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DY: Dynamik und Statistische Physik
DY 34: Poster
DY 34.72: Poster
Montag, 7. März 2005, 15:30–18:00, Poster TU D
Self-excited drop oscillations in electrowetting experiments — •Jean-Christophe Baret1 und Frieder Mugele2 — 1Philips Research, Eindhoven (NL) — 2University of Twente, Physics of Complex Fluids; 7500 AE Enschede (NL)
In electrowetting experiments the contact angle of a droplet on a substrate is modified by an external voltage applied between
an electrode immersed in the drop and a substrate coated with an insulating layer. When the electrode is barely immersed into
the droplet, a capillary neck develops between the droplet and the electrode upon applying a voltage. At a certain threshold
voltage, the capillary neck breaks.
Working with AC voltage U at frequency f from 1 to 30 kHz, and electrical conductivity of the drop from 0.1 to 10 mS/cm,
two regimes have been observed above the critical voltage: A regime of fast and regular oscillations of the drop (10-120 Hz)
at high frequency f or low conductivity and a regime of eratic and slow oscillations at low frequency f or
high conductivity.
The difference in the two regimes is explained by a simple electrical model based on the divergence of the resistance of capillary neck at the break-up r(t)=r0 (t/t*)γ, r0 being representing the conductivity of the drop. From this analysis an dimensionless number A= 2 π r0 C νγ+1 t*γ is defined , C being the capacitance of the drop. For A>>1, the charge kept in the drop after the break up is 0 which explains that the relaxation of the drop is purely hydrodynamical leading to regular oscillations, while for A<<1 the charge is randomly disctributed between 0 and CU explaining the eratic behavior.