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.
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.
