Unveiling Defects in Amorphous Solid Behavior

Researchers from the Indian Institute of Science Education and Research (IISER) Pune and the CSIR-National Chemical Laboratory (NCL) Pune have uncovered that the macroscopic deformation of amorphous solids is significantly influenced by structural defects within these materials.

This collaborative study, spearheaded by Dr. Vijayakumar Chikkadi at IISER Pune and Dr. Sarika Bhattacharyya at CSIR-NCL, integrates experimental work on colloidal glasses—which serve as models for amorphous solids—with a theoretical framework that employs the structural order parameter. Their results, published in the Proceedings of the National Academy of Sciences, tackle a long-standing issue in materials science and condensed matter physics.

It is known that all materials undergo deformation when subjected to external stresses. In 1934, G.I. Taylor, M. Polanyi, and E. Orowan independently identified that macroscopic deformation stems from defect dynamics within materials. While locating these defects in crystalline solids is generally easy due to observable lattice distortions, doing so in amorphous solids is much more complex due to their lack of long-range order.

To address this complexity, Ratimanasee Sahu and Dr. Chikkadi carried out experiments using dense colloidal suspensions, effectively modeling amorphous solids. By monitoring the movement of nearly 100,000 individual colloidal particles in three dimensions over time with advanced microscopy techniques, they achieved unprecedented insights into the microscopic behavior of these systems.

The study leveraged the structural order parameter developed by Mohit Sharma and Dr. Bhattacharyya, which helps identify soft and hard regions in amorphous suspensions and pinpoint structural defects. Dr. Bhattacharyya emphasized that this order parameter, based on detailed microscopic theory, is particularly valuable for experimental applications, making it more user-friendly compared to other theoretical measures.

For the first time, the team demonstrated that macroscopic deformation in colloidal glasses results from localized deformations that occur mainly in areas with structural defects when exposed to external stress. Dr. Chikkadi noted that this significant advancement deepens our understanding of how defects impact the mechanical properties of disordered solids. Additionally, it sets the stage for the development of enhanced rheological models based on structural features, applicable across a range of materials, including soft matter such as granular materials and emulsions, as well as metallic glasses.