Modeling and simulation of droplet translocation and fission by electrowetting-on-dielectrics (EWOD)

doi:10.1007/s11465-010-0104-z

Front Mech Eng Chin

2010, Vol. 5

Issue (4) : 376-388 https://doi.org/10.1007/s11465-010-0104-z

RESEARCH ARTICLE

Modeling and simulation of droplet translocation and fission by electrowetting-on-dielectrics (EWOD)

Nathan HOWELL, Weihua LI(

)

School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia

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Abstract

This paper discusses methods of microfluidic droplet actuation by means of electrowetting-on-dielectrics (EWOD) and provides a technique for modeling and simulating a microfluidic device by using the computational fluid dynamics (CFD) program, Flow3D. Digital or droplet microfluidics implies the manipulation of droplets on a scale of nanoliters (10^-9 L) to femtoliters (10^-15 L), as opposed to continuous microfluidics that involve the control of continuous fluid within a channel. The two operations in focus here are droplet translocation (moving) and droplet fission (splitting), in which the pressures and velocities within the droplet are analyzed and compared to existing works, both theoretical and experimental. The variation in the pressure of the leading and trailing faces of a droplet indicates the variation in surface energy—an important parameter that explains how a droplet will move toward a region of higher electric potential. The higher voltage on one side of a droplet reduces surface energy, which leads to an induced pressure drop, thus resulting in fluid motion. Flow3D simulations are for both water and blood droplets at voltages between 50 V and 200 V, and the droplet size, surface properties (Teflon coated), and geometry of the system are kept constant for each operation. Some peculiarities of the simulation are brought to light, such as instabilities of the system to higher voltages and fluids with higher dielectric constants, as well as the creation of a tertiary droplet when the applied voltage causes a large enough force during fission. The force distribution across the droplet provides a general understanding of the electrowetting effect and more specifically allows for a comparison between the effects that different voltages have on the forces at the droplet surface. The droplet position and mean kinetic energy of the droplet are also investigated and compared to other works, proving the dynamics of a droplet motion found here.

Keywords electrowetting-on-dielectrics (EWOD) electrowetting microfluidics droplet translocation droplet fission Flow3D dielectric constant

Corresponding Author(s): LI Weihua,Email:weihuali@uow.edu.au

Issue Date: 05 December 2010

Cite this article:

Weihua LI,Nathan HOWELL. Modeling and simulation of droplet translocation and fission by electrowetting-on-dielectrics (EWOD)[J]. Front Mech Eng Chin, 2010, 5(4): 376-388.

URL:

https://academic.hep.com.cn/fme/EN/10.1007/s11465-010-0104-z
https://academic.hep.com.cn/fme/EN/Y2010/V5/I4/376

Fig.1 Electrowetting on dielectrics (EWOD) phenomenon. (a) Fluid head before a voltage is applied; (b) depicts a decrease in contact angle where the fluid contacts the channel wall at the electrode

Fig.2 Theoretical comparison between contact angle and applied voltage

Fig.3 Typical simulation geometry. (a) Translocation; (b) fission

Fig.4 Successful droplet translocation from this simulation, water at 100 V, = 100 μm. (a) Center electrode active; (b) 0.0045 s after right electrode becomes active, droplet centered; (c) 0.006 s after right electrode becomes active; (d) 0.008 s after right electrode becomes active, translocation complete

Fig.5 Successful droplet translocation at 40 V, = 70 μm. (a) =0 s; (b) =0.066 s; (c) =0.132 s; (d) =0.33 s[]

Tab.1 Simulation parameters for translocation

Fig.6 Successful droplet fission from this simulation—blood at 150 V, = 100 μm. (a) Center electrode active; (b) 0.0019 s after outer electrodes become active, necking beginning; (c) 0.004 s outer electrodes become active; (d) 0.009 s outer electrodes become active

Fig.7 Successful droplet fission at 60 V, = 70 μm. (a) =0 s; (b) =0.099 s; (c) =0.12 s[]

Tab.2 Simulation parameters for fission.

Fig.8 Pressure distribution for a droplet during translocation. (a) = 0 s; (b) = 0.005 s; (c) = 0.01 s; (d) = droplet centered between electrodes

Fig.9 Pressure distribution for a droplet during fission. (a) = 0.01 s; (b) = necking beginning

Fig.10 Velocity distribution for a droplet during translocation. (a) = 0 s; (b) = 0.005 s; (c) = 0.01 s; (d) = droplet centered between electrodes

Fig.11 Velocity distribution for a droplet during fission. (a) = 0.01 s; (b) = necking beginning

Fig.12 Velocity versus voltage for systems scaled to different channel heights, electrode dimensions, and droplet diameters in same ratio

Fig.13 Force distribution on droplet at different voltages. (a) Force upon the actuation of center electrode, Flow3D simulation with channel gap of 100 μm; (b) experimental results with a channel gap of 10 μm []; (c) channel gap of 100 μm []

Fig.14 Droplet position during translocation. (a) Left to right; (b) right to left

Fig.15 Estimated mean kinetic energy for droplet translocation

Fig.16 Stages of fission showing creation of tertiary droplet—water at 200 V. (a) Center electrode active; (b) 0.001 s after outer electrodes become active, necking beginning; (c) 0.0025 s after outer electrodes become active; (d) 0.005 s after outer electrodes become active, creation of a third droplet

Fig.17 Young-Lipmann model obtained from simulation []

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