A. Bettucci

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 ...

Indentation modulus and hardness of viscoelastic thin films by atomic force microscopy: A case study

We propose a nanoindentation technique based on atomic force microscopy (AFM) that allows one to deduce both indentation modulus and hardness of viscoelastic materials from the force versus penetration depth dependence, obtained by recording the AFM cantilever deflection as a function of the sample vertical displacement when the tip is pressed against (loading phase) and then removed from (unloading phase) the surface of the sample. Reliable quantitative measurements of both indentation modulus and hardness of the investigated sample are obtained by calibrating the technique through a set of different polymeric samples, used as reference materials, whose mechanical properties have been previously determined by standard indentation tests. By analyzing the dependence of the cantilever deflection versus time, the proposed technique allows one to evaluate and correct the effect of viscoelastic properties of the investigated materials, by adapting a post-experiment data processing procedure well-established for standard depth sensing indentation tests. The technique is described in the case of the measurement of indentation modulus and hardness of a thin film of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate), deposited by chronoamperometry on an indium tin oxide (ITO) substrate.

Quantitative measurement of indentation hardness and modulus of compliant materials by atomic force microscopy

An atomic force microscopy (AFM) based technique is proposed for the characterization of both indentation modulus and hardness of compliant materials. A standard AFM tip is used as an indenter to record force versus indentation curves analogous to those obtained in standard indentation tests. In order to overcome the lack of information about the apex geometry, the proposed technique requires calibration using a set of reference samples whose mechanical properties have been previously characterized by means of an independent technique, such as standard indentation. Due to the selected reference samples, the technique has been demonstrated to allow reliable measurements of indentation modulus and hardness in the range of 0.3-4.0 GPa and 15-250 MPa, respectively.

Microstructural evidence for seismic and aseismic slips along clay‐bearing, carbonate faults

In this multi-methodological study, microstructural observations of fault rocks are combined with micromechanical property analyses (Contact Resonance Atomic Force Microscopy, CR-AFM) and with rotary friction experiments (SHIVA apparatus) to find evidence of seismic to aseismic slip and understand the nanoscale rheology of clay-bearing, carbonate-hosted faults. Fluidized structures, truncated clasts, pores and vesicles, and phyllosilicate nano-sized spherules and tubes suggest fast deformation events occurred during seismic slip, whereas clay-assisted pressure-solution processes, clumped clasts, foliation surfaces, and mantled clasts indicate slow deformation events occurred during postseismic/interseismic periods. CR-AFM measurements show that the occurrence of ~5 wt.% of clay within the carbonate-hosted gouges can significantly reduce the fault core stiffness at nanoscale. In addition, during high-velocity friction experiments simulating seismic slip conditions, the presence of ultra-thin phyllosilicate-bearing (≤3 wt.%) layers within calcite gouges, as those observed in the natural fault, show faster dynamic weakening than that of pure calcite gouges. The weak behavior of such layers could facilitate the upward propagation of seismic slip during earthquakes, thus possibly enhancing surface faulting. Microstructural observations and experimental evidence fit some well-known geophysical and geodetic observations on the short- to long-term mechanical behavior of faults such as post-seismic/interseismic aseismic creep, interseismic fault locking, and seismic slip propagation up to the Earths surface.

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.

Periodic Oscillations within the Chaotic Region in Finite Piezoelectric Structures

We report experimental observation of a window of periodic oscillations within the chaotic oscillations of a finite piezoelectric structure electrically forced with a frequency equal to—or close to—a normal mode. Continuously sampled spectra of the oscillation as the level of the driving voltage is increased, reveal the general behaviour of the system as well as features that suggest that the periodic oscillation forms a distinct island within the chaotic region in the parameter space. A period‐doubling sequence routing to chaotic oscillations is also observed.