Holographic acoustic tweezers

Acoustic tweezers use sound radiation forces to manipulate matter without contact. They provide unique characteristics compared with the more established optical tweezers, such as higher trapping forces per unit input power and the ability to manipulate objects from the micrometer to the centimeter scale. They also enable the trapping of a wide range of sample materials in various media. A dramatic advancement in optical tweezers was the development of holographic optical tweezers (HOT) which enabled the independent manipulation of multiple particles leading to applications such as the assembly of 3D microstructures and the probing of soft matter. Now, 20 years after the development of HOT, we present the realization of holographic acoustic tweezers (HAT). We experimentally demonstrate a 40-kHz airborne HAT system implemented using two 256-emitter phased arrays and manipulate individually up to 25 millimetric particles simultaneously. We show that the maximum trapping forces are achieved once the emitting array satisfies Nyquist sampling and an emission phase discretization below π/8 radians. When considered on the scale of a wavelength, HAT provides similar manipulation capabilities as HOT while retaining its unique characteristics. The examples shown here suggest the future use of HAT for novel forms of displays in which the objects are made of physical levitating voxels, assembly processes in the micrometer and millimetric scale, as well as positioning and orientation of multiple objects which could lead to biomedical applications.This project has been funded by the UK Engineering and Physical Science Research Council (EP/N014197/1)

Acoustic levitation in mid-air: recent advances, challenges, and future perspectives

Mid-air acoustic levitation is becoming a powerful tool to suspend and manipulate millimetric objects. Because of its unique characteristics, acoustic levitation is suitable to trap a wide variety of materials such as liquids, solids, soap bubbles, and even living creatures. Acoustic levitation can also be combined with noncontact measurement systems, allowing contactless analysis and characterization of levitating samples. In this article, we review some of the advances that have been made over the last decade. We also present the technical challenges that must be overcome in order to extend the capability of current acoustic levitation devices and, finally, we point out future directions for acoustic levitation.This work was supported by the São Paulo Research Foundation–FAPESP (Grant No. 2017/27078-0) and by the Government of Navarre through Fondo Europeo de Desarrollo Regional (FEDER) Project No. 0011-1365-2019-000086

Experimental investigation of the particle oscillation instability in a single-axis acoustic levitator

Single-axis acoustic levitators are employed in biomedicine, chemistry and physics experiments due to their ability to trap in mid-air objects of a wide range of materials and sizes. Although this type of levitator has been studied for decades, there are effects that are not well understood. One of these effects is the particle oscillation instability, in which the levitating particle starts to oscillate with increasing amplitude until it is ejected out of the levitator. Most of the operations performed with acoustic levitation require high accuracy regarding the positioning of the particle, thus a lack of stability severely hinders the experiments. In this paper, we present an experimental setup that consists of a single-axis levitator, a mechanized stage to control the separation between the emitter and the reflector, a scale to measure the radiation force and a high-speed camera. We experimentally investigate the effect of the distance between the emitter and the reflector on the apparatus resonant frequency and on levitation stability. In accordance with previous theoretical studies, three types of levitation behavior were experimentally identified: stable levitation, oscillation of constant amplitude and unstable oscillation. We also show that the type of levitation behavior can be controlled by changing the distance between the emitter and the reflector.This research was supported by the Sao Paulo Research Foundation - FAPESP (Grant No. 2017/27078-0) and by the European Union's Horizon 2020 research and innovation programme under the FET-Open Scheme with grant agreement No 737087. Gianluca Memoli's time is partially funded by his UKRI fellowship (EP/S001832/1)

Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator

The following article appeared in Andrade, Marco A.B., Marzo, Asier, (2018). Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator. Review of Scientific Instruments December 2018 89 (12), doi: 10.1063/1.5063715 and may be found at http://dx.doi.org/doi: 10.1063/1.5063715.We present an acoustic levitation system that automatically injects, transports, merges and ejects liquid droplets in mid-air. The system consists of a phased array operating at 40 kHz on top of a plane reflector. The phase array generates multiple focal points at independent positions that form standing waves between the array and the reflector. In the reflector there is an inlet for a piezoelectric droplet injector which automatically inserts liquid droplets at the lower pressure nodes of the standing waves, and a hole that serves as an outlet for ejecting the processed droplets out of the system. Simulations of the acoustic radiation potential acting on the levitating droplets are in good agreement with the experiments. High-speed footage captured the functioning of the system in four fluidic operations: injection, transport, merging and ejection of liquid droplets. Having these operations integrated reliably into a single automatic system paves the way for the adoption of mid-air acoustophoretic processing in biological, chemical and pharmaceutical applications.This research was supported by the São Paulo Research Foundation—FAPESP (Grants Nos. 2017/27078-0 and 2018/04101-0)

Laser-induced breakdown spectroscopy of samples of astrochemical interest handled as individual particles by means of non-inertial acoustic confinement.

The present communication will show experiments performed with an acoustic levitator capable of trapping individual solid particles of different sizes (preferably in the range between 0.1 - 5 mm), shapes, and chemical properties in air. The levitator offers significant advantages over conventional handling approaches for particulate matter as it just requires picking up the desired particle, place it in the levitation device, and performing fine adjustment to bring the particle to the focal point of the laser beam. Single-shot or accumulative shots can be performed depending on the laser energy required, allowing the recording of excellent signal-to-noise ratio LIBS spectra.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech