Glasses are often celebrated for their strength, stability and resistance to deformation. Yet the very features that make a glass mechanically robust can also create a dangerous weakness: once the material is pushed beyond its limit, it may fail suddenly along a single narrow plane. A new computational study suggests that this trade-off between strength and ductility may not be permanent. By introducing a small number of self-propelled particles into a glass while it is being sheared, researchers have shown that catastrophic failure can be transformed into a slower, more distributed process. Instead of allowing damage to concentrate in one dominant shear band, the active particles redirect deformation into a connected network of smaller bands, enabling the material to withstand greater stress before yielding.
The work, led by Rashmi Priya and Smarajit Karmakar at the Tata Institute of Fundamental Research in Hyderabad, in collaboration with Jürgen Horbach of Heinrich Heine University Düsseldorf, addresses a longstanding problem in the mechanics of amorphous solids. Unlike crystals, glasses lack a repeating atomic structure. Their constituent particles are disordered and remain trapped in configurations inherited from the liquid state. The degree of stability depends strongly on how the glass was prepared. A slowly cooled or extensively aged glass settles into a deep, stable energy minimum, making it difficult to disturb. That stability generally produces high strength, but it also makes the material brittle. A rapidly cooled glass occupies a shallower energy minimum and is easier to deform, giving it greater ductility but lower load-bearing capacity.
This contrast becomes especially clear when a glass is subjected to shear, the type of deformation produced when the bottom of a block is held fixed while its top is pushed sideways. In a stable glass, particles resist rearrangement until the applied stress reaches a large yield point. At that moment, a collective rearrangement suddenly develops into a thin shear band, a plane across which particles undergo intense localized motion. Once the band spans the sample, stress drops sharply and the material can fail almost instantaneously. Less stable glasses tend to deform more gradually because rearrangements are distributed throughout the material, but they yield at lower stress. For decades, this apparent strength–ductility trade-off has limited the design of metallic glasses, amorphous coatings, soft solids and engineered metamaterials.
The researchers explored whether the mechanical behavior of a glass could be changed after preparation rather than by altering its cooling history or aging process. Their simulations introduced a small fraction of self-propelled particles, sometimes called active particles, into an otherwise passive amorphous solid. These particles continuously exert forces on their surroundings and move in a direction that changes after a characteristic interval. Real-world analogues include swimming microorganisms, fuel-driven colloids and synthetic particles activated by light or chemical reactions. In the model, the active particles were not simply sources of random heating. Their motion supplied directional forces that interacted with the glass’s own structural rearrangements and introduced a new internal timescale into the material.
Under appropriate conditions, the active particles did not weaken the glass as might be expected. Instead, they altered the shape of its stress–strain response. In a conventional brittle glass, stress rises with increasing deformation until it reaches a sharp maximum, followed by an abrupt drop associated with the formation of a dominant shear band. With short-lived, rapidly reorienting activity, the simulated stress–strain curve became more rounded. The material remained able to carry stress for longer, reached a higher yield stress and then relaxed through a sequence of distributed rearrangements. The result was not the elimination of failure, but a change in the way failure developed: one catastrophic plane was replaced by many smaller bands that gradually formed, interacted and connected across the sample.
The mechanism depends strongly on the persistence time of the active motion, defined as the period during which an active particle continues pushing in approximately the same direction before reorienting. When this persistence time is short, an active particle repeatedly pushes against the cage formed by its neighbors but does not travel far enough to escape. Its motion resembles rattling within a crowded environment. Those local force fluctuations can disrupt the conditions needed for a single shear band to grow uninterrupted, encouraging deformation to appear at multiple locations instead. In effect, the active particles redistribute the damage before it can collapse into one dominant path. The glass retains its high stability while acquiring a more ductile mode of failure.
The same particles can produce the opposite effect when their persistence time becomes long. If an active particle continues pushing in one direction for sufficiently long, it can break out of its local cage and drive a larger rearrangement. Rather than merely perturbing the glass locally, it can help initiate sustained flow. In that regime, activity makes the material easier to deform and may reduce the advantage gained from its initial stability. The simulations therefore identify a non-monotonic role for activity: weak, rapidly changing propulsion can strengthen and toughen the response, while persistent propulsion can promote mobility and yielding. The critical factor is not simply how much active force is present, but how that force is timed relative to the material’s structural dynamics.
The researchers found that this timing creates an equivalence between external shear and internal activity. A glass deformed rapidly experiences a strong drive toward rearrangement, while a slowly sheared glass has more time to relax between deformation events. Activity can compensate for these differences. In the simulations, a rapidly sheared glass with weak activity could display behavior similar to a slowly sheared glass with stronger activity. This relationship reflects a competition among three timescales: the time imposed by the external shear, the persistence time of the active particles and the time required for a shear band to propagate through the material. When propagation wins, deformation localizes. When activity and external driving interfere with that propagation, strain spreads through multiple bands.
The effect also appeared in creep simulations, in which a constant stress is applied and the resulting deformation is monitored over time. Increasing activity under the appropriate conditions delayed the onset of sustained flow and reduced the rate at which the material deformed. This is significant because creep failure can be as important as sudden yielding in practical applications. Components made from amorphous materials may experience loads below their apparent yield stress for long periods, gradually accumulating deformation. If active doping can regulate that slow flow, it could offer a new strategy for designing materials whose mechanical response is adjustable during operation rather than fixed during manufacturing. The findings may be relevant to dense colloids, soft composite materials, active suspensions and, eventually, biological tissues that use internal activity to respond to mechanical stress.
The study remains theoretical and computational, so experimental confirmation is the next major challenge. Dense colloidal systems containing photoswitchable active particles could provide a promising testing ground because light can control propulsion and persistence. However, current experimental methods are more readily applied to gels and colloidal suspensions than to dense solid glasses. If the predictions are verified, active-particle doping could become a new way to separate strength from brittleness, allowing materials to support high stresses without concentrating damage into a single fatal crack-like band. The broader implication is that the failure of disordered solids may be governed not only by their static structure or preparation history, but also by controllable internal motion. By adding particles that push, stop and turn on their own, researchers may have found a route to make some of the strongest glasses fail less catastrophically.
Subject of Research: Active particle doping and tunable yielding in amorphous glasses
Article Title: Tunable yielding and emergent rheology in amorphous solids with active particle doping
Web References: https://doi.org/10.1038/s41467-026-75709-y; https://www.tifrh.res.in; https://www.hhu.de/en/
References: Nature Communications, DOI: 10.1038/s41467-026-75709-y
Image Credits: Rashmi Priya, TIFR Hyderabad
Keywords
active matter, active particles, amorphous solids, glasses, shear bands, material failure, ductility, brittleness, computational simulation, rheology, self-propelled particles, colloids, Nature Communications
