Surface acoustic wave concentration of particle and bioparticle suspensions

A rapid particle concentration method in a sessile droplet has been developed using asymmetric surface acoustic wave (SAW) propagation on a substrate upon which the droplet is placed. Due to the asymmetry in the SAW propagation, azimuthal bulk liquid recirculation (acoustic streaming) is generated. Once the local particle concentration is sufficiently high within a particular streamline of the acoustic streaming convective flow, shear-induced migration gives rise to an inward radial force that concentrates the particles at the centre of the droplet. In this paper, a SAW device consists of a 0.75-mm thick, 127.68° Y–X-axis-rotated cut, X-propagating LiNbO₃ for a substrate and an interdigital transducer electrode (IDT) with 25 straight finger pairs in a simple repeating pattern, 12 mm aperture, and a wavelength of λ=440 μm was patterned on the substrate. The IDT was then driven with a sinusoidal signal at the resonance frequency ƒ0 of 8.611 MHz. To investigate the effect of particle type and size on the concentration process, three types of particles were used in this study, including fluorescent particles (1 μm), polystyrene microspheres (3, 6, 20, 45 μm), and living yeast cells (10–20 μm). Different RF powers were applied ranging from 120 to 510 mW. The concentration processes occurs within 2 to 20 s, depending on the particle size, type and input radio frequency (RF) power, much faster than currently available particle concentration mechanisms due to the large convective velocities achieved using the SAW device. Moreover, this concentration method is efficient, concentrating the particles into an aggregate one-tenth the size of the original droplet. Most importantly, bioparticles can also be concentrated by this method; we have verified that yeast cells are not lysed by the SAW radiation during concentration. By using the rapid concentration process described in this work, the breadth of applications and measurement sensitivity of SAW biosensor systems should be greatly enhanced.