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

Very-high-frequency surface acoustic waves, generated and transmitted along singlecrystal 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 10mm/s were obtained in centimetres-long rectangular channels with crosssectional 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. Copyright c © EPLA, 2009 Introduction. – The dominance of surface tension and viscous forces at small scales enormously complicates efficient transport and mixing in microfluidic technologies. In searching for solutions to this problem, we describe and explain three physical aspects peculiar to very-highfrequency (VHF) surface acoustic wave (SAW) excitation of fluids in a microchannel: an ability to drive fluids along the channel at 1–10mm/s [1], the appearance of vortices and concomitant mixing under specific conditions, and the collection and transport of particle suspensions. The first two phenomena offer —by merely changing the SAW frequency— controlled switching between uniform flow for fluid delivery and vortex-laden flows for mixing in the same microchannel, eliminating the complex architectures [2] required to induce mixing. In a similar vein, the manipulation of particle-laden flows is directly applicable to sorting or patterning. The SAW is a Rayleigh wave [3] generated by a sinusoidal electric potential applied to an interdigital transducer (IDT) on the surface of a 127.68◦ y–x cut, x-propagating lithium niobate (LN) single-crystal piezoelectric substrate (fig. 1) formed using standard UV photolithography. While the SAW is confined in its propagation from the transducer along the anisotropic (a)E-mail: [email protected] Fig. 1: (Color online) The cut channel, indicating the different planes used to obtain experimental images. The dot at the corner of the channel indicates the origin of the coordinates: x1, x2, and, x3 point along the channel’s length Lch, width Wch, and into the substrate along the channel’s depth Dch, respectively. substrate’s x-axis at a velocity vSAW ≈ 3990m/s, contact with a fluid medium atop the substrate causes some of the SAW’s energy to be radiated into the fluid. With sufficient intensity, this acoustic energy propagates as finite amplitude sound radiation traveling at a speed c0 < vSAW (c0 ≈ 1450m/s in water at room temperature) in a direction defined by the Rayleigh angle θSAW = sin (c0/vSAW). This forms an (Eckart [4]) acoustic streaming force in the fluid due to the nonzero and temporally phase-shifted distribution of the pressure and velocity [5] over centimetre-order length scales. Boundary layer streaming [5] may also arise due to the