Acoustic radiation torque exerted on a subwavelength spheroidal particle by a traveling and standing plane wave

Abstract

The nonlinear interaction of ultrasonic waves with a nonspherical particle may give rise to the acoustic radiation torque on the particle. This phenomenon is investigated here considering a rigid prolate spheroidal particle of subwavelength dimensions that is much smaller than the wavelength. Using the partial wave expansion in spheroidal coordinates, the radiation torque of a traveling and standing plane wave with arbitrary orientation is exactly derived in the dipole approximation. We obtain asymptotic expressions of the torque as the particle geometry approaches a sphere and a straight line. As the particle is trapped in a pressure node of a standing plane wave, its radiation torque equals that of a traveling plane wave. We also find how the torque changes with the particle aspect ratio. Our findings are in excellent agreement with previous numerical computations. Also, by analyzing the torque potential energy, we determine the stable and unstable spatial configuration available for a particle. arXiv:1912.12933v1 [physics.flu-dyn]

Controlled orientation and sustained rotation of biological samples in a sono-optical microfluidic device

“…For small particles of spheroidal shape (with long axis a and small axis b) one can derive an explicit expression for the acoustic radiation torque. 50 Such a particle will experience a maximal torque when oriented at 45°to a plane wave, but no torque when ‡ Please note that here T refers to a non-zero component of the torque in some specific direction, the full angular dependence of the torque vector in 3D is not addressed at the moment. placed at an angle of 0°as well as 90°.…”

“…45 Non-zero time-averaged torques on an object mediated by acoustic fields have been studied for many years since the 1950s, [46][47][48][49] theoretically as well as in view of lab-on-a-chip applications. The components of an acoustically generated torque follow the general scaling rule, 36,50…”

“…The contribution known as acoustic radiation torque [46][47][48][49][50] depends on the relative orientation of the particle and the incident wave(s). It can be depicted as the torque arising due to the acoustic radiation forces 52 on individual parts of an extended, asymmetric particle.…”

“…For small particles of spheroidal shape (with long axis a and small axis b) one can derive an explicit expression for the acoustic radiation torque. 50 Such a particle will experience a maximal torque when oriented at 45°to a plane wave, but no torque when placed at an angle of 0°as well as 90°. When the object shape approaches a sphere the torque is found to vanish.…”

Mean-acoustic fields exerted on a subwavelength axisymmetric particle

“…The analytic solutions for the mean-acoustic fields depend, apart from the incoming wave, on the geometric shape [3][4][5][6][7][8][9][10][11][12][13][14] and material properties [15][16][17][18][19][20] of the particle. Moreover, the viscous [21][22][23][24] and thermodynamic 25 properties of the surrounding fluid can significantly change the mean-acoustic fields on the particle.…”