Biomedical Engineering Reference
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measured through the water) were small. As contact line speed decreased, during
the dewetting of the hydrophobic substrate,
θ
steadily increased and molecular dis-
sipation became dominant.
E. Conclusion
Droplets of ionic liquids (Rmim.BF
4
and bmim.X) were immersed in an immiscible
liquid (
n
-hexadecane) and electrowet on a flat electrode insulated with a micro-
scopic layer of an amorphous fluoropolymer (Teflon AF1600). The static contact
angles at zero voltage are very large (
150
◦
) and decrease when voltage is applied
according to the Young-Lippmann equation. The lowest contact angles achievable
through electrowetting were 48
◦
(with DC voltage) and 15
◦
(with AC voltage). The
reversibility of the electrowetting curves was excellent and contact angle hysteresis
was very small (
∼
2
◦
). AC electrowetting is very effective and robust, and thus offers
significant advantages in microfluidic devices. The saturation contact angle could
not be predicted with the zero-interfacial tension theory most probably because
of the existence of an alkane film trapped between the droplet and the insulator.
Experiments with bmim.BF
4
-water mixtures of various compositions showed that
electrowetting with dilute and concentrated electrolytes proceeds macroscopically
in a similar fashion.
Electrowetting was very fast (initial contact line speed reached 0.08 m/s) dur-
ing spreading and retraction. The base area of the droplet (at a given DC voltage)
increased exponentially during advancing (exponential saturation) and decreased
exponentially during receding (exponential decay, after the voltage was switched
off). A characteristic time was related to the viscosity of the ionic liquid and
was shorter during spreading (wetting) and longer during retraction (dewetting).
The dynamics of wetting was examined by comparing the dependence of the dy-
namic contact angle on the speed of the contact line with the hydrodynamic model
(Voinov's equation) and the molecular-kinetic model (Blake's equation). Viscous
dissipation dominated at small dynamic contact angles and molecular dissipation
prevailed at large angles in agreement with the scheme outlined by Brochard-Wyart
and de Gennes.
F. Acknowledgements
The experimental results discussed here were obtained by Mani Paneru during
his PhD candidature. Financial support from the Australian Research Council is
gratefully acknowledged (Special Research Centre Scheme and DP110103391).
RS acknowledges the support obtained from the Department of Further Education,
Employment and Training (Government of South Australia) and the University of
South Australia (RLDP 2009-10 and TRGGS 2009-10). This work was also sup-
ported by the Department of Innovation, Industry, Science and Research (Australian
Government) through the Australia-India Strategic Research Fund (ST020050).
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