Acoustophoretic Contactless Elevation, Orbital Transport and Spinning of Matter in Air

Abstract

We present the experimental demonstration and theoretical framework of an acoustophoretic concept enabling contactless, controlled orbital motion or spinning of droplets and particles in air. The orbital plane is parallel to gravity, requiring acoustophoretic lifting and elevation. The motion (spinning, smooth, or turnstile) is shown to have its origin in the spatiotemporal modulation of the acoustic field and the acoustic potential nodes. We describe the basic principle in terms of a superposition of harmonic acoustic potential sources and the intrinsic tendency of the particle to locate itself at the bottom of the total potential well.

Controllable Micro-Particle Rotation and Transportation Using Sound Field Synthesis Technique

“…Theoretical calculation of the acoustic radiation torque on a rigid rectangular disk was first proposed by Maidanik, in which the general expression was derived by an approach analogous to the radiation force [36]. Nowadays, the theory has been further developed with the influence of particle shape and material [37][38][39][40]44,48].…”

“…Besides the conventional vortex-based rotation, controlling the object orientation is also achievable with this kind of acoustic torque. Changing the orientation of the sound field, by the manner of mechanically shifting the wave direction or modulating the wave parameters (amplitude or frequency), the fibers, and other non-spherical particles will be driven to rotate and finally reach an equilibrium angular position [38][39][40][41][42][43][44]. However, the resolution and controllability are limited for these demonstrated methods.…”

“…However, the resolution and controllability are limited for these demonstrated methods. For the method adjusting the propagation direction of standing wave with alternate excitation of several pairs of opposed transducers, the particle could only rotate like a step motor and the step angle 180 • /N is limited by the number of transducer pairs N [38,39]. Particle rotation based on phase or frequency modulation strongly relies on the resonant modes of the employed acoustic chambers.…”

Dynamical stability of slip-stacking particles

“…Acoustic standing waves can be used to trap small spheres or droplets in a sinusoidal potential originally described by Gor'kov [33]. This technique has grown in sophistication and application [34,35]. Equation (24) would also determine the stability of objects (with mass M) in a possible acoustic slip-stacking configuration.…”

A One-Sided View of Acoustic Traps

“…Recent improvements to standing acoustic wave schemes led to levitating and translating single or multiple particles in air [15], and acoustophoresis provides advanced particle, cell and organism separation in complex microfluidic environments [16,17]. Standing wave schemes have recently been proposed to accurately manipulate particles in two dimensions using surface [18,19] or bulk acoustic waves [20] with phase or frequency shifts in order to demonstrate capabilities similar to OTs.…”