Visualization of a Standing Lattice of Scattered Light on Spherical Water Droplets under an Acoustic Field Using Biogenic Microparticles

Micromanipulation using acoustic sound is a promising technique for drug delivery, cell manipulation, biosensors, and microfluidic devices. Additionally, the visualization of acoustic fields by advanced optical measurement techniques can be combined with this micromanipulation technique. The present study reveals that a lattice pattern of reflected light appears on the surface of water droplets containing microparticles when the droplets are exposed to audible sound in the range of 1900–10000 Hz. A piezoelectric membrane providing an audible acoustic field induced a stream of microparticles on which the lattice pattern overlapped, with the appearance of a standing wave. The effects of microparticles, including BaSO 4 , TiO 2 , and guanine platelets derived from fish scales, on the formation of the lattice pattern were investigated. These three types of microparticles in water enabled a visualization of the vortex streams and generated a lattice pattern of reflected light. The guanine platelets exhibited the most precise lattice pattern over the droplet surface, with a lattice width of 100–200 μm. This phenomenon may provide a new tool for detecting and manipulating micro vortex flows in the aqueous chamber of a microfluidic device combined with an acoustic transducer.