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

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.

Figure 1

Schematic of the handling system and its driving signal $A_i$ (t). The $x$, $y$, and $z$ axes represent the reference system relative to the LE, while the $x^{\prime }$, $y^{\prime }$, and $z^{\prime }$ axes represent the global reference system.

Figure 2

(a) Orbit 0, at the center, red. Orbits 1 and 2 are denoted by green and white, respectively. The expanded polystyrene particles ($R_s\approx 0.6\mathrm{mm}$, ${\rho }_s=0.04\mathrm{g}/{\mathrm{cm}}^3$) are highlighted with a circle of the color of the corresponding orbit. (b) Water droplet spinning in orbit 0. The acoustic field distribution stretches the droplet to its typical ellipsoidal shape. (c) Two water droplets simultaneously handled. The droplet at orbit 0 rotates at its axis (spinning), while the droplet in orbit 1 is transported along a circular orbit. Scale bar: (a) 5, (b) 1, and (c) 5 mm.

Figure 3

(a) Contour plot of the nondimensional Gorkov potential within the acoustophoretic manipulator. The white circles represent the two levitated particles during the experiment. (b) Cartesian ($x^{\prime }$, $z^{\prime }$, normalized values) and (c) radial ($\theta$, degree) coordinates of the particle position predicted by the numerical and analytical models are plotted against the experimental data along orbit 1 $T=26\mathrm{s}$). Movies were recorded with a digital camera and processed using the open source package ImageJ 1.46a. (d) Evolution of the acoustic potential between emitter—reflector—emitter. The continuous motion and the periodic discontinuities are explained by the acoustic potential pattern evolution, resembling a tilted washboard.

Figure 4

Particle dynamics. The particle follows the acoustic node and shows strong oscillations at the discontinuities ($R_s=0.6\mathrm{mm}$, ${\rho }_s=0.04\mathrm{g}/{\mathrm{cm}}^3$, $T=26\mathrm{s}$, air viscosity $\mu =20\mu \mathrm{Pas}$, $V_0=1.45\mathrm{m}/\mathrm{s}$).