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1. Acoustic tweezer with complex boundary-free trapping and transport channel controlled by shadow waveguides

   https://doi.org/10.1126/sciadv.abi5502  

   Li, Junfei; Shen, Chen; Huang, Tony Jun; Cummer, Steven A. (August 2021, Science Advances)

   null (Ed.)

   Acoustic tweezers use ultrasound for contact-free, bio-compatible, and precise manipulation of particles from millimeter to submicrometer scale. In microfluidics, acoustic tweezers typically use an array of sources to create standing wave patterns that can trap and move objects in ways constrained by the limited complexity of the acoustic wave field. Here, we demonstrate spatially complex particle trapping and manipulation inside a boundary-free chamber using a single pair of sources and an engineered structure outside the chamber that we call a shadow waveguide. The shadow waveguide creates a tightly confined, spatially complex acoustic field inside the chamber without requiring any interior structure that would interfere with net flow or transport. Altering the input signals to the two sources creates trapped particle motion along an arbitrary path defined by the shadow waveguide. Particle trapping, particle manipulation and transport, and Thouless pumping are experimentally demonstrated. 

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2. Airborne Acoustic Vortex End Effector‐Based Contactless, Multi‐Mode, Programmable Control of Object Surfing

   https://doi.org/10.1002/admt.202400564  

   Li, Teng; Li, Jiali; Bo, Luyu; Brooks, Michael R; Du, Yingshan; Cai, Bowen; Pei, Zhe; Shen, Liang; Sun, Chuangchuang; Cheng, Jiangtao; et al (June 2024, Advanced Materials Technologies)

   Abstract Tweezers based on optical, electric, magnetic, and acoustic fields have shown great potential for contactless object manipulation. However, current tweezers designed for manipulating millimeter‐sized objects such as droplets, particles, and small animals exhibit limitations in translation resolution, range, and path complexity. Here, a novel acoustic vortex tweezers system is introduced, which leverages a unique airborne acoustic vortex end effector integrated with a three‐degree‐of‐freedom (DoF) linear motion stage, for enabling contactless, multi‐mode, programmable manipulation of millimeter‐sized objects. The acoustic vortex end effector utilizes a cascaded circular acoustic array, which is portable and battery‐powered, to generate an acoustic vortex with a ring‐shaped energy pattern. The vortex applies acoustic radiation forces to trap and spin an object at its center, simultaneously protecting this object by repelling other materials away with its high‐energy ring. Moreover, The vortex tweezers system facilitates contactless, multi‐mode, programmable object surfing, as demonstrated in experiments involving trapping, repelling, and spinning particles, translating particles along complex paths, guiding particles around barriers, translating and rotating droplets containing zebrafish larvae, and merging droplets. With these capabilities, It is anticipated that the tweezers system will become a valuable tool for the automated, contactless handling of droplets, particles, and bio‐samples in biomedical and biochemical research. 

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3. Optical trapping and manipulation for single-particle spectroscopy and microscopy

   https://doi.org/10.1063/5.0086328  

   Chen, Zhenzhen; Cai, Zhewei; Liu, Wenbo; Yan, Zijie (July 2022, The Journal of Chemical Physics)

   Optical tweezers can control the position and orientation of individual colloidal particles in solution. Such control is often desirable but challenging for single-particle spectroscopy and microscopy, especially at the nanoscale. Functional nanoparticles that are optically trapped and manipulated in a three-dimensional (3D) space can serve as freestanding nanoprobes, which provide unique prospects of sensing and mapping the surrounding environment of the nanoparticles and studying their interactions with biological systems. In this perspective, we will first describe the optical forces underlying the optical trapping and manipulation of microscopic particles, then review the combinations and applications of different spectroscopy and microscopy techniques with optical tweezers. Finally, we will discuss the challenges of performing spectroscopy and microscopy on single nanoparticles with optical tweezers, the possible routes to address these challenges, and the new opportunities that will arise. 

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4. Ring-shaped photoacoustic tweezers for single particle manipulation

   https://doi.org/10.1364/OL.447861  

   Zhao, Zijie; Xia, Jianping; Huang, Tony Jun; Zou, Jun (January 2022, Optics Letters)

   This Letter reports ring-shaped photoacoustic (PA) tweezers that are capable of manipulating single or multiple micron-sized particles. By illuminating a thin layer of an optically absorptive liquid medium with a focused annular pulsed laser beam and a higher pulse repetition rate (e.g., 800 Hz), both acoustic radiation force and instantaneous vaporization repulsion are generated within a certain distance of the illumination region. This makes it possible to conduct continuous and versatile locomotion of single or multiple microparticles. In this Letter, interactions between two or more particles are demonstrated, such as separation, attachment, and grouping of microparticles. The PA tweezers combine some of the advantages of conventional optical and acoustic tweezers and are expected to be a useful alternative approach for the manipulation of microscale objects. 

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5. Hydrodynamic coupling melts acoustically levitated crystalline rafts

   https://doi.org/10.1073/pnas.2301625120  

   Wu, Brady; VanSaders, Bryan; Lim, Melody X.; Jaeger, Heinrich M. (July 2023, Proceedings of the National Academy of Sciences)

   Going beyond the manipulation of individual particles, first steps have recently been undertaken with acoustic levitation in air to investigate the collective dynamical properties of many-body systems self-assembled within the levitation plane. However, these assemblies have been limited to two-dimensional, close-packed rafts where forces due to scattered sound pull particles into direct frictional contact. Here, we overcome this restriction using particles small enough that the viscosity of air establishes a repulsive streaming flow at close range. By tuning the particle size relative to the characteristic length scale for viscous streaming, we control the interplay between attractive and repulsive forces and show how particles can be assembled into monolayer lattices with tunable spacing. While the strength of the levitating sound field does not affect the particles’ steady-state separation, it controls the emergence of spontaneous excitations that can drive particle rearrangements in an effectively dissipationless, underdamped environment. Under the action of these excitations, a quiescent particle lattice transitions from a predominantly crystalline structure to a two-dimensional liquid-like state. We find that this transition is characterized by dynamic heterogeneity and intermittency, involving cooperative particle movements that remove the timescale associated with caging for the crystalline lattice. These results shed light on the nature of athermal excitations and instabilities that can arise from strong hydrodynamic coupling among interacting particles. 

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