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1.
This paper describes a flow reversal phenomenon for fluids with moderate conductivity. Fluids with conductivities of 2 × 10−4 S/m, 0.02 S/m and up to 0.1 S/m were experimented at frequencies ranging from 1 to 110 kHz. Flow reversal was observed only
at ~1 kHz and 5.3 V
rms for σ = 0.02 S/m, and our analysis indicates that AC electrothermal effect could be responsible. Analysis of the system impedance
and simulation of power consumption show that the distribution of electric power consumption is dependent on conductivity
and AC frequency. At low frequencies, possibly more electric power is consumed at surface/electrolyte interface rather than
within the fluid, which consequently changes the location of temperature maximum and the directions of temperature gradients.
The direction of AC electrothermal force is reoriented, causing the flow reversal. Numerical simulation is also performed
and agrees within the experiments. 相似文献
2.
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
3.
B. S. Kwak B. S. Kim H. H. Cho J. S. Park H. I. Jung 《Microfluidics and nanofluidics》2008,5(2):255-262
This article presents a new design of a silicon-based microcalorimeter made with dual thermopiles and a microchannel. The
dual thermopile was fabricated with chromium and copper using a microelectromechanical system (MEMS) technique, and the microchannel
was made of PDMS using soft-lithography. Each thermopile consists of 26 thermocouple pairs and 50 μm wide electrodes. The
total sensitivity of thermopile is 428 μV/K. The dual thermopile system enables the microcalorimeter to acquire reliable data
in a rapid and convenient manner because it detects the reaction and reference temperatures simultaneously. This self-compensation
allows our device to analyze a few microliters of sample solution without the need for a surrounding adiabatic vacuum. 相似文献
4.
In this work we combined numerical simulation with molecular-diffusion effect, high-tempo micro-particle image velocimetry
(μ-PIV), and probability distribution function (PDF) analysis to investigate the chaotic mixing and hydrodynamics inside a
droplet moving through a planar serpentine micromixer (PSM). Robust solutions for the distributions of interface and concentration
of the droplets were obtained via computational fluid dynamics. The simulated fluid patterns are consistent with those measured
with μ-PIV, which serves as a powerful nonintrusive diagnostic approach to observe the droplets. Two mechanisms are proposed
to enhance the performance of mixing in a PSM—the deformation of droplets and the asymmetric recirculation within the droplets.
On introducing alternating cross sections into a winding channel, this specific design of PSM is found to amplify the fluid
disturbance and maximum vorticity difference. Data show that the PDF of the vorticity fields is modified and the fraction
with larger vorticity is increased. Accordingly, the PSM is capable of achieving a mixing index 90% within 700 μm (Re = 2), which is eight times better than for a straight microchannel. The results not only demonstrate explicitly the fluid
patterns within the droplets but also provide significant insight into the factors dominating the mixing efficiency. 相似文献
5.
Goran T. Vladisavljevi? Isao Kobayashi Mitsutoshi Nakajima 《Microfluidics and nanofluidics》2011,10(6):1199-1209
Uniformly sized droplets of soybean oil, MCT (medium-chain fatty acid triglyceride) oil and n-tetradecane with a Sauter mean diameter of d
3,2 = 26–35 μm and a distribution span of 0.21–0.25 have been produced at high throughputs using a 24 × 24 mm silicon microchannel
plate consisting of 23,348 asymmetric channels fabricated by photolithography and deep reactive ion etching. Each channel
consisted of a 10-μm diameter straight-through micro-hole with a length of 70 μm and a 50 × 10 μm micro-slot with a depth
of 30 μm at the outlet of each channel. The maximum dispersed phase flux for monodisperse emulsion generation increased with
decreasing dispersed phase viscosity and ranged from over 120 L m−2 h−1 for soybean oil to 2,700 L m−2 h−1 for n-tetradecane. The droplet generation frequency showed significant channel to channel variations and increased with decreasing
viscosity of the dispersed phase. For n-tetradecane, the maximum mean droplet generation frequency was 250 Hz per single active channel, corresponding to the overall
throughput in the device of 3.2 million droplets per second. The proportion of active channels at high throughputs approached
100% for soybean oil and MCT oil, and 50% for n-tetradecane. The agreement between the experimental and CFD (Computational Fluid Dynamics) results was excellent for soybean
oil and the poorest for n-tetradecane. 相似文献
6.
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
7.
