This paper reviews the principles, operations, and applications of bubble-based electrowetting-on-dielectric (EWOD). EWOD has proved to be an efficient tool in digital microfluidics that employs discrete droplets, and various applications that use the principles of EWOD have been developed from lab-on-a-chip to optical systems. Similar to its use with droplets, EWOD can also be applied to gaseous bubbles. This review begins with a discussion of the principles of EWOD for a bubble on an electrode covered with a hydrophobic dielectric layer. It then addresses EWOD actuation and the transportation of a bubble in an aqueous medium, along with a physical explanation of bubble motion. The operation of EWOD is then extended to the on-chip creation/elimination and splitting of bubbles. In particular, micro-mixers and pumps are discussed as potential applications of these operations. Unlike droplets, bubbles can be easily oscillated by external excitation, which provides additional functionalities. By integrating EWOD with external excitation, a number of new advanced applications are introduced, including the capture/separation of particles and the propulsion of objects. In these advanced operations, cavitational microstreaming flows and acoustic radiation forces are mainly responsible for the physical mechanisms. This paper also discusses these advanced operations along with their underlying physics. It is expected that in addition to bubble oscillation, other bubble actuation modes will create new functionalities and new potential applications.
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- ρ :
- μ :
fluid dynamic viscosity
- V :
mean fluid velocity
- L :
- θ :
contact angle under an applied electrical potential
- θ e :
equilibrium contact angle
- V :
- ɛ :
permittivity of a dielectric layer
- γ :
- t :
thickness of a dielectric layer
- θ R :
contact angle on the right side of a bubble
- θ L :
contact angle on the left side of a bubble
- θ adv :
advancing contact angle
- θ rec :
receding contact angle
- w :
width of a bubble base
- R :
radius of a bubble
- F driving :
bubble driving force
- Ψ :
streaming function of a cavitational streaming flow
- ɛ :
amplitude of bubble oscillation normalized by a radius of a bubble
- ω :
angular frequency of an applied acoustic wave
- r :
distance from a bubble center
- Δϕ :
phase shift between volume and translational oscillations
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Chung, S.K., Rhee, K. & Cho, S.K. Bubble actuation by electrowetting-on-dielectric (EWOD) and its applications: A review. Int. J. Precis. Eng. Manuf. 11, 991–1006 (2010). https://doi.org/10.1007/s12541-010-0121-1