Particle assembly and object propulsion using acoustic holograms

Abstract

The contact-free manipulation of particles using ultrasound fields is an active field of research promising a number of applications. Conventional acoustic tweezers use strongly focused beams or higher order Bessel beams to provide a trap for single particles to be manipulated with. Other more mature methods use resonators to create elongated potential wells for collective particle trapping or separation. The resulting assemblies have a limited complexity, because the fields are highly symmetric. We recently introduced the acoustic hologram as an alternative way to create arbitrary ultrasound fields. In this talk, I will present two concepts, one for particle trapping and one for propulsion of objects, that have been enabled by this new method. The first is parallel assembly of microparticles at a surface in the shape of a projected acoustic image. Using a global trigger, these particles can be fused together to form a mechanically stable object. The second demonstration is a seemingly dynamic effect resulting from our static hologram. By defining the phase gradient (essentially the wave vector) along the water-air interface, it is possible to continuously propel objects along predefined trajectories. The necessary complexity to create such ultrasound fields with defined amplitude and phase distribution is easily managed using acoustic holograms.