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Research Papers: Heat and Mass Transfer

A Numerical Study on Electrowetting-Induced Droplet Detachment From Hydrophobic Surface

[+] Author and Article Information
Md Ashraful Islam

Department of Mechanical and
Aerospace Engineering,
University of Texas at Arlington,
Arlington, TX 76019
e-mail: mdashraful.islam@mavs.uta.edu

Albert Y. Tong

Department of Mechanical and
Aerospace Engineering,
University of Texas at Arlington,
Arlington, TX 76019
e-mail: tong@uta.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received November 15, 2016; final manuscript received October 1, 2017; published online February 21, 2018. Editor: Portonovo S. Ayyaswamy.

J. Heat Transfer 140(5), 052003 (Feb 21, 2018) (11 pages) Paper No: HT-16-1745; doi: 10.1115/1.4038540 History: Received November 15, 2016; Revised October 01, 2017

Electrowetting-induced microwater droplet detachment from hydrophobic surface has been studied numerically. The governing equations for transient microfluidic flow are solved by a finite volume scheme with a two-step projection method on a fixed computational domain. The free surface of the droplet is tracked by the volume-of-fluid method with the surface tension force determined by the continuum surface force (CSF) model. The static contact angle has been implemented using a wall-adhesion boundary condition at the solid–liquid interface, while the dynamic contact angle is computed assuming a fixed deflection from the static contact angle. The results of the numerical model have been validated with published experimental data and the physics of stretching, recoiling, and detachment of the droplet have been investigated. A parametric study has been performed in which the effects of droplet volume, voltage amplitude, and voltage pulse width have been examined.

Copyright © 2018 by ASME
Topics: Drops
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References

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Figures

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Fig. 1

Schematic of wall adhesion model and no-slip boundary condition

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Fig. 8

Energy analysis during recoiling and detachment

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Fig. 2

Schematic of experimental setup for electrowetting-induced droplet detachment [7]

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Fig. 3

Schematic of a DC square voltage pulse and corresponding contact angle

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Fig. 4

Numerical modeling of various static contact angles

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Fig. 5

Various stages of droplet detachment: (a) experimental [7] and (b) present numerical study

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Fig. 6

Base radius and apex height versus time during a 5 μL droplet detachment; numerical simulation at 135 V (left); experimental results [7] (right)

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Fig. 7

Pressure and velocity fields during droplet stretching

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Fig. 9

Pressure and velocity fields during droplet recoiling

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Fig. 10

Pressure and velocity fields during droplet detachment

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Fig. 12

Base radius at the instant of voltage release and maximum apex height versus pulse width (5 μL droplet with 116 deg/62 deg change in contact angle)

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Fig. 13

Pressure and velocity fields during droplet recoiling: (a) 4.6 ms pulse width (lower limit) and (b) 10.35 ms pulse width (upper limit)

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Fig. 14

Apex height and base radius versus time with 116 deg/62 deg change in contact angle. (Starting time shifted to the instant of voltage release.)

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Fig. 15

Droplet spreading spectrum for various droplet volumes: (a) without and (b) with normalization

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Fig. 17

Apex height versus time for various droplet volumes

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Fig. 18

Threshold voltage for droplet detachment for various droplet volumes

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Fig. 16

Effect of actuating contact angle on the droplet detachment process for a 5 μL droplet

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Fig. 19

Droplet detachment with various grid sizes

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Fig. 11

Droplet spreading spectra for a 5 μL droplet for various actuating contact angles

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