Acoustic energy measurement for a standing acoustic wave in a micro-channel
Laboratoire de Physique et Mécanique des Milieux Hétérogènes (PMMH) UMR7636 CNRS - ESPCI - UPMC Univ. Paris 6 - UPD Univ. Paris 7, École Supérieure de Physique et de Chimie Industrielles - 10, rue Vauquelin, F-75231 Paris Cedex 5, France, EU
Accepted: 18 January 2012
An optical method to measure the acoustic energy density of standing acoustic waves in microfluidic devices has been proposed and applied by Barnkob et al. (Lab Chip, 10 (2010) 563) to a 2 MHz standing acoustic wave in a 250 μm high micro-channel, with the acoustical axis orthogonal to the optical axis. The method is based on the motion of a particle under Acoustic Radiation Force (ARF). However, this configuration is not always possible. We present a new optical method to measure the acoustic energy density when acoustical and optical axis are parallel. The main difference is that it is not possible to track directly the trajectory of the particles anymore, since they move along the optical axis. The method is based on the information contained in the defocused particle images. The variation of the diameter of defocused particle images is used to measure the vertical velocity along the vertical direction. From the axial velocity profile induced by ARF, it is possible to evaluate the acoustic energy density in our experimental setup. This result is important for every application using ultrasounds in micro-channels.
PACS: 43.25.Gf – Standing waves; resonance / 43.25.Uv – Acoustic levitation / 43.25.Qp – Radiation pressure
© EPLA, 2012