Liquid streaming and droplet formation caused by leaky Rayleigh waves

Abstract

sented. It is found that the SAW streaming depends on the SAW power, frequency, pulse Various liquid motions such as width, the liquid volume and surface chemstreaming and droplet formation have been ical conditions of the substrate surface. observed when a liquid was put on the If the surface is hydrophobic, liquid surface of SAW propagation substrate. In droplets are effectively ejected from the this report we call these phenomena "SAW surface into the air at an oblique angle. streaming" and experimental and theoreti-Under the proper condition, very fine fogs cal studies are presented. Surface treat-are produced. ment of SAW substrate by metalization and Theoretically, a general differential by silanization is employed to demonstrate equation of second order which is used for that the liquid motion is strongly influ-the acoustic streaming theory3) is develenced by the chemical condition of the oped. It is shown that the calculated substrate surface. Experiments using 128' force of SAW streaming due to leaky rot. Y-X LiNb03 at the frequency of 50 MHz Rayleigh wave is much greater than that of with various pulse width are described.acoustic streaming due to general attenu-Theoretically, SAW streaming force ated plane traveling waves in bulk liquid. can be derived using the acoustic streaming theory. The calculated SAW streaming force is found to be very large and strong enough to expel the droplet from the liquid surface.0090-5607/89/0000-0643 $1 .OO 0 1989 IEEE

Acoustic Wave Based Microfluidics and Lab-on-a-Chip

“…Under an external body force, Fj, induced by acoustic waves, the fundamental hydrodynamics of a steady viscous fluid is governed by Navier-stoke equation [47],…”

“…Streaming is a 3D phenomenon, especially for streaming inside a droplet; therefore displacements in all directions have to be considered. The displacements in the x and y directions are expressed by [47,53];…”

“…Kr=2 / is a real number, where is the wave length and the leaky SAW wave number (kL=kr+jki) is complex with the imaginary part, jki, representing the SAW energy dissipation within the liquid. The leaky SAW wave number can be obtained by extending the method of Campbell and Jones [54,47] into the solid-liquid structure assuming both stress and displacement to be continuous at y=0, and VL=1500 m/s for water (Most of the biofluid is waterbased, VL changes if other liquid is used.). If the wave displacements (ux, uy) are replaced by the wave velocities using…”

“…(7), the two components of streaming force can be obtained for an incompressible fluid as follows [47,53]; 22 2 11 (1 ) exp 2( )…”

Surface Acoustic Wave Motors and Actuators: Structure, Mechanism, Characteristic and Application

“…When the fluid volume is small and the acoustic power increases above a certain threshold, the fluid begins to move in the direction of the SAW propagation, which provides an observable pattern of the SAW energy flow. Moreover, if the substrate surface is hydrophobic, a lot of droplets are expelled from the liquid surface as shown in Fig.10 ( Shiokawa et al, 1989;Uchida et al, 1995). …”