Andrew B. Croll, PhD, North Dakota State University
Over this second year
of our grant, we have made significant progress towards our goal of creating
and exploring the behaviour a soft-colloidal, tissue-like material. We have
continued our exploration of soft spheres and collections of spheres and begun
to explore new off-shoots from this preliminary work. Our scientific progress
was a success because the grant enabled talented graduate students and
undergraduates to push the research forward.Damith Rozairo, also
funded last year, is now in his third year as a Ph.D. candidate in NDSU’s
material science and nanotechnology program. He is primarily focused on the
mechanics of emulsion droplets protected by an amphiphilic diblock copolymer.
In the past year Damith has made progress in his development of a new
measurement technique and has just submitted the first paper on the work. The
technique allows him to make a mechanical measurement of a soft sphere’s
resistance to deformation that is applicable to any soft sphere. He
demonstrated the technique with a novel polyelectrolyte spherical shell that he
has synthesized. The polyelectrolyte shell was designed to match the mechanical
properties of the fluid emulsion drops in order to show that a solid shell can
often behave in the same manner as a simple fluid droplet.The
measurement that we have developed uses buoyancy to deform a shell, fluid
droplet or any sphere against a freshly cleaved mica sheet (a sessile drop). To
model the shell, Damith has adapted the Bashforth-Adams model to accommodate
the elasticity of the shell and simplified the overall theory in
order to fit contact area as a function of drop size. This is remarkably
useful, because any ordinary microcope can be used to image contact area and
drop size. Figure 1. Shows the basic outcome of the experiment for a fluid
drop and the elastic shell designed to match. We show both the modified
Bashforth-Adams fit, and a simple scaling argument we developed.Damith is currently
working on an interesting additional discovery. In a typical experiment, Damith
creates a drop with a pipette and it floats up to the mica surface. When the
droplet nears the surface, it traps some fluid under its contact patch. This
is a well known drainage problem which is critically important to understanding
the coarsening dynamics of emulsions and foams. Damith discovered that the
process is so strongly linked with the interfacial properties, that he can
measure the surface rheological properties of the emulsion drops. This work is
ongoing, but we will start presenting the measurements at national meetings
over the next year.Bekele
Gurmessa was partially funded for a period of this award and is currently
working to write his Ph.D. thesis. Bekele has made many contributions outside
of this grant, and his funding with the PRF-DNI has allowed him to explore the
role of discreteness in buckling structures. His work involves compressing a
layer of hard colloid scale particles and examining the pattern of the
resulting buckled structure. This work has already lead to two publications,
and is having impact in the granular physics community. The instability that
Bekele has been studying, we believe, is related to the ‘buckling force chains’
which are the first steps in granular failure (landslides). Understanding this
buckling process on a fundamental level will be a major contribution. He is
currently examining the role of heterogeneities in the failure process, and we
believe another publication will be forthcoming.We would
also like to highlight the work of the summer student that was funded by this
grant. This year we had a summer student modify our micropipette measurement
system with the addition of an op-amp amplifier and some added automation. We
hope that his rebuilt system will be capable of accurately measuring the
dynamic surface tension of the block copolymer emulsion drops. When complete,
it will enable a comparison with our drainage technique. The student is currently
analysing what we think is good data created with the system. Understanding
the dynamics is a critical part of understanding how the overall soft colloidal
system will function.In closing,
this PRF Doctoral New Investigator grant has already contributed 2 published
works, one is under review and we see at least 4 more publications down the
road. The grant has allowed my career to begin, has allowed my research to be
done and has created useful results for the soft colloidal community. I have
been able to mentor several graduate students, and helped several
undergraduates experience the exiting world of research. The bedrock research
we have been able to begin will lead to many more important results down the
road Undoubtedly, the PRF-DNI awards effects will be long lasting in the
training of the students and our contributions to the literature.
Figure 1. Contact
radius (r) as a function of drop radius (R) for a fluid drop (red triangles)
and an elastic shell (green squares). The dashed curve is a power law related
to a scaling argument we have constructed. The solid curve is the output from
a modified Bashforth-Adams equation.
printer friendly
