Research Papers: Flows in Complex Systems

Investigation of the Effect of Geometric Parameters on EWOD Actuation in Rectangular Microchannels

[+] Author and Article Information
Sajad Pooyan

Department of Mechanical Engineering,
Ferdowsi University of Mashhad,
Mashhad 9177948974, Iran
e-mail: sajad.pooyan@mail.um.ac.ir

Mohammad Passandideh-Fard

Department of Mechanical Engineering,
Ferdowsi University of Mashhad,
Mashhad 9177948974, Iran
e-mail: mpfard@um.ac.ir

1Corresponding author.

2The general term “droplet-based microfluidic” system emphasizes the use of discrete and distinct volumes of liquids in contrast with the continuous nature of other systems. In the literature, sometimes the term “digital microfluidics system” is used to refer to the systems in which circular microdroplets sandwiched between two parallel plates are manipulated whereas the term “droplet microfluidics” is used for the systems in which discrete volumes of liquids are transported in microchannels of circular or rectangular cross section. However, as this may cause confusion among readers, we use “droplet-based microfluidic systems” to refer to the entire field.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received November 2, 2017; final manuscript received February 20, 2018; published online April 19, 2018. Assoc. Editor: Daniel Maynes.

J. Fluids Eng 140(9), 091104 (Apr 19, 2018) (9 pages) Paper No: FE-17-1701; doi: 10.1115/1.4039512 History: Received November 02, 2017; Revised February 20, 2018

Efficient actuation of liquid slugs in microfluidic circuits is a matter of interest in droplet-based microfluidic (DMF) applications. In this paper, the electrowetting on dielectric (EWOD) actuation of a liquid slug fully confined in a microchannel is studied. A set of experiments are conducted in which the mean transport velocity of a liquid slug enclosed in a microchannel of rectangular cross section and actuated by EWOD method is measured. A printed circuit board-based (PCB-based) microfluidic chip is used as the platform, and the transport velocity of the slug is measured by processing the images recorded by a high-speed camera while the slug moves in the channel. To investigate the effect of microchannel geometry on the mean transport velocity of the slugs, different channel heights and widths (ranging between 250440μm and 1–2 mm, respectively) as well as different liquid volumes (ranging between 2.94and5.15μL) are tested and slug velocities up to 14.9 mm/s are achieved. A theoretical model is also developed to analyze the effect of involved parameters on the transport velocity. The results show that, within the range of design parameters considered in this study, for a constant slug volume and channel width, increasing the channel height enhances the velocity. Moreover, keeping the slug volume and channel height fixed, the transport velocity is increased by enlarging the channel width. An inverse proportionality between the slug length and velocity is also observed. These results are also shown to agree with the theoretical model developed.

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Fig. 1

A schematic view of a liquid slug moving inside a microchannel: (a) overall view, (b) top view, and (c) side view

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Fig. 2

The fabrication process for the PCB used as the bottom (base) plate

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Fig. 3

Schematic representation of the experimental setup

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Fig. 4

The sequential images of the liquid slug meniscus displacement for microchannels of various cross section dimensions: (a) w = 2 mm, h = 440 μm, (b) w = 1.5 mm, h= 340 μm, and (c) w = 1 mm, h= 250 μm. For all cases, the volume of slug is 3.68 μl and dimensionless electrowetting number (η) is 0.11.

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Fig. 5

Transport velocity as a function of channel width (w) and channel height (h): (a) liquid volume 2.94μL, (b) liquid volume 3.68μL, (c) liquid volume 4.41μL, and (d) liquid volume 5.15μL. The legend for all diagrams is the same and as displayed in part (a).

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Fig. 6

Side view images of a circular droplet sandwiched between parallel plates. The vertical gap between plates is 340 μm: (a) droplet at rest and (b) droplet under EWOD actuation and moving to left.

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Fig. 7

The transport velocity as a function of slug length: (a) h = 250 μm, (b) h = 340 μm, and (c) h = 440 μm

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Fig. 8

The slug transport velocity as a function of the slug length for various channel dimensions



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