Abstract
Acoustic tweezers use ultrasound for contact-free manipulation of particles from millimeter to sub-micrometer scale. Particle trapping is usually associated with 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 three-dimensional (3D) trapping without a complex transducer array and significant constraints on the trapped particle properties. Here we demonstrate a 3D acoustic tweezer in fluids 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 operating at 500 kHz, 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 an additional twist to the approach and expands the range of particles that can be manipulated.
Highlights
Acoustic tweezers use ultrasound for contact-free manipulation of particles from millimeter to sub-micrometer scale
Focusing an acoustic vortex increases its spatial selectivity, and makes the 2D trap stronger. 3D trapping with radiation force in a focused acoustic vortex requires careful selection of material properties and sizes, and yet the achieved axial force is several orders magnitude weaker than lateral forces
Such a mechanism offers three advantages: i) it doesn’t require resonance modes of the particle to provide lifting force along z axis, as the levitation is provided by the drag in the steady flow, so it sets less constraints on particle size and materials; ii) the steady flow velocity can reach several centimeters per second, especially suitable for large and heavy particles that cannot be handled with radiation forces; and iii) the drag force and trapping position can be tuned by controlling the streaming flow velocity
Summary
The focal plane section results indicate that the simulated flow magnitude is weaker along the axis itself but reaches the maximum in the surrounding cylindrical region, forming a fluid vortex where the acoustic vortex is located (Fig. 3b) These combined effects will trap the particle in the x–y plane, but provide a strong localized drag flow for levitation and trapping in the z direction. The small discrepancy is due to the slight change in field distribution, the existence of PIV particles, and that the viscosity of water changes over time due to the dissolved gas, dust Such a result confirms that the required levitation force is providing by acoustic streaming. The wave field is generated by a single transducer and passive lens instead of the transducer array, which provides an inexpensive and reliable route for contact-free particle and fluid manipulation
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