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Surface acoustic wave ͑SAW͒ devices were fabricated on ZnO thin films deposited on Si substrates. Effects of ZnO film thickness on the wave mode and resonant frequency of the SAWs have been investigated. Rayleigh and Sezawa waves were detected, and their resonant frequencies decrease with increase in film thickness. The Sezawa wave has much higher acoustic velocity and larger signal amplitude than those of Rayleigh mode wave. Acoustic streaming for mixing has been realized in piezoelectric thin film SAWs. The Sezawa wave has a much better efficiency in streaming, and thus is very promising for application in microfluidics.

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.

Microfluidic systems are part of an emerging technology which deals with minute amount of liquids (biological samples and reagents) on a small scale. They are fast, compact and can be made into a highly integrated system to deliver sample purification, separation, reaction, immobilization, labelling, detection, etc, and thus are promising for applications such as lab-on-a-chip and handheld healthcare devices. Miniaturized micropumps typically consist of a moving-part component, such as a membrane structure, to deliver liquids, and they are unreliable, complicated in structure and difficult to be integrated with other control electronics circuits. The trend of new-generation micropumps is moving-part-free micropumps operated by advanced techniques, such as electrokinetic force, surface tension/energy, acoustic waves, etc. This paper reviews the development and advances of the relevant technologies, and introduces the electrowetting-on-dielectrics and acoustic wave-based microfluidics. The programmable electrowetting micropump has been realized to dispense and manipulate droplets in 2D with up to 1000 addressable electrodes and electronics built underneath. The acoustic wave-based microfluidics can be used not only for pumping, mixing and droplet generation but also for biosensors, suitable for single-mechanism-based lab-on-chips application.

This paper provides a detailed study on surface acoustic wave (SAW) induced acoustic streaming and pumping, focusing on the effects of the wave mode and surface modification. SAW devices with wavelengths of 32 and 64 µm were fabricated on 128° Y-cut lithium niobate substrates with aluminium interdigitated transducers. A higher order harmonic mode wave appears in addition to the fundamental Rayleigh wave for samples with metallization ratios less than 0.6. Both waves have demonstrated the ability to induce acoustic streaming and to pump liquid. A high streaming velocity, and hence a high mixing efficiency and a higher acoustic force, can be obtained using the fundamental Rayleigh wave as the high harmonic waves have large propagation losses. A linear relationship between the streaming velocity and RF signal voltage has been obtained, and effective mixing can be achieved. An acoustic wave has also been utilized to manipulate and pump droplets with sizes up to 5 µl, and a moving speed of ∼1.4 cm s−1 has been obtained on an octadecyltrichlorosilane-treated SAW device using a signal voltage of 40 V.

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High quality, c-axis oriented zinc oxide (ZnO) thin films were grown on silicon substrate using RF magnetron sputtering. Surface acoustic wave (SAW) devices were fabricated with different thickness of ZnO ranging from 1.2 to 5.5 µm and the frequency responses were characterized using a network analyzer. Thick ZnO films produce the strongest transmission and reflection signals from the SAW devices. The SAW propagation velocity is also strongly dependent on ZnO film thickness. The performance of the ZnO SAW devices could be improved with addition of a SiO 2 layer, in name of reflection signal amplitude and phase velocity of Rayleigh wave.

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