108 research outputs found
Deblocking of interacting particle assemblies: from pinning to jamming
A wide variety of interacting particle assemblies driven by an external force
are characterized by a transition between a blocked and a moving phase. The
origin of this deblocking transition can be traced back to the presence of
either external quenched disorder, or of internal constraints. The first case
belongs to the realm of the depinning transition, which, for example, is
relevant for flux-lines in type II superconductors and other elastic systems
moving in a random medium. The second case is usually included within the
so-called jamming scenario observed, for instance, in many glassy materials as
well as in plastically deforming crystals. Here we review some aspects of the
rich phenomenology observed in interacting particle models. In particular, we
discuss front depinning, observed when particles are injected inside a random
medium from the boundary, elastic and plastic depinning in particle assemblies
driven by external forces, and the rheology of systems close to the jamming
transition. We emphasize similarities and differences in these phenomena.Comment: 20 pages, 8 figures, submitted for a special issue of the Brazilian
Journal of Physics entitled: Statistical Mechanics of Irreversible Stochastic
Models - I
A micromechanical model of collapsing quicksand
The discrete element method constitutes a general class of modeling
techniques to simulate the microscopic behavior (i.e. at the particle scale) of
granular/soil materials. We present a contact dynamics method, accounting for
the cohesive nature of fine powders and soils. A modification of the model
adjusted to capture the essential physical processes underlying the dynamics of
generation and collapse of loose systems is able to simulate "quicksand"
behavior of a collapsing soil material, in particular of a specific type, which
we call "living quicksand". We investigate the penetration behavior of an
object for varying density of the material. We also investigate the dynamics of
the penetration process, by measuring the relation between the driving force
and the resulting velocity of the intruder, leading to a "power law" behavior
with exponent 1/2, i.e. a quadratic velocity dependence of the drag force on
the intruder.Comment: 5 pages, 4 figures, accepted for granular matte
Searching for superspreaders of information in real-world social media
A number of predictors have been suggested to detect the most influential
spreaders of information in online social media across various domains such as
Twitter or Facebook. In particular, degree, PageRank, k-core and other
centralities have been adopted to rank the spreading capability of users in
information dissemination media. So far, validation of the proposed predictors
has been done by simulating the spreading dynamics rather than following real
information flow in social networks. Consequently, only model-dependent
contradictory results have been achieved so far for the best predictor. Here,
we address this issue directly. We search for influential spreaders by
following the real spreading dynamics in a wide range of networks. We find that
the widely-used degree and PageRank fail in ranking users' influence. We find
that the best spreaders are consistently located in the k-core across
dissimilar social platforms such as Twitter, Facebook, Livejournal and
scientific publishing in the American Physical Society. Furthermore, when the
complete global network structure is unavailable, we find that the sum of the
nearest neighbors' degree is a reliable local proxy for user's influence. Our
analysis provides practical instructions for optimal design of strategies for
"viral" information dissemination in relevant applications.Comment: 12 pages, 7 figure
The Role of the IRA in Twitter during the 2016 US Presidential Election: Unveiling Amplification and Influence of Suspended Accounts
The impact of the social media campaign conducted by the Internet Research
Agency (IRA) during the 2016 U.S. presidential election continues to be a topic
of ongoing debate. While it is widely acknowledged that the objective of this
campaign was to support Donald Trump, the true extent of its influence on
Twitter users remains uncertain. Previous research has primarily focused on
analyzing the interactions between IRA users and the broader Twitter community
to assess the campaign's impact. In this study, we propose an alternative
perspective that suggests the existing approach may underestimate the true
extent of the IRA campaign. Our analysis uncovers the presence of a notable
group of suspended Twitter users, whose size surpasses the IRA user group size
by a factor of 60. These suspended users exhibit close interactions with IRA
accounts, suggesting potential collaboration or coordination. Notably, our
findings reveal the significant role played by these previously unnoticed
accounts in amplifying the impact of the IRA campaign, surpassing even the
reach of the IRA accounts themselves by a factor of 10. In contrast to previous
findings, our study reveals that the combined efforts of the Internet Research
Agency (IRA) and the identified group of suspended Twitter accounts had a
significant influence on individuals categorized as undecided or weak
supporters, probably with the intention of swaying their opinions.Comment: 13 Tables, 12 Figure
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