20 research outputs found
Anomalous diffusion: A basic mechanism for the evolution of inhomogeneous systems
In this article we review classical and recent results in anomalous diffusion
and provide mechanisms useful for the study of the fundamentals of certain
processes, mainly in condensed matter physics, chemistry and biology. Emphasis
will be given to some methods applied in the analysis and characterization of
diffusive regimes through the memory function, the mixing condition (or
irreversibility), and ergodicity. Those methods can be used in the study of
small-scale systems, ranging in size from single-molecule to particle clusters
and including among others polymers, proteins, ion channels and biological
cells, whose diffusive properties have received much attention lately.Comment: Review article, 20 pages, 7 figures. arXiv admin note: text overlap
with arXiv:cond-mat/0201446 by other author
Percolation and cooperation with mobile agents: Geometric and strategy clusters
We study the conditions for persistent cooperation in an off-lattice model of
mobile agents playing the Prisoner's Dilemma game with pure, unconditional
strategies. Each agent has an exclusion radius rP, which accounts for the
population viscosity, and an interaction radius rint, which defines the
instantaneous contact network for the game dynamics. We show that, differently
from the rP=0 case, the model with finite-sized agents presents a coexistence
phase with both cooperators and defectors, besides the two absorbing phases, in
which either cooperators or defectors dominate. We provide, in addition, a
geometric interpretation of the transitions between phases. In analogy with
lattice models, the geometric percolation of the contact network (i.e.,
irrespective of the strategy) enhances cooperation. More importantly, we show
that the percolation of defectors is an essential condition for their survival.
Differently from compact clusters of cooperators, isolated groups of defectors
will eventually become extinct if not percolating, independently of their size
On the Modeling of Droplet Evaporation on Superhydrophobic Surfaces
When a drop of water is placed on a rough surface, there are two possible
extreme regimes of wetting: the one called Cassie-Baxter (CB) with air pockets
trapped underneath the droplet and the one characterized by the homogeneous
wetting of the surface, called the Wenzel (W) state. A way to investigate the
transition between these two states is by means of evaporation experiments, in
which the droplet starts in a CB state and, as its volume decreases, penetrates
the surface's grooves, reaching a W state. Here we present a theoretical model
based on the global interfacial energies for CB and W states that allows us to
predict the thermodynamic wetting state of the droplet for a given volume and
surface texture. We first analyze the influence of the surface geometric
parameters on the droplet's final wetting state with constant volume, and show
that it depends strongly on the surface texture. We then vary the volume of the
droplet keeping fixed the geometric surface parameters to mimic evaporation and
show that the drop experiences a transition from the CB to the W state when its
volume reduces, as observed in experiments. To investigate the dependency of
the wetting state on the initial state of the droplet, we implement a cellular
Potts model in three dimensions. Simulations show a very good agreement with
theory when the initial state is W, but it disagrees when the droplet is
initialized in a CB state, in accordance with previous observations which show
that the CB state is metastable in many cases. Both simulations and theoretical
model can be modified to study other types of surface.Comment: 23 pages, 7 figure
Symbiotic behaviour in the Public Goods game with altruistic punishment
Finding ways to overcome the temptation to exploit one another is still a
challenge in behavioural sciences. In the framework of evolutionary game
theory, punishing strategies are frequently used to promote cooperation in
competitive environments. Here, we introduce altruistic punishers in the
spatial public goods game. This strategy acts as a cooperator in the absence of
defectors, otherwise it will punish all defectors in their vicinity while
bearing a cost to do so. We observe three distinct behaviours in our model: i)
in the absence of punishers, cooperators (who don't punish defectors) are
driven to extinction by defectors for most parameter values; ii) clusters of
punishers thrive by sharing the punishment costs when these are low iii) for
higher punishment costs, punishers, when alone, are subject to exploitation but
in the presence of cooperators can form a symbiotic spatial structure that
benefits both. This last observation is our main finding since neither
cooperation nor punishment alone can survive the defector strategy in this
parameter region and the specificity of the symbiotic spatial configuration
shows that lattice topology plays a central role in sustaining cooperation.
