Willis Coupling-Induced Acoustic Radiation Force and Torque Reversal

Area of Science:

• Acoustics
• Metamaterials
• Particle Manipulation

Background:

• Acoustic meta-atoms are fundamental to metamaterials, enabling functions like beam steering and focusing.
• Metamaterials shape acoustic fields, influencing acoustic radiation force and torque for applications like acoustic tweezers.
• Object shape significantly impacts scattering properties and thus acoustic radiation force and torque.

Purpose of the Study:

• To demonstrate control over nonlinear acoustic effects (radiation force and torque) by designing linear properties of objects using metamaterial concepts.
• To extend existing models of acoustic scattering to include object shape asymmetry and Willis coupling terms.
• To investigate the impact of shape asymmetry on acoustic radiation force and torque, and its implications for particle manipulation.

Main Methods:

• Extended scattering models to include a polarizability tensor with Willis coupling terms to quantify asymmetry.
• Applied the model to a 3D subwavelength meta-atom exhibiting maximal Willis coupling.
• Analyzed the influence of shape asymmetry on acoustic radiation force and torque.

Main Results:

• Demonstrated that nonlinear acoustic effects can be controlled by designing object properties with metamaterial concepts.
• Showed that maximal Willis coupling in an asymmetric meta-atom can reverse acoustic force and torque compared to a symmetric particle.
• Confirmed that shape asymmetry shifts stable trapping locations in acoustic fields.

Conclusions:

• Extended Gorkov's theory of acoustophoresis by incorporating shape asymmetry into acoustic radiation force and torque calculations.
• Highlighted the potential for tunable, shape-dependent particle sorting through the control of acoustic fields via asymmetric meta-atoms.