Universal Relations of Energy Flow, Acoustic Spin and Torque for Near-Field Acoustic Tweezers

TL;DR

This paper establishes universal geometric relations among acoustic spin, torque, energy flow, and reactive power in near-field acoustics, verified across various systems, revealing fundamental principles and new locking relations beyond known wave phenomena.

Contribution

It proves universal geometric relations among key acoustic quantities, independent of wave structure, and demonstrates their implications in near-field acoustic sources and manipulations.

Findings

Universal relations verified in multiple acoustic systems

Acoustic spin and torque linked to energy flow and reactive power

Demonstrated multipole torques in near-field acoustic sources

Abstract

Acoustic spin, radiation torque, energy flow, and reactive power are of significant importance from both fundamental and practical aspects, responsible for flexible tweezer manipulations and near-field sound directionality. Nevertheless, the intrinsic relations among these physical quantities are far from clear. Here, we prove the universal geometric relations among them in acoustics, independent on wave structure details. Particularly, we connect acoustic spin and torque to the cross product of time-averaged energy flow and reactive power, as well as to the local vorticity of energy flow. These relations are universally valid, verified in a variety of different acoustic systems. We also demonstrate the multipole mechanical torques and forces generated in three acoustic near-field sources: Janus, Huygens and Spin sources, applying on small lossy particles. These universal geometric relations uncover hidden locking relations beyond simple spin-momentum locking of near-field waves, and show the basic principles between the acoustic spin, radiation torque, and energy flow, reactive power.