Particles in complex fluids
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Date
2010Author
Zand, Daniëlle D. van ’t
Metadata
Abstract
This thesis describes experimental studies of colloidal particles dispersed in solvents
which themselves have phase transitions. One common definition of soft matter is: a
material characterized by a mesoscopic length scale. This length scale is, for example,
the colloid size or the ordered domain size. Here we combine a complex host with one
characteristic length scale with dispersed particles that have a different size. It might
be anticipated that new behaviour will occur. Two limits of particle characteristics
are probed: the case of dilute sterically-stabilized particles and the case of a weak gel
of attractive particles. The two systems are polymer particles dispersed in a phaseseparating
microemulsion and silica nanoparticles dispersed in a low molecular weight
liquid crystal. In each system a temperature driven phase transition plays a crucial role.
In the microemulsion case we observe how transitional and pre-transitional phenomena
create effective interactions between particles and how new behaviour emerges in the
host solvent in the late stage of the phase separation. We show that the pre-transitional
clustering of the PMMA particles is due to an adsorbed layer of dodecane. Subsequently
heterogeneous nucleation of the gas phase is seen. After phase separation has occurred
in off-critical samples the particles remain in either the continuous or dispersed phase
depending on the original microemulsion composition. In the late stage of the phaseseparation
the coalescence and coarsening behaviour changes significantly, after more
material exchange between the phases has taken place. This behaviour is reminiscent
of viscoelastic phase separation in polymer based samples.
In the liquid crystal case we discover the anisotropy of the liquid crystal persists over
large length scales and modifies the local dynamics of the gel. Using electron microscopy
and scattering techniques we demonstrate that the silica embedded in the liquid crystal
forms agglomerates with a fractal structure. Rheological characterization demonstrates
that the resulting composite is a gel. Investigation of the composite’s local dynamics
using x-ray photon correlation spectroscopy shows anisotropy and intermittency in the
dynamics on significant length scales.
In both systems we have studied new behaviour seen due to the influence of one
component on the dynamic characteristics of the other The pre- and post- phase
transition phenomena are only crucial in the microemulsion case where the particles
have purely repulsive interactions. Our results illustrate the subtle balances that occur
in soft composite systems.