VIBRATION INDUCED DE-WETTING TRANSITION ON SUPERHYDROPHOBIC BUBBLE MEMBRANE
Sukumar Rajauria, Ozgur Ozsun, John Lawall, Victor Yakhot and Kamil Ekinci
In recent years, oscillating flows in Newtonian fluids due to high frequency M/NEMS resonators have gained increasing attention . A Newtonian fluid is known to follow a no-slip boundary condition at the solid-fluid boundary. This leads to a friction force, also called the ‘viscous drag’, which is responsible for large damping of M/NEMS resonators in water.
In this work, we have studied the fluidic damping on resonators such as micromechanical membranes, which were coated with a super-hydrophobic layer. Super-hydrophobic surfaces tend to shrink the solid-liquid interface area by creating air pockets on surface. The trapped air on the surface acts as lubricant for viscous drag, thus reducing the fluidic dissipation.
The superhydrophobic resonators are fabricated out of thin (~ 200 nm) Silicon-nitride membranes by etching holes in them and then coating the resulting structure with hydrophobic perfluro-octyl-trichloro-silane. The holes provide a reservoir of trapped air pockets on the membrane surface. The contact angle (or hydrophobicity) increases with the density of etched holes, with a maximum angle of 160o.
Superhydrophobic bubble membranes are characterized first in vacuum and then in water to extract mass loading and drag coefficient in water. By varying the wet solid area fraction (ΦS), we have observed a reduction in the fluidic drag. At an intermediate solid fraction ΦS ~ 0.6, there is a sudden decrease in the drag coefficient and mass loading, which can be explained by the de-wetting transition .
 K.L. Ekinci, V. Yakhot, S. Rajauria, C. Colosqui and D. M. Karabacak, Lab on a Chip 10, 3013 (2010).
 S. Rajauria et al in preperation