Surface acoustic waves (SAWs) are nanometre order electroelastic analogues of earthquake waves that propagate along the surface of a

piezoelectric substrate. Due to the fluid-structural coupling between the undulations along the substrate surface and the fluid above the substrate, substantial acoustic energy is transferred into the fluid, which can then be exploited to drive a wide range of microfluidic phenomena.

As shown above, a drop placed on the substrate in the pathway of the SAW propagation begins to vibrate at low powers due to SAW beneath it. This has interesting implications for colloidal patterning at the free surface of the drop. As the power is increased, body forces on the drop induced by the acoustic energy leaked into the drop causes it to translate at high speeds, typically around 1 cm/s. Further increases in the power leads to a peculiar jetting phenomenon, in which liquid jets that persist up to 1-2 cm in length are observed. Finally, at extremely high powers, the large surface accelerations on the substrate rapidly destabilise the interface, causing the entire drop to atomise.

The following pages describe the various SAW-driven microfluidic manipulations and associated applications that we have discovered:

1. 1.Surface acoustic wave vibration induced colloidal patterning
   

2. 2.Surface acoustic wave driven microcentrifugation
   

3. 3.Surface acoustic wave droplet actuation
   

4. 4.Surface acoustic wave microchannel actuation
   

5. 5.Surface acoustic wave interfacial jetting
   

6. 6.Surface acoustic wave atomisation and associated phenomena
   

1. 1.LY Yeo, JR Friend. Ultrafast Microfluidics Using Surface Acoustic Waves. Invited paper: Biomicrofluidics, 3, 012002 (2009) (PDF).
   

2. 2.RP Hodgson, M Tan, L Yeo, J Friend. Transmitting High Power RF Acoustic Radiation via Fluid Couplants into Superstrates for Microfluidics. Appl Phys Lett 94, 024102 (2009) (PDF).