Evaluation of the thermal aging of aeronautical composite materials using Lamb waves.

This work aims to demonstrate the ability to evaluate bulk thermal aging of aeronautical composite materials using Lamb waves. These composite materials are bi-dimensional woven composite structures with polymers matrix. More than 20 different thermal aging profiles are obtained by modulating the temperature and curing duration. The impact on material properties is evaluated using normalized destructive characterization and x-ray computed tomography. Lamb waves are generated and detected using a single 3 MHz phased array transducer in contact with the composite structure. In that way, a spatio-temporal image of elastic wave propagation in the composite material is obtained. The dispersion curves are calculated by using a bi-dimensional discrete Fourier transform or singular value decomposition to project the experimental data into the wavenumber-frequency domain. Both methods allow for accurate estimation of the Lamb wave dispersion curves. Thermal aging is then evaluated by comparing experimental data with a set of dispersion curves that correspond to different controlled thermal aging profiles. Our promising results open the door toward quantitative evaluation of composite material thermal aging on the runway.

Probing single-cell mechanics with picosecond ultrasonics.

The mechanical properties of cells play a key role in several fundamental biological processes, such as migration, proliferation, differentiation and tissue morphogenesis. The complexity of the inner cell composition and the intricate meshwork formed by transmembrane cell-substrate interactions demands a non-invasive technique to probe cell mechanics and cell adhesion at a subcell scale. In this paper we review the use of laser-generated GHz acoustic waves--a technique called picosecond ultrasonics (PU)--to probe the mechanical properties of single cells. We first describe applications to vegetal cells and biomimetic systems. We show how these systems can be used as simple models to understand more complex animal cells. We then present an opto-acoustic bio-transducer designed for in vivo measurements in physiological conditions. We illustrate the use of this transducer through the simultaneous probing of the density and compressibility of Allium cepa cells. Finally, we demonstrate that this technique can quantify animal-cell adhesion on metallic surfaces by analyzing the acoustic pulses reflected off the cell-metal interface. This innovative approach allows investigating quantitatively cell mechanics without fluorescent labels or mechanical contact to the cell.

Laser-induced ultrafast demagnetization in the presence of a nanoscale magnetic domain network.

Femtosecond magnetization phenomena have been challenging our understanding for over a decade. Most experiments have relied on infrared femtosecond lasers, limiting the spatial resolution to a few micrometres. With the advent of femtosecond X-ray sources, nanometric resolution can now be reached, which matches key length scales in femtomagnetism such as the travelling length of excited 'hot' electrons on a femtosecond timescale. Here we study laser-induced ultrafast demagnetization in [Co/Pd](30) multilayer films, which, for the first time, achieves a spatial resolution better than 100 nm by using femtosecond soft X-ray pulses. This allows us to follow the femtosecond demagnetization process in a magnetic system consisting of alternating nanometric domains of opposite magnetization. No modification of the magnetic structure is observed, but, in comparison with uniformly magnetized systems of similar composition, we find a significantly faster demagnetization time. We argue that this may be caused by direct transfer of spin angular momentum between neighbouring domains.