Isao Kobayashi Yoichi Murayama Takashi Kuroiwa Kunihiko Uemura Mitsutoshi Nakajima 《Microfluidics and nanofluidics》2009,7(1):107-119
This paper reports the production of monodisperse water-in-oil (W/O) emulsions using new microchannel emulsification (MCE)
devices, asymmetric straight-through MC arrays that were hydrophobically modified. The silicon asymmetric straight-through
MC arrays consisted of numerous pairs of microslots and circular microholes whose cross-sectional sizes were 10 μm. This paper
primarily focused on investigating the effect of the osmotic pressure of a dispersed phase (Πd) on MCE. This paper also investigated the effects of the type of continuous-phase oils and the dispersed-phase flux (J
d) on MCE. The dispersed phases were Milli-Q water and Milli-Q water solutions containing sodium chloride. The continuous phases
were decane (as control), hexane, medium chain triacylglyceride (MCT), and refined soybean oil (RSO) solutions containing
tetraglycerin monolaurate condensed ricinoleic acid ester (TGCR) as a surfactant. At Πd of exceeding threshold, highly uniform aqueous droplets with coefficients of variation of less than 3% were stably generated
via hydrophobic asymmetric straight-through MCs. Monodisperse W/O emulsions with average droplet diameters between 32 and
45 μm were produced using the alkane–oil and triglyceride–oil solutions as the continuous phase. This work also demonstrated
that the hydrophobic asymmetric straight-through MC array had remarkable ability to produce highly uniform aqueous droplets
at very high J
d of up to 1,200 L m−2 h−1. 相似文献
8.
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
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.
Cheng-Che Chung I-Fang Cheng Chi-Chang Lin Hsien-Chang Chang 《Microfluidics and nanofluidics》2011,10(2):311-319
We studied an imaging-based technique for the rapid quantification of bio-particles in a dielectrophoretic (DEP) microfluidic
chip. Label-free particles could be successively sorted and trapped in a continuous flow manner under the applied alternating
current (AC) conditions. Both 2 and 3 μm polystyrene beads at a concentration of 1.0 × 107 particles ml−1 could be rapidly quantified within 5 min in our DEP system. Capturing efficiencies higher than 95% could be 2 μm polystyrene
beads with a linear flow speed, applied voltage and frequency of 0.89 mm s−1, 20 Vp-p and 5 MHz. Yeast cells (Candida glabrata and Candida albicans) could also be captured even at a lower concentration of 2.5 × 105 cells ml−1. Images of aggregative particles taken from the designed trapping area were further processed based on the intensity of relative
greyscale followed by correction of the particle numbers. The imaging-based quantification method showed higher agreement
than that of the conventional counting chamber method and proved the stability and feasibility of our AC DEP system. 相似文献
11.
Experiments on opto-electrically generated microfluidic vortices 总被引:1,自引:1,他引:0
Strong microfluidic vortices are generated when a near-infrared (1,064 nm) laser beam is focused within a microchannel and
an alternating current (AC) electric field is simultaneously applied. The electric field is generated from a parallel-plate,
indium tin oxide (ITO) electrodes separated by 50 μm. We present the first μ-PIV analysis of the flow structure of such vortices.
The vortices exhibit a sink-type behavior in the plane normal to the electric field and the flow speeds are characterized
as a function of the electric field strength and biasing AC signal frequency. At a constant AC frequency of 100 kHz, the fluid
velocity increases as the square of the electric field strength. At constant electric field strength fluid velocity does not
change appreciably in the 30–50 kHz range and it decreases at larger frequencies (>1 MHz) until at approximately 5 MHz when
Brownian motion dominates the movement of the 300 nm μ-PIV tracer particles. Presence of strongly focused laser beams in an
interdigitated-electrode configuration can also lead to strong microfluidic vortices. When the center of the illumination
is focused in the middle of an electrode strip, particles experiencing negative dielectrophoresis are carried towards the
illumination and aggregate in this area. 相似文献
12.
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
13.
E. Leclerc H. Kinoshita T. Fujii D. Barthès-Biesel 《Microfluidics and nanofluidics》2012,12(5):761-770
The study deals with a microfluidic method to investigate the transient behavior of microcapsules in flow. The technique consists
of investigating ovalbumin microcapsules passing through a convergent–divergent microchannel made of PolyDiMethylSiloxane.
We work with three types of square microchannel with, respectively, cross section values of h × h = 30 × 30, 50 × 50 and 70 × 70 μm. The microchannels length is L = 3h. We analyze the kinetics of deformation of the microcapsules in the microchannels for velocity ranging from 2 to 5 cm/s and
for microcapsule size ratio d/h ranging from 0.9 to 2.5. The relaxation process at the pore outlet is modeled using an exponential relaxation law. We show
that that the relaxation time at the divergent outlet depends on the microcapsule size ratio d/h. Thanks to the analytical expression of the relaxation, we extract a shear modulus of the membrane equal to 0.04 N/m. This
value is consistent with the value of 0.07 N/m that we found using the steady state analysis performed in cylindrical glass
capillaries. Thus, it is interesting to notice that the microcapsule behavior based on a simple analytical model can be successfully
described despite the complex flow situation consisting of deformable microcapsule in confined square microchannels. 相似文献
14.
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
15.
I. S. Akhatov J. M. Hoey O. F. Swenson D. L. Schulz 《Microfluidics and nanofluidics》2008,5(2):215-224
A micro-capillary system capable of generating a focused collimated aerosol beam (CAB) is demonstrated both theoretically
and experimentally. The approach is based on a manifestation of the Saffman force where high velocity (∼100 m/s) aerosol particles,
flowing through a micro-capillary (d ∼ 100 μm and l ∼ 1 cm), migrate perpendicular to the centerline of the capillary. Upon exiting the micro-capillary system, the particles
maintain momentum, and when the aerosol is comprised of solid-in-liquid dispersions such as Ag nanoparticle ink, the CAB approach
enables printing of advanced materials features with linewidth ≤ 10 μm. 相似文献
16.
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
17.
S. Prentner D. M. Allen L. Larcombe S. Marson K. Jenkins M. Saumer 《Microsystem Technologies》2010,16(7):1091-1096
The behaviour of blood flow in relation to microchannel surface roughness has been investigated. Special attention was focused
on the techniques used to fabricate the microchannels and on the apparent viscosity of the blood as it flowed through these
microchannels. For the experimental comparison of smooth and rough surface channels, each channel was designed to be 10 mm
long and rectangular in cross-section with aspect ratios of ≥100:1 for channel heights of 50 and 100 μm. Polycarbonate was
used as the material for the device construction. The shims, which created the heights of the channels, were made of polyethylene
terephthalate. Surface roughnesses of the channels were varied from R
z of 60 nm to 1.8 μm. Whole horse blood and filtered water were used as the test fluids and differential pressures ranged from
200 to 5,000 Pa. The defibrinated horse blood was treated further to prevent coagulation. The results indicate that a surface
roughness above an unknown value lowers the apparent viscosity of blood dramatically due to boundary effects. Furthermore,
the roughness seemed to influence both water and whole blood almost equally. A set of design rules for channel fabrication
is also presented in accordance with the experiments performed. 相似文献
18.
We report a new technique for conducting immuno-diagnostics on a microfluidic platform. Rather than handling fluid reagents
against a stationary solid phase, the platform manipulates analyte-coated magnetic beads through stationary plugs of fluid
reagents to detect an antigenic analyte. These isolated but accessible plugs are pre-encapsulated in a microchannel by capillary
force. We call this platform microfluidic inverse phase enzyme-linked immunosorbent assay (μIPELISA). μIPELISA has distinctive
advantages in the family of microfluidic immunoassay. In particular, it avoids pumping and valving fluid reagents during assaying,
thus leading to a lab-on-a-chip format that is free of instrumentation for fluid actuation and control. We use μIPELISA to
detect digoxigenin-labeled DNA segments amplified from E. coli O157:H7 by polymerase chain reaction (PCR), and compare its detection capability with that of microplate ELISA. For 0.259 ng μl−1 of digoxigenin-labeled amplicon, μIPELISA is as responsive as the microplate ELISA. Also, we simultaneously conduct μIPELISA
in two parallel microchannels. 相似文献
19.
We present the fabrication and characterization of nanoscale fluidic channels with embedded electrodes. Arrays of 2.25 μm
long and 60 nm tall nanochannels with widths ranging from 60 to 500 nm were microfabricated in SiO2 with Au electrodes embedded inside and outside of the nanochannels. The built-in electrodes were able to probe nanochannel
conductance via a redox reaction of
\textFe(\textCN)63 - /4 - {\text{Fe}}({\text{CN}})_{6}^{3 - /4 - } . Amperometric characterization showed that conductance of nanochannel arrays varied linearly both with the width and number
of nanochannels and was in the 10–100 pS range. Further, we show that electrical current was largely diffusion based and could
be predicted from channel geometry using standard diffusion equations. We also discuss the potential of such nanochannel arrays
as electronic biomolecular sensors and show preliminary streptavidin detection results. 相似文献
20.
Jayaprakash Sivasamy Yong Cai Chim Teck-Neng Wong Nam-Trung Nguyen Levent Yobas 《Microfluidics and nanofluidics》2010,8(3):409-416
This article reports a design that reliably adds reagents into droplets by exploiting the physics of fluid flow at a T-junction
in the microchannel. An expanded section right after the T-junction enhances merging of a stream with a droplet, eliminates
the drawbacks such as extra droplet formation and long mixing time. The expanded section reduces the pressure buildup at the
T-junction and minimizes the tendency to form extra droplets; plays the role in creating low Laplace pressure jump across
the interface of the droplet forming from the T-junction which reduces the probability of forming extra droplet in the merging
process; provides space for droplet coalescence if there is an extra droplet due to droplet break-up before merging. In this
design, after merging, the reactants are in axial arrangement inside the droplets which lead to faster mixing. Reliable addition
of reagent to the droplets happens for the combination of flow rates in a broad range from 25 to 250 μl/h, for both DI water
(Q
DI) and fluorescent (Q
fluo) streams. 相似文献