Résumé
In order to study the viscoelastic behaviour of cell wall layers and the influence of hygroscopicity on these layers, acoustic microscopy and atomic force microscopy were used. Reflection acoustic microscopy had limited application due to high ultrasound damping in biological materials such as wood. A new transmission acoustic microscope allowing a resolution of .5 µm at 1 GHz, using microtome slides of 10 to 20 µm thickness was designed and built. Mapping of the sound velocity gave an "elastic" image of the cell wall in the green state that can be used to quantitatively estimate the relative values of the Modulus of Elasticity for different layers of the cell wall. Atomic force microscopy in the tapping mode can be used to give 3 D images of the cell wall on a transverse section of small wood blocks. This can be done successively on freshly cut wet wood and on wood under different stages of drying. For example, measurements of the differential shrinkage between cell walls layers showed that the G layer had a higher shrinkage in the longitudinal direction than the other layers. Using atomic force microscopy in the force modulation mode, the resonance frequencies found for each cell wall layer can be linked to local mechanical behaviour of this layer in the dry state. In addition, maps of vibration amplitudes at given frequencies gave images of mechanical behaviour that can be used to study differential viscoelasticity between layers at different conditions of relative humidity and temperature. (Texte intégral)