Abraham P Lee

John Collins

Yung-Chieh Tan

CiteSeerX

DownloaProceedings of IMECE2003 
ASME International Mechanical Engineering Congress and RD&D Expo 
November 15-21, 2003 Washington, D.C. 
IMECE2003-41983 
 
Optimization of Shear Driven Droplet Generation in a Microfluidic Device 
 
John Collins 
Department of Biomedical 
Engineering 
University of California at Irvine 
Irvine, CA. 
Yung-Chieh Tan 
Department of Biomedical 
Engineering 
University of California at Irvine 
Irvine, CA. 
Abraham P. Lee 
Departments of Biomedical Engineering and 
Mechanical & Aerospace Engineering 
University of California at Irvine 
Irvine, CA. 
 
Proceedings of IMECE’03 
2003 ASME International Mechanical Engineering Congress 
Washington, D.C., November 15–21, 2003 
 
 
IMECE2003-41983 
 
ABSTRACT 
 
Hydrophilic and lipophilic interfaces of fluids 
play an important role in the formation of 
droplets. A large collection of droplets constitutes 
emulsions of water dispersive phase into oil 
continuous phase. Since droplet generation forms 
the basis of the manufacturing of emulsion, great 
efforts have been made to understand the 
science, technological and industrial problems 
associated with the generation of droplets. This 
paper presents the optimization of a novel method 
of droplet generation [1] in a microchannel 
resulting from the laminar co-flow of water and oil 
in a T type channel. Water in oil droplets are 
formed with olive oil (interfacial tension 28mN/m, 
viscosity 84mPa/s, density 918Kg/m3). At the T- 
junction, the water stream sent through the 
middle channel is sheared and cut by the oil 
stream sent through the outer channel. 
Competition between interfacial tension and the 
Laplace pressure at the oil/water interface results 
in droplets of finite diameter. Fluid properties such 
as density, viscosity and surface tension and the 
flow parameters such as pressure, mass flow rate 
and velocity are varied at the inlets and outlets to 
optimize size, frequency and periodicity of 
droplets using CFD-ACE+, a multiphysics modeling 
tool (CFDRC, Huntsville, AL) 
 
INTRODUCTION 
 
Microdroplets have been generated over the last decades 
primarily through microporous membrane[1] and jet pinch 
off[2]. The advancement of microfluidics led to form droplets 
at the microchannels[3]. Formation of droplets with coflow of 
oil and water is reported[4]. At the T- junction, the water  
ded From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of Ustream sent through the middle channel is sheared and cut by 
the oil stream sent through the outer channel. Competition 
between interfacial tension and the Laplace pressure at the 
oil/water interface results in droplets of finite diameter. 
Symmetrically sheared droplets at the T shaped junction are 
generated and manipulated[5]. The paper is directed towards 
design and optimization of droplets generated in the 
microfluidic T channel using CFD simulation. 
NOMENCLATURE 
F Liquid volume fraction 
T Time 
V velocity 
P Pressure 
ρ Density 
Q1 Flowrate of oil phase 
Q2 Flow rate of water phase 
σ Interfacial tension 
θ Contact angle 
 
METHOD 
 
For two immiscible incompressible fluids, the liquid volume 
fraction, F evolves in the direction of the velocity, v and is 
determined by solving the following passive transport equation 
along with the Navier-Stokes Equation. 
 
 
 
vvPvv
t
v rrrrr 21)( ∇+∇−=∇⋅+
∂
∂
ρ
 
0=•∇+
∂
∂ Fv
t
F r
Volume of Fluid method[6] is used for tracking the interface 
implemented with ‘piecewise linear interface calculation’ for 
surface reconstruction using CFD-ACE+, a multiphysics 
modeling tool[7] (CFDRC, Huntsville, AL). A 2-dimensional 
flow domain was used in the simulation with orthogonal 
meshing. The boundary condition is set by fixing mass flow 
rate at the channel inlets. The outlet is set to a constant pressure 1 Copyright © 2003 by ASME 
se: http://www.asme.org/about-asme/terms-of-use
 
