Yielding dynamics of a Herschel-Bulkley fluid: a critical-like fluidization behaviour

The shear-induced fluidization of a carbopol microgel is investigated during long start-up experiments using combined rheology and velocimetry in Couette cells of varying gap widths and boundary conditions. As already described in [Divoux et al., {\it Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], we show that the fluidization process of this simple yield stress fluid involves a transient shear-banding regime whose duration $\tau_f$ decreases as a power law of the applied shear rate \gp. Here we go one step further by an exhaustive investigation of the influence of the shearing geometry through the gap width $e$ and the boundary conditions. While slip conditions at the walls seem to have a negligible influence on the fluidization time $\tau_f$, different fluidization processes are observed depending on \gp and $e$: the shear band remains almost stationary for several hours at low shear rates or small gap widths before strong fluctuations lead to a homogeneous flow, whereas at larger values of \gp or $e$, the transient shear band is seen to invade the whole gap in a much smoother way. Still, the power-law behaviour appears as very robust and hints to critical-like dynamics. To further discuss these results, we propose (i) a qualitative scenario to explain the induction-like period that precedes full fluidization and (ii) an analogy with critical phenomena that naturally leads to the observed power laws if one assumes that the yield point is the critical point of an underlying out-of-equilibrium phase transition.Comment: 16 pages, 14+2 figures, published in Soft Matte

Tainted law? The Italian Penal Code, fascism and democracy

Author's draft. Final version Copyright © Cambridge University Press 2011. Available online at http://journals.cambridge.org/The current Italian Penal Code is the direct descendant of the 1930 Rocco Code. Originally a hybrid of authoritarian and liberal elements, but revised and reinterpreted in the post-war Republic, the Code was nevertheless introduced under the Fascists and has not been definitively reformed or renamed. Given such roots, this article argues that the Code’s legitimacy can be questioned by considering the significance of the Fascist past in terms of the Code’s symbolic, contextually narrative and memorial dimensions. On this basis the article develops a concept of tainted law in order to ground and direct analysis of law in relation to the anti-democratic past, arguing that critical engagement with the connections between law and the darker episodes of twentieth-century politico-legal history is vital to the construction and conservation of democratic legal systems today

Flux Flow, Pinning, and Resistive Behavior in Superconducting Networks

Numerical simulators are used to study the behavior of interacting quantized vortices and vortex lines in superconducting networks, films, and three dimensional bulk samples. An emphasis is on the explanation of the phenomenological behavior of the ''high-Tc'' copper-oxide superconductors and related model systems

Dynamica phase transitions in the 2D XY model

Thesis (B.S.)--University of Rochester. Dept. of Physics and Astronomy, 1990.The 2D XY model is studied by Monte Carlo simulations in which two different configurations, subjected to the same thermal noise, are compared. By using a Rotationally Invariant Dynamics, we observe only two temperature regimes: a high-temperature phase where the two configurations are random with respect to each other and a low-temperature phase where the spins in each configuration tend to align in certain direction. This contrasts with the three phases reported recently in a work by Golinelli and Derrida. We also find that our transition temperature is close to the Kosterlitz-Thouless transition temperature

Statistical properties of disordered jammed packings of frictionless disks

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2015.We numerically simulate mechanically stable packings of soft-core, frictionless particles in two dimensions interacting with a short range contact potential for the purpose of studying the statistical properties in such disordered systems. To avoid crystallization of the particles, we use a mixture of equal numbers of big and small particles. To prepare a mechanically stable packing, we use the Conjugate Gradient Method to minimize the total energy of the system U(r) to its local minimum from randomly initialized particle positions. For our system with Lees-Edwards periodic boundary conditions, U implicitly depends on the box parameters (box length in x, y directions Lx, Ly and the skew ratio γ in the x direction). we define a modified total energy Ũ(r, Lx, Ly, γ) so that when Ũ is brought to its local minimum, not only the net force on each particle vanishes, but the total stress tensor of the system will simultaneously be the desired, isotropic stress tensor. We optimize our program so that an ensemble of configurations consisting of a large number of particles can be efficiently generated. Therefore we can have good accuracy on the statistics of the quantities that we want to measure. We study a set of conserved quantities, in particular the stress ΓC, the Maxwell-Cremona forcer-tile area AC, the Voronoi volume VC, the number of particles NC, and the number of small particles NsC on subclusters of particles C. These subclusters are sampled from non-overlapping clusters embedded in the systems with the fixed isotropic global system stress. We defined our circular subclusters in two ways; (i), clusters with fixed radius R; (ii), clusters with fixed number of particles M. We compute the averages, variances and correlations of the conserved quantities on the clusters. We find significantly different behavior of the conserved quantities for the two cluster ensembles. The cluster ensemble with fixed radius R has important advantages and is therefore selected for the study of stress distribution on clusters with the maximum entropy hypothesis. We then show that the maximum entropy hypothesis can successfully explain the stress distribution on clusters for our system with isotropic total stress. In contrary to the previous claim that the stress alone as a conserved quantity is enough to explain the stress distribution on clusters, we find that an additional conserved quantity, called the Maxwell-Cremona force-tile area, also needs to be taken into consideration. We show that the joint distribution of the stress and force-tile area can be successfully explained by the maximum entropy hypothesis subject to constraints on the average values of the conserved quantities. Finally, we investigate the fluctuation of local packing fraction ϕ(r) to test whether our configurations display the hyperuniformity that has beed claimed to exit exactly at ϕJ. For our configurations with fixed isotropic global stress, generated by a rapid quench protocol, we find that hyperuniformity persists only out to a finite length scale, and that this length scale doesn’t appear to increase as the system stress decreases towards zero, i.e., towards the jamming transition. Our results suggests that the presence of hyperuniformity at jamming may be sensitive to the specific protocol used to constructed the jammed configurations

Cell based modeling of arteriosclerosis

Thesis (B.S.)--University of Rochester. Dept. of Physics and Astronomy, 2005.Studying how mesenchymal cells organize to cause arteriosclerosis, a disease where the mesenchymal cells on the vascular wall of the arteries or on the cardiac valves turn to bone tissue and harden, could give insight into its cellular mechanisms. Mesenchymal cells secrete the morphogen activator bone morphogenic protein (BMP-2) and the inhibitor matrix carboxyglutamaic acid protein (MGP) that drive the formation of striped patterns. Previous continuum models (Garfinkel et al.) [3] can predict this pattern formation by displaying high concentrations of BMP-2. This model uses the reaction-diffusion equations to adjust the concentrations of the morphogens over time by having them fuel their own production autocatalytically. However, the Garfinkel et al. model omits the cells and assumes cellular signals even in the absence of cells, thus neglecting the cells' feedback. In order to make the model closer to how a biological system actually works a cell based model is needed. Our cellular Potts model substitutes autocatalytic production of BMP-2 for chemotactic recruitment of mesenchymal cells

A single institution anesthetic experience with catheterization of pediatric pulmonary hypertension patients.

Cardiac catheterization remains the gold standard for the diagnosis and management of pediatric pulmonary hypertension (PH). There is lack of consensus regarding optimal anesthetic and airway regimen. This retrospective study describes the anesthetic/airway experience of our single center cohort of pediatric PH patients undergoing catheterization, in which obtaining hemodynamic data during spontaneous breathing is preferential. A total of 448 catheterizations were performed in 232 patients. Of the 379 cases that began with a natural airway, 274 (72%) completed the procedure without an invasive airway, 90 (24%) received a planned invasive airway, and 15 (4%) required an unplanned invasive airway. Median age was 3.4 years (interquartile range [IQR] 0.7-9.7); the majority were either Nice Classification Group 1 (48%) or Group 3 (42%). Vasoactive medications and cardiopulmonary resuscitation were required in 14 (3.7%) and eight (2.1%) cases, respectively; there was one death. Characteristics associated with use of an invasive airway included age <1 year, Group 3, congenital heart disease, trisomy 21, prematurity, bronchopulmonary dysplasia, WHO functional class III/IV, no PH therapy at time of case, preoperative respiratory support, and having had an intervention (p < 0.05). A composite predictor of age <1 year, Group 3, prematurity, and any preoperative respiratory support was significantly associated with unplanned airway escalation (26.7% vs. 6.9%, odds ratio: 4.9, confidence interval: 1.4-17.0). This approach appears safe, with serious adverse event rates similar to previous reports despite the predominant use of natural airways. However, research is needed to further investigate the optimal anesthetic regimen and respiratory support for pediatric PH patients undergoing cardiac catheterization