Microfluidic pumps employing surface acoustic waves generated in ZnO thin films

Abstract

ZnO thin film based surface acoustic wave (SAW) devices have been utilized to fabricate microfluidic pumps. The SAW devices were fabricated on nanocrystalline ZnO piezoelectric thin films deposited on Si substrates using rf magnetron sputtering and use a Sezawa wave mode for effective droplet motion. The as-deposited ZnO surface is hydrophilic, with a water contact angle of ∼75°, which prevents droplet pumping. Therefore, the ZnO surface 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 ZnO surface 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 SAW device structure is demonstrated in which the ZnO piezoelectric 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.

Slippery Liquid-Infused Porous Surfaces and Droplet Transportation by Surface Acoustic Waves

“…Various hydrophobic polymer coatings, such as octadecyltrichlorosilane (OTS), Teflon AF ® and CYTOP TM have previously been used to increase the contact angle of the droplet on the surface [13,[27][28][29]. Most of the hydrophobic polymers coated onto SAW device surfaces have contact angles of around 90-120° [13,[29][30]. Surfaces with such contact angles retain a sufficiently large footprint for the applied SAW power to be dissipated into the droplet through the wave/liquid interaction areas.…”

“…This reduces the length of the droplet-solid surface contact line, but also reduces the solid-liquid contact area critical to the acoustic wave-droplet interaction. Various hydrophobic polymer coatings, such as octadecyltrichlorosilane (OTS), Teflon AF ® and CYTOP TM have previously been used to increase the contact angle of the droplet on the surface [13,[27][28][29]. Most of the hydrophobic polymers coated onto SAW device surfaces have contact angles of around 90-120° [13,[29][30].…”

“…The energy and momentum of the longitudinal wave dissipated into the droplet provide a pressure or a body force on the droplet in the direction of propagation of the SAW. This provides the basis for droplet movement, pumping and mixing, streaming, manipulation and jetting/nebulisation [13,15,26,27,29].…”

AlScN thin film based surface acoustic wave devices with enhanced microfluidic performance

“…silicon, glass, metal, or polymer). 7,[10][11][12][13] Currently, piezoelectric thin films such as ZnO have been extensively explored for SAW based sensors and microfluidics. 6,7,14 AlN, another extensively used piezoelectric film materials in microelectro mechanical system (MEMS), has great advantages such as high acoustic velocity, outstanding stability at high temperatures, excellent chemical inertness and good compatibility with CMOS process.…”

“…[1][2][3][4][5][6][7] Most of these SAW devices were made on bulk piezoelectric materials such as LiNbO3 due to their good performance. 5,7 However, the relatively brittle nature and high cost of substrate materials, as well as difficulty in integration with CMOS process could limit their applications. Thin film acoustic waves based on piezoelectric thin film materials, such as zinc oxide (ZnO) and aluminum nitride (AlN), are considered to be the future technology for the acoustic wave based sensors, microfluidics and lab-on-chips.…”