M. Germano

Local indentation modulus characterization of diamondlike carbon films by atomic force acoustic microscopy two contact resonance frequencies imaging technique

Two contact resonance frequencies atomic force acoustic microscopy imaging technique has been used to evaluate local indentation modulus of a diamondlike carbon film deposited on a molybdenum foil by laser ablation from glassy carbon target. Acoustic images were obtained by measuring both first and second contact resonance frequency at each point of the scanned area, and then numerically evaluating local contact stiffness and reconstructing an indentation modulus bidimensional pattern. The wide difference of the indentation modulus values allows to detect the presence of residual glassy carbon agglomerates in the diamondlike carbon film.

Effect of tip geometry on local indentation modulus measurement via atomic force acoustic microscopy technique

Atomic force acoustic microscopy (AFAM) is a dynamical AFM-based technique very promising for nondestructive analysis of local elastic properties of materials. AFAM technique represents a powerful investigation tool in order to retrieve quantitative evaluations of the mechanical parameters, even at nanoscale. The quantitative determination of elastic properties by AFAM technique is strongly influenced by a number of experimental parameters that, at present, are not fully under control. One of such issues is that the quantitative evaluation require the knowledge of the tip geometry effectively contacting the surface during the measurements. We present and discuss an experimental approach able to determine, at first, tip geometry from contact stiffness measurements and, on the basis of the achieved information, to measure sample indentation modulus. The reliability and the accuracy of the technique has been successfully tested on samples (Si, GaAs, and InP) with very well known structural and morphological ...

Anomalous propagation characteristics of evanescent waves

An analysis is done of the crossing of a forbidden region in a thin plate by a backward propagating Lamb wave: the refraction/reflection effects undergone by the coupled modes produced at each boundary of the forbidden region are taken into consideration, as well as the penetration of the backward wave as an evanescent wave. The outcome of the acoustic perturbation is analysed for a few angles of incidence and experiments are performed that confirm the theoretical predictions.

A numerical method for studying nonlinear bubble oscillations in acoustic cavitation

Abstract A numerical method is developed for the study of the behaviour of a gas bubble in ultrasonically induced cavitation. This method is based on the solution of the full Navier-Stokes equation for the two-fluid system consisting of the gas inside the bubble and the liquid surrounding it, following ideas originally introduced for the analysis of multi-component fluid flows. Analysis of acoustic cavitation must be done taking into account the compressibility of the gas bubble and for this purpose the Navier-Stokes equation is coupled with an equation of state for the gas; our model also considers the presence of viscosity and surface tension, thus allowing surface oscillations of the bubbles. To avoid numerical problems in the solution of the Navier-Stokes equation two different grids are introduced, an Eulerian one for the ‘background’, where the Navier-Stokes equation is solved, and another moving one for the interface; this second grid is explicitly tracked and properly modified during motion and is responsible for the behaviour of the bubble. The transfer of information between the Eulerian grid and the interface grid is obtained with the aid of a lattice modified distribution function. The method is tested analyzing forced oscillations of cavitation bubbles excited by ultrasonic standing waves at different frequencies and pressure amplitudes, showing characteristic behaviour of nonlinear dynamical systems; frequency spectra are obtained, stability analysis is performed and strong dependence from initial conditions is showed; comparisons with previous different approaches are also performed.

Water temperature dependence of single bubble sonoluminescence threshold

Water temperature dependence of single bubble sonoluminescence (SBSL) threshold has been experimentally measured to perform measurements at different temperatures on the very same bubble. Results show lower thresholds, i.e. an easier prime of mechanism, of sonoluminescence at lower water temperatures. Dependence is almost linear at lower temperatures while between 14 degrees C and about 20 degrees C the curve changes its slope reaching soon a virtual independence from water temperature above about 20 degrees C.

