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My doctoral research
has focused on rheometry
which is the art or practical
science of measuring a material
function like a
viscosity of a material
as it flows or deforms.
A major challenge in rheometry
is the characterization
of viscoelastic liquids at
very large deformation rates,
on the order of a million
inverse seconds, which
are found in applications
from the chewing of food stuff
to industrial applications
like coating flows or flows
through porous mediums.
Conventional
rheometric techniques
are ill suited to
these high deformation
rates on the account of the
onset of flow instabilities
like inertial turbulence,
which confound
the accuracy of measurements
at these high deformation rates
and generally limit the
ability of these techniques
to deformation rates at most of
around 1,000 inverse seconds.
To overcome these
limitations, I focused
on using microfluidic devices
for high deformation rate
rheometry.
A major advantage
of these devices
is their small length
skills, generally as small
as the size of a
human hair, which
generally insurers that
inertial effects are negligible.
On the other hand,
a major challenge
in using these
devices for rheometry
is that it's often quite
difficult-- at leas compared
to conventional techniques--
to relate a strain
rate in the material--
or quickly the material
is deformaing-- to a
stress in the material--
or how much internal
friction or resistance
that material has
to being deformed.
My solution to this challenge
is to combine velocity
measurements-- which enable me
to see inside the micro channel
and how the material
is deforming--
pressure measurements--
which enable me to measure
the pumping power or energy
dissipation associated
with deforming the material--
and finally, flow induced
birefringence measurements,
which provides
an additional measure of stress
and molecular orientation
of the material based on
polarized light microscopy.
I have also collaborated with
researchers at the Harvard
Medical School on the topic of
flow induced particle migration
in channels, often
called inertia focusing.
This phenomenon has been
proposed as a breakthrough
technology for applications
in flow cytometry,
or also the isolation of
rare and diseased cells
in the bloodstream.
The challenge however is that
real physiological fluids
like blood are non-Newtonian.
And therefore both the
fluid inertia and elasticity
are simultaneously relevant
in governing the particle
migration behavior.
This is a regime that has
not been thoroughly studied,
and so we have used
the same techniques
I have been using for studying
microfluidic rheometry to study
this phenomenon.
In our experiments, we
see a clear difference
between the migration
behavior in a Newtonian fluid
like water or a buffer solution
and in a viscoelastic polymer
solution.
In a Newtonian
fluid, inertia tends
to drive the particles
or cells toward the wall
where the sheer rates are very
high, generating a three streak
image when visualized from
below on a microscope.
On the other hand,
polymer solutions
offer the potential
benefit that they usually
drive particles towards
the channel center,
away from regions of high
sheer rate and high stress,
which could be
beneficial for long term
viability of the cells.
Overall my doctoral
research has focused
on developing a basic tool
set and framework for using
microfluidic devices
to characterize
viscoelastic liquids at
large deformation rates.
Having this tool set is
of significant importance
for the oil industry,
in the production
of food and consumer
products, and in point of care
diagnostics for biomedicine.
