Mid-Air Single-Sided Acoustic Levitation in High-Pressure Regions of Zero-Order Bessel Beams

Abstract

Acoustic levitation enables non-contact manipulation using sound waves. While conventional methods entrap particles at pressure nodes (zero-pressure region surrounded by high-pressure), we demonstrate stable acoustic levitation and translation in mid-air within a high-pressure axial core of a single-sided zero-order Bessel beam for the first time. The trap operates at a long working distance, up to 397 mm ($46.6 λ$), supports simultaneous multi-particle levitation, and maintains stability over obstacles. Our work establishes a new paradigm for single-sided acoustic manipulation in mid-air.

Figures (4)

• Figure 1: (a) Illustration of particle levitation at the central core of a zero-order Bessel beam. The beam is generated using a PAT. (b)–(e) Calculated acoustic pressure field of a zero-order Bessel beam from the PAT ($\beta = 20^\circ$, $V_\text{in}=8\,\text{V}$) and the radiation force on an EPS sphere (radius $a=0.75\,\text{mm}$). (b) Transverse and (d) axial pressure profiles. (c) Transverse force $F_x$. For the axial plots (d) and (e), solid vertical lines indicate the two evaluation planes, $z_1=110\,\text{mm}$ (red) and $z_2=220\,\text{mm}$ (blue, near the experimental levitation height). (e) Net axial force. The black line shows the balance between radiation force and gravity. The orange and green dashed lines include the drag from acoustic streaming calculated with thermoviscous hu2015sound and atmospheric bass1995atmospheric attenuation models, respectively. The range between these two models is consistent with experimentally observed streaming effects stone2025characterisingstone2025experimental.
• Figure 2: Particle translation by (a)-(c) horizontal beam tilting ($\theta_\text{tilt}$ from -10$^{\circ}$ to 10$^{\circ}$) and (d)-(f) vertical cone angle modulation ($\beta$ from 25$^{\circ}$ to 15$^{\circ}$). The experimental trajectories in (a) and (d) are composites of images taken every 0.05 s, visualized with a 30--100% transparency ramp. The corresponding simulated pressure profiles are shown in (b, c) and (e, f). (g) Experimental comparison of the working ranges for a Bessel beam and a conventional twin trap (average of 5 trials).
• Figure 3: Simultaneous levitation of multiple particles. (a) Parallel trapping of two particles (left: $a=0.79\,\text{mm}$, $\rho_p=32.3\,\text{kg\,m}^{-3}$; right: $a=0.79\,\text{mm}$, $\rho_p=31.6\,\text{kg\,m}^{-3}$) in replicated Bessel beams with Damman grating. (b) Vertically aligned trapping of two particles (top: $a=0.77\,\text{mm}$, $\rho_p=35.5\,\text{kg\,m}^{-3}$; bottom: $a=0.77\,\text{mm}$, $\rho_p=34.3\,\text{kg\,m}^{-3}$) in separated zones with bottle signature . The cyan lines indicate the range of positional fluctuations, which is significantly smaller for the vertically aligned trap.
• Figure 4: Experimental demonstration and numerical simulation of levitation through an obstacle. (a) Simulated sound pressure field using BEM, showing the reconstruction of the Bessel beam profile beyond the obstacle. The mesh size for BEM was set to approximately $\frac{\lambda}{11}$. The cyan lines indicate the range of positional fluctuations. (b) Photograph of a particle levitated above the obstacle.