Liquid streaming and droplet formation caused by leaky Rayleigh waves

Various liquid motions such as streaming and droplet formation have been observed when a liquid was put on the surface of a SAW (surface acoustic wave) propagation substrate. Results of experimental and theoretical studies on these so-called SAW streaming phenomena are presented. Surface treatment of the SAW substrate by metallization and by silanization is employed to demonstrate that the liquid motion is strongly influenced by the chemical condition of the substrate surface. Experiments using 128° rotated Y-X LiNbO₃ at 50 MHz with various pulse widths are described. In a hydrophilic case, such as an Al-deposited metal surface, the pattern of the SAW power flow can be observed. This indicates that one can quickly visualize the SAW radiation by merely putting a small volume of liquid on the surface. If the substrate surface is hydrophobic, small droplets are ejected from the water surface. Using this phenomenon makes it possible to develop new fluid devices. Theoretically, the SAW streaming force can be derived using the theory of acoustic streaming. The calculated SAW streaming force is found to be very large and strong enough to expel the droplet from the liquid surface