Precise, contact-free manipulation of objects is crucial in various fields, including nanofabrication and bioengineering. Acoustic tweezers, using ultrasound, offer a promising approach. Two main types exist: radiation force tweezers and acoustic-streaming tweezers. Radiation force tweezers, further divided into standing-wave and traveling-wave types, typically use multiple transducers to create standing wave nodes or manipulate a single beam's phase to create pressure nodes. While effective, these methods often lack selectivity and/or require complex transducer arrays. Traveling-wave tweezers using acoustic vortices, which impart angular momentum, show promise for single-particle trapping, but 3D trapping remains challenging due to the axial node line. Acoustic-streaming tweezers utilize nonlinear Rayleigh streaming to manipulate particles indirectly, offering simplicity but lower spatial resolution. This research proposes a hybrid approach, combining radiation force and acoustic streaming using a single transducer and a passive lens to achieve 3D trapping with enhanced capabilities.

Existing acoustic tweezers face limitations in 3D trapping. Standing-wave tweezers, while capable of manipulating particle groups, lack selectivity due to their grid-like node structure and often require multiple transducers. Traveling-wave tweezers using acoustic vortices offer increased selectivity but generally only achieve 2D trapping because of the axial node line. Although some advances have been made in 3D trapping using acoustic vortices, these methods often have limitations in terms of particle parameters or require complex transducer arrays. Acoustic streaming-based tweezers, while simple, suffer from poor spatial resolution. The existing methods either lack 3D capability, require complex setups, are limited by particle size, or are not highly selective. This study attempts to overcome these limitations.

This study proposes a hybrid 3D acoustic tweezer using a single transducer and a passive polydimethylsiloxane (PDMS) lens to focus an acoustic vortex. The lens design combines Fresnel lens and vortex phase signatures, calculated using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimization method. The acoustic field and streaming field were simulated using Code Aster and OpenFOAM, respectively. The acoustic field simulation solved the Helmholtz equation, while the acoustic streaming simulation involved three stages: time-domain wave propagation, time-averaging to calculate the body force driving streaming, and incompressible steady-state CFD to calculate the streaming velocity field. Experimental measurements of the acoustic field were conducted using a hydrophone and a 3D positioning stage. Particle Image Velocimetry (PIV) was used to measure the acoustic streaming field using polyamide seeding particles. 3D trapping was demonstrated using cellulose acetate polymer spheres and cylindrical particles. The levitation force was calculated by comparing the difference between gravity and buoyancy. Radiation forces were calculated semi-analytically using angular spectrum analysis and Gorkov potential.

The researchers successfully demonstrated a 3D acoustic tweezer using a single transducer and a PDMS lens to focus an acoustic vortex. Finite element simulations and experimental measurements showed good agreement in both acoustic and streaming fields. The experimental levitation force reached 5.2 µN for a 1.5 mm diameter cellulose acetate sphere, three orders of magnitude greater than previous 3D traps based solely on radiation forces. This enhanced levitation force enables manipulation of a wider range of particle sizes, shapes, and material properties, as demonstrated with both spherical and cylindrical particles. The streaming velocity along the z-axis, measured by PIV, confirmed the significant contribution of streaming to levitation. The tweezer also demonstrated the ability to move particles along 3D trajectories by scanning the transducer. The trap remained stable even with a tilt angle of up to 21°. The lateral stiffness was calculated as 0.52 mN/m, and the vertical stiffness as approximately 0.064 mN/m.

The results demonstrate the efficacy of the hybrid approach combining radiation force and acoustic streaming for 3D particle manipulation. The significantly increased levitation force compared to previous radiation force-based 3D tweezers highlights the advantages of harnessing acoustic streaming. This method's ability to handle a wider range of particle parameters, along with its simpler single-transducer setup, offers considerable advantages. The ability to control the levitation force by adjusting ultrasound amplitude provides versatility. This technology has potential applications in areas requiring precise 3D manipulation of various particles, such as biomedical studies involving cell manipulation and microfluidics.

This research presents a novel 3D acoustic tweezer that leverages acoustic streaming to achieve significantly improved levitation force and expand the range of manipulatable particles. The use of a single transducer and passive lens simplifies the setup, while the enhanced levitation opens doors for various applications. Future work could focus on improving axial resolution, exploring different lens designs to further optimize trapping, and investigating the use of pulsed signals to enhance control. The integration of this technology into microfluidic devices for biological studies is a particularly promising avenue.

The current design has limitations. The axial trapping stiffness is lower than the radial stiffness due to the persistence of streaming flow. The size of trapped particles is limited by the acoustic and streaming vortex. The trap relies on the balance of streaming and gravity, functioning best in an upward orientation. The relationship between levitation force and acoustic amplitude is non-linear, requiring careful amplitude tuning for different particles. More precise models considering fluid-structure interaction are needed for better theoretical predictions.