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Microfluidics and Nanofluidics

, Volume 2, Issue 6, pp 471–480 | Cite as

Capillary and electrostatic limitations to the contact angle in electrowetting-on-dielectric

  • Kazimierz Adamiak
Research Paper

Abstract

The shape of a conducting liquid droplet placed on a hydrophobic dielectric surface is simulated numerically by solving the Laplace–Young capillary equation. The electric force, acting on the conducting surface, distorts the droplet shape leading to a change in the apparent contact angle; its variation is compared with a theoretical Young–Lippman prediction. At sufficiently large values of voltage, applied to the droplet, the numerical algorithm fails to converge, which is interpreted as the break-up of the droplet surface with small droplets being ejected from the surface. These highly charged droplets, as well as any other electric charges near the triple contact line, generated for example by the electric corona discharge, cause a change of the distribution of the electric forces. This effect can be helpful in explaining saturation of the apparent contact angle: an appropriately selected surface charge near the contact line can completely stop droplet distortion, and the contact angle variation, despite the increased droplet voltage. Furthermore, the simulation results show the effect of the permittivity of the medium surrounding the droplet, on the contact angle variation.

Keywords

Electrowetting Electrocapillary effect Contact angle Droplets Electric field 

List of symbols

1/R1 + 1/R2

sum of the principal radii of curvature

d

thickness of the dielectric layer

g

gravity constant

n

unit vector normal to the surface (versor)

r

radial coordinate in the spherical set of coordinates

fφ

derivative of function f with respect of φ

D

electrostatic displacement vector

E

electric field intensity

R

radius of an undistorted droplet

V

scalar electric potential

V0

voltage applied to the droplet

ε, εm

absolute electrical permittivity of the solid insulating layer and ambient medium

εr, \(\varepsilon _{{{\text{rm}} }}\)

relative electrical permittivity of the solid insulating layer and ambient medium

γ

surface tension

θ

apparent contact angle (also called Lippman’s angle)

θ0

local contact angle (Young’s angle)

ρ

fluid density

σs

surface charge density

σ

space charge density

φ

azimuthal coordinate in the spherical coordinate system

Δp

pressure difference between any pair of points inside and outside of the droplet

Indices

s

solid

l

liquid

n

normal direction

Notes

Acknowledgment

The work was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC).

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  1. 1.Department of Electrical and Computer EngineeringThe University of Western OntarioLondonCanada

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