How do crystals flow? The plastic flow of crystalline materials – irreversible deformation of their shape – requires disrupting the long-range crystalline order, which happens through the nucleation, motion, and interaction of topological line defects in the crystalline structure called dislocations. To understand the complex collective behavior of these defects and the emergence of macroscopic plastic deformation, we use colloidal crystals. Colloidal crystals share basic properties with a broad range of atomic crystalline materials; the micron size of the colloidal particles allows us to visualize the deformation process with unprecedented detail using optical rather than electron microscopy.
Instability in epitaxial growth of colloidal crystals
Epitaxial growth of thin films on a lattice-mismatched substrate – as happens in many technologically important applications – is unstable: when the film reaches a critical thickness, the misfit strain is relaxed by the nucleation and elongation of dislocations. We use fast confocal microscopy to resolve the position of ∼10 million particles and to understand the interplay between the collective behavior of dislocations and the relaxation process. Our recent work reveals the critical role of dislocation interactions during the plastic deformation of thin films and can be readily generalized from the colloidal to the atomic scale despite the apparent simplicity of the hard sphere particle-particle interactions.
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- I. Svetlizky, S. Kim, D. A. Weitz, and F. Spaepen “Dislocation interactions during plastic relaxation of epitaxial colloidal crystals” Nat. Commun. 14, 5760 (2023).
Plastic shear flow of colloidal crystals
We are developing an experimental setup that allows us an unprecedented real-time look at how single crystals flow under imposed shear. We have recently demonstrated that colloidal crystals exhibit work hardening in direct analogy to their atomic counterparts: increasing stresses are required to sustain plastic flow. We now aim to understand the underlying mechanisms of work hardening, what new mechanisms emerge when crystals are hardened to the theoretical limit, and how memory and the deformation history are encoded within the structure of the dislocation networks.
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- S. Kim, I. Svetlizky, D. A. Weitz, and F. Spaepen “Work hardening in colloidal crystals” Nature (2024)
Multi-scale imaging of dislocations
We are developing a multi-scale visualization approach to resolve in space and time the fast kinetics of dislocations: Visualisation at the single-particle level achieved by confocal microscopy is complemented by direct imaging of dislocations by using TEM-inspired diffraction-based imaging methods. We are developing a deep convolutional neural network to address the inverse problem of inferring the structure of the dislocation network from multiple complex diffraction images. This approach will be instrumental in probing the collective behavior of dislocations over significantly shorter times and larger areas.
Previous research
Spatio-temporal dynamics of fracture
Solids can break when they are sufficiently strained. The fracture process – the growth of the fracture surface – is mediated by a dynamically propagating crack front – a line that defines the fracture surface. The dynamics of the crack front in a fluid-driven fracture are particularly interesting due to the strong coupling between the fluid and the elastic medium. We visualize circular fluid-driven fracture using high-speed imaging and find that the crack propagation is unstable: long pauses of no motion are followed by rapid forward jumps. The forward propagation occurs through a nucleation event, followed by rapid transverse expansion of the crack front. These results highlight the importance of transverse dynamics in the forward propagation of extended fractures.
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- T. Cochard, I. Svetlizky, G. Albertini, R. C. Viesca, S. M. Rubinstein, F. Spaepen, C. Yuan, M. Denolle, Y-Q. Song, L. Xiao, D. A. Weitz ”Propagation of Extended Fractures by Local Nucleation and Rapid Transverse Expansion of Crack-Front Distortion” Nat. Phys. 2024
Onset of friction
The onset of frictional motion is mediated by dynamically propagating fronts, akin to earthquakes, that rupture the discrete contacts forming the rough frictional interface. These rupture fronts separate the sticking and sliding regions; macroscopic motion is initiated only after such a front has traversed the entire frictional interface. Describing the coupling between the propagating ruptures, frictional interface, and macroscopic sliding is key to understanding friction. Our work experimentally established the mapping of frictional rupture fronts to cracks: The classical fracture mechanics framework describes the singular elastic fields in the rupture tip vicinity, the equation of motion of these ruptures, their arrest, and stress-wave radiation. This fracture-based description of friction replaces the idea of a characteristic static friction coefficient.
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- I. Svetlizky, J. Fineberg “Classical shear cracks drive the onset of dry frictional motion“ Nature, 509, 205–208 (2014)
- E. Bayart, I. Svetlizky, J. Fineberg “Fracture mechanics determine the lengths of interface ruptures that mediate frictional motion” Nat. Phys. 12, 166–170 (2016)
- I. Svetlizky, E. Bayart, J. Fineberg “Brittle fracture theory describes the onset of frictional motion” Annu. Rev. Condens. Matter Phys. 253-273 (2019)