Microfluidic pumps employing surface acoustic waves generated in ZnO thin films

ZnOthin film based surface acoustic wave(SAW)devices have been utilized to fabricate microfluidic pumps. The SAWdevices were fabricated on nanocrystalline ZnOpiezoelectricthin films deposited on Si substrates using rf magnetron sputtering and use a Sezawa wave mode for effective droplet motion. The as-deposited ZnOsurface is hydrophilic, with a water contact angle of ∼75°, which prevents droplet pumping. Therefore, the ZnOsurface was coated using a self-assembled monolayer of octadecyltrichlorosilane which forms a hydrophobic surface with a water contact angle of ∼110°. Liquid droplets between 0.5 and 1 μl in volume were successfully pumped on the hydrophobic ZnOsurface at velocities up to 1 cm s−1. Under acoustic pressure, the water droplet on an hydrophilic surface becomes deformed, and the asymmetry in the contact angle at the trailing and leading edges allow the force acting upon the droplet to be calculated. These forces, which increase with input voltage above a threshold level, are found to be in the range of ∼100 μN. A pulsed rf signal has also been used to demonstrate precision manipulation of the liquid droplets. Furthermore, a SAWdevice structure is demonstrated in which the ZnOpiezoelectric only exists under the input and output transducers. This structure still permits pumping, while avoiding direct contact between the piezoelectric material and the fluid. This is of particular importance for biological laboratory-on-a-chip applications.