Non-Newtonian Fluids - Current Interests
- Rheology of yield stress
materials: We measure the behavior of yield-stress fluids in response to an applied stress or
strain, to characterize the materials and to learn about how their bulk properties depend on things
like composition, temperature, and time. We can use rheological measurements to study the storage and
dissipation of energy in these fluids, and this in turn gives us some clues as to the processes that
take place on the microscopic scale.
- Viscoplastic Flows: The flow of
viscoplastic materials is important in areas ranging from the development of cosmetics, to
bioreactors, to the study of lava flows and avalanches. Because of the existence of a yield stress,
viscoplastic flows are quite different from flows in Newtonian fluids like water. We are doing
experiments using several flow visualization techniques to understand the flow of yield-stress
materials in a variety of situations: drainage from the bottom a cylinder, flow around a sphere, flow
down an inclined plane. We are also studying the motion of bubbles through yield-stress fluids.
- Microrheology of yield-stress fluids: We study the viscoelastic properties of
structured fluids on micro-meter length scales using the techniques of microrheology. We track the
diffusive motion of small suspended fluorescent particles using a fluorescence microscope. We also do
dynamic light scattering, which involves determination of the motion of small tracer particles from
fluctuations in the intensity of laser light scattered by the particles. In both cases, the data
allows us to extract the microscopic viscous and elastic moduli and infer information about the
micron-scale structure of the material.
- Rheology of polymeric
materials: We study the structure, properties and rheological behaviour of a range of polymer
fluids. We have looked at associative polymers, in which the polymer molecules interact via to form a
network structure. We are also interested in composites made up of polymer molecules and
nanometer-sized particles such as carbon nanotubes or clay particles. We used light scattering,
rheometry, microrheology, and other techniques to study these interesting and complex materials.
- Materials for biomedical phantoms: It is often important to use materials which mimic the
propertied of human tissue in the testing of medical devices. Wtih Blaine Chronik , I am
investigating the properties of polymer-based materials to be used as phantoms for the testing of
devices for magnetic resonance imaging.
- Onset of yield stress: Typically yield
stress develops in a material as the concentration of the suspended particle is increased. In some
cases yield stress develops over time as the suspended particles aggregate. We are interested in the
transition from the fluid state, with no yield stress, to the solid, which has a yield stress.
This is a gelation transition in the case of clay suspensions, but can take other forms in other
materials. We have been studying this transition in a variety of different materials using both
microscopic techniques and bulk rheological methods.
- Rayleigh-Benard convection in complex fluids: A thin layer of fluid heated from
below will develop flow in a pattern of convective rolls or cells when the temperature difference
across the layer becomes sufficiently large. In small fluid samples, the flow pattern is very simple,
but in large samples it can be extremely complex. We have studied convection in Newtonian fluids and
are now planning to look at flow patterns that develop in complex fluids, including yield-stress
fluids and viscoelastic materials.