A theoretical potential-well model of acoustic tweezers

Standing wave acoustic tweezers have been popularly used in non-invasive and non-contact particle manipulation. With better penetration ability in biological tissue, acoustic tweezers has the promising potential for in-vivo study. However, the dual-beam configuration has many limits on operation and system setup. A single-beam trapping model is thus preferable. According to the concept of optical vortex, we propose an acoustics-vortex trapping model of acoustic tweezers by using one four-element planar transducer. Each element is a 5.8 by 5.8-mm square, driven by 1-MHz and 1-MPa sine wave at a phase increment of ¿/2. The kerfs are set to be 0.51 mm. An acoustical vortex with an axial null and spiral wavefronts is produced. By applying Gor´kov´s theory in the Rayleigh regime, the potential energy and radiation force exerted on a particle can be obtained. In transverse aspect, the acoustical vortex behaves as a series of potential wells. After overcoming the repulsion in the outer acoustical vortex, particles are confined within beam´s axis but not limited in a certain depth; the trapping effect of acoustical vortex is only considered in transverse section. For 13-¿m polystyrene particles, the trapping force is 52.5 pN. The trapping capacity reaches 10⁶ particles within a plane. Most stiff and dense particles are suitable in this model. The results also suggest the ideal trapping depth locate in the near field of the transducer. This model is advantageous for 2-D manipulation particularly when utilized on an in-vivo blood vessel. The study also provides the discussion of trapping properties in an acoustical vortex model.