Polarization conversion of surface acoustic waves for enhanced microscale actuation applications

Surface acoustic wave (SAW) based actuators used for active microfluidic applications, most often utilize Rayleigh-type SAW with significant out-of-plane surface displacements. Besides the desired momentum transfer to the liquid in contact with the active surface, an additional, unwanted momentum transfer into the polymer wall of the microfluidic vessel also occurs. The energy dissipation inside the vessel wall decreases actuator efficiency and may result in leakage and degradation of the wall material. Our first investigations show that boundary polarized SAW modes with comparatively small out-of-plane displacement can also be used for microfluidic actuation purposes to overcome this drawback. The boundary polarized mode is capable to pass the vessel wall-substrate interface with minimum of acoustical loss, but needs to be converted finally inside the vessel into a vertical polarized mode in order to ensure the intended acoustofluidic interaction. Such a mode or polarization conversion can be realized by an appropriate scattering structure arranged inside the vessel area.