Manipulation and Patterning of Micro-objects Using Acoustic Waves

Acoustics is a commonly used external field to manipulate microparticles and, increasingly, micro-robots within different liquid and gaseous mediums. The establishment of pressure fields within these microfluidic systems via an acoustic excitation, known as acoustofluidics, has been used extensively to trap, pattern, sort and steer particles, droplets and biological matter over the past few decades. Predominantly, acoustic fields are created using piezoelectric transducers and electroactive polymers via two distinct driving systems known as bulk acoustic waves and surface acoustic waves. Recently, acoustic tweezers, a subset of acoustofluidics, have gained traction enabling the accurate manipulation of either individual or a cluster of particles and cells within an arbitrary field established via three distinct approaches, namely, acoustic vortices, phased arrays and acoustic structures akin to optical kinoforms. This chapter introduces the relevant forces, excitation methods and applications enabled by the use of this gentle, non-contact approach for object manipulation.