The effect of echoes interference on phonon attenuation in a nanophononic membrane.

Nanophononic materials are characterized by a periodic nanostructuration, which may lead to coherent scattering of phonons, enabling interference and resulting in modified phonon dispersions. We have used the extreme ultraviolet transient grating technique to measure phonon frequencies and lifetimes in a low-roughness nanoporous phononic membrane of SiN at wavelengths between 50 and 100 nm, comparable to the nanostructure lengthscale. Surprisingly, phonon frequencies are only slightly modified upon nanostructuration, while phonon lifetime is strongly reduced. Finite element calculations indicate that this is due to coherent phonon interference, which becomes dominant for wavelengths between ~ half and twice the inter-pores distance. Despite this, vibrational energy transport is ensured through an energy flow among the coherent modes created by reflections. This interference of phonon echos from periodic interfaces is likely another aspect of the mutual coherence effects recently highlighted in amorphous and complex crystalline materials and, in this context, could be used to tailor transport properties of nanostructured materials.

Simulation of multi-curve active catheterization for endovascular navigation to complex targets.

The recent development of endovascular therapies has been accompanied by increasingly accurate navigation simulations to assist surgeons in decision making processes or to produce training tools. However, they have been focused mostly on targets within the aortic vasculature. In order to reach complex targets such as cerebral arteries by endovascular navigation, an active guidewire made of a Shape Memory Alloy (SMA) was recently proposed. The active part becomes deformed by the Joule effect and this deformation induces a bending of the guidewire. This setup is particularly suited for facilitating the access to Supra-Aortic Trunks (SATs) and, in our case, especially the left carotid artery. A complete characterization of the endovascular active navigation was conducted. In this framework, a test bench was developed to obtain an order of magnitude of the velocities applied on the guidewire as well as on the passive catheter going along with it in endovascular navigation. A numerical model was developed and validated in the case of navigation in a complex phantom aorta. We succeeded in representing crucial phenomena observed experimentally: snapping, active curvatures, interactions between the tools. In the last part of this study, it was demonstrated that adapting the guidewire design made it possible to hook the left carotid on three complex aortas.

Thermal Transport in a 2D Nanophononic Solid: Role of bi-Phasic Materials Properties on Acoustic Attenuation and Thermal Diffusivity.

Nanophononic materials have recently arisen as a promising way for controlling heat transport, mirroring the results in macroscopic phononic materials for sound transmission, filtering and attenuation applications. Here we present a Finite Element numerical simulation of the transient propagation of an acoustic Wave-Packet in a 2D nanophononic material, which allows to identify the effect of the nanostructuration on the acoustic attenuation length and thus on the transport regime for the vibrational energy. Assuming elastic behavior in the matrix and in the inclusions, we find that the rigidity contrast between them not only tunes the apparent attenuation length of the wave packet along its main trajectory, but gives rise to different behaviours, from weak to strong scattering, and waves pinning. As a consequence, different energy transport regimes can be identified in the three-parameter space of the excitation frequency, inclusions size and rigidity contrast, leading to the identification of a combination of parameters allowing for the shortest attenuation distance. These results could have applications both in the field of acoustic insulation, and for the control of heat transfer.

Interacting Cracks Obey a Multiscale Attractive to Repulsive Transition.

The observed repulsive behavior of two initially collinear cracks growing towards each other and leading to a hook-shaped path questioned recently the validity of the principle of local symmetry within linear elastic fracture mechanics theory. Our theoretical and numerical work solves this dilemma, providing the precise geometric conditions for the existence of this repulsive phase. We moreover reveal a multiscale behavior of the repulsive-attractive transition, explaining its ubiquitous occurrence, but also the difficulty to predict the final cracks' paths.