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 submillimeter range. A beam intensity less than $50\mathrm{W}/{\mathrm{cm}}^2$ was required, ensuring damage-free trapping conditions. The large spectrum of frequencies covered by coherent ultrasonic sources provides a wide variety of manipulation possibilities from macroscopic 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.

Figure 1

(a) Sketch of the ultrasonic emitter (1.15 MHz) immersed in a water tank. The beam is focused with a concave acoustical lens of focal ratio $\simeq 0.7$ upon polystyrene particles lying on an acoustically transparent polyethylene film, (b)–(e) Measured acoustic field. (b) Color plot of the normalized acoustic intensity in the direction of propagation (negative to positive $z$). (c,d) Field’s phase and intensity in the focal plane ($z=0$). (e) Comparison of the measured and predicted pressure fields (intensity) in the focal plane $z=0\mathrm{mm}$.

Figure 2

(a) Photograph of a particle of radius $a=170\mu \mathrm{m}$ trapped in three dimensions. The large negative axial radiation force lifts the particle from a distance $\simeq 2.6\mathrm{mm}$ below the equilibrium position. A second particle lying on the film is not attracted to the focus. The scale bar represents $200\mu \mathrm{m}$. (b) Theoretical prediction of the axial acoustical radiation force as a function of the particle’s position on the propagation axis.

Figure 3

Measurement of the lateral force as a particle is accelerated towards the propagation axis. The force is obtained as a function of the lateral position $x$ by calculating the lateral acceleration from the instantaneous position of the center of the sphere. The snapshots are displayed every 2 ms.

Figure 4

Systems of one or two particles can be trapped. (a) Radius $a=189\mu \mathrm{m}$. (b) Radii $a=177$ and $108\mu \mathrm{m}$. (c) Radii $a=98$ and $67\mu \mathrm{m}$. The scale bar represents $200\mu \mathrm{m}$. Their self-arrangement exhibits the effect of secondary acoustic radiation forces.