Active particles could make stable glasses stronger without catastrophic brittle failure

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The strongest glasses have an Achilles' heel that causes them to fail catastrophically when pushed past their limit. They do not bend or stretch, as all damage concentrates into a single plane and the material fails in an instant.

This brittleness has long capped the usefulness of high-stability amorphous solids, from bulk metallic glasses to engineered metamaterials. Rashmi Priya and Smarajit Karmakar from the Tata Institute of Fundamental Research (TIFR), Hyderabad, in collaboration with Jürgen Horbach from Heinrich Heine University (HHU), Düsseldorf, report a potential way around this problem: "lacing" the glass with self-propelled particles while it is being sheared markedly reduces its brittleness. This also allows the material to bear higher stress, making it stronger than before.

This theoretical framework strengthens our understanding of brittleness in glasses and links shear to self-propulsion. Unlike a crystal, a glass has no repeating atomic pattern. Its particles are disordered, much as they were in the liquid from which the glass formed, but are trapped in place.

How firmly they are trapped depends on how the glass is prepared. Let us picture a rugged landscape of hills and valleys, where each valley represents one possible arrangement of the particles. A slowly cooled or well-aged glass settles into a deep valley.

It is stable and difficult to perturb, but brittle when pushed past its limit. A rapidly cooled glass is caught in a shallower valley. Being less stable, it is weaker but more ductile.

The difference shows up under shear: Imagine fixing the bottom of a block and pushing its top sideways. In a brittle glass, deformation remains suppressed until the stress reaches a large yield value. The particles suddenly organize into a shear band: a thin plane of intense rearrangement slicing across the whole sample, with a sudden drop in stress.