Transport of ultrasonic waves in strongly scattering or absorbing heterogeneous media
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Propagation of ultrasonic waves in disordered heterogeneous media was studied experimentally, using samples composed of aluminum beads and viscoelastic samples consisting of noodle dough. Even if they look very different, all these samples have in common some resonating constituents: the aluminum beads are resonators as are the bubbles trapped inside a dough. The strong influence of the resonances on the propagation of ultrasonic waves is observed in two different contexts. In the case of aluminum bead samples, the resonators are coupled together and enable a propagation pathway that leads to anomalous transport. For noodle dough, the propagation in the viscoelastic matrix is modified by the resonances of the bubbles. Measurements of the transport of ultrasound in a 3D sample of loose aluminum beads immersed a liquid has led to the first observation and quantitative analysis of multiply scattered waves travelling together through two weakly coupled pathways. A model was developed to explain and understand the properties of wave propagation in this sample, showing that one of the two propagating waves is diffusive while the other is sub-diffusive. The diffusive wave travels in the liquid and scatters off the beads whereas the sub-diffusive wave uses the network of beads as its medium of propagation. One of the key features of sub-diffusive transport is the dependence on position and frequency of the diffusion coefficient. A second experiment was designed to study this effect. This experiment was performed on a single layer of brazed aluminum beads, allowing ultrasonic measurements to be performed as a function of position inside this 2D sample (this is not possible inside a 3D aluminum sample). The wave field on the surface of the sample was measured using a laser interferometer, giving access to the three components of displacement. The same set of measurements was also used to investigate energy equipartition in a multiply scattering medium. In a complementary set of experiments, a non-contact ultrasonic technique was developed to characterize the mechanical properties of Asian noodle dough and investigate the influence of dough composition and processing parameters. This work demonstrated the potential for online quality control during noodle production.