Abstract: Inspired by the versatility and vast range of applications of optical tweezers, acoustic tweezers have attracted significant recent research interest. They have potential advantages over optical tweezers in the range of materials that can be manipulated and their requirement for lower power densities. They also typically operate on a somewhat larger length scale so there is a strong case for their use as a complementary manipulation modality.  However, relative to optical counterparts, the development of acoustic tweezers is in its infancy. 
The use of arrays of active piezoelectric elements enables a wide variety of acoustic pressure fields to be generated.  In essence the piezo-array has a similar functionality to the spatial light modulator in optics. Given that the acoustic trapping force is governed by the spatial distribution of acoustic energy around a particle, controlling the acoustic pressure means controlling the forces. The use of arrays enables reconfigurability so that a given device can generate multiple field patterns. Critically we make at least one dimension non-resonant and this leads to acoustic fields are not dependent solely on device geometry. The ultimate device would be able to generate an arbitrary acoustic field, but the theoretical and practical solution to this problem remains an open question. However, ideas from optical tweezers such as vortex fields can be exploited in acoustic tweezers as they enable trapping, translation and rotation.
Applications of acoustic tweezers are wide ranging and include bio-assays based on the response of cells to an external force, 3D tissue engineering as well as the assembly of engineering materials such as composites. Recent work has also shown that these array devices act as versatile acousto-optic devices capable of, for example, imparting orbital angular momentum to optical beams.