in biological analysis have received much attention. Herein, parallel laminar flow (PLF) in the microchannel can provide an
effective platform to enhance mass transfer and preserve separate phases simultaneously. As fundamentals in feasible and convenient
sampling of PLF for ATPS, the phase separation methods and rules in branched microchannel were studied in this work, selecting
PEG 4000 + Na2SO4 + H2O as a model system. The exploration of flow pattern showed that a stable PFL was easily to form in the shallow microchannel
of 200 μm (depth) × 600 μm (width), as long as the velocity of lower phase was higher than 0.51 mm/s. The phase interface
of PLF could be easily controlled by the flow ratio of two phases. Single-phase separation could be reliably achieved in T-junction
outlets when the flow rate of outlet ascertains to be smaller or larger than that of inlet on the same side. The trifurcate
outlets with an extra middle channel could help realize a simultaneous two-phase separation. The flow rate of the extra channel
is the key for the phase separation performance, the range of which available for simultaneous two-phase separation is determined
by the resistance balance and the flow rates deviation offsetting as well. It is favor for increasing phase separation efficiency
to make the products of flow rate, viscosity, and the length of corresponding outlet channel close with each other for the
upper phase and the lower phase. The adjustable lengths of three channels can provide flexible choices to enhance simultaneous
two-phase separation of diversified ATPSs at various operating flow ratios. A multiport microchip with T-junction inlets and
trifurcate outlets designed for adjusting the lengths of branched channels on-chip is a convenient tool for PLF contact and
in situ phase separation of ATPSs in varieties of application. 相似文献
20.
An integrated flow-cell for full sample stream control 总被引:1,自引:1,他引:0
In this study, we present a novel three-dimensional hydrodynamic sheath flow chip that allows full control of a sample stream.
The chip offers the possibility to steer each of the four side sheath flows individually. The design of the flow-cell exhibits
high flexibility in creating different sample stream profiles (width and height) and allows navigation of the sample stream
to every desired position inside the microchannel (vertical and horizontal). This can be used to bring the sample stream to
a sensing area for analysis, or to an area of actuation (e.g. for cell sorting). In addition, we studied the creation of very
small sample stream diameters. In microchannels (typically 25 × 40 μm2), we created sample stream diameters that were five
to ten times smaller than the channel dimensions, and the smallest measured sample stream width was 1.5 μm. Typical flow rates
are 0.5 μl/min for the sample flow and around 100 μl/min for the cumulated sheath flows. The planar microfabricated chip,
consisting of a silicon–glass sandwich with an intermediate SU-8 layer, is much smaller (6 × 9 mm2) than the previously presented
sheath flow devices, which makes it also cost-effective. We present the chip design, fluidic simulation results and experiments,
where the size, shape and position of the sample stream have been established by laser scanning confocal microscopy and dye
intensity analysis. 相似文献