Acoustic Virtual Vortices with Tunable Orbital Angular Momentum for Trapping of Mie Particles

Acoustic vortices can transfer angular momentum and trap particles. Here, we show that particles trapped in airborne acoustic vortices orbit at high speeds, leading to dynamic instability and ejection. We demonstrate stable trapping inside acoustic vortices by generating sequences of short-pulsed vortices of equal helicity but opposite chirality. This produces a “virtual vortex” with an orbital angular momentum that can be tuned independently of the trapping force. We use this method to adjust the rotational speed of particles inside a vortex beam and, for the first time, create three-dimensional acoustics traps for particles of wavelength order (i.e., Mie particles).

Figure 1

Simulated amplitude fields of a RV1 (topological $\text{charge}=1$) generated with the SC array using 15 Vpp (a),(b),(e),(f),(i),(j). Arrows represent the simulated forces ($z$ component not to scale) exerted on particles of radius $0.5\mathrm{mm}=0.058\lambda$ (a),(b), $1\mathrm{mm}=0.116\lambda$ (e),(f), and $4\mathrm{mm}=0.232\lambda$ (i),(j). Simulated trajectories that the particles follow (c),(d),(g),(h),(k),(l). The $0.058\lambda$ particle remains trapped, whereas the others get ejected. Axes and radius of the particles are in wavelengths ($\lambda$), ($k=\text{wave number}$, $a=\text{radius}$).

Figure 2

Time to ejection as a function of the particle size for a RV1 and a VV1 generated with the SC array and an excitation signal of 15 Vpp. The particle is stably trapped after 2 s. Radius of the particles are in wavelengths ($k=\text{wave number}$, $a=\text{radius}$).

Figure 3

Decoupling OAM from trapping force in a VV1. (a),(c) Angular velocity (in rpm) and (b),(d) axial drop against gravity of a $0.6\mathrm{mm}=(0.069\lambda )$ radius particle as a function of (a),(b) number of cycles of ${\mathit{N}}^{-}$ (counterclockwise vortex) with clockwise vortex $({\mathit{N}}^{+})=40$ periods and (b),(d) the amplitude of the excitation signal for a VV1 (${\mathit{N}}^{-}=80$, ${\mathit{N}}^{+}=40$). Experiments were repeated 5 times and the error bars represent the standard deviation.

Figure 4

(a),(b) An EPS particle of $16\mathrm{mm}=1.86\lambda$ diameter trapped in a VV5 (topological $\text{charge}=5$) in the OSS array. (c) Simulated force along $X$ or $Y$ directions centered on the focus; horizontal axis is in terms of wavelengths $\lambda$. (d) Simulated forces along the $Z$ direction centered on the focus. Simulated amplitude fields in the $XY$ (e) and $XZ$ (f) planes. Simulated phase fields in the $XY$ (g) and $XZ$ (h) planes. The gray dashed circle in (e) and (f) represent the sphere.