Work hardening in colloidal crystals
Nature volume 630, pages 648–653 (2024 )Cite this article
Colloidal crystals exhibit interesting properties1,2,3,4 that are in many ways analogous to their atomic counterparts. They have the same crystal structures2,5,6,7, undergo the same phase transitions8,9,10, and possess the same crystallographic defects11,12,13,14. In contrast to these structural properties, the mechanical properties of colloidal crystals are quite different from those of atomic systems. For example, unlike in atomic systems, the elasticity of hard-sphere colloidal crystals is purely entropic15; as a result, they are so soft that they can be melted just by stirring16,17. Moreover, crystalline materials deform plastically when subjected to increasing shear and become stronger because of the ubiquitous process of work hardening18; but this has so far never been observed in colloidal crystals, to our knowledge. Here we show that hard-sphere colloidal crystals exhibit work hardening. Moreover, despite their softness, the shear strength of colloidal crystals can increase and approach the theoretical limit for crystals, a value reached in very few other materials so far. We use confocal microscopy to show that the strength of colloidal crystals increases with dislocation density, and ultimately reaches the classic Taylor scaling behaviour for atomic materials19,20,21, although hard-sphere interactions lack the complexity of atomic interactions. We demonstrate that Taylor hardening arises through the formation of dislocation junctions22. The Taylor hardening regime, however, is established only after a transient phase, and it ceases when the colloidal crystals become so hard that the strain is localized within a thin boundary layer in which slip results from an unconventional motion of dislocations. The striking resemblance between colloidal and atomic crystals, despite the many orders of magnitude difference in particle size and shear modulus, demonstrates the universality of work hardening.
The properties of atomic crystals under high strain are well established18; when subjected to stresses beyond the yield stress, they exhibit plastic flow, which causes an irreversible change in their shape. This is mediated by nucleation and motion of topological line defects called dislocations18,23. Increasing plastic deformation requires an increasing flow stress because of the interactions between dislocations. This is work, or strain, hardening. This phenomenon is ubiquitous, yet, owing to the many ways dislocations can interact, our understanding of the mechanism that governs work hardening is still incomplete20,22,24,25,26. In contrast to atomic crystals, colloidal crystals have much simpler interparticle interactions; they consist of solid particles in a fluid and can exhibit purely hard-sphere interactions. Here we show that, despite the simplicity of these interactions, hard-sphere colloidal crystals exhibit work hardening. The micron scale of the particles enables the structure and dynamics of colloidal crystals to be investigated on a particle-by-particle level using optical microscopy5,8,9,12,13,14,27,28,29. Dislocations in these colloidal crystals can, therefore, be directly visualized in three dimensions and in real time. Thus, these measurements provide insight into the general nature of work hardening.
