We have studied the diffusion of a fluorescent molecule in porous silica of various porosities. The measured diffusion coefficient depends on the length scale. At small scales we observe free diffusion; at large scales the diffusion is slowed down because of the obstruction effects created by the porous structure. The crossover region begins at scales comparable to the pore size. This is to our knowledge the first measurement of scale-dependent diffusion where the limits of free diffusion and large-scale diffusion are clearly evidenced. The analysis of the data leads to an original description of the structure of the porous media.

The plasma polymerization technique used on mica provided us with ultrathin hydrophobic polymer layers which were homogeneous and water stable but not resistant to peeling in water. These layers did not seem to alter the molecular smoothness of the mica cleavage planes. It was therefore possible to measure force/distance profiles between such polymer surfaces with a comparable accuracy to that usually obtained with multiple beam interferometry. Moreover, such surfaces did not produce any detectable contact angle hysteresis.

From the knowledge of piezoelectric material tensorial components and using the finite-element method, it is possible to calculate the electrical impedance vs. frequency and simulate the mechanical deformation of piezoelectric bars. To check the validity of the simulation, interferrometric measurements of the mechanical deformation amplitude were performed. It is shown that these measurements can reveal the homogeneity of the materials under study and that a small error in the tensorial parameter absolute values leads to an inconsistent picture of simulated mechanical deformation.

Results are reported of comparative experimental studies between pulse-echo and correlation imaging systems. The latter is optimized by computer simulation to reduce the signal-to-clutter ratio. The experimental results in A mode show that in correlation experiments, the emitted power is about 10/sup 4/ times lower than the power needed in the classical pulse-echo technique to extract an echo signal from ambient noise.

The ultrasonic pulse-echo technique, widely used in nondestructive testing and medicine, uses a part of the echo content, yielding only a qualitative image. In order to extract quantitative information from echographic signals, characterization techniques have been developed. Most of them are based on the power-spectrum estimation of received signals. However, in most cases data processing is time consuming and prevents real-time display of the data. In order to shorten this time the authors assembled a Fourier transformer which processes a 1024-point Fourier transform in 2 ms. The whole data-processing system can be connected to any commercial acoustic-imaging system, if necessary through a radio-frequency signal digitalizer.