Study of acoustic spatio-temporal vortices for particle and cell manipulation

Spatio-temporal vortices (STVs) represent a cutting-edge class of structured fields. Introduced a decade ago in optics, they have been experimentally observed and analytically described in acoustics only several years ago. They represent acoustic beams carrying a phase singularity in the mixed space–time domain, resulting in orbital angular momentum L whose direction is not necessarily aligned with the propagation axis and can be engineered through spatio-temporal structuring. This is in sharp contrast with the conventional acoustic beams where L is aligned with the propagation direction. Some conventional acoustic beams enable elliptical motion of the particles and thus induce acoustic spin angular momentum S, a property formally resembling the polarization of electromagnetic waves. The STVs generically carry S the orientation of which can be independently engineered.

Exhibiting these angular momenta, STVs offer unprecedented opportunities for particle manipulation. Transfer of L and S from the beam to the irradiated particles induces their motion through the time-dependent radiation forces and torques enabling particle dynamics impossible with steady beams. With these additional degrees of freedom, particle manipulation via STVs may find applications in biological cells distinguishing  different response to the field by the cells of different types having different stiffness.

Being rather complex polychromatic structured fields, STVs possess certain challenges from both the analytical and experimental points of view. Most of the elaborated theoretical descriptions of acoustic fields and their interaction with particles, as well as the conservation relations, rely on the monochromaticity of the fields, thus the revision of the theoretical basis is needed. From the generation point of view, construction of sources enabling both space-time coupling and broad bandwidth is rather difficult. Another problem is combining the source with the microfluidic environment where the particles are embedded.

Within this PhD thesis we aim at analytical description, numerical optimization and potential experimental observation of the particle manipulation by the acoustic STVs. Through the optimization of the field-particle interaction, namely the induced torque, the necessary parameters of the beam should be obtained, revealing the properties of the generating system. The long-term goal is to distinguish between real cancer cells observing the induced torque.

IEMN

Most of the research will have an analytical and numerical nature. In particular, three main theoretical tasks need to be solved: (i) an appropriate shape of the field, (ii) description of the particle keeping in mind the biologically-oriented implementation of the project, (iii) description of the beam interaction with the particle. Then the numerical optimization of the STV should be performed to maximize its manipulation capacities and particle distinguishing potential. A captivating perspective of taking part in the process of manufacturing the source and experimental measurements is open as well. We are thus searching for a highly motivated candidate with a strong theoretical and numerical background in acoustics and microfluidics, willing to broaden his/her expertise to the experimental implementation and characterization of advanced acoustic wave fields viewing experimentation as an essential complement to theory.