XXII ICTAM, 25–29 August 2008, Adelaide, Australia MODELING CHAOTIC DROPLET ROTATION THROUGH HIGH-FREQUENCY ACOUSTIC EXCITATION

The application of microfluidics to biomedicine has been touted [1] as a means to deliver improved and inexpensive medical care. Though there are many methods for transporting fluids within these devices [2], from the passive capillary forces that draw a fluid along a hydrophilic channel to the actively driven forces like electrophoresis, none would appear to be as powerful as surface acoustic wave (SAW) irradiation [3].Using a piezoelectric substrate with an interdigital electrode (IDT) [4] deposited upon its surface, a traveling strain or Rayleigh [5] wave may be generated on the substrate surface from the IDT. As the wave passes, a point on the surface will move in a transversely-polarized elliptical fashion at less than 10 nm but with velocity of nearly 1 m/s. At 10 MHz, the accelerations of the surface easily exceed one million m/s, and so researchers have moved beyond the original application of SAW in telecommunications [6] to consider what SAW can do in microfluidics [3, 7, 8]. In particular, a rapid particle concentration method in sessile droplets and confined fluid chambers has been developed [9] using asymmetric surface wave propagation on a substrate upon which the droplet is placed. The SAW generates a combination of forces upon suspended particles and the fluid droplet itself via diffraction to drive rapid collection or dispersion of the particles, far faster than other currently available methods due to the large convective velocities achieved using the device. Strangely, a droplet will also rotate if placed completely in the path of the SAW (see Fig. 1); there should be no torque on the droplet due to the symmetric SAW radiation. Further, several transition regions exist with respect to IDT input power, from local particle collection at <1 mW, particle collection and dispersion blinking phenomena at intermediate (1–5 mW) power, to complete particle collection toward the center of the droplet at high (>5 mW) power, evidently representing a Hopf bifurcation with O(2) symmetry [10]. Though the fluid dynamics behavior in particle collection through localized and acoustically-driven agglomeration, followed by shear vortex migration has been verified [9], the reasons for the rotation of a droplet completely immersed in SAW irradiation was left unexplained.