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RESEARCH (5)
Mechanical properties of curved particle shells
Fullerene-like structures, such as colloidosomes, composite particles, and hollow particle shells offer new opportunities for drug encapsulation and delivery. Because of their small scale and topological features, the mechanical properties of these structures are special. We investigate the microstructural processes underlying the deformation of  these curved structures under different loading conditions. The ground state of spherical crystals contains a finite number of topological defects that accommodate some of the stress induced by curvature. We study their dynamic behavior and mechanical implications under driving conditions.

References:

Deformation and failure of curved colloidal crystal shells,
C. Negri, A.L. Sellerio, S. Zapperi & M.C. Miguel
PNAS 112 , No. 47, 14545 (2015).


in collaboration with:
  • Carlotta Negri
  • Alessandro Sellerio
  • Stefano Zapperi
Dislocations and Disclinations in the Corbino Disk Geometry
When we slowly shear a liquid, the local fluid velocity is proportional to the local force. This flow is possible because the molecules in a liquid are not ordered. Crystals are instead ordered and therefore do not flow at small stress, but deform elastically until the stress is large enough to cause plastic or irreversible flow that is usually localized on shear bands. At even larger stresses the flow can become laminar because of shear melting. Simulations of two-dimensional vortex crystals at low temperature now show that laminar flow can occur without melting: the crystal retains most of its ordered structure. This process is made possible by a suitable arrangement of topological defects in the lattice such as disclinations (a particle with five or seven neighbours) and dislocations (a pair of adjacent five-seven disclinations). As shown in the figure disclinations (shaded cells) migrate in the interior of the disk while dislocations (pairs of dots) form radial walls or scars. Similar scars were observed in crystals arranged on curved surfaces, yielding an intriguing analogy between a sheared crystal in flat space and an equilibrium crystal in curved space.

References:


Laminar flow of a sheared vortex crystal: Scars in flat geometry,
M.C. Miguel, A. Mughal & S. Zapperi
Phys. Rev. Lett. 106 , 245501 (2011).

Tearing transition and plastic flow in crystalline thin films,
M.C. Miguel & S. Zapperi
Nature Mater. 2, 477 (2003).

in collaboration with:
  • Adil Mughal
  • Stefano Zapperi