1. Y. Murakami1, T. Arakawa1, E. H. Jeong2, J. S. Go2 and S. Shoji1
HIGH GENERATION RATE OF UNIFORM OIL-IN-WATER DROPLETS FORMED BY MULTI-STAGE DIVERGENCE MICROFLOW DEVICE
Y. Murakami1, T. Arakawa1,
E. H. Jeong2, J. S. Go2 and S. Shoji1
1 Major in Nano-science and Nano-engineering, Waseda University
Ohkubo Shinjuku, Tokyo, Japan
2School of Mechanical Engineering, Pusan National University
Instructor: Professor 劉承賢 CHENG-HSIEN LIU
Reporter : 葉致成 CHIH-CHENG YEH
TRANDUCERS & EUROSENSORS’07
The 14th international Conference on Solid-State Sensor. Actuator and Microsystems, Lyon. France. June 10-14,2007

2. methods to generate the smaller size of droplets
Flow focusing geometry.
Geometrically mediated breakup of droplets.
Before I introduce this paper. I will show some methods to generate the smaller size of droplets .

3. Flow focusing geometry
Parameters
The depth of the channels
h=117 μ m
Water flows in the central channel, Wi=197 μ m
Oil flows in the two outer channels, Wo=278 μ m.
The width of the orifice .
D=43.5 μ m.
Distance downstream of three coaxial inlet streams.
Hf=161 μ m
The is structure has three inlet and one orifice.
They used a cross-junction channel ,in which the dispersed water flow and continuous oil flow are focused into narrow orifice,
And also a satellite droplet.
The droplet size depends on the flow rate and the flow rate ratio.
Flow-focusing geometry implemented in a microfluidic device
Satellite droplet

4. Flow focusing geometry
Qo :flow rate of the outer fluid (oil).
Qi :the flow rate of the inner fluid
(Water).
Qi/Qo:The flow rate ratio.
Such collisions can occur at low
flow rates when the droplets cannot travel downstream
quickly enough that will let the droplets combine together.
The vertical axis is the flow rate of the outer fluid. The transverse axis is the flow rate ratio.
This condition can reduced by appropriate use of surfactants to stabilize the droplets. Or generally corresponding to higher oil flow rates and lower flow ratio , just like the figure (g) to (h)
When the oil flow rate become larger and the flow rate ratio become lower, the droplets generation rate will increase and the droplets size get smaller.

5. Geometrically mediated breakup of droplets
There is a critical flow rate which the drops always break into two daughter droplets.
The length of l1 and l2 , control the relative flow resistance of the paths and, hence, the relative sizes of the two daughter drops.
q1/q2 ≒l2/l1 ≒V1/V2
Water droplet
l1:l2=1:1
l1:l2=1:5.2
l1:l2=1:8.1
This structure separated droplets into two daughter droplet by using the T-junction passive breakup.
A significant advantage of T-junction passive breakup is that it can be modified to facilitate asymmetric breakup
of droplets.
It can also use in multi-stage structure to generate smaller droplets.

6. Geometrically mediated breakup of droplets
a:b =1:1.2
a:b=1:2.7
When a two-layer configuration of droplets
encounters an obstacle placed off center, only drops in one of the layers breaks, resulting in a regular sequence of three different sizes of drops.
The channel width is 120 μm.
The obstacle is a 60 μm square.
This is another type of droplet breakup .An obstruction is placed downstream of the region where drops are first formed.
The transverse placement of the obstacle, indicated by the distances a and b from each
sidewall, allows control over the degree of breakup and
the relative size distribution of the resulting dispersion.

7. MULTI-STAGE DIVERGENCE MICROFLOW DEVICE
The depth of the microchannel was 40 μm.
In this paper , the author combined above two methods to make this device. The channel had three inlet and one outlet.
And It has a mediated breakup at each stage.

8. Flow velocity difference by the channel resistance is not observed by using these structures. In other words, these structures enable the same inlet pressure down stream of the bifurcation point in each channel and uniform droplet separation is obtained.
Between a bifurcation point and the next point, the width of the microchannel
was extended in order to ensphere the droplet. The droplet moved from oval shape to sphere.

9. RESULTS
At the fixed organic phase flow condition (3μl/min), the generation rate became larger when the water flow rate is increased.
By using four series separation point and optimally designed microchannels, that
achieved high generation rate with dividing one
droplet into 16 pieces.

10. RESULTS
The left-hand side figure shows droplets size and deviation at each position of micro flow channel. The droplets size become smaller and smaller.
The deviation in each bifurcation point was smaller than 3%.
And look at the right-hand side figure . The vertical axis is generation rate and droplet size ,the transverse axis is water phase flow rate.
The generation rate increased with water phase flow rate. And the droplets size decrease with water phase flow rate.
Water phase flow rate : 9 μl/min
Organic phase flow rate:3 μl/min

11. Conclusions
High generation rate of uniform water-in-oil droplets was realized by multi-stage divergence microflow device.(Max generation rate was 1760 droplets/sec ,and the average diameter of the droplets was 37 μm)
This device can be applied to product microbeads and microcapsules which used in drug delivery.

12. Reference
[1] S. L. Anna, N. Bontoux, and H. A. Stone, "Formation of dispersions using “flow focusing” in microchannels," Applied Physics Letters, vol. 82, pp , 2003.
[2] D. R. Link, S. L. Anna, D. A. Weitz, and H. A. Stone, "Geometrically Mediated Breakup of Drops in Microfluidic Devices," Physical Review Letters, vol. 92, p , 2004.

13. THANKS FOR YOUR ATTENTION.