SMM Lab

Stefano Zapperi receives an Advanced Grant from the European Research Council. The project SIZEFFECTS is devoted to explain size effects in fracture and plasticity through theory and numerical simulations.

Applications for postdoctoral position within this project are still open!

Does knowing how things break help predict how likely it is that they will break? Our latest papers points in that direction. If a dump truck filled with rocks drives over a bridge with 1000 beams, how likely is it that one of the beams will break? In the past, the answer has been estimated using a mathematical truth – the weakest link (like that beam) in a large system (like that bridge) will fail with a likelihood that is mostly independent of the reasons for failure. In this old approach, each of the 1000 beams is itself a large system of millions of ‘beam pieces’. Engineers could test how many beams fail under huge loads and use these results to try to predict the number of weak beams in a bridge.  Now, using supercomputer simulations we argue that knowing why the beams break could improve on these predictions. We showed that understanding the lengths of existing cracks in beams dramatically improved on the old methods. The old theory did work for very big beams, but the beams needed to be larger than the observable Universe before it worked well. The new theory works even in microscopic systems, making it far more useful to engineers who make sure bridges on earth are safe…

Read the paper: Claudio Manzato, Ashivni Shekhawat, Phani K. V. V. Nukala, Mikko J. Alava, James P. Sethna, and Stefano Zapperi, Fracture Strength of Disordered Media: Universality, Interactions, and Tail Asymptotics, Phys. Rev. Lett. 108, 065504 (2012)

In collaboration  with Caterina La Porta (from the University of Milano, Italy) and James sethna (from Cornell University) we have shown that cell senescence occurs spontaneously in melanoma cells but does not arrest their growth, which  is sustained by a small population of cancer stem cells. The results published in PLoS Computational Biology explain why it is difficult to treat cancer by inducing senescence.

Normal cells can duplicate for a finite number of times in vitro, after which they turn senescent and stop replicating. Since cancer cells grow indefinitely, it is commonly believed that senescence could act as a barrier for tumor growth and could thus be used to treat cancer. We followed the long-term evolution of melanoma cell populations, monitoring  the fraction of senescent cells. After three months, growth slowed and most of the cells turned senescent. Yet growth did not stop, but  eventually resumed at the initial rate while senescent cells almost disappeared. We modeled the experimental data, using the cancer stem cell hypothesis. The model assumes a small population of cancer cells with  stem cell properties that  are able to self-renew and give rise to more differentiated cells indefinitely and and a larger population of cancer cells that can duplicate only a finite number of times before turning senescent. The model yields an indirect confirmation of the presence of cancer stem cells in melanoma, an issue that is still controversial in the literature.

Since a large fraction of cancer cells are susceptible to senescence, but those cells are irrelevant for tumor growth, the researchers conclude that inducing senescence is unlikely to provide a successful therapeutic strategy. Targeting cancer stem cells would appear to be more promising, but would face challenges from strong resistance to drug induced senescence in these cells.

Paper (Open access):

La Porta CAM, Zapperi S, Sethna JP (2012) Senescent Cells in Growing Tumors: Population Dynamics and Cancer Stem Cells. PLoS Comput Biol 8(1): e1002316. doi:10.1371/journal.pcbi.1002316

Apply at: http://www.cecam.org/workshop-6-751.html

Understanding materials that transform from insulators to metals as they are heated combines the challenges of strongly-correlated electron physics and disorder. Experiments studying the transition under an applied voltage show a broad range of resistance jumps — adding the challenge of non-equilibrium physics to the mix. In a collaboration with Cornell University we have developed a surprisingly simple explanation for these experiments, building on models originally developed to study lightning bolts and sparks. When a current passes through air, it creates ions which lower the resistance to further current: like a hole in a dike, this leads to `dielectric breakdown’, discharging all the current along a hot channel forming the bolt. Using this model the physicists are able to predict the phase diagram as a function of voltage and temperature obtaining big bolts at high voltages, random transforming regions at low voltages, and bolt-avalanches of all sizes at the transition voltage separating these two regimes.

Publised in A. Shekhawat et al. Phys. Rev. Lett. 107, 276401 (2011)

We anticipate the opening of postdoctoral positions in 2012 to work on statistical aspects of fracture and plasticity. We look for candidates with a strong background in one or more of the following areas: statistical mechanics (scaling and critical phenomena), numerical modeling fracture, dislocation dynamics simulations, molecular dynamics simulations of the mechanical properties of solids. Interested candidates please send an email with their CV.

One of our papers has been selected by CNR for the Highlights 2009-2010.
Reference: M. Reguzzoni, M. Ferrario, S. Zapperi and M. C. Righi, “Onset of frictional slip in an adsorbed monolayer”, PNAS 107, 1311 (2010)

Read the section devoted to our paper.

The intermittent transition between slow growth and rapid shrinkage in polymeric assemblies is termed ‘‘dynamic instability’’, a feature observed in a variety of biochemically distinct assemblies including microtubules, actin, and their bacterial analogs. The existence of this labile phase of a polymer has many functional consequences in cytoskeletal dynamics, and its repeated appearance suggests that it is relatively easy to evolve. Here, we consider the minimal ingredients for the existence of dynamic instability by considering a single polymorphic filament that grows by binding to a substrate, undergoes a conformation change, and may unbind as a consequence of the residual strains induced by this change. We identify two parameters that control the phase space of possibilities for the filament: a structural mechanical parameter that characterizes the ratio of the bond strengths along the filament to those with the substrate (or equivalently the ratio of longitudinal to lateral interactions in an assembly), and a kinetic parameter that characterizes the ratio of timescales for growth and conformation change. In the deterministic limit, these parameters serve to demarcate a region of uninterrupted growth from that of collapse. However, in the presence of disorder in either the structural or the kinetic parameter the growth and collapse phases can coexist where the filament can grow slowly, shrink rapidly, and transition between these phases, thus exhibiting dynamic instability.

see: S. Zapperi and L. Mahadevan, Biophysical Journal Volume 101 July 2011 267–275 267

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.

Reference:
Laminar Flow of a Sheared Vortex Crystal: Scars in Flat Geometry
M.-Carmen Miguel, Adil Mughal, and Stefano Zapperi
Phys. Rev. Lett. 106, 245501 (2011)

A letter is published online in Nature Physics on January 23, 2011. Power-law scaling of critical phenomena has been most powerful for predictions near a critical point. By averaging the noise emitted by avalanches of a given duration, however, universal scaling functions can extend the predictive power of scaling far from the critical point.

see: Universality beyond power laws and the average avalanche shape.
Stefanos Papanikolaou, Felipe Bohn, Rubem Luis Sommer, Gianfranco Durin, Stefano Zapperi, James P. Sethna
Nature Physics (2011) doi:10.1038/nphys1884