Download Experimental Setup Abstract Graduate Category: Engineering and Technology

Graduate
Category: Engineering and Technology
Degree Level: Ph.D.
Abstract ID# 1343
Multiscale Thermal Fluids Laboratory
Pooyan Tirandazi – Carlos H. Hidrovo
Abstract
Over the last few years, microfluidic systems known as Lab-on-a-Chip (LOC) and
micro total analysis systems (μTAS) have been increasingly developed as essential
components for numerous biochemical applications. Droplet microfluidics,
however, provides a distinctive attribute for delivering and processing discrete as
well as ultrasmall volumes of fluid, which make droplet-based systems more
beneficial over their continuous-phase counterparts. Droplet generation in its
conventional scheme usually incorporates the injection of a liquid (water) into a
continuous immiscible liquid (oil) medium. In this study we demonstrate a novel
scheme for controlled droplet generation in confined gas-liquid microflows. We
experimentally investigate the manipulation of water droplets in flow-focusing
configurations using a high inertial air stream. Different flow regimes are observed
by varying the gas and liquid flow rates, among which, the “dripping regime”
where monodisperse droplets are generated is of great importance. The controlled
size and generation rate of droplets in this region provide the capability for precise
and contaminant-free delivery of microliter to nanoliter volumes of fluid.
Furthermore, the high speed droplets generated in this method represent the basis
for a new approach based on droplet pair collisions for fast efficient micromixing
which provides a significant development in modern LOC and μTAS devices.
Introduction
Conventional droplet microfluidics usually rely on contact of two immiscible
liquids (e.g. water in oil)
Liquid-in-gas droplet microfluidics present a lot of new perspectives for
aerosol applications and aerobiology.
A novel method is presented which enables the generation of monodisperse
droplets in a high-speed gaseous microflow and subsequent collection of the
droplets in a second liquid medium after being generated in the gas phase.
The high speed droplets generated in this method, in contrast to
conventional liquid-in-liquid generation, represent the basis for a new
micromixing approach based on droplet pair collisions for implementation in
modern LOC and μTAS devices.
Experimental Setup
Fig. 2 Schematic of the
experimental setup used for
droplet generation and
subsequent collection of the
drops. Air is controlled and
regulated in multiple steps
previous to entering the
microfluidic chip inlets. Outlet
of the chip is immersed in an oil
bath composed of Hexadecane
mixed with 2.5wt% of a
nonionic surfactant (span 80).
Fig. 6 Images of the collected droplets are captured and a set of image
processing techniques are performed in order to obtain the droplet sizes
present in each image.
B
A
Results & Discussion
Fig. 3 Microchannel configuration used
for formation of droplets in air. A flowfocusing junction is utilized with liquid
stream in the middle and two gas
streams on the sides. The height of all
the channels are 40 µm.
Fig. 7 The two generation mode which occur within the dripping regime. Liquid flow rate for both
cases is 1µL/min A) Monodisperse generation mode. In this mode by having a constant liquid and
gas flow rate the size of the generated droplets are almost the same. Gas Reynolds number this
mode is 150. B) Satellite generation mode. In this mode small daughter droplets are generated
beside the main droplet which results in polydisperse distribution of the droplets. Gas Reynolds
number in this mode is 300.
Conclusion & Future Works
Fig. 4 Representation of the flow regimes observed in the experiments. The above images were taken
with the speed of 10000 fps for the constant water flow rate of 1 µl/min. As the air flow rate increases
three different patterns are distinguished which are (a) co-flow regime (Re~10), (b) jetting regime
(Re~30), and (c) dripping regime (Re~100)
A novel method has been presented for the first time for microfluidic
generation of monodisperse droplets in air down to 50 µm in diameter.
We are actively working on synchronized generation of airborne particles
and subsequent processing of the droplets all in an integrated microfluidic
device.
Chip 1
Chip 2
LW
LG
LO
H
40 µm
40 µm
100 µm
80 µm
180 µm
120 µm
40 µm
40 µm
Fabrication Process
Fig. 1 Process flow for fabrication of microfluidic chips. A photomask containing the microchannels
features is designed using a CAD software. This mask is used in Photolithography process with SU-8 to
fabricate a Silicon master mold. The fabricated mold is then used in standard soft lithography process
with PDMS. After casting the PDMS solution on the mold, each microfluidic chip is peeled off the mold.
Required holes are punched and the chip is bonded to a pre-cleaned glass slide using a plasma cleaner.
Fig. 5 (Top) Actual images of droplets moving inside the microchannel and their size difference
for a range of gas Reynolds numbers corresponding to the dripping regime. For the same rate of
liquid input, increasing gas Reynolds number result in smaller droplets. (Bottom) Experimental
data of detached droplet size for different liquid flow rates and gas Reynolds numbers for a
channel with aspect ratio of 1.25. Data has been nondimensionalized by the channel hydraulic
diameter.
Fig. 7 Actual images and
size distributions of
collected droplets for
two microfluidic chips
with the dimension
shown in Table. Liquid
flow rate and gas flow
rate for both cases are
2µL/min and 15mL/min
respectively. In both
cases more than %60 of
the drops have the
same average diameter
which are 86µm and
54µm for each case.
References
1. S.-Y. Teh, R. Lin, L.-H. Hung, and A. P. Lee, “Droplet microfluidics.,” Lab Chip, vol. 8, pp. 198–220, 2008.
2. B. Carroll and C. Hidrovo, “Droplet collision mixing diagnostics using single fluorophore LIF,” Exp. Fluids, vol. 53, no. 5, pp.
1301–1316, Aug. 2012.
3. J. H. Xu, S. W. Li, J. Tan, Y. J. Wang, and G. S. Luo, “Preparation of highly monodisperse droplet in a T-junction microfluidic
device,” AIChE J., vol. 52, no. 9, pp. 3005–3010, Sep. 2006.
4. P. Garstecki, M. J. Fuerstman, H. a Stone, and G. M. Whitesides, “Formation of droplets and bubbles in a microfluidic Tjunction-scaling and mechanism of break-up.,” Lab Chip, vol. 6, pp. 437–446, 2006.
This project is currently being supported by an NSF CAREER Award grant CBET- 1151091.