Rapid fluid flow and mixing induced in microchannels using surface acoustic waves

Very-high-frequency surface acoustic waves, generated and transmitted along single-crystal lithium niobate, are used to drive homogeneous aqueous suspensions of polystyrene nanoparticles along microchannels. At a few hundred milliwatts, uniform and mixing flows with speeds of up to 10 mm/s were obtained in centimetres-long rectangular channels with cross-sectional dimensions of tens to a few hundreds of microns. A transition from uniform to mixing flow occurs as the channel width grows beyond the wavelength of sound in the fluid at the chosen excitation frequency. At far lower input powers, the suspension agglomerates into equally spaced, serpentine lines coincident with nodal lines in the acoustic pressure field. We expose the physics underlying these disparate phenomena with experimental results aided by numerical models.