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Browsing by discipline "Modeling Molecules and Nanosystems (MoMoNano)"

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  • Hilal, Comak (2019)
    Acoustic microscopy is a rising technology which uses phonons for obtaining images and measuring the mechanical properties of the samples. As opposed to other devices such as atomic force microscopy, light microscopy or X-ray spectroscopy, acoustic microscopy is non-invasive and can measure the subsurface of the material without any contact. Commonly, water is used as a coupling medium in the acoustic microscope experiments of the biological samples. It is vital for the living organisms and necessary if one desires to conduct measurements while they are still alive. In order to improve the resolution of the images, we are interested in hypersound range of the phononic spectrum. However at this range, sound attenuation in water is one of the limitations. Sound attenuation is described as the decrease of the sound intensity, or loss of sound flux, during the propagation of sound through a medium. In most fluids, it increases with the square of the frequency. Consequently, there is a maximum frequency for a given distance between the lens and the sample and this affects the resolution. In this thesis, we studied the hypersound attenuation in water with Molecular Dynamics Simulations. We used two different potentials mW and TIP4P/2005f for the simulations and we compared the results. In order to investigate the attenuation coefficient, we introduced varying sound signals with different frequencies 125 GHz, 250 GHz, 500 GHz, 750 GHz, 1 THz, 1.25 THz at the different amplitudes of 0.5 Å and 1 Å with both mW and TIP4P/2005f potentials and 0.25 Å with mW potential. We analysed the results by extracting the stresses from the simulation results and conducting Fast Fourier Transform Method for the signal content. Following, we obtained the Power Spectrum and plotted to observe how it changes with distance and calculated the attenuation coefficients for each cases. The most obvious finding to emerge from this study is that at high frequency range, the sound signal attenuates in a couple of Å. Moreover, we observed that mW and TIP4P/2005f potentials give different results, but show similar trends. We also noticed some indications for a possible structural change and a stream.