Results were obtained by means of Monte Carlo simulations on a square lattice
and subsequently confirmed by a pairwise comparison of different strategies'
payoffs in diverse group compositions, leading to a phase diagram of the
possible states
Geometrical distribution of Cryptococcus neoformans mediates flower-like biofilm development
Microbial biofilms are highly structured and dynamic communities in which phenotypic diversification allows microorganisms to adapt to different environments under distinct conditions. The environmentally ubiquitous pathogen Cryptococcus neoformans colonizes many niches of the human body and implanted medical devices in the form of biofilms, an important virulence factor. A new approach was used to characterize the underlying geometrical distribution of C. neoformans cells during the adhesion stage of biofilm formation. Geometrical aspects of adhered cells were calculated from the Delaunay triangulation and Voronoi diagramobtained fromscanning electronmicroscopy images (SEM). A correlation between increased biofilm formation and higher ordering of the underlying cell distribution was found. Mature biofilm aggregates were analyzed by applying an adapted protocol developed for ultrastructure visualization of cryptococcal cells by SEM. Flower-like clusters consisting of cells embedded in a dense layer of extracellular matrix were observed as well as distinct levels of spatial organization: adhered cells, clusters of cells and community of clusters. The results add insights into yeast motility during the dispersion stage of biofilm formation. This study highlights the importance of cellular organization for biofilm growth and presents a novel application of the geometrical method of analysis
Spatio-temporal conjecture for diffusion
We present here a conjecture about the equivalence between the noise density
of states of a system governed by a generalized Langevin equation and the
fluctuation in the energy density of states in a Hamiltonian system. We present
evidence of this for a disordered Heisenberg system.Comment: 6 pages, 1 figure. Submitted to Physica
Heterogeneous contributions can jeopardize cooperation in the Public Goods Game
When studying social dilemma games, a crucial question arises regarding the
impact of general heterogeneity on cooperation, which has been shown to have
positive effects in numerous studies. Here, we demonstrate that heterogeneity
in the contribution value for the focal Public Goods Game can jeopardize
cooperation. We show that there is an optimal contribution value in the
homogeneous case that most benefits cooperation depending on the lattice. In a
heterogeneous scenario, where strategy and contribution coevolve, cooperators
making contributions higher than the optimal value end up harming those who
contribute lower. This effect is notably detrimental to cooperation in the
square lattice with von Neumann neighborhood, while it can have no impact in
others lattices. Furthermore, in parameter regions where a higher-contributing
cooperator cannot normally survive alone, the exploitation of lower value
contribution cooperators allows their survival, resembling a parasitic
behavior. To obtain these results, we employed various distributions for the
contribution values in the initial condition and conducted Monte Carlo
simulations
A Simple Non-Markovian Computational Model of the Statistics of Soccer Leagues: Emergence and Scaling effects
We propose a novel algorithm that outputs the final standings of a soccer
league, based on a simple dynamics that mimics a soccer tournament. In our
model, a team is created with a defined potential(ability) which is updated
during the tournament according to the results of previous games. The updated
potential modifies a teams' future winning/losing probabilities. We show that
this evolutionary game is able to reproduce the statistical properties of final
standings of actual editions of the Brazilian tournament (Brasileir\~{a}o).
However, other leagues such as the Italian and the Spanish tournaments have
notoriously non-Gaussian traces and cannot be straightforwardly reproduced by
this evolutionary non-Markovian model. A complete understanding of these
phenomena deserves much more attention, but we suggest a simple explanation
based on data collected in Brazil: Here several teams were crowned champion in
previous editions corroborating that the champion typically emerges from random
fluctuations that partly preserves the gaussian traces during the tournament.
On the other