Shear strain assisted grain coarsening and solid-solid phase transitions in colloidal crystals

Abstract

Micrometer-sized colloidal particles can serve as "model" of atoms, whereby the larger length scales (10⁴ times larger than atoms) and the slower time scales (10¹² 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 𝛾 ≥ 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 ≤ 𝛾 < 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 𝜑_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 (𝛾 < 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 ≤ 𝛾 < 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 ≤ 𝛾 < 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 (𝛾 < 0.01) as shear plays significant roles in inducing melting and align the △ nuclei.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology