Research Experiences for Undergraduates
Materials Research at Carnegie Mellon University
Mohammad F. Islam
Assistant
Professor
Department
of Chemical Engineering & Materials Science and
Engineering
Carnegie
Mellon University
“Soft and Nanomaterials to
study Phase Transitions”
Wednesday, June 20, 2007
12 PM Seminar in Hamerschlag B131
Refreshments will be served.
I will describe recent experiments on phase transitions
in colloidal rods and spheres using temperature sensitive
gels. In one set of experiments we explore the
isotropic-to-nematic transition of rods, in this case
single wall carbon nanotubes, by dispersing nanotubes
in a temperature sensitive cross-linked polymer gel,
and then inducing a temperature-dependent volume-compression
transition of the gel [1]. We also observe the
following striking features due to the coupling of
nematic order and elasticity of the polymer matrix:
(i) undulations and then cusping of the gel sidewalls,
(ii) a nematic director field that evolves as the gel
sidewalls deform, (iii) networks of surface cracks
that are orthogonal to the nematic director field,
and (iv) fissures at the sidewall cusps and associated
topological defects that would not form in liquid nematics
[1,2]. In a second set of experiments we investigate
the melting mechanisms of three dimensional (3-D) colloidal
crystals using thermally responsive monodisperse microgel
spheres [3]. The particle diameter decreases
with increasing temperature and leads to volume fraction
changes that drive phase-transitions. Interestingly,
by imaging the motions of individual particles in 3-D
colloidal crystals during the melting process, we find
premelting near grain boundaries and dislocations. |
