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Mechanism and flow measurement of AC electrowetting propulsion on free surface

Abstract

A free surface in contact with a floating object can be vertically oscillated by applying an alternating current electrowetting-on-dielectric (AC EWOD). The oscillation of the free surface generates a propelling force on the centimeter-sized floating object. This paper describes a propulsion mechanism in free-surface oscillation along with its experimental results. Flow visualizations, wave patterns measured by the free-surface synthetic schlieren method, and PIV measurements show that the oscillation generates a capillary Stokes drift on the water surface and two counter-rotating spiral underwater vortices, leading to an ejecting flow (streaming flow) normal to the wall of the boat. The momentum of the ejecting flow produces a reaction force on the wall and ultimately propels the floating boat. The propulsion speed of the boat highly depends on the amplitude, frequency, and shape of the AC EWOD signal. Curve fittings based on the Stokes drift well match the experimental measurements of propulsion speed. The width of the EWOD electrode also has significant effects on the boat speed.

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Abbreviations

A :

Wave amplitude (m)

b :

Width of electrode (m)

A boat :

Projected area of submerged boat part (m2)

C D :

Drag coefficient

f :

Frequency of applied AC signal (Hz)

F drag :

Drag force (N)

F prop :

Propulsion force (N)

g :

Gravitational acceleration (m/s2)

h :

Water depth (m)

k :

Wavenumber (1/m)

t :

Thickness of dielectric layer (m)

u drift :

Time-averaged drift velocity (m/s)

u boat :

Boat propulsion speed (m/s)

V :

Voltage applied to EWOD system (V)

V d :

Voltage across dielectric layer (V)

z :

Vertical position from free surface (m)

γ LV :

Air–water surface tension (N/m)

ε 0 :

Vacuum permittivity (8.854 × 10−12 F/m)

ε r :

Relative permittivity of dielectric layer

ω :

Angular frequency of wave (rad/s)

ρ :

Density (kg/m3)

θ :

Contact angle (°)

θ 0 :

Initial contact angle with no voltage applied (°)

C :

Coefficient in Eq. (8)

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Acknowledgments

This work is in part supported by the NSF Grant (ECCS-1029318).

Author information

Correspondence to Sung Kwon Cho.

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Yuan, J., Cho, S.K. Mechanism and flow measurement of AC electrowetting propulsion on free surface. Exp Fluids 56, 67 (2015). https://doi.org/10.1007/s00348-015-1936-4

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Keywords

  • Contact Angle
  • Free Surface
  • Particle Image Velocimetry
  • Contact Line
  • Oscillation Amplitude