The mode conversion of acoustic surface waves contacted by liquid droplets has been utilized for the propulsion of nanoliter droplets with respect to lab-on-a-chip applications. In this case, however, piezoelectric substrates equipped with interdigital transducers on the surface on which the droplets are moving have been used for the excitation of surface acoustic waves. Our approach is aiming at the acceleration of comparatively large droplets such as raindrops on non-piezoelectric substrates, e. g. glass plates. Following this approach, we have excited Lamb waves on glass plates by piezoelectric interdigital transducers attached at the rear side of the substrate, which are not in contact with the liquid. A propulsive action on water droplets of μl size has been observed, which could be improved by hydrophobic surface coatings. With continuous wave excitation at a frequency of 1 MHz and an amplitude of 200 V_(pp) an acceleration of about 1 m/s~2 has been achieved so far with water droplets of 10 μl on a 1 mm thick glass plate. This concept allows numerous technical applications, including two-dimensional droplet relocation or "intelligent" droplet removal from glass panels in combination with droplet detection via a Lamb wave transmission line established by a second interdigital transducer.
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