Fig. 2. Schematic representation of general acoustofluidic separation techniques.

a Standing acoustic waves are excited in the half-wavelength resonator formed by a silicon-based microfluidic channel. b Microfluidic channels are formed by stainless steel or other high acoustic impedance materials by stacking several layers together. A transducer is attached to the channel to excite vibrations. Standing waves are generated by the reflection of waves at steel/fluid interface. c PDMS channels are bonded in between two IDTs. A standing SAW is formed on the piezoelectric substrate by the interference of oppositely propagating SAWs. Upon contact, SAWs leak into the fluid domain in the form of leaky waves. d One pair of IDTs generate traveling SAWs that leak into the fluid inside the PDMS microfluidic channel. e Particles are directed towards lower (higher) acoustic pressure regions through the effect of acoustic radiation force (Fr) if the acoustic contrast factor (Φ) is larger (smaller) than zero