Current Research

Anomalous dispersion in strongly scattering media
Resonant scattering can cause both the phase and group velocities to vary dramatically with frequency (see our logo). We investigate these effects experimentally by extracting the weak pulse that propagates coherently through the medium from the much larger multiply scattered signals. Our results are well explained by a theoretical model which overcomes previous limitations of CPA effective medium theories.

Diffusion and Localization of Sound
How do sound waves diffuse? What happens when the scattering is so strong that the waves scatter before they have travelled a single wavelength? We use ultrasonic techniques to investigate these and other questions relating to the diffusive propagation and localization of sound in very strongly scattering materials. Because of the way our detectors work (we measure the full wave field), the different ways sound interacts with matter, and new theoretical models to interpret our experimental data, we are in a unique position to address these very basic questions.

Ultrasonic Correlation Spectroscopy
We are developing two new ultrasonic techniques, Dynamic Sound Scattering (DSS) and Diffusing Acoustic Wave Spectroscopy (DAWS), to investigate the dynamics of strongly scattering materials. Possible uses of these techniques range from fundamental studies of hydrodynamics interactions in slurries (mixtures of particles in a fluid), to new applications in process control and monitoring of chemical slurry-bed reactors, to seismic probes of the evolution of underground deposits during oil recovery.

Phononic Band Gap Materials
Crystalline arrays of mm-sized beads in a fluid or solid matrix cause ultrasonic waves to be Bragg scattered, leading to the formation of phononic band gaps in which wave propagation is forbidden. We measure both the dispersion relations and the transmission in these phononic crystals, which are analogous, but complementary, to the much more studied case of photonic band gap materials.

Porous Media
We investigate the ballistic and diffusive transport of elastic waves (both longitudinal and transverse polarizations) in porous solids. Examples of these materials include foams and sintered networks of beads, both of which can exhibit very strong scattering in the intermediate frequency regime. We are currently exploring the effects of this very strong scattering on wave diffusion.

Fluidized Suspensions
Non-Brownian particles can be suspended in a fluid by flowing the fluid upward to counteract gravity-induced sedimentation. Even though the ensemble average velocity of the particles is zero, the fluctuations in the particle velocities are remarkably large. We study the temporal and spatial correlations of the velocity fluctuations as a function of concentration and Reynolds number using ultrasonic correlation spectroscopy.

Bubbly Media
Concentrated suspensions of gas bubbles in a liquid or gel profoundly influence the propagation of ultrasonic waves, leading to intriguing wave dispersion and multiple scattering effects. We use a combination of ballistic and diffusive pulse propagation experiments to investigate this behaviour, and Diffusing Acoustic Wave Spectroscopy to probe the dynamics of gas bubbles. We are interested in learning how bubbles nucleate, grow and coalesce, and how bubbles influence the flow properties of liquids in which they are suspended.

Biological Materials in Food Science
We use ultrasonic techniques to study the mechanical and structural properties of inhomogeneous biological materials that make up the foods we eat. Examples with important structural characteristics at ultrasonic wavelengths are bread, dough and potatoes. We are aiming for a better understanding of their complex physical properties, a long-term goal being the use of new ultrasonic techniques to assess food quality. This interdisciplinary project is being carried out in collaboration with Dr. Martin Scanlon in the Department of Food Science.

To learn more about what we are doing, see our publications or contact us - we'd be happy to talk with you.