Radiation forces exerted on arbitrarily located sphere by acoustic tweezer

Abstract

In a previous paper acoustic radiation force on a lipid sphere in a 100-MHz focused Gaussian field was calculated to demonstrate the acoustic tweezer effect near the focus. The theoretical formulation was based on the situation where the sphere is centered along the beam axis. Given intensity distribution independent of the x axis, it was then approximated by a cylindrical model for the sake of simplicity. Only the axial forces were considered because no lateral forces exist due to an object's symmetry. However, it was difficult to employ the same technique to the more general case when it is off the beam axis. To overcome the limitation, in this paper the previous model is modified to compute two additional lateral forces by carrying out the projection over arbitrary incident planes to restrict the integration limits. For different sizes of the sphere, the magnitudes of the net forces in three orthogonal directions are computed. The results show that the acoustic tweezer can be realized more easily in the lateral directions than in the axial directions. Differing from the axisymmetric case, the spheres of small sizes tend to be more strongly attracted than the larger ones in the lateral directions.

Acoustic radiation force on a rigid cylinder in a focused Gaussian beam

“…These studies have shown that Gaussian beams can be used for trapping particles in both axial and transverse directions, although the latter is expectedly much easier. These theoretical findings were also experimentally confirmed using a single focused Gaussian ultrasound beam, for entrapment of very fine lipid droplets (in order of 125 m) [41][42][43]. Besides the transverse entrapment, it was also shown that a high frequency focused beam could be used to laterally move micro-droplets towards the focus point.…”

“…The above observations suggest that negative axial and lateral radiation forces can be produced generally when a focused Gaussian beam is concentrated on the very top (or bottom) of the particle. To complete the discussion, it is worth mentioning that the problem of lateral movement of spherical particles using Gaussian beams, to pull them towards the beam axis has been previously reported by other authors [41][42][43]. The magnitude of such attractive transverse forces is understandably greater than the negative axial forces, as seen in Figs.…”

“…In some recent experimental and theoretical investigations [20,[41][42][43], it has been shown that Gaussian beams can effectively be used for axial and transverse entrapment of miro-and nano-scale particles. Preliminary analytical studies of Lee et al [20] and Lee and Shung [41] using ray acoustics approach also showed the feasibility of acoustic tweezing and trapping of arbitrarily located spherical objects using a highly focused high frequency Gaussian beam. These studies have shown that Gaussian beams can be used for trapping particles in both axial and transverse directions, although the latter is expectedly much easier.…”

Manipulation of particles in two dimensions using phase controllable ultrasonic standing waves

“…Initially, these 'acoustical tweezers' used opposing focused transducers to produce a trap at their, common, focal point (Wu 1991). More recent theoretical (Lee et al 2005;Lee & Shung 2006) and experimental (Lee et al 2009) work has demonstrated the feasibility of using a single focused transducer working at a high frequency (30 MHz) to trap particles much larger than a wavelength in diameter against a surface. Particle manipulation has been achieved by moving the transducer and hence the focal position.…”

Modulation of ultrasound to produce multifrequency radiation force

“…[8][9][10] The manipulation of spheres, drops, bubbles, and other objects using radiation force has also been explored. [11][12][13][14][15][16] Methods based on radiation force have been developed to characterize different materials such as soft gelatin phantoms and soft tissue. [17][18][19] One of the emerging uses of ultrasound radiation force is its use in ultrasound-based elasticity imaging.…”