Downlosame as atmospheric pressure. Water in oil droplets are formed 
with olive oil (interfacial tension 28 mN/m, viscosity 84 mPa/s, 
density 918 Kg/m3). Figure 1 shows the meshes at the junction 
of inlet and outlet channels. The mesh size is optimized so that 
the each simulation is completed within 20 hours in a P4 
/3.06GHz /1GB DELL computer keeping the good accuracy. 2-
d axisymmetric case is considered in the simulation. 
Optimization of flow rates of oil and water, interfacial surface 
tension, and geometry of the microfluidic device are considered 
in this paper. 
Fig 2. CFD Simulation of Droplets of different sizes 
generation  
(a) Q1=23e-3 kg/s and Q2=6e-4 kg/s 
(b) Q1=7e-3 kg/s and Q2=6e-4 kg/s 
(c) Q1=5e-3 kg/s and Q2=4e-4 
 
RESULTS AND DISCUSSION 
 
Droplet generation 
 
Figure 2 shows the simulation of droplets at a volumetric flow 
rate of 0.1µl/min (mass flow rate=5e-4Kg/sec) of oil in the 
outer channels and 0.833µl/min (mass flow rate=6e-4Kg/sec) of 
water in the inner channel. The droplets are periodic and are 
sensed by measuring the disturbance due to the droplets flow 
that occurs in the flow rate at the outlet. The flow rate produces 
a spike as every drop flows as given in Fig. 3a. The peaks are 
picked and the corresponding time values are replotted for 
every drop. The plot is a straight line as seen in Fig. 3b and the 
slope is the time for the generation of a single drop. The radius 
of the drop can be calculated from this time. 
 
 
 
 
 
 
Optimization of Flow Properties 
 
 Flow in the outer and inner channels are adjusted with 
mass flow rates. The flow rate of oil (Q1) in the outer channels 
are kept constant and the flow rate (Q2) of water in the inner 
channel is varied in the in the range realized in the experiment. 
Similarly simulations are carried out with Q2 constant and 
varying Q1. The time for the generation of a single drop is 
plotted against the flow rates as given in Fig.4. Figure 4a has a 
double exponential decay of time as the flow rate of water 
increases. It is noted that the discontinuity in Fig 4b is due to 
Raleigh’s instability [8]. Very fine droplets with times of the  
aded From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of UseFig 3. Droplets flow detected and measured from the flow 
rate at the outlet channel. a) Flow rate calculated from the 
start of the experiment. b) The corresponding time of the 
peaks of the flow rate graph is plotted for every drop. The 
slope of the straight line fit gives the time for generation of 
a single drop. 
order of microseconds are generated at the discontinuous 
window. The fine droplets due to Raleigh’s instability are 
observed in Fig 5.  
0.0 0.1 0.2 0.3 0.4
-7.0x10-6
-6.5x10-6
-6.0x10-6
-5.5x10-6
 
 
Fl
ow
 R
at
e 
(m
3 /s
ec
)
Time (sec)
 
a
 
0 2 4 6 8 10 12 14 16 18
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
Y= 0.05362 + 0.02174 X
 
 
Ti
m
e 
fo
r s
uc
ce
ss
iv
e 
D
ro
pl
et
s 
(s
ec
)
Droplet Number
 
b
 
 
0.0000 0.0002 0.0004 0.0006
0.2
0.4
0.6
0.8
1.0
 
 
Ti
m
e 
(s
ec
)
Q2 (Kg/sec)
Q1=5E-3 Kg/sec a2 Copyright © 2003 by ASME 
: http://www.asme.org/about-asme/terms-of-use
 
Downloa0.005 0.010 0.015
0.005
0.010
0.015
0.020
0.025
0.030
0.035
 
 
Ti
m
e 
(s
ec
)
Q1 (Kg/sec)
Q2=6E-4 Kg/sec
 
Fig 4. Time of a droplet generation with 
various flow rates of (a) water and (b) oil  
0.010 0.012 0.014 0.016 0.018 0.020
-0.0000235
-0.0000230
-0.0000225
-0.0000220
-0.0000215
 
