Rotational Mechanical Effects of Sound

Introduction

In wave physics, the destructive interference occurs along lines or at points, resulting in threads of darkness. A particular wave phenomenon, the occurrence of phase singularties, was unveiled in 1974 [J F Nye and M V Berry, Dislocations in wave trains, Proc. R. Soc. London, Ser. A 336, 165 (1974)], which refers to the fact that the phase is undefined at those places of null intensity. Accordingly, phase singularities are associated with helical wavefronts which has important consequences on the mechanical wave properties. Indeed, light, sound or electron beam having on-axis phase singularity—vortex beams—carry orbital angular momentum. This allows a wave to exert rotational mechanical actions on matter via wave-matter interaction.

Here, the proposal deals with the case of acoustic vortex beams, in the ultrasonic regime. Our global aim is to address experimentally unexplored yet fundamental aspects of rotational mechanical effects using ultrasonic acoustic vortex beams. In practice, these effects can be angular displacement, spinning or orbiting motion depending on the nature of the sound-matter interaction that may involve wave absorption and/or wave scattering phenomena. Specifically, we are defining a limited number of objectives that will be address during the project, that all serve a common goal: to set an experimental pathway to benefit from the orbital angular momentum (OAM) of sound for novel options to manipulate matter without contact.

Our long-term ambition is to contribute to future developments of echography imaging techniques that are so far restricted to translational mechanical effects of sound in tissues. Indeed, we anticipate that the mechanical control of the rotational degrees of freedom of matter work will allow developing novel imaging techniques by monitoring rotational echoes instead of translational ones, which will give extra information on elastic properties of the probed materials.

The idea is to combine my longstanding experience in the analysis acoustic wave properties involving absorption and scattering of sound as the wave interact with an object with the state-of-the-art expertise of Dr. E. Brasselet from the university of Bordeaux in France (UBx). The chosen host has been selected because its research activities cover the investigation of the mechanical effects of acoustic waves on matter, especially acoustic vortex beams and their mechanical effects on matter resulting from the transfer of OAM of sound. This will provide me with a favorable scientific environment to develop my career both by benefiting of the expertise and resources of the host group and by seeding the latter with new ideas and specific experience on handling and processing acoustic waves.

The basic science objective of the project is to detect and measure unexplored yet rotational mechanical effects of sound towards future developments of practical applications of acoustic waves. In practice, the experimental approach will be based on using high-power piezoelectric ultrasonic transducer, hydro/micro-phone (depending on the media, liquids or air, that will be used) and various objects that will be selected for the absorbing and scattering properties. Noteworthy, being already a proficient user of all these tools that I used and developed during my previous research investigations, it is expected that the research collaboration with Etienne Brasselet will be both original and efficient and will suit well two-way transfer of knowledge.

In order to give a clear picture of the underlying physics and to better grasp the originality of the proposal, we provide in the next subsection a state-of-the-art of the proposed research topic. There, we will present the two mechanisms by which the orbital angular momentum of sound can induce rotational mechanical effects and explain why the present project is timely regarding the present international research context.

State-of-the-art

In acoustics, the transfer of OAM from vortex beams to matter was experimentally revealed in 2008, using acoustic vortex beams [K Volke-Sepulveda, A O Santillan and R R Boullosa, Transfer of angular momentum to matter from acoustical vortices in free space, Phys. Rev. Lett. 100, 024302 (2008)], [K D Skeldon, C Wilson, M Edgar and M J Padgett, An acoustic spanner and its associated rotational Doppler shift, New J. Phys. 10, 013018 (2008)]. The physics of such a process may involve two distinct mechanism: the transfer of orbital angular momentum from sound to matter occurs either via absorption or conversion of the incident OAM: Absorption—the material gain angular momentum by absorbing that of the incident wave; Conversion—the gain of angular momentum results from a change of the angular momentum carried by the wave as it interacts with matter. From a mechanical point of view, the result is the same in both cases: matter experiences a torque from the acoustic wave.

Noteworthy, after the experimental discovery of the transfer of OAM from acoustic vortex beams to matter via absorption mechanism, all subsequent experimental works were also dealing with absorption [C E M Demore, Z Yang, A Volovick, S Cochran, M P MacDonald and G C Spalding, Mechanical evidence of the orbital angular momentum to energy ratio of Vortex Beams, Phys. Rev. Lett. 108, 194301 (2012)], [A Anhauser, R Wunenburger and E Brasselet, Acoustic rotational manipulation using orbital angular momentum transfer, Phys. Rev. Lett. 109, 034301 (2012)]. It was only in 2015 that the host group published the first experimental study relying on the conversion mechanism [R Wunenburger, J I V Lozano and E Brasselet, Acoustic orbital angular momentum transfer to matter by chiral scattering, New J. Phys. 17, 103022 (2015)] though the effect was hindered by material absorption. The quantitative assessment of pure conversion mechanical effects has only been achieved very recently by the host group [B Sanchez-Padilla, L Jonusauskas, M Malinauskas, R Wunenburger and E Brasselet, Direct mechanical detection and measurement of wave-matter orbital angular momentum transfer by nondissipative vortex mode conversion, Phys. Rev. Lett. 123, 244301 (2019)] [B Sanchez-Padilla and E Brasselet, Torsional mechanical oscillator driven by the orbital angular momentum of sound, Phys. Rev. Applied 13, 064069 (2020)]. Finally, we note very recent theoretical developments unveiled a novel source of acoustic angular momentum—of a ‘spin’ nature [K Y Bliokh and F Nori, Spin and orbital angular momenta of acoustic beams, Phys. Rev. B 99, 174310 (2019)] that involves strongly focused acoustic vortex beams, and such fundamental predictions remains experimentally unexplored so far.

Interdisciplinarity

At first, it should be stress that our work has fundamentally a multi-physics background as it deals with the mechanical of wave physics. In particular, the contactless manipulation of matter with waves has a lot to gain once being based on acoustic waves, as articulated in the review paper [J Thomas, R Marchiano and D Baresch, Acoustical and optical radiation pressure and the development of single beam acoustical tweezers, J. Quant. Spectrosc. RA 195, 55 (2017)] and by the Nobel Prize 2018 in Physics delivered to A. Ashkin “for the optical tweezers and their application to biological systems”. Indeed, since the radiation torque exerted by a vortex beam (be it made of sound or light) scales as the acoustic power divided by the wave angular frequency, the use of sound instead of light brings an immense benefit. Quantitatively, considering a given applied torque on an object, the needed power from an ultrasound wave at 100kHz is 1010 times less that an optical wave at typical wavelength of 1μm. Specifically, our work—developing technological acoustic beam shaping techniques, demonstrating novel acoustic rotational manipulation capabilities and contributing to the progress of knowledge — is characterized by:

- A short-term applicative potential can be foreseen from our beam shaping approach based on the use of 3D printing technology to transform any standard flat piezoelectric transducer into an acoustic vortex beam with arbitrary focusing parameter and topological charge instead of using very expensive arrays of transducer as is done nowadays. This should encourage widespread dissemination of the use of acoustic angular momentum to end-users (e.g., in contactless sensing applications and rheology, where translational degree of freedom [acoustic radiation force] is already being exploited).

A long-term innovation potential for the development of novel medical imaging technique, namely, acoustic vortex elastography, as recently introduced by Etienne Brasselet.

Future Career Prospects

My interaction with a highly-qualified supervisor and through work in a highly-equipped laboratory will substantially increase my field of knowledge and a number of practical skills, making me a more versatile specialist in related disciplines of wave physics that will consolidate my professional maturity towards becoming an independent researcher. More precisely, by working on acoustic orbital angular momentum and its rotational mechanical effects on matter, I will further strengthen already acquired expertise in sound-wave interaction. Moreover, being hosted in a group for its contributions in the field of light-matter interaction in presence of OAM, I will have the opportunity to extend my knowledge beyond my actual frontiers, which promises to be intellectually stimulating and career-fruitful. Therefore, besides the expected scientific results, the Marie Curie fellowship will allow me to drastically boost my career.

Indeed, the completion of this project will provide me the opportunity to establish as an independent and leading young researcher with an extensive theoretical and experimental experience on sound-matter interaction. In particular, this includes in-depth knowledge of acoustic angular momenta and quantitative awareness allowing me to consider further implementation of practical applications in contactless manipulation and imaging of matter in future research proposals,especially with the aim at reaching an academic position. Indeed, the implementation of an original research project under a prestigious Marie Sklodowska-Curie Fellowship will provide me with an academic outreach and re-enforced self-confidence in the capability to establish my own research team in any high-level research institution. I will naturally find ways to cooperate with various research teams, especially that of Prof. R. Wunenburger (Paris, France) and that of D. Baresch at UBx, with whom the supervisor already conducted joint projects on mechanical effects of acoustic waves in fluids.

Also, through the mentoring of the host and its network, I will learn how to develop cooperation with industrial partners who are interested in the development of fundamental research related to well-defined practical application. Concretely, this will be made by being introduced me to Aquitaine Science Transfert, which is located on the UBx campus, and with whom the supervisor develops demonstrators for the industry. In the present case, it is the objective 1 that has potential for developing a portfolio of 3D printed solutions for beam shaping upgrades adapted to existing flat ultrasonic transducers. It is also important to mention that the successful achievement of the scientific challenges associated with objectives 2 and 3 will constitute a basic academic enhancement factor after the fellowship as the planned first-time experimental demonstration are expected to lead to impactful high-profile publications, at first in the acoustics community. Since my long-term prospect is the creation and management of my own research group, it is useful in all respects to work with such an experienced mentor as E. Brasselet within the Marie Sklodowska-Curie Fellowship.

Use and Dissemination of Results

The most important results will be published in high-impact scientific journals and we will specifically target the following ones at the first place: Nature Physics, Physical Review Letters, Applied Physics Letters, and Nature Communications. Indeed, all these journals are dedicated to broad readerships, including acoustics community. According to the Horizon 2020 policy about open access of scientific publications, we will also deposit our publications in standard scientific repositories Hal and arXiv, respectfully from embargo policy of the scientific journals is any.

In addition, we will be committed to disseminate take-home scientific and technological messages of our results via internet resources made to reach wide audience. Basically, we will exploit the possibility offered by the following free social networks created for the cooperation of scientists of all scientific disciplines and the establishment of business contacts: ResearchGate, Academia.edu, and LinkedIn.

At the national level, we will exploit the communication resources of CNRS and UBx in order to advertise our results on the institutional web. Moreover, I plan to write and administrate a Blog on RMES project, which will promote Marie Curie Fellowship program as well as enhancing the visibility of my own scientific development all along the project duration. The success of the Blog will be worked out by regular updates as often as possible and by creating links on host laboratory website, hence easily disseminated through UBx students at Bachelor and Master levels, with the aim at attracting them for internship within RMES research activities, who I will co-supervise with the supervisor.

Another channel of dissemination of the innovative application potential of our results is the direct interaction with potential end-users in various fields, such as medical imaging or acoustic rheology. Importantly, we will pay attention to intellectual property in order to properly protect and exploit our developments having potential for innovative acoustic technologies (e.g., 3D printed solutions for acoustic beam shaping add-ons toolbox) before advertising on them. This will be made with the help of Aquitaine Science Transfert at UBx, that will help us to write patents whenever it is found relevant, noting that patent writing experience with Aquitaine Science Transfert is already available from the supervisor himself. Clearly, this can also be considered as part of the training objectives for the researcher.

Communication Activities

• I will give lectures on physics in general and the field of acoustics in particular, which the proposed project is devoted to, based on classical knowledge, as well as the results of my research, in a simplified form for children in schools and on the relevant activities of similar public organizations. To reach this aim, I will benefit from the presence of “Maison pour la Science en Aquitaine” at UBx, an institute supported by the French Academy of Sciences that is dedicated to the promotion of science at various levels (from schools, including the teachers, to science education training at UBx). The scientific staff at the host laboratory LOMA, including the supervisor, is indeed already involved in such actions, which will facilitate the setting up of my contributions.
• I will propose to participate to the constant efforts of the host laboratory that is actively involved in the annual events “Days of science” and “Nuit des chercheur.e.s”, during which research laboratories open their doors to the public, inviting them to participate in experimental demonstrations held at Cap Science in Bordeaux (Cap Science – center for scientific, technical and industrial culture, based in Bordeaux) and at LOMA.
• Based on my previous experience to create ones for my previous laboratory (LIMU) website, I will create educational material for the popularization of science in the appropriate format and posted on the RMES website and social networks (YouTube, Facebook, Twitter, Instagram and WhatsApp), where the developed communication will quickly spread to a very large audience. Adopting a story telling strategy for consecutive material, I expect to secure and increase the number of followers as time goes by.