Browsing by Author "Zapperi, S."

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  • Comment on “Roughness of Interfacial Crack Fronts: Stress-Weighted Percolation in the Damage Zone”
    (2004) Alava, Mikko J.; Zapperi, S.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A Comment on the Letter by Jean Schmittbuhl, Alex Hansen, and G. George Batroun, Phys Rev. Lett. 90, 045505 (2003). The authors of the Letter offer a Reply.
  • Damage accumulation in quasi-brittle fracture
    (2014) Manzato, C.; Alava, M.J.; Zapperi, S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The strength of quasibrittle materials depends on the ensemble of defects inside the sample and on the way damage accumulates before failure. Using large-scale numerical simulations of the random fuse model, we investigate the evolution of the microcrack distribution as the applied load approaches the fracture point. We find that the distribution broadens mostly due to a tendency of cracks to coalesce in a way that increases with system size. We study how the observed behavior depends on the disorder present in the sample and relate the results with fracture size effects.
  • Ground state optimization and hysteretic demagnetization: the random-field Ising model
    (2005) Alava, M.J.; Basso, V.; Colaiori, F.; Dante, L.; Durin, G.; Magni, A.; Zapperi, S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We compare the ground state of the random-field Ising model with Gaussian distributed random fields, with its nonequilibrium hysteretic counterpart, the demagnetized state. This is a low-energy state obtained by a sequence of slow magnetic-field oscillations with decreasing amplitude. The main concern is how optimized the demagnetized state is with respect to the best-possible ground state. Exact results for the energy in d=1 show that in a paramagnet, with finite spin-spin correlations, there is a significant difference in the energies if the disorder is not so strong that the states are trivially almost alike. We use numerical simulations to better characterize the difference between the ground state and the demagnetized state. For d⩾3, the random-field Ising model displays a disorder induced phase transition between a paramagnetic and a ferromagnetic state. The locations of the critical points R(DS)c and R(GS)c differ for the demagnetized state and ground state. We argue based on the numerics that in d=3 the scaling at the transition is the same in both states. This claim is corroborated by the exact solution of the model on the Bethe lattice, where the critical points are also different.
  • Navigation Strategies of Motor Proteins on Decorated Tracks
    (2015) Bertalan, Z.; Budrikis, Z.; La Porta, C.A.M.; Zapperi, S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Motor proteins display widely different stepping patterns as they move on microtubule tracks, from the deterministic linear or helical motion performed by the protein kinesin to the uncoordinated random steps made by dynein. How these different strategies produce an efficient navigation system needed to ensure correct cellular functioning is still unclear. Here, we show by numerical simulations that deterministic and random motor steps yield different outcomes when random obstacles decorate the microtubule tracks: kinesin moves faster on clean tracks but its motion is strongly hindered on decorated tracks, while dynein is slower on clean tracks but more efficient in avoiding obstacles. Further simulations indicate that dynein’s advantage on decorated tracks is due to its ability to step backwards. Our results explain how different navigation strategies are employed by the cell to optimize motor driven cargo transport.
  • Overshoot during phenotypic switching of cancer cell populations
    (2015) Sellerio, A.L.; Ciusani, E.; Ben-Moshe, N.B.; Coco, S.; Piccinini, A.; Myers, C.R.; Sethna, J.P.; Giampietro, C.; Zapperi, S.; La Porta, C.A.M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The dynamics of tumor cell populations is hotly debated: do populations derive hierarchically from a subpopulation of cancer stem cells (CSCs), or are stochastic transitions that mutate differentiated cancer cells to CSCs important? Here we argue that regulation must also be important. We sort human melanoma cells using three distinct cancer stem cell (CSC) markers — CXCR6, CD271 and ABCG2 — and observe that the fraction of non-CSC-marked cells first overshoots to a higher level and then returns to the level of unsorted cells. This clearly indicates that the CSC population is homeostatically regulated. Combining experimental measurements with theoretical modeling and numerical simulations, we show that the population dynamics of cancer cells is associated with a complex miRNA network regulating the Wnt and PI3K pathways. Hence phenotypic switching is not stochastic, but is tightly regulated by the balance between positive and negative cells in the population. Reducing the fraction of CSCs below a threshold triggers massive phenotypic switching, suggesting that a therapeutic strategy based on CSC eradication is unlikely to succeed.
  • Role of the Number of Microtubules in Chromosome Segregation during Cell Division
    (2015) Bertalan, Z.; Budrikis, Z.; La Porta, C.A.M.; Zapperi, S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Faithful segregation of genetic material during cell division requires alignment of chromosomes between two spindle poles and attachment of their kinetochores to each of the poles. Failure of these complex dynamical processes leads to chromosomal instability (CIN), a characteristic feature of several diseases including cancer. While a multitude of biological factors regulating chromosome congression and bi-orientation have been identified, it is still unclear how they are integrated so that coherent chromosome motion emerges from a large collection of random and deterministic processes. Here we address this issue by a three dimensional computational model of motor-driven chromosome congression and bi-orientation during mitosis. Our model reveals that successful cell division requires control of the total number of microtubules: if this number is too small bi-orientation fails, while if it is too large not all the chromosomes are able to congress. The optimal number of microtubules predicted by our model compares well with early observations in mammalian cell spindles. Our results shed new light on the origin of several pathological conditions related to chromosomal instability.
  • Universality classes and crossover scaling of Barkhausen noise in thin films
    (2014) Laurson, L.; Durin, G.; Zapperi, S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Wrinkle motifs in thin films
    (2015) Budrikis, Z.; Bertalan, Z.; Sellerio, A.L.; Zapperi, S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    On length scales from nanometres to metres, partial adhesion of thin films with substrates generates a fascinating variety of patterns, such as ‘telephone cord’ buckles, wrinkles, and labyrinth domains. Although these patterns are part of everyday experience and are important in industry, they are not completely understood. Here, we report simulation studies of a previously-overlooked phenomenon in which pairs of wrinkles form avoiding pairs, focusing on the case of graphene over patterned substrates. By nucleating and growing wrinkles in a controlled way, we characterize how their morphology is determined by stress fields in the sheet and friction with the substrate. Our simulations uncover the generic behaviour of avoiding wrinkle pairs that should be valid at all scales.