Observation of a single-beam gradient force acoustical trap for elastic particles: acoustical tweezers

We demonstrate the trapping of elastic particles by the large gradient force of a single acoustical beam in three dimensions. Acoustical tweezers can push, pull and accurately control both the position and the forces exerted on a unique particle. Forces in excess of 1 micronewton were exerted on polystyrene beads in the sub-millimeter range. A beam intensity less than 50 Watts/cm$^2$ was required ensuring damage-free trapping conditions. The large spectrum of frequencies covered by coherent ultrasonic sources provide a wide variety of manipulation possibilities from macro- to microscopic length scales. Our observations could open the way to important applications, in particular in biology and biophysics at the cellular scale and for the design of acoustical machines in microfluidic environments.