Shear induced topological changes of local structure in dense colloidal suspensions

Abstract

Understanding the structural origins of glass formation and mechanical response remains a central challenge in condensed matter physics. Recent studies have identified the local caging potential experienced by a particle due to its nearest neighbors as a robust structural metric that links microscopic structure to dynamics under thermal fluctuations and applied shear. However, its connection to locally favored structural motifs has remained unclear. Here, we analyze structural motifs in colloidal crystals and glasses and correlate them with the local caging potential. We find that icosahedral motifs in glasses are associated with deeper caging potentials than crystalline motifs such as face-centered cubic (FCC) and hexagonal close-packed (HCP) structures. Both crystalline and amorphous systems also contain large number of particles belonging to stable defective motifs, which are distortions of the regular motifs. Under shear, large clusters of defective motifs fragment into smaller ones, driving plastic deformation and the transition from a solid-like to a liquid-like state in amorphous suspensions. Particles that leave clusters of stable motifs are associated with shallower caging potentials and are more prone to plastic rearrangements, ultimately leading to motif disintegration during shear. Our results thus reveal that the loss of mechanical stability in amorphous suspensions is governed by the topological evolution of polytetrahedral motifs, uncovering a structural mechanism underlying plastic deformation and fluidization.