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Research Papers: Fundamental Issues and Canonical Flows

Droplet Detachment Mechanism in a High-Speed Gaseous Microflow

[+] Author and Article Information
Carlos Hidrovo

e-mail: hidrovo@mail.utexas.edu
The University of Texas at Austin,
Multiscale Thermal Fluids Laboratory,
Austin, TX 78712

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received Juy 27, 2012; final manuscript received March 14, 2013; published online May 17, 2013. Assoc. Editor: Ali Beskok.

J. Fluids Eng 135(7), 071206 (May 17, 2013) (8 pages) Paper No: FE-12-1348; doi: 10.1115/1.4024057 History: Received July 27, 2012; Revised March 14, 2013

This paper experimentally investigates the mechanism of water droplet detachment in a confined microchannel under highly inertial (10 < Re < 200) air flow conditions. Experimental observations show that as the Reynolds number of the continuous phase is increased, the droplet transitions from an elongated slug to a nearly uniform aspect ratio droplet. Supporting scaling arguments are then made that examine the relevant forces induced by the continuous phase on the droplet at the point of detachment. The inertial, viscous, and hydrodynamic pressure forces that result as the air flow is confined in the small gap between droplet and channel walls are compared to the surface tension force pinning the droplet at the injection site. The results indicate that the dominant detachment mechanism transitions from the hydrostatic pressure difference to inertial drag as the continuous phase velocity is increased.

Copyright © 2013 by ASME
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References

Figures

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Fig. 1

Water droplets detaching into oil flow (upper left) and changes in water droplet size for increasing water to oil flow rate ratios (lower left) [20]. Note that a thin oil film separates the droplet from the channel walls. Water droplet detaching into gaseous flow (upper right) and changes in water droplet size for decreasing gas Re number (lower right). Even for small air Re numbers, the detached droplet never fills the channel width as compared to the oil-based flow and is in direct contact with the channel walls.

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Fig. 2

CAD image (not to scale) of the gas-liquid microfluidic device used for droplet detachment and droplet collisions studies. Four different devices were tested with the following aspect ratio: 1.75, 2.0, 2.0, and 4.25.

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Fig. 3

Process flow for PDMS soft lithography: mask CAD design, photolithography, molding, bonding, and fluid and gas port integration

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Fig. 4

Schematic of hardware and instruments for gas and liquid flow control: facility compressed air supply (1), desiccant dryer (2), submicron particulate filter (3), rough pressure regulator (4), pressure regulators with transducer feedback control (5), sealed liquid reservoir containing reactants to be mixed (6), mass flow sensors (7), miniature inert solenoid valve (8), and microfluidic device under test (DUT)

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Fig. 5

Experimental data of detached droplet length versus air Reynolds number for three different channel aspect ratios and four different devices

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Fig. 6

Experimental data of detached droplet height versus air Reynolds number for three channel aspect ratios (channel height / channel depth) and four different devices

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Fig. 7

Experimental data showing how the ratio of droplet length to height decreases with increasing air Reynolds number for three different channel aspect ratios and four different devices

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Fig. 8

Experimental data of detached droplet volume versus air Reynolds number for three different channel aspect ratios and four different devices

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Fig. 9

High speed camera images of droplet geometry for different gas Reynolds numbers (ReDh) for a channel 100 μm high and 50 μm deep

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Fig. 10

High speed camera images of droplet growth and detachment for ReDh = 118

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Fig. 11

Schematic of rectangular microchannel with a detached droplet/slug on the lower wall. The channel dimensions are: height, H, and depth, W. The droplet dimensions are height, h, and length, l. The Cartesian coordinate system used for the detachment analysis is also indicated.

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Fig. 12

Experimental data of detached droplet length versus air Reynolds number for three different channel aspect ratios and four different devices. Data has been nondimensionalized by channel hydraulic diameter.

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Fig. 13

Experimental data of detached droplet height versus air Reynolds number for three different channel aspect ratios and four different devices. Data has been nondimensionalized by channel hydraulic diameter. Note that the droplet height ratio can exceed unit using this nondimensionalization.

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Fig. 14

Plot showing how the ratio of gas-phase inertial force to droplet surface tension, WeMod, changes with increasing ReDh. The ratio exceeds unity past a ReDh 80 and indicates a transition to inertial droplet detachment.

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Fig. 15

Plot showing how the ratio of the imposed gas-phase viscous force to droplet surface tension, CaMod, changes with increasing ReDh. The ratio is less than unity for all channel aspect ratios considered.

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Fig. 16

Plot showing how the ratio of pressure drop across the droplet to the surface tension securing the droplet at the injection site changes with increasing ReDh. The ratio is largest for the channel with the smallest aspect ratio and decreases for all channel sizes with increasing ReDh.

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Fig. 17

Comparison of experimental data showing the viscous, pressure, and inertial detachment ratios for the 100 × 50 μm channel. The detachment mechanism transitions from pressure to inertial near ReDh of 100. The still images of detached droplets shown in the lower portion of the figure were captured at the indicated Reynolds number indicated and clearly show how droplet shape transitions from elongated slugs to more spherical droplets with increasing gas-phase inertia.

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