Robustness of stable trapping equilibria in conical shape acoustic tweezers

Single-axis acoustic tweezers employing multi-element arrays have become a low-cost and easily implemented tool for containerless manipulation in air across various applications. Planar and/or spherical transducer geometries, or a combination thereof, have been primarily used to structure the stationary or progressive acoustic field generating the intended trapping. Typically, the primary objective is to attain the highest possible sound pressure level with minimal infrastructure while maintaining the object under study at a few stable equilibrium points. However, in numerous applications, it is imperative to simultaneously maximize both the number of stable equilibrium points and the trapping forces without compromising longitudinal and transverse trapping capabilities, or increasing the infrastructure or excitation level. In this work, we assess the potential offered by acoustic sources with a conical geometry. This approach leverages their higher beam focus depth, which surpasses that provided by spherical radiators. Simulation results, validated through experiments, are presented to quantify the effect of misalignment, both angular and parallel. Furthermore, the robustness of stable equilibria in this type of trapping device when axial and transverse disturbances are applied is discussed.