Secondary vortex induced by gigahertz acoustic streaming and its applications for 3D particle manipulation

Acoustic streaming serves as a fast-developing contactless tool for manipulation of micro/nanoparticles. However, traditional Eckart streaming has low manipulation precision and Rayleigh streaming only works in a very close distance to the solid boundaries, which limits their applications over a large spatial distance in fluid. In this paper, we propose a novel boundary-independent secondary vortex induced by the high frequency (GHz) acoustic streaming and demonstrate its application for precise three-dimensional (3D) particle manipulation. The size of the Eckart streaming generated by the micro-fabricated acoustic transducer can be efficiently compressed by increasing the viscosity of the solution, which results in a more stable and controllable secondary convection streaming in the chamber. Arbitrary and precise manipulation of different types of particles in 3D space is approached by controlling the position and power of the transducer. Moreover, multilayer assembly of cell-encapsulating hydrogels is achieved to validate the high throughput and good biocompatibility of the secondary vortex. This secondary vortex-based manipulation tool exhibits versatility toward different objects, highly efficient assembly, and good biocompatibility for handling viable samples, and shows potential in bio-fabrication, material synthesis, tissue engineering, <i>etc.</i>