Three dimensional acoustic tweezers with acoustic vortex streaming
Acoustic tweezers use ultrasound for contact free manipulation of particles from millimeter to sub micrometer scale. Particle trapping originated in either radiation forces or acoustic streaming fields. Acoustic tweezers based on single beam focused acoustic vortices have attracted considerable attention due to their selective trapping capability, but have proven difficult to use for 3D trapping without a complex transducer array and significant constraints on the trapped particle properties. Here we demonstrate the first 3D acoustic tweezer that uses a single transducer and combines the radiation force for trapping in two dimensions with the streaming force to provide levitation in the third dimension. The idea is demonstrated in both simulation and experiments, and the achieved levitation force reaches three orders of magnitude larger than for previous 3D trapping. This hybrid acoustic tweezer that integrates acoustic streaming adds a new twist to the approach and expands the range of particles that can be manipulated.