Dynamic Acoustic Levitator Based On Subwavelength Aperture Control.

Acoustic levitation provides a means to achieve contactless manipulation of fragile materials and biological samples. Most acoustic levitators rely on complex electronic hardware and software to shape the acoustic field and realize their dynamic operation. Here, the authors introduce a dynamic acoustic levitator that is based on mechanically controlling the opening and (partial) closing of subwavelength apertures. This simple approach relies on the use of a single ultrasonic transducer and is shown to permit the facile and reliable manipulation of a variety targets ranging from solid particles, to fluid and ferrofluidic drops. Experimental observations agree well with numerical simulations of the Gor'kov potential. Remarkably, this system even enables the generation of time-varying potentials and induces oscillatory and rotational motion in the levitated objects via a feedback mechanism between the trapped object and the trapping potential. This is shown to result in long distance translation, in-situ rotation and self-modulated oscillation of the trapped particles. In addition, dense ferrofluidic droplets are levitated and transformed inside the levitator. Controlling subwavelength apertures opens the possibility to realize simple powerful levitators that nevertheless allow for the versatile dynamic manipulation of levitated matter.