Work hardening makes soft colloidal crystals ‘stronger’ than most metals

Colloidal crystals are fascinating materials. They exhibit a rich behavior that is in many ways similar to their atomic counterparts. However, the mechanical properties of colloidal crystals are quite distinct from those of atomic systems. For example, unlike in atomic systems, the elasticity of hard-sphere colloidal crystals is purely entropic; as a result, they are so soft that they can be melted just by stirring. In addition, atomic crystals deform plastically when subjected to increasing shear and become stronger due to work hardening – a phenomenon never observed in colloidal crystals. Here we show that colloidal crystals exhibit work hardening. Using confocal microscopy, we show that the relationship between strength and dislocation density aligns with the classic Taylor scaling behavior for atomic materials. We visualize in-situ the dislocation interaction mechanism responsible for work hardening: the formation of dislocation junctions. Remarkably, despite being intrinsically soft materials, the normalized strength of colloidal crystals exceeds that of most metals and approaches the theoretical limit. The observed high strength and high dislocation density could be explained by the vanishingly small stacking fault energy of hard-sphere crystals. Observation of work hardening in colloidal crystals not only provides new insights to this soft matter system, but also offers an opportunity to gain insights into the underlying mechanisms of work hardening itself.