Structural and topological defects governing plastic deformation in colloidal glasses under shear

Abstract

Defects play a fundamental role in determining the mechanical behavior of materials. In crystalline solids, their nature and dynamics are well understood, as dislocations and vacancies mediate plastic deformation. However, identifying analogous structural defects in amorphous materials remains an open challenge. In this thesis, we explore the microscopic origin of relaxation and plasticity in amorphous systems using dense colloidal glasses as a model, aiming to connect their macroscopic mechanical response to the underlying local structure. We define a structural order parameter (SOP) based on the mean-field caging potential that a particle experiences from its surrounding neighbors. This parameter, grounded in density functional theory, quantifies local structural stability and successfully identifies weak, defect-like regions in the glass. Experiments under quiescent conditions show that particles with higher SOP values are more prone to rearrangement, demonstrating a strong correlation between structure and dynamics. Under shear, we extend this structural framework to examine how mechanical loading modifies local stability and triggers plastic rearrangements. The mean SOP increases with strain, indicating shear-induced softening, while the non-affine displacement grows correspondingly, showing that structural evolution and dynamics are closely coupled. The SOP also accurately identifies the locations of shear transformation zones (STZs), revealing that plastic activity preferentially originates and proliferates in structurally weak regions. We have further investigated the structural changes induced by shear using an additional approach based on local structural motifs. The underlying idea is that rigidity in supercooled liquids and glasses arises from the presence and growth of locally stable motifs as the system approaches the glass transition. Under applied shear, however, this rigidity is weakened as these motifs undergo structural rearrangements. Such changes manifest as variations in the population, spatial distribution, and size of the locally stable motifs, providing a complementary structural measure of how shear disrupts the underlying amorphous order. We employ topological cluster classification to assess the stability of different local configurations. Icosahedral clusters exhibit the deepest caging potentials, identifying them as the most stable motifs, while 10B defective icosahedra and 11F clusters emerge as the predominant locally favored structures. Under shear, these motifs progressively break down, demonstrating how glasses lose rigidity through structural reorganization. Interestingly, even within the same topological cluster, the caging potential governs the particle stability, the particles with lower caging potentials are more prone to undergo larger displacements and consequently alter or disrupt their topology under shear. In the final section, we present the investigation of topological defects that govern plasticity in a colloidal monolayer that is driven using an optical vortex. We use the cage relative displacement of particles to detect topological singularities mediating plasticity. We identify the topological defects of charge +1 and -1 in the coarse-grained displacement field and show that negatively charged defects act as the primary carriers of plastic deformation. The number of defects increase during the shear and their number decreases at secession of shear. The -1 defect aligns at 45◦ to the shear direction during shear to relieve the stress and the defects orient parallel or perpendicular to make their existence energetically favorable. Their proliferation under shear, alignment, and annihilation upon cessation of shear mirror the defect dynamics known in crystalline materials, offering a unifying perspective of plasticity across ordered and disordered systems. Overall, this work provides a coherent picture linking local structural stability, plastic rearrangements, and topological defect dynamics in amorphous materials. It demonstrates that relaxation in glasses originate from structurally weak regions whose evolution and dynamics under stress governs the way a solid undergoes transitions from a solid irreversible plastic flow.