The physical acoustics of acoustic tweezers

The operating principles of acoustic tweezers have attracted significant recent research. In parallel, applications of this technology are growing rapidly. For an acoustic tweezer to be dexterous it must be able to not only to trap particles, but to manipulate them flexibly, for example by moving different particles or groups of particles independently and producing a variety of particle distributions. This paper will survey the physical principles behind these developments: acoustic radiation forces on particles large and small, the design of dexterous tweezers and the control of acoustic fields and hence the forces and dexterity possible.

Two key concepts are explored, firstly the modelling of acoustic forces on arbitrary shaped and sized particles of arbitrary composition. This requires numerical methods such as finite volume models that include all the relevant physics. Various assumptions are possible under certain circumstances such that simple analytical models can be used. However, biological applications involving soft and asymmetrical particles (i.e. cells) necessitates these more comprehensive numerical models or a clear understanding of the assumptions inherent in the simpler analytical models. The second concept is the use of arrays of active elements that enable a wide variety of acoustic pressure fields to be generated. Critically, arrays enable reconfigurability so that a given device can generate multiple and movable field patterns - it is this that leads to dexterous manipulation. Importantly, the design of these devices requires them to be non-resonant in at least one dimension so that the fields are not dependent solely on device geometry.

In this paper it is shown that the acoustic radiation force on particles of arbitrary size and shape can be modeled using finite volume time domain methods. It is also shown that in many practically significant cases, much simpler analytical models can be used with only a small loss of accuracy. Maximum dexterity is only obtained if an arbitrary acoustic field is generated within the device and this requires solution of an inverse problem: how to best excite the sources to achieve the desired field? Ideas from optical tweezers such as vortex fields are also shown to be applicable to acoustic devices as they enable trapping, translation and rotation. A number of possible applications of the emerging dexterous acoustic tweezers, such as new bio-assays based on the response of cells to an external force and tissue engineering are also described. Finally, future prospects are discussed, with particular attention to the complimentary developments of optical tweezers and how together these devices might find even more widespread application.