Electronic Supplementary Material for Microfluidics and Nanofluidics
SUPPLEMENTARY MATERIAL
On-demand, parallel droplet merging method with
non-contact droplet pairing in droplet-based microfluidics
Sanghyun Lee1 · Hojin Kim1 · Dong-Joon Won1 · Jaehyung Lee2 · Joonwon Kim1
1 Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH),
San 31 Hyoja-dong, Nam-gu, Pohang, Kyungbuk, 790-784, Republic of Korea
2 Stratio, Inc., 998 Hamilton Ave., Menlo Park, CA 94025
Joonwon Kim (Corresponding author)
Tel: +82-54-279-2185
Fax: +82-54-279-2960
E-mail: Joonwon@postech.ac.kr
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Electronic Supplementary Material for Microfluidics and Nanofluidics
S1. Characterization of the effect of the front neck width and droplet diameter on the
required suction pressure to droplet trap or squeeze
Effect of front neck width on the suction pressure required to squeeze a droplet
As expected from Eq. (2), the net Laplace pressure increases as the front neck width decreases. The reason is that the
smaller neck width generates a greater difference in the Laplace pressure between the front and back of the droplet.
Thus, a higher net hydrodynamic pressure (corresponding to suction pressure) is required to squeeze the droplet into
a smaller trap entrance. The oil and aqueous phases were mineral oil containing 5% (w/w) Span 80 and Dulbecco’s
phosphate buffered saline (DPBS), respectively. A droplet diameter of 70 μm was chosen and the interfacial tension
was measured to be approximately 2.2 mN/m using a SmartDrop_Lab HS (Femtofab Co., Ltd.). The net Laplace
pressure was calculated for different front neck widths (30 μm, 35 μm, 40 μm, 45 μm, and 50 μm) and the suction
pressures required to squeeze the droplets were characterized (Fig. S1). It can be seen from Fig. S1 that a higher
suction pressure is required to squeeze a droplet to pass through a front neck with smaller front neck width, which is
consistent with the expectation. While maintaining the main geometry of the merging device, the range of suction
pressures required to squeeze a droplet of 70 μm diameter was from 8.5±0.5 kPa (for a 50-μm-wide trap) to 52.67±2.05
kPa (for a 30-μm-wide trap).
Fig. S1 Calculated net Laplace pressure and experimental values of the suction pressure required to squeeze droplets for different
front neck widths.
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Electronic Supplementary Material for Microfluidics and Nanofluidics
Effect of droplet diameter on the suction pressure required to squeeze a droplet
The effect of droplet diameter on required suction pressure was characterized while keeping the oil-water system
constant. The suction pressure required to squeeze the droplets was investigated by varying the droplet diameter (60
μm, 70 μm, and 80 μm) at a front neck width of 40 μm (Fig. S2). As expected from Eq. (2), the net Laplace pressure
is greater for a larger droplet diameter. For the same front neck width, a larger droplet has a greater difference between
the radii of curvature at the front and back of the droplet, which results in a higher net Laplace pressure. Therefore, a
higher suction pressure is required to squeeze the larger droplet; this trend can be seen in Fig. S2. The range of suction
pressure required to squeeze the droplets was from 19.67±1.25 kPa (for a droplet diameter of 60 μm) to 31.67±2.05
kPa (for a droplet diameter of 80 μm).
Fig. S2 Calculated net Laplace pressure and experimental values of suction pressure required to squeeze droplets for different
droplet diameters.
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Electronic Supplementary Material for Microfluidics and Nanofluidics
S2. Design of droplet generation device
The droplet generation device (flow focusing geometry) was fabricated using soft lithography. All droplets used in
this study (DPBS, KSCN, FeCl3) were prepared using this device.
Fig. S3 Design of droplet generation device. Droplets are prepared in the flow-focusing device using two immiscible fluids; oil
phase and aqueous phase are supplied by syringe pumps. Generated droplets are collected at the outlet of the device and re-injected
into the droplet merging device
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Electronic Supplementary Material for Microfluidics and Nanofluidics
S3. Experimental setup
The experimental setup consisted of a droplet generation device, droplet merging device, pneumatic pressure supply
system, and syringe pump. Generated droplets were collected, re-injected into the droplet merging device, and then
transported toward downstream by a pneumatic pressure supply system. The pneumatic pressure supply system
comprised a diaphragm pump, pressure monitor, and regulator. A PC-controllable solenoid valve was integrated to
enable precise control of the pressure amplitude and duration.
Fig. S4 Experimental setup for this study
References
Anna SL, Bontoux N, Stone HA (2003) Formation of dispersions using “flow focusing” in microchannels Applied
Physics Letters 82:364-366
Xia Y, Whitesides GM (1998) Soft lithography Annual Review of Materials Science 28:153-184
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