THESIS
2019
xix, 59 pages : illustrations ; 30 cm
Abstract
Micrometer-sized colloidal particles can serve as "model" of atoms, whereby the
larger length scales (10
4 times larger than atoms) and the slower time scales
(10
12 times slower than atoms) allow us to directly observe their thermal dynamics
under optical microscopes. In this thesis, we employ thermosensitive N-isopropylacrylamide
(NIPA) micro-gel colloidal spheres and video microscopy to
explore crystal structure evolutions under an oscillatory shear strain with single-particle
resolution. Specifically, we study the polycrystal grain growth during
annealing and solid-solid phase transitions in colloidal crystals under oscillatory
shear strains.
In chapter 1, we report a 'melting-recrystallization' process in the polycrystal
annealing under a large oscillatory shear strain (st...[
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Micrometer-sized colloidal particles can serve as "model" of atoms, whereby the
larger length scales (10
4 times larger than atoms) and the slower time scales
(10
12 times slower than atoms) allow us to directly observe their thermal dynamics
under optical microscopes. In this thesis, we employ thermosensitive N-isopropylacrylamide
(NIPA) micro-gel colloidal spheres and video microscopy to
explore crystal structure evolutions under an oscillatory shear strain with single-particle
resolution. Specifically, we study the polycrystal grain growth during
annealing and solid-solid phase transitions in colloidal crystals under oscillatory
shear strains.
In chapter 1, we report a 'melting-recrystallization' process in the polycrystal
annealing under a large oscillatory shear strain (strain amplitude ? ≥ 1.0). The
colloidal crystals catastrophically melt rather than via a classical liquid nucleation
process, and then the melted parts recrystallized into a new polycrystal
with larger grain sizes and were better aligned along the shear direction.The crystallization has the same three distinguishing stages as the observation of
supercooled water. The growth of crystallites can be described by the Avrami
equation with exponent n = 0.81, which corresponding the lowest growth rate
in crystallization of supercooled water, indicating that oscillatory shear environment
slows down the crystallization.
Chapters 2 and 3, report a two-stage polycrystalline grain growth under the annealing
of a smaller oscillatory shear strain amplitude 0.01 ≤ ? < 0.5. Chapter
2 shows that the early stage is a normal grain growth (NGG) dominated by the
shear coupled grain boundary migration via gliding of disconnections. A novel
grain rotation from a large-misorientation angle (ϑ) to a small ϑ is observed at
the single-particle level. This provides the first experimental observation of the
annihilation of dislocations from opposite grain boundaries previously suggested
in theory.
In chapter 3, we discuss the later stage of the grain growth under a smaller
shear strain. The initial-stage NGG was replaced by a dynamic abnormal grain
growth (DAGG) featured by a few rapidly growing grains with extremely large
size. Such DAGG has been observed in metals, but the mechanism is not clear.
The slow dynamics in our system enables to resolve that the DAGG arises from
the melting and recrystallization of grains with large mismatch angle, β (the angle
between grain orientation and shear direction). The melting volume fraction
of a grain ?
m ∝ −cos(6β).
In chapter 4, we studied the solid-solid phase transitions driven by an oscillatory
shear. For a crystal free of shear stress (? < 0.01), the nucleation kinetics of
a 5 layer square lattice transforms into a 4 layer triangular lattice (5☐ ⟶ 4△) is a two-step nucleation with an intermediate liquid state. At 0.01 ≤ ? < 0.05,
the kinetic pathway of nucleation changes to martensitic nucleation via particle
inserting from neighboring layer. Such shear stress that suppresses the formation
of intermediate liquid phase may arise from the shear coupled GB migration in
which shear stress built up. At 0.05 ≤ ? < 0.15, we found the nucleation kinetics
turns back to a new two-step nucleation with an intermediate liquid state. However,
this two-step nucleation is different from the nucleation kinetics at stress
free (? < 0.01) as shear plays significant roles in inducing melting and align the
△ nuclei.
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