THESIS
2013
xxvi, 117 p. : ill. (some col.) ; 30 cm
Abstract
In this thesis, we employ colloids and video microscopy to study phase transitions
with single particle dynamics. Specifically, we explore the melting transitions and
nucleation of solid-solid transitions in thin-film colloidal crystals.
In the first set of experiments, we studied the melting of multilayer colloidal
crystals composed of diameter tunable microgel spheres with short-ranged repulsive
interactions confined between two walls. We tune the temperature to
change volume fraction of samples to drive the melting transition. The film
thickness effects on the melting process and on the phase behaviors of single
crystal and polycrystalline films are explored: Thick films ( 4 layers) are observed
to melt heterogeneously, while thin films (≤ 4 layers) melt homogeneously,
ev...[
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In this thesis, we employ colloids and video microscopy to study phase transitions
with single particle dynamics. Specifically, we explore the melting transitions and
nucleation of solid-solid transitions in thin-film colloidal crystals.
In the first set of experiments, we studied the melting of multilayer colloidal
crystals composed of diameter tunable microgel spheres with short-ranged repulsive
interactions confined between two walls. We tune the temperature to
change volume fraction of samples to drive the melting transition. The film
thickness effects on the melting process and on the phase behaviors of single
crystal and polycrystalline films are explored: Thick films (> 4 layers) are observed
to melt heterogeneously, while thin films (≤ 4 layers) melt homogeneously,
even for polycrystalline films. Grain-boundary melting dominates other types
of melting processes in polycrystals when films are thicker than 12-layers. A
novel heterogeneous melting at dislocation is discovered. Thin film melting is
qualitatively different: thin films homogeneously melt by generating many small
defects which need not nucleate at grain boundaries or dislocations without hexatic
phase. The melting of 3D single crystals and buckled phases is also studied
by direct observation.
In another set of experiments, we investigated the nucleation of solid-solid transitions
between square (⬜) and triangular (△) lattices in colloidal thin films
composed of thermally sensitive micro-spheres. We superheated the interior of
crystals, including with vacancies, with dislocations and on a grain boundary,
by local heating techniques. Two types of nucleation processes were directly
observed by video microscopy and studied at the single-particle level. Without
flow, the nucleation is a two-step diffusive process: square-lattice(⬜) crystal →
a liquid nucleus → triangle-lattice(△) nucleus and its precursor is local particle-exchange
loops. The middle liquid nuclei are well explained by small liquid-⬜
interface tension. A post-critical liquid nucleus is circular but become faceted
after transforming to △. The growth rate of different facets depends on their
coherence. However, under small flow rates the nucleus of the triangle lattice
forms by a martensitic ( diffusionless) mechanism and its precursor is a pair of
dislocations. Our observation shows the microscopic process of generating a pair
of dislocations and triggering more pairs. The initial nucleus is semi-coherent and
loses coherency as nucleus growth. Furthermore, we studied the growth stage of
post-critical nuclei and found that incoherent facets with higher interfacial energy
grow much faster than coherent facets. Nearby nuclei can merge with each
other by devoloping an liquid-like channel, distorting ⬜-lattice between them
and attracting smaller nuclei by the strain field in the parent matrix.
In addition, I will report my theoretical work on the self-similarity of phase space
networks in four models. These models includes: (1) the antiferromagnetic Ising
model on a two-dimensional triangular lattice (1a) at the ground states and
(1b) above the ground states and (2) the six-vertex model. The two lattice
gas models were (3) the one-dimensional lattice gas model and (4) the two-dimensional
lattice gas model. We mapped their phase spaces into networks
and used box-covering and cluster-growing methods to illustrate their self-similar
properties. According to the box-covering method, fractal phase spaces are found in Models 1a, 2, and 3 with long-range power-law correlations in real space,
while models 1b and 4 with short-range exponential correlations in real space
exhibit nonfractal phase spaces. This behavior agrees with one of the untested
assumptions in Tsallis nonextensive statistics.
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