Inducing rapid fluid flows in microchannels with surface acoustic waves

We investigate uniform fluid and mixing flows with linear speeds up to 1-10 mm/s, actuated by surface acoustic waves. Under strong acoustic excitation, transition from uniform to oscillatory mixing flow occurs when the width of the channel increases beyond one acoustic wavelength of sound in the fluid ¿_f. In this high velocity regime, particles of one micrometer in diameter suspended in aqueous solution are observed to follow the streamlines. Under weak acoustic excitation, particles aligning into equally-spaced lines (with a separation of ¿_f/2) due to the presence of acoustic standing waves across the channel and move slowly in the reverse direction due to the slow streaming. We developed a numerical model of the system that accounts for the acoustic streaming in the fluid with treatment of viscous and solid-fluid coupling effects, and the results qualitatively support the observed phenomena in the experiments.