Articles 2025

Résumé: Microstructures arrayed over a substrate have shown increasing interest due to their ability to provide advanced 3D cellular models, which open up new possibilities for cell culture, proliferation, and differentiation. Still, the mechanisms by which physical cues impact the cell phenotype are not fully understood, hence the necessity to interrogate cell behavior at the highest resolution. However, cell 3D high-resolution optical imaging on such microstructured substrates remains challenging due to their complexity as well as axial calibration issues. In this work, we address this issue by leveraging the geometrical characteristics of fractal-like structures, which serve as axial calibration tools and modulate cell growth. To this end, we use multiscale 3D SiO2 substrates consisting of spatially arrayed octahedral features of a few micrometers to hundreds of nanometers. Through optimizations of both the structures and optical imaging conditions, we demonstrate the potential of these 3D multiscale structures as an alternative to electron microscopy for material imaging but also as calibration tools for 3D super-resolution microscopy. We used their multiscale and known geometry to perform lateral and axial calibrations in 3D single-molecule localization microscopy (SMLM) and assess imaging resolutions. We then utilized these substrates as a platform for high-resolution bioimaging. As a proof of concept, we cultivate human mesenchymal stem cells on these substrates, revealing very different growth patterns compared to flat glass. Specifically, the spatial distribution of cytoskeleton proteins is vastly modified, as we demonstrate with a 3D SMLM assessment.

Résumé: Understanding the mechanisms behind the remote triggering of landslides by seismic waves at micro-strain amplitude is essential for quantifying seismic hazards. Granular materials provide a relevant model system to investigate landslides within the unjamming transition framework, from solid to liquid states. Furthermore, recent laboratory experiments have revealed that ultrasound-induced granular avalanches can be related to a reduction in the interparticle friction through shear acoustic lubrication of the contacts. However, investigating slip at the scale of grain contacts within an optically opaque granular medium remains a challenging issue. Here, we propose an original coupling model and numerically investigate two-dimensional dense granular flows triggered by basal acoustic waves. We model the triggering dynamics at two separated time scales - one for grain motion (milliseconds) and the other for ultrasound (10μs) - relying on the computation of vibrational modes with a discrete element method through the reduction of the local friction. We show that ultrasound predominantly propagates through the strong-force chains, while the ultrasound-induced decrease of interparticle friction occurs in the weak contact forces perpendicular to the strong-force chains. This interparticle friction reduction initiates local rearrangements at the grain scale that eventually lead to a continuous flow through a percolation process at the macroscopic scale - with a delay depending on the proximity to the failure. Consistent with experiments, we show that ultrasound-induced flow appears more uniform in space than pure gravity-driven flow, indicating the role of an effective temperature by ultrasonic vibration.

Résumé: We present a semi-analytical approach to compute quasi-guided elastic wave modes in horizontally layered structures radiating into unbounded fluid or solid media. This problem is of relevance, e.g., for the simulation of guided ultrasound in embedded plate structures or seismic waves in soil layers over an elastic half-space. We employ a semi-analytical formulation to describe the layers, thus discretizing the thickness direction by means of finite elements. For a free layer, this technique leads to a well-known quadratic eigenvalue problem for the mode shapes and corresponding horizontal wavenumbers. Incorporating the coupling conditions to account for the adjacent half-spaces gives rise to additional terms that are nonlinear in the wavenumber. We show that the resulting nonlinear eigenvalue problem can be cast in the form of a multiparameter eigenvalue problem whose solutions represent the wave numbers in the plate and in the half-spaces. The multiparameter eigenvalue problem is solved numerically using recently developed algorithms. Matlab implementations of the proposed methods are publicly available.