Dynamics of an acoustically trapped sphere in beating sound waves

Acoustic manipulation of multi-body structures and dynamics

“…The precise spatio-temporal force profile exerted on the suspended particles can then be tuned by shaping the beam of light. Indeed, recent advances in acoustic manipulation have recapitulated some of the library of technologies developed for advanced optical manipulation, including acoustic holography [147,148], controlled rotation via the transfer of orbital angular momentum [54,[149][150][151][152][153], and acoustic tweezers [154][155][156][157].…”

“…Specially shaped acoustic modes have also been shown to carry orbital angular momentum, which tends to cause levitated objects to orbit special phase singularity points in the acoustic field. Similar to optical beams, orbital angular momentum is commonly produced by structuring helical phase gradients in cylindrical transducer arrays [54,[149][150][151][152][153], such that particles acquire orbital angular momentum about the center of the trap. Acoustic spin and orbital angular momentum transfers are in practice difficult to distinguish, as they often occur together and in the same direction [187].…”

“…The extremely helpful simplification to conservative dynamics occurs when the acoustic mode exciting the system has no net momentum, as in the case of an ideal standing wave. Indeed, traveling waves, or waves with angular momentum, can be used to transfer momentum directly to acoustically manipulated objects [54,[149][150][151][152][153]. In these cases, computing the primary force exerted by the acoustic mode on a point particle includes terms involving the gradient of the mode's phase (a cyclic quantity) [167].…”

Conservative and nonconservative forces for Mie particles in acoustic trapping

“…And ATs have been implemented using various techniques. For instance, they have been achieved through the utilization of single beam [9], hologram [12], ultrasonic attenuation [16], beating sound waves [17], and other methods [10]. Due to the orders of magnitude slower wave speed, a sound wave is generally able to exert much larger forces than its optical counterpart at the same intensity.…”

“…It is possible for Rayleigh particles [24,25], but not for larger particles. And for deeply subwavelength Rayleigh particles where the SF is vanishing small, the acoustic force field can be considered conservative [17] and is associated with the Gor'kov potential [26,27]. For larger particles, we also emphasize the crucial role of SF, which can greatly affect the trapping stability.…”