Photoacoustic cell for ultrasound contrast agent characterization

Photoacoustics has emerged as a tool for the study of liquid gel suspension behavior and has been recently employed in a number of new biomedical applications. In this paper, a photoacoustic sensor is presented which was designed and realized for analyzing photothermal signals from solutions filled with microbubbles, commonly used as ultrasound contrast agents in echographic imaging techniques. It is a closed cell device, where photothermal volume variation of an aqueous solution produces the periodic deflection of a thin membrane closing the cell at the end of a short pipe. The cell then acts as a Helmholtz resonator, where the displacement of the membrane is measured through a laser probe interferometer, whereas photoacoustic signal is generated by a laser chopped light beam impinging onto the solution through a glass window. Particularly, the microbubble shell has been modeled through an effective surface tension parameter, which has been then evaluated from experimental data through the shift of the resonance frequencies of the photoacoustic sensor. This shift of the resonance frequencies of the photoacoustic sensor caused by microbubble solutions is high enough for making such a cell a reliable tool for testing ultrasound contrast agent, particularly for bubble shell characterization.

Non linear behaviour of cell tensegrity models

Tensegrity models for the cytoskeleton structure of living cells is largely used nowadays for interpreting the biochemical response of living tissues to mechanical stresses. Microtubules, microfilaments and filaments are the microscopic cell counterparts of struts (microtubules) and cables (microfilaments and filaments) in the macroscopic world: the formers oppose to compression, the latters to tension, thus yielding an overall structure, light and highly deformable. Specific cell surface receptors, such as integrins, act as the coupling elements that transmit the outside mechanical stress state into the cell body. Reversible finite deformations of tensegrity structures have been widely demonstrated experimentally and in a number of living cell simulations. In the present paper, the bistability behaviour of two general models, the linear bar oscillator and the icosahedron, is studied, as they are both obtained from mathematical simulation, the former, and from larger scale experiments, the latter. The dis...

Resonant condition in second harmonic generation of water surface waves

Works on nonlinear effects in water surface waves are complementary stimulated by interest in understanding the spectral composition of oceanic waves and feasibility of local detection of physical parameters. On the other side, water surface waves are composite waves contributed both by surface tension and gravity, such that study of nonlinear phenomena are theoretically severe and different approaches are available, that variously attempt to consider transverse or dilational stresses, mode resonant interactions, solitons etc. In the present work, a phenomenological approach is done, that simply moves from the detection of amplitude and phase of the harmonic components present along the propagation direction of the waves, to highlight the local nonlinear interaction between the fundamental and second harmonic propagating modes. The effect is here put into evidence by detecting harmonic production at different frequencies around the so-called resonant condition, where fundamental and second harmonic share ...

Pattern Matching of Harmonic Vibrations in Nonlinearly Generated Acoustic Modes in Bovine Bone Samples

Local probing of nonlinear generation of harmonic vibrations has been done on bone plate samples and the evaluation of the nonlinear term is derived from a limited number of cases of bovine thigh bones, that shows that a low level of nonlinearity is present in bone structures. This is consistent with the assumption that in low level nonlinear samples the distribution of harmonic vibrations matches the corresponding power distribution of the fundamental mode.

Photoacoustical evaluation of thermal response of microbubbles

Photoacoustic technique has emerged as a tool for the study of liquid, gel, suspension behaviour and has been recently employed in a number of new biomedical applications. In the present paper, a new photoacoustic sensor is presented which was designed and realized to analysing photoacoustic signals from solution filled with microbubbles, usually used as contrast agents in echotomography imaging. The device is a closed cell where photothermal expansion in acqueous solution samples causes the deflection of a thin membrane closing a short pipe 0.7 mm by radius. The overall set up acts as a Helmoltz resonator, where the solution in the pipe is the mass oscillating under the driving force produced by the alternating expansion of the solution in the cell body. Displacement of the membrane was measured using a laser probe interferometer, whereas photoacoustic signal was generated by chopping a laser light beam impinging into the solution through a glass window in the cell. The response of the cell filled with s...