Polydispersity in Colloidal Phase Transitions
dc.contributor.advisor
Poon, Wilson
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dc.contributor.author
Fairhurst, David John
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dc.date.accessioned
2007-02-09T16:01:47Z
dc.date.available
2007-02-09T16:01:47Z
dc.date.issued
1999-06
dc.description.abstract
I have studied the effects of polydispersity on the phase behaviour of suspensions
of PMMA colloidal spheres on their own and in the presence of non-adsorbed
polymer.
I systematically explored the volume fraction-polydispersity phase behaviour of hard spheres (with radii R =167, 244, 300 and 303nm) through direct
observations and crystallography measurements. I observed normal crystallisation for sigma <
7:5%, and no crystals at sigma >
18%. Samples at sigma ~~ 9.5% showed
crystal-fluid coexistence between 0:52 <
phi < 0:56 but no fully crystalline be-
haviour above this region. This may be explained by slow particle diffusion in the dense metastable fluid and a glass transition, possibly involving only the
larger particles.
The addition of random coil polymer (radius of gyration rg) to a suspension of
single-sized spherical colloidal particles induces an attractive depletion potential
which, for size ratios Xi = rg=R <
0:2, has the effect of expanding the crystal-fluid
coexistence region. Surprisingly, when such a polymer solution (with Xi = 0:1),
with a range of concentrations cp, is added to a polydisperse colloidal suspension
(sigma ~~ 10%), crystal formation is actually suppressed. This can be explained by the
fact that the polymer compresses the nascent crystal phase to volume fractions
greater than the maximum phi permitted for polydisperse spheres. By modifying
existing free energy equations to include the effects of colloidal polydispersity we
also succeed in reproducing the observed phase diagram.
Larger added polymer (Xi >
0:3) introduces a region of stable gas-liquid coexistence. In systems where crystallisation is suppressed due to polydispersity, this
will theoretically be the only transition. By preparing many samples over a range
of phi and cp this prediction was observed experimentally for Xi = 0:5. Fractionation studies on coexisting phases enabled verification of a recent universal law of fractionation in slightly polydisperse systems.
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dc.format.extent
1167539 bytes
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dc.format.mimetype
application/pdf
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dc.identifier.uri
http://hdl.handle.net/1842/1496
dc.language.iso
en
dc.publisher
The University of Edinburgh: College of Science and Engineering: School of Physics
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dc.subject.other
colloidal spheres
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dc.subject.other
condensed matter
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dc.title
Polydispersity in Colloidal Phase Transitions
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dc.type
Thesis or Dissertation
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dc.type.qualificationlevel
Doctoral
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dc.type.qualificationname
PhD Doctor of Philosophy
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