Orbital Angular Momentum Transfer to Stably Trapped Elastic Particles in Acoustical Vortex Beams

Area of Science:

• Acoustofluidics
• Particle manipulation
• Rheology

Background:

• Acoustic vortex beams can trap and manipulate particles in fluids.
• Understanding particle rotation dynamics is key for applications like micro-rheology.

Purpose of the Study:

• To investigate the controlled rotation of solid particles trapped by ultrasonic vortex beams.
• To analyze the torque balance and identify dissipation mechanisms responsible for particle rotation.

Main Methods:

• Controlled trapping and rotation of single polystyrene beads and clusters using ultrasonic vortex beams.
• Comparison of experimental torque measurements with a torque balance model.
• Analysis of acoustic response and dissipation mechanisms.

Main Results:

• Particles were trapped against gravity and simultaneously rotated by the acoustic beam.
• Measured torque suggests two dominant dissipation mechanisms: bulk absorption within the particle and viscous dissipation in the fluid boundary layer.
• Dissipation affects both dipolar and quadrupolar vibration modes, invalidating the Rayleigh scattering regime for torque modeling.

Conclusions:

• The study elucidates the primary mechanisms driving acoustic orbital angular momentum dissipation and particle rotation.
• Accurate knowledge of particle elastic absorption is crucial for rheological measurements using trapped spheres.
• External rotational flow can significantly influence particle dynamics in viscous liquids.