Microparticle collection and concentration via a miniature surface acoustic wave device

The ability to detect microbes, pollens and other microparticles is a critically important ability given the increasing risk of bioterrorism and emergence of antibiotic-resistant bacteria. The efficient collection of microparticles via a liquid water droplet moved by a surface acoustic wave (SAW) device is demonstrated in this study. A fluidic track patterned on the SAW device directs the water droplet's motion, and fluid streaming induced inside the droplet as it moves along is a key advantage over other particle collection approaches, because it enhances microparticle collection and concentration. Test particles consisted of 2, 10, 12 and 45 mm diameter monodisperse polystyrene and melamine microparticles; pollen from the Populus deltoides, Kochia scoparia, Secale cerale, and Broussonetia papyrifera (Paper Mulberry) species; and Escherichia coli bacteria. The collection efficiency for the synthetic particles ranged from 16 to 55%, depending on the particle size and surface tension of the collection fluid. The method was more effective in collecting pollen and the bacteria with an efficiency of 45-68% and 61.0-69.8%, respectively. Pollen collection was strongly influenced by its diameter, size, and surface geometry in a manner contrary to initial expectations. Reasons for the consistent yet unexpected collection results include leaky SAW pressure boundary segregation and shear-induced concentration of larger particles, and the subtle effects of wetting interactions. These results demonstrate a new method for collecting microparticles requiring only about one second per run, and illustrate the inadequacy of using synthetic microparticles as a substitute for their biological counterparts in experiments studying particle collection and behavior.