 
Fl
ow
 R
at
e 
(m
3 /s
ec
)
Time (sec)
 
Fig 5. Fine Droplets due to Raleigh's Instability.  The 
droplet generation time is 0.2 msec 
 
 
 
 
 
 
 
Optimization of Fluid Properties 
 
Droplets of fine sizes are generated by adding surfactants to the 
oil phase and so by reducing the interfacial tension (σ) of 
oil/water. Simulations predicted that the time per droplet 
generation exponentially reduces with the decreases of 
interfacial tension as shown in Fig. 5a. Most of the other 
simulations are done for olive oil whose interfacial tension with 
water is 22.8 mN/m and oleic acid (15.6 mN/m). On the other 
hand, increase of contact angle (θ) of water/oil/wall interface 
reduces the droplet generation time as presented in Fig. 5b. At 
high value of contact angle the time goes to saturation. Most of 
the other simulations are carried out with wall made of PDMS 
material (the contact angle is ~110o for oil/water/PDMS).  
 
Optimization of Geometry 
 
 In order to optimize the geometry, the channel width 
of the outer channel at the junction and the angle of expansion 
of the outer channel are varied (See Fig. 7). Figure 6a shows a 
b  
ded From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of UsFig 6. Time of a droplet generation with various (a) 
interfacial tension of water/oil and contact angle of 
air/water/wall interface. 
pair of straight lines with slopes 0.595 and 3.11 sec/mm, (one is 
5 times that of the other). Beyond 25 um, fine droplets are 
generated due to Raleigh’s instability. The angle of expansion 
of the outlet channel shows (Fig 6b) an oscillatory behavior 
with the droplet generation time. The oscillation has a period of 
1.66º and the amplitude of the oscillation decays exponentially 
as seen in the inserted figure. At larger angles of expansion 
droplet fusion take place along the length of the channel. 
However, the droplet generation time changes slightly with the 
angle of expansion of the channels. 
 
18 20 22 24 26 28 30 32 34 36
0.028
0.030
0.032
0.034
0.036
0.038
0.040
0.042
0.044
0.046
 
 
Ti
m
e 
(s
ec
)
Interfacial tension (mN/m)
Q1= 5E-3 Kg/sec
Q2=6E-4 Kg/sec
contact angle=30o
0 20 40 60 80 100 120 140 160
0.015
0.020
0.025
0.030
0.035
0.040
0.045
Q1= 5E-3 Kg/sec
Q2= 6E-4 Kg/sec
Interfacial Tension=22.8 mN/m
 
 
Ti
m
e 
(s
ec
)
Contact Angle (Deg)
 
a
b
 
5 10 15 20 25
0.015
0.020
0.025
0.030
0.035
0.040
Q1= 5E-3 Kg/sec
Q2= 6E-4 Kg/sec
 
 
Ti
m
e(
se
c)
Width of the outlet channel at the junction (µm)
a 3 Copyright © 2003 by ASME 
e: http://www.asme.org/about-asme/terms-of-use
DownFig 6. Optimization of the channel width of the 
outer channel at the junction and the angle of 
expansion of the outer channel 
0 1 2 3 4 5
0.020
0.021
0.022
0.023
0.024
0.025
0.026
1 2 3
0.0215
0.0220
0.0225
0.0230
0.0235
0.0240
0.0245Q1= 5E-3 Kg/sec
Q2= 6E-4 Kg/sec
 
 
Ti
m
e 
(s
ec
)
Angle of expansion of the outlet channel (Deg)
P
ea
k 
am
pl
itu
de
Peak Number
 
b 
 
 
 
 
 
