  This paper presents a microfluidic device that implements standing surface
acoustic waves in order to handle single cells, droplets, and generally
particles. The particles are moved in a very controlled manner by the
two-dimensional drifting of a standing wave array, using a slight frequency
modulation of two ultrasound emitters around their resonance. These acoustic
tweezers allow any type of motion at velocities up to few 10mm/s, while the
device transparency is adapted for optical studies. The possibility of
automation provides a critical step in the development of lab-on-a-chip cell
sorters and it should find applications in biology, chemistry, and engineering
domains.
