Abstract Acoustic tweezers use ultrasound for contact-free manipulation of particles from millimeter to sub-micrometer scale. Particle trapping is usually associated with either radiation forces or acoustic streaming fields. based on single-beam focused vortices have attracted considerable attention due their selective capability, but proven difficult three-dimensional (3D) without a complex tr...

In the second part of the thematic tutorial series "Acoustofluidics--exploiting ultrasonic standing waves forces and acoustic streaming in microfluidic systems for cell and particle manipulation", we develop the perturbation theory of the acoustic field in fluids and apply the result in a study of acoustic resonance modes in microfluidic channels.