Speaker
Michele Starnini

Description

The temporal dimension of social systems is fundamental in shaping the topological properties of the corresponding networked representation, and it deeply impacts the behaviour of dynamical processes running on top of them, such as epidemic spreading or information diffusion. In this talk I will give an overview of some recent works addressing the role of the temporal dimension in social network modelling. First, I will focus on a data-driven approach, by considering empirical face-to-face interaction networks, recorded in different social gatherings. I will present a simple model, based on the social attractiveness of individuals, able to quantitatively reproduce most statistical features of these human contact networks. The importance of the network's dynamics will be also illustrated
by analysing the behaviour of simple diffusion processes on top of empirical time-varying networks. The case of social networks formed by multiple layers, representing different kinds of social interactions, is particularly interesting in order to show the effects of temporal correlations between layers on coupled spreading processes.
Finally, I will briefly present an analytical approach in modelling the bursty nature of human dynamics. I will focus on two examples: temporal network modelling that incorporates the burstiness of social interactions, and the Non-Markovian dynamics of epidemic spreading processes. Our contributions in social dynamics modelling shed light on the temporal features of human interactions, and may be of interest to researchers in the broad filed of time-evolving complex systems.

 

Speaker
Ian Hatton, ICTA-UAB

Description

A more general understanding of growth at all levels of organization could have implications for distinct processes from tumor and disease proliferation, to resource management and global carbon cycling. Recent work has uncovered striking regularities in growth-mass scaling, spanning the tree of life over distinct levels of organization. I will summarize these results, which show similar power law growth exponents near 3⁄4 (sub-exponential growth) spanning all major taxa and extending from embryos, to individuals and populations, up to whole ecosystems across terrestrial and aquatic biomes. These largest scales are exemplified by the relationship between predators and prey, such as lion and buffalo from the dry desert to the lush tropics. We do not yet understand where this pattern stems from. I will show that these patterns have deep implications for ecological and evolutionary theory and discuss a general dynamical perspective that attempts to integrate growth processes across hierarchies.

Speaker
Alberto Fernández-Nieves (Physics; Georgia Tech and ICREA)

Description

Motivated by classic thermodynamic experiments with dilute fluids, we explore the free and constrained expansion of fire-ant aggregations. In the latter case, we confine the ants to 2D vertical columns; hence, as the ants expand, they do work against the gravitational field. Surprisingly, we often observe the spontaneous generation of density waves; these propagate at a speed that depends on both the width and the amplitude of the wave, and occur cyclically. We also perform experiments in horizontal cells and find that the ants exhibit activity cycles, where the density homogeneity and mechanical properties of the aggregation change with activity. We believe that these cycles together with the large ant densities in our vertical columns are responsible for the generation of the observed waves. Finally, since the average ant density is larger at the bottom of the vertical column than at the top, we follow our temptation and attempt at interpreting the results in lieu of sedimentation equilibrium to seek for an equation of state. Despite our results are still highly preliminary, they provide interesting phenomenology that could perhaps be seen in active systems other than fire-ant aggregations. 

Speaker
Alberto Fernández-Nieves (Physics, Georgia Tech and ICREA)

Description

We will discuss our recent results with active nematics on toroidal surfaces and show how, despite the intrinsic activity and out-of-equilibrium character of our system, we still observe remnants of the expected curvature-induced defect unbinding predicted for nematics in their ground state. In our experiments, however, the number of defects is far larger than what one would expect for conventional nematics. In addition, these defects move throughout the toroidal surface and explore "phase space", bringing about interesting analogies with what we could call the high-temperature limit of a nematic liquid crystal. We unravel the role of activity by comparing our results to numerical simulations, which additionally allows us to perform defect microrheology to obtain the material properties of the active nematic. 

Speaker
Alberto Fernández-Nieves (Physics, Georgia Tech and ICREA)

Description

Toroidal droplets transform into spherical droplets to minimize their surface area. They do so either by breaking via the Rayleigh-Plateau instability or by shrinking; in this case, the "hole" progressively disappears eventually resulting in the formation of a single spherical droplet. Shrinking is always present for an uncharged toroidal droplet due to the variation of the Laplace pressure around the circular cross-section of the torus. The presence of charge can qualitatively change this behavior and result in the expansion of the torus; this happens as a result of the electric stress on the surface, which competes with the surface tension stress. In this talk, we will describe these different instabilities. We will also show that the expansion can result in the formation of fingers that are reminiscent of those formed via Saffman-Taylor instabilities. Finally, we will discuss how to stabilize the toroidal shape using yield-stress materials, which opens the door to a novel way to 3D print.