Experimental Verification 
 
In order to realize the droplet generation, T channel is 
fabrication in PDMS channel is fabricated using SU-8 molds 
and is sealed against a microscopic glass slide through oxygen 
plasma binding [9].  The inlets are attached with plastic tubing 
connected to syringes loaded with water and oleic acids.  The 
water is dyed with napthol green to indicate the water phase 
under light microscopy.  The syringes inject at constant flow 
rates controlled by three Picoplus Syringe pumps from Harvard 
Apparatus.  The outlet flow in the PDMS channel is sucked by 
vacuum at atmospheric pressure. The variation in flow rates of 
water and oleic acids is changed in steps and the corresponding 
video microscopic images are recorded at 500 to 10,000 
frames/sec using a Photran camera. The radii of the droplets are 
measured using ‘NIH image’ software from the recorded 
images and are showed in Fig. 7. The sizes of droplets 
generated at various oil flow rates are presented in  Fig 8. 
 
 
 
 
 
 
 
Fig 7. Droplet Generation Experiments  
(a) Q1=1µl/min, Q2=15µl/min, camera speed=5000 fps  
(b) Q1=1µl/min, Q2=5µl/min, , camera speed 2000 fps 
(c)  Q1=1µl/min, Q2= 1µl/min, caera speed =2000 fps
 
loaded From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of Us0
10
20
30
40
0 5 10 15 20 25
Oil flow rates (microliter / min)
ra
di
us
 o
f d
ro
pl
et
s 
(m
ic
ro
m
et
er
)
 
 
 Fig 8. Size of the droplets generated at various 
oil flow rates 
CONCLUSION 
 
These results predict the flow, fluid and geometry parameters 
for large number of small sized droplets through CFD 
simulation. The time of a droplet generation is proportional to 
the radius of the droplets. The size variation of the generated 
droplets using a particular dimension of the geometry can be 
found out from this study. The correlation of geometry 
parameters to the flow and fluid parameters of the microfluidic 
droplet generator is analyzed. Qualitative and quantitative 
verification of different sizes of droplets generation is presented 
with experiment using a microdevice. This study also leads to a 
wide area of research understanding the Raleigh’s instability of 
the nonlinear fluid dynamics in microsystem.  
 
ACKNOWLEDGMENTS 
We acknowledge CFDRC for supplying the multiphysics 
simulation tool CFD-ACE+ and UCI for funding. 
REFERENCES 
 
1 V.Schroder, O.Behrend, H. Schubert, Effect Of Interfacial 
Surface Tension On The Emulsification Process Using 
Microporous, Ceramic Membranes.  J. Colloids Int. Sci, 
202, 334 (2002). 
2 D.R.Webster, E.K.Longmire, Jet Pinch-Off And Drop 
Formation In Immiscible Liquid-Liquid Systems, 
Experiments in Fluids, 30, 47 (2001). 
3 S.P. Swierkowski, A Micromachined Droplet Dispenser for 
Biofluidics, Biomedical Devices, 4, 55 (2002). 
4 Thorson, T., et al., Dynamic Pattern Formation in a Vesicle-
Generating Microfluidic Device. Physical Review Letters, 
2001. 86(18): p. 4163-4166. 
5 C. W. Hirt, B. D. Nichols, Volumer of Fluid (VOF) Method 
for the Dynamics of Free Boundaries, J. Computational, 
Physics, 39, 201 (1981). 
6 Y.C. Tan, J. Collins, A.P. Lee, Controlled Fission of Droplet 
Emulsion in Bifurcating Microfluidic Channels, Submitted 
to µTAS 2003, Lake Tahoe. 
7 Module Manual, CFD-ACE+, CFDRC, Huntsville, 
AL. 
8 Lord Raleigh, ‘On the instability of jets’, Proc. 
London Mathematical Society, 10, 4 (1878). 
9  McDonald, J. C., D. C. Duffy, et al. (2000). 
"Fabrication of microfluidic systems in 
poly(dimethylsiloxane)." Electrophoresis 21(1): 27-40. 4 Copyright © 2003 by ASME 
e: http://www.asme.org/about-asme/terms-of-use


Optimization of Shear Driven Droplet Generation in a Microfluidic Device

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Optimization of Shear Driven Droplet Generation in a Microfluidic Device

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