SHORT COMMUNICATION Parametric study of acoustic focusing of particles in a micro-channel in the perspective to improve micro-PIV measurements Olivier Dron Æ Claire Ratier Æ Mauricio Hoyos Æ Jean-Luc Aider Received: 27 March 2009 / Accepted: 27 June 2009 / Published online: 30 July 2009 Ó Springer-Verlag 2009 Abstract In this study, we propose a new technique to bring most of the particles in a horizontal plane at mid- height of a micro-channel in order to improve the quality of the micro-PIV measurements. The basic principle is to create a stationary acoustic wave along the channel height so that the resulting acoustic force moves the particles toward the pressure node. A parametric study has been carried out without mean flow to characterize the motion of the particles toward the nodal-plane. We found that focusing speed grows with the acoustic pressure amplitude, with the concentration of particles in the suspension and with the particles diameter. We also led a preliminary investigation of acoustic focusing together with a mean stationary flow. We still observed an important focusing of particles for low freestream velocities. Nevertheless, acoustic focusing is inefficient beyond a given critical freestream velocity U 0 for a given set of acoustic param- eters and a given type of particles. It was also shown that other phenomena, like clumps formation, can be observed without mean flow if the acoustic focusing lasts long enough. Keywords Flow field velocimetry  Acoustic streaming  Standing wave 1 Introduction While the last 10 years have seen enormous progress in the general areas of Micro Electro Mechanical Systems (MEMS) design and fabrication (Gad-el-hak 1999), the understanding of fundamental physical processes on these small scales is hampered by the lack of suitable quantita- tive measurement and simulation tools. Velocity measurements at small scale is a challenge and requires dedicated techniques. Several common macro- scopic measurement techniques have been extended to microscopic length scales, like micro-LDV (Laser Doppler Velocimetry) (Thieu et al. 1995), micro-MTV (Molecular Tagging Velocimetry) (Garbe et al. 2008), and micro- PIV (Particle Image Velocimetry) (Santiago et al. 1998; Meinhart and Santiago 1999). Micro-LDV will give accu- rate time dependant velocity measurements in a given point of the volume, while the micro-MTV technique allows mainly for velocity measurements along a line. For in-plane velocity measurements, one has to use micro-PIV, which is actually similar to the macroscopic PIV method, which means bulk seeding and auto-correla- tion calculations to get velocity fields. However, the laser sheet illumination is irrelevant for micro-PIV since the laser sheet thickness (the laser Waist) and the micro-channel height are roughly of the same value. Instead, the whole volume of the flow is illuminated at the same time, which means all the tracer particles are illuminated, even the ones that are not in the measurement plane. Those particles generate background noise on the images, and lower the resolution. Several techniques to improve micro-PIV have O. Dron (&)  J.-L. Aider Team ‘‘Instability, Control and Turbulence’’ ESPCI, Laboratoire de Physique et Me ´canique des Milieux He ´te ´roge ´nes, 10 rue Vauquelin, Paris Cedex 05 75231, France e-mail: dron@pmmh.espci.fr J.-L. Aider e-mail: aider@pmmh.espci.fr C. Ratier  M. Hoyos Team ‘‘Separation’’ ESPCI, Laboratoire de Physique et Me ´canique des Milieux He ´te ´roge ´nes, 10 rue Vauquelin, Paris Cedex 05 75231, France 123 Microfluid Nanofluid (2009) 7:857–867 DOI 10.1007/s10404-009-0477-7