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DY: Dynamik und Statistische Physik
DY 15: Fluids I
DY 15.2: Talk
Monday, March 8, 2004, 14:45–15:00, H3
Dynamics of self-excited droplet oscillations in electrowetting — •Jean-Christophe Baret1,2, Dagmar Steinhauser1, Ralf Seemann1, Stephan Herminghaus1,3, and Frieder Mugele1 — 1Universitaet Ulm, Abt. Angewandte Physik, D-89069 Ulm — 2Philips Research, Eindhoven (NL) — 3Max Planck Institut fuer Stroemungsforschung, D-37018 Goettingen
Under suitable conditions, liquid droplets in electrowetting experiments can perform self-excited oscillations, as shown recently in Ref. 1. Here, we present an extension of these previous experiments to higher and much better defined oscillation frequencies. We deposited droplets of a conductive liquid (≈ 1 nl) on top of a Si wafer, with an insulation layer (1 µm SiO2) and a monolayer of octadecyltrichlorosilane. We applied an AC voltage (10 kHz) between the substrate and a Pt wire that was immersed into the droplet at a distance d above the surface. Upon increasing the voltage U from zero to 100 V, the contact angle decreased from 130 deg to 70 deg. If d was chosen close to the height of the droplet at zero voltage, the droplet detached from the Pt wire upon increasing the voltage. Within a certain range of d and U, droplets periodically jumped on and off the wire. The oscillation frequency varied between 50 Hz and 120 Hz for droplet sizes between 1mm and 0.1mm. Close to the critical values of d and U, where the oscillations cease, we found chaotic behavior. Furthermore, we demonstrate microfluidic mixing in oscillating droplets.
(1) A. Klingner, S. Herminghaus, and F. Mugele, Appl. Phys. Lett. 82, 4187 (2003)