Des tourbillons sonores pour manipuler des objets à l’échelle microscopique
Level 1 :
Manipulating micro-robots in the human body, structuring different cell strains in 3D, and immobilising and imaging micro-organisms in 3D are just a few examples of the vast range of applications opened up by acoustic tweezers. Researchers at the IEMN and INSP have demonstrated that it is possible to manipulate micro-objects without physical contact by trapping them at the heart of special wave structures called acoustic vortices. These vortices are generated by miniature chips combining the principles of holograms and active materials.
Level 2:
The capabilities offered by optical tweezers have opened up new fields of investigation in physics, crowned by the two Nobel Prizes of S. Chu and A. Ashkin. Acoustic tweezers could spark a similar revolution in the fields of biology and health, as they are non-invasive, biocompatible and can operate in opaque environments, making it possible to manipulate microscopic objects in vivo.
Acoustic clamps, like their optical analogues, are based on an average force produced by a wave, called radiation pressure. The first evidence of the existence of this force dates back to Kepler's observations of the orientation of comet tails in the direction of propagation of sunlight. In both cases (optical and acoustic), radiation pressure is proportional to the intensity of the wave irradiating a particle divided by the speed of the wave in question. As the speed of sound (around 1500 ms-1 in a liquid) is several orders of magnitude lower than that of light (around 3×108 ms-1), this gives acoustic tweezers a clear advantage: it is possible to exert considerably higher forces with the same injected power, or conversely to use beams of very low intensity to apply the same force. This considerably reduces the risk of damage when handling biological objects.
But one difficulty has long held back the development of acoustic tweezers: manipulating an object precisely and selectively requires localising the energy of the wave in a small portion of space. This can be achieved using focused wave beams, like those used in optical tweezers. But in acoustics, most objects of interest (solid particles, cells, etc.) are not attracted but expelled from the centre of these focused waves. Another constraint is successfully trapping a 3D object with a beam from a single direction. In fact, like the tail of a comet, the particles tend to be pushed in the direction of propagation of the wave, which makes it difficult to trap them axially. These two paradoxes have been resolved by using special wave structures called acoustic vortices.
However, there remained a major obstacle to manipulating microscopic objects: succeeding in producing these ultrasonic vortices on microscopic scales with a system that is sufficiently miniature and transparent to be integrated into a standard microscope. This is the challenge that was recently taken up by teams from the IEMN and the INSP. They combined the principles of holographic field synthesis, active materials and manufacturing methods inherited from the semiconductor industry to produce a miniature chip capable of producing acoustic vortices and thus manipulating microscopic particles.
Bibliography :
[1] M. Baudoin, J.-L. Thomas, Acoustic Tweezers for particles and fluid micromanipulation, Annual Review of Fluid Mechanics, 52: 205-234 (2020)
[2] M. Baudoin, J.-C. Gerbedoen, A. Riaud, O. Bou Matar, N. Smagin, J.-L. Thomas, Folding a focalized acoustical vortex on a flat holographic transducer: miniaturized selective acoustical tweezer, Science Advances 5: eaav1967 (2019)
