Webinar 3: Acoustic Vortex Beams - From Particle Levitation in Air to Cell Manipulation

Electroactive Diffraction Gratings: An Efficient Alternative for High-Quality Airborne Ultrasound Vortex Beam Generation

Acoustic vortex beams (AVB) have shown great potential in different emerging applications, such as contactless particle manipulation, transfer of angular momentum to matter, acoustic imaging, communications, and more recently, in biomedical applications. Different methods for AVB generation have been proposed, e.g., physically spiral sources, phase-driven sources, passive metastructured devices and electroactive spiral gratings. Notably, most of them have been developed in water, whereas airborne technologies offer interesting advantages, such as the free access to the manipulated sample.

In this work, an efficient technique to easily generate high-quality structured acoustic beams in air is presented. This new approach eliminates reflection losses included in previously reported passive approaches that combine an acoustic source with a diffraction grating. This is possible by converting the grating into an emitter. To demonstrate the versatility of this approach, active spiral-shaped diffraction gratings were fabricated to generate AVB over a broad range of frequencies, i.e., from 70 kHz to beyond 300 kHz, in air. By varying the excitation frequency, a fine and continuous tuning of the focal length of the resultant AVB can be achieved, i.e., the beam can be axially steered while their spatial distribution is preserved. Experimental results from two grating prototypes are included: an Archimedean spiral able to generate simultaneous higher order Bessel beams with different topological charges along the propagation axis and a spiral-Fresnel Zone Plate that allows for sharply focused AVG. The experiments show a good agreement with simulations. The versatility and simplicity of the fabrication process make this technique highly suitable for emerging applications such as particle manipulation, imaging, and transfer of angular momentum to matter. A comparison between the different fabrication methods reported for AVB generation is included in this presentation.

3D Printed Acoustofluidic Devices for Biospectroscopy Applications

Acoustofluidic devices combine the forces and torques produced by ultrasonic waves with microfluidics technology to separate, enrich, pattern, and rotate cells, bacteria, and other microorganisms. In this talk, I will discuss the physical principles of 3D printed acoustofluidic devices used for cell levitation and aggregation. The cellular aggregate is formed in a controlled microenvironment at hundreds of micrometers above the device substrate. This is particularly suitable for Raman biospectroscopy, where signal interferences from the substrate are avoided. In turn, the Raman spectrum reveals the chemical bonds present in a cell. Also, the aggregation stability allows for individual cell Raman-spectrum acquisition. Acoustofluidic-assisted biospectroscopy of murine macrophages and red blood cells is illustrated with preferred quality over ordinary Raman strategies.