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1.
Effect of viscosities of dispersed and continuous phases in microchannel oil-in-water emulsification
Koen van Dijke Isao Kobayashi Karin Schroën Kunihiko Uemura Mitsutoshi Nakajima Remko Boom 《Microfluidics and nanofluidics》2010,9(1):77-85
Although many aspects of microchannel emulsification have been covered in literature, one major uncharted area is the effect
of viscosity of both phases on droplet size in the stable droplet generation regime. It is expected that for droplet formation
to take place, the inflow of the continuous phase should be sufficiently fast compared to the outflow of the liquid that is
forming the droplet. The ratio of the viscosities was therefore varied by using a range of continuous and dispersed phases,
both experimentally and computationally. At high viscosity ratio (η
d/η
c), the droplet size is constant; the inflow of the continuous phase is fast compared to the outflow of the dispersed phase.
At lower ratios, the droplet diameter increases, until a viscosity ratio is reached at which droplet formation is no longer
possible (the minimal ratio). This was confirmed and elucidated through CFD simulations. The limiting value is shown to be
a function of the microchannel design, and this should be adapted to the viscosity of the two fluids that need to be emulsified. 相似文献
2.
Microchannel emulsification for mass production of uniform fine droplets: integration of microchannel arrays on a chip 总被引:1,自引:0,他引:1
Isao Kobayashi Yoshihiro Wada Kunihiko Uemura Mitsutoshi Nakajima 《Microfluidics and nanofluidics》2010,8(2):255-262
We present a novel microchannel emulsification (MCE) system for mass-producing uniform fine droplets. A 60 × 60-mm MCE chip
made of single-crystal silicon has 14 microchannel (MC) arrays and 1.2 × 104 MCs, and each MC array consists of many parallel MCs and a terrace. A holder with two inlet through-holes and one outlet
through-hole was also developed for simply infusing each liquid and collecting emulsion products. The MCE chip was sealed
well by physically attaching it to a flat glass plate in the holder during emulsification. Uniform fine droplets of soybean
oil with an average diameter of 10 μm were reliably generated from all the MC arrays. The size of the resultant fine droplets
was almost independent of the dispersed-phase flow rate below a critical value. The continuous-phase flow rate was unimportant
for both the droplet generation and the droplet size. The MCE chip enabled mass-producing uniform fine droplets at 1.5 ml h−1 and 1.9 × 109 h−1, which could be further increased using a dispersed phase of low viscosity. 相似文献
3.
Straight-through microchannel devices for generating monodisperse emulsion droplets several microns in size 总被引:1,自引:1,他引:0
Isao Kobayashi Takayuki Takano Ryutaro Maeda Yoshihiro Wada Kunihiko Uemura Mitsutoshi Nakajima 《Microfluidics and nanofluidics》2008,4(3):167-177
The authors recently proposed a promising technique for producing monodisperse emulsions using a straight-through microchannel
(MC) device composed of an array of microfabricated oblong holes. This research developed new straight-through MC devices
with tens of thousands of oblong channels of several microns in size on a silicon-on-insulator plate, and investigated the
emulsification characteristics using the microfabricated straight-through MC devices. Monodisperse oil-in-water (O/W) and
W/O emulsions with average droplet diameters of 4.4–9.8 μm and coefficients of variation of less than 6% were stably produced
using surface-treated straight-through MC devices that included uniformly sized oblong channels with equivalent diameters
of 1.7–5.4 μm. The droplet size of the resultant emulsions depended greatly on the size of the preceding oblong channels.
The emulsification process using the straight-through MC devices developed in this research had very high apparent energy
efficiencies of 47–60%, defined as (actual energy input applied to droplet generation/theoretical minimum energy input necessary
for making droplets) × 100. Straight-through MC devices with numerous oblong microfluidic channels also have great potential
for increasing the productivity of monodisperse fine emulsions. 相似文献
4.
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
5.
Takashi Kuroiwa Hisato Kiuchi Kazuki Noda Isao Kobayashi Mitsutoshi Nakajima Kunihiko Uemura Seigo Sato Sukekuni Mukataka Sosaku Ichikawa 《Microfluidics and nanofluidics》2009,6(6):811-821
We investigated a preparation method of giant vesicles using monodisperse water-in-oil (W/O) emulsions stabilized by bilayer-forming
emulsifiers. A mixture of phosphatidylcholine, cholesterol and stearylamine was used both to stabilize the water droplets
formed in the emulsion and to form the vesicles. Using this lipid mixture, we obtained monodisperse W/O emulsions with mean
droplet diameters of 10–40 μm and coefficients of variation as small as ca 5% by means of the microchannel (MC) emulsification
technique. Utilization of an asymmetric straight-through MC array device enabled a monodisperse droplet productivity of up
to 80 ml/h. The obtained water droplets were converted to giant vesicles via evaporative removal of the continuous-phase solvent
followed by addition of an aqueous buffer solution. The resulting vesicles were similar in size to their starting water droplets,
and a hydrophilic fluorescent marker was entrapped inside the vesicles. 相似文献
6.
Simulations have been carried out for water flow in a square microchannel with a miter bend. The simulation considered a pressure-driven
flow in a channel-hydraulic diameter of 5 μm for series of Reynolds number (Re) range from 0.056 to 560, in order to investigate
water flow at bends. The result shows formation of two vortices after the miter bend, which are more discernible above Re
5.6. The critical inlet velocity for the generation of vortex in this particular geometry occurs at 1 m/s. A simple energy
mechanism is postulated to explain the vortex formation as well as core skew direction. The high pressure region at the outer
wall before and after the bend is a major factor for vortex formation since the liquid needs to reduce the additional energy
effected by the high pressure region. Navier–Stokes equation is utilized with a no slip boundary condition for a total microchannel
centerline length of 795 μm which is sufficient to produce a laminar flow pattern at the outlet. 相似文献
7.
For further understanding the dispersion process in the T-shaped microfluidic device, a double-pore T-shaped microchannel
was designed and tested with octane/water system to form monodispersed plugs and droplets in this work. The liquid–liquid
two-phase flow patterns were investigated and it was found that only short plugs, relative length L/w < 1.4, were produced. Additionally, the droplets flow was realized at phase ratios (F
C
/F
D) just higher than 0.5, which is much smaller than that in the single-pore T-shaped microchannels. A repulsed effect between
the initial droplets was observed in the droplet formation process and the periodic fluctuation flow of the dispersed phase
was discussed by analyzing the resistances. Besides, the effect of the two-phase flow rates on the plug length and the droplet
diameter was investigated. Considering the mutual effect of the initial droplets and the equilibrium between the shearing
force with the interfacial tension, phase ratio and Ca number were introduced into the semi-empirical models to present the plug and droplet sizes at different operating conditions. 相似文献
8.
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
9.
Correlations of droplet formation in T-junction microfluidic devices: from squeezing to dripping 总被引:2,自引:1,他引:1
In this work, we have systematically analyzed the scaling law of droplet formation by cross-flow shear method in T-junction
microfluidic devices. The droplet formation mechanisms can be distinguished by the capillary number for the continuous phase
(Cac), which are the squeezing regime (Cac < 0.002), dripping regime (0.01 < Cac < 0.3), and the transient regime (0.002 < Cac< 0.01). Three corresponding correlations have been suggested in the different range of Cac. In the dripping regime, we developed a modified capillary number for the continuous phase (Cac′) by considering the influence of growing droplet size on the continuous phase flow rate. And the modified model could predict
droplet diameter more accurately. In the squeezing regime, the final plug length was contributed by the growth and ‘squeeze’
stages based on the observation of dynamic break-up process. In the transient regime, we firstly suggested a mathematical
model by considering the influences of the above two mechanisms. The correlations should be very useful for the application
of controlling droplet size in T-junction microfluidic devices. 相似文献
10.
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
11.
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
12.
Recently, microbead generation and microencapsulation of cells using microfluidic technology have been actively pursued for
various applications. However, most of the proposed systems are not only technically demanding, but might also be harmful
to the encapsulated cells. To tackle these issues, this study reports a microfluidic alginate microbead generator consisting
of a polydimethylsiloxane (PDMS) microfluidic chip and an integrated quartz microcapillary tube. The working principle is
based on the use of a pulsed airflow to segment a continuous alginate suspension flow to form suspension fragments in a microchannel
and then alginate microbeads when they were delivered out the microfluidic system to a sterile calcium chloride solution through
a microcapillary tube. In this study, the alginate suspension fragments with varied sizes in the microchannel can be generated
either by modulating the alginate suspension flow rate or the pulsation frequency of airflow injection. By fine tuning the
size of them, the alginate microbeads can be generated in a size-controllable manner. Results showed that alginate microbeads
with the size ranging from 150 to 370 μm in diameter can be generated at the suspension flow rate and airflow injection frequency
ranges of 2–4 μl/min and 0.6–35 Hz, respectively. Besides, the alginate microbeads generated by the system were tested with
excellent size uniformity (CV: 3.1–5.1%). Moreover, its application for the microencapsulation of chondrocytes in alginate
microbeads was also demonstrated with high cell viability (96 ± 2%). As a whole, the proposed device has paved an alternative
route to perform alginate microbead generation or microencapsulation of cells in a simple, continuous, controllable, uniform,
cell friendly, and less contaminated manner. 相似文献
13.
Katerina Butron Fujiu Isao Kobayashi Kunihiko Uemura Mitsutoshi Nakajima 《Microfluidics and nanofluidics》2011,10(4):773-783
Microchannel (MC) emulsification is a promising technique to produce monodisperse emulsions by spontaneous interfacial-tension-driven
droplet generation. The purpose of this study was to systematically characterize the effect of temperature on droplet generation
by MC emulsification, which is a major uncharted area. The temperature of an MC emulsification module was controlled between
10 and 70°C. Refined soybean oil was used as the dispersed phase and a Milli-Q water solution containing sodium dodecyl sulfate
(1 wt%) as the continuous phase. Monodisperse oil-in-water (O/W) emulsions with a coefficient of variation below 4% were produced,
and at all the operating temperatures, their average droplet diameter ranged from 32 to 38 μm. We also investigated the effect
of flow velocity of the dispersed phase on droplet generation characteristics. The maximum droplet generation rate (frequency)
from a channel at 70°C exceeded that at 10°C by 8.1 times, due to the remarkable decrease in viscosity of the two phases.
Analysis using dimensionless numbers indicated that the flow of the dispersed phase during droplet generation could be explained
using an adapted capillary number that includes the effect of the contact angle of the dispersed phase to the chip surface. 相似文献
14.
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
15.
Numerical simulation of emergence of a water droplet from a pore into a microchannel gas stream 总被引:1,自引:1,他引:0
A numerical investigation on the dynamic behavior of liquid water entering a microchannel through a lateral opening (pore)
in the wall is reported in this paper. The channel dimensions, flow conditions and transport properties are chosen to simulate
those in the gas channel of a typical proton exchange membrane fuel cell (PEMFC). Two-dimensional transient simulations employing
the volume of fluid method are used to explicitly track the liquid–gas interface, and to gain understanding into the dynamics
of a water droplet subjected to airflow in the bulk of the microchannel. A series of parametric studies, including the effects
of static contact angle, dimensions of the pore, air-inlet velocity, and water-inlet velocity are performed with a particular
focus on the effect of hydrophobicity. The simulations show that the wettability of the microchannel surface has a major impact
on the dynamics of the water droplet. Flow patterns are presented and analyzed showing the splitting of a droplet for a hydrophobic
surface, and the tendency for spreading and film flow formation for a hydrophilic surface. The time evolution of the advancing
and receding contact angles of the droplet are found to be sensitive to the wettability when the gas diffusion layer surface
is hydrophilic, but independent of wettability when the surface is hydrophobic. The critical air velocity at which a droplet
detaches is found to decrease with increasing hydrophobicity and with increasing initial dimension of the droplet. The critical
air velocity found in the present study by taking into account the water transport and evolution of the droplet from a pore
are found to differ significantly from previous works which consider a stagnant droplet sitting on the surface. 相似文献
16.
Hirofumi Shintaku Takeo Kuwabara Satoyuki Kawano Takaaki Suzuki Isaku Kanno Hidetoshi Kotera 《Microsystem Technologies》2007,13(8-10):951-958
In this paper, we propose a cell encapsulation and hydrogel-beads production method using droplet formation in a microchannel. The hydrogel-beads produced by the microfluidic device developed here have smaller diameter and narrower distribution in their diameter compared to the conventional method, such as the droplet extrusion and the emulsification. The effects of the flow velocity and microchannel wall were analyzed based on fluid dynamical analysis. The results revealed that the wall effect of the microchannel strongly affected to the diameter of the droplet and the shape of the hydrogel-beads. 相似文献
17.
This study successfully uses the micro-mixers and flow-focusing devices, which are integrated into a gradient-microfluidic
droplet generator, to generate the different sizes of the droplets with different concentrations simultaneously and applies
these microcapsules for drug release. The sizes of these four types of droplet with different concentrations are uniformity
with a coefficient of variation less than 5% and can be precisely controlled by adjusting the water phase flow rate and oil
phase flow rate. Moreover, Ca-alginate microcapsules with different concentrations of the bovine serum albumin are used for
uniform size drug release, and the Ca-alginate microcapsule size is from 60 to 105 μm in diameter. This developed microfluidic
chip has the advantages of actively controlling the droplet diameter, simultaneously generating uniform-sized droplets with
different concentrations, and having a simple process and a high throughput. This preparation approach for Ca-alginate microcapsules
of four different concentrations will provide many potential applications for drug delivery and pharmaceutical area. 相似文献
18.
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
19.
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
20.
Timo Veijola 《Microfluidics and nanofluidics》2007,3(3):359-368
A compact model for oscillatory flow in a long microchannel with a circular cross-section is derived from the linearised Navier–Stokes
equations. The resulting two-port model includes the effects of viscosity due to rarefied gas in the slip flow regime, inertia,
compressibility and losses due to heat exchange. Both an acoustic impedance T network and an acoustic admittance Π network
are presented for implementation in system level and circuit simulation tools. Also, reduced T and Π networks with constant
component values are given to be used in the low frequency region. They are useful in time domain simulations, too. To verify
the analytical model, simulations with a harmonic finite element solver for acoustic viscous flow are performed for microchannels
exploiting the axisymmetry. The simulation results with both open and closed outlet conditions are compared with the two-port
model with excellent agreement. Contribution of the slip conditions and the accuracy of the simple model are demonstrated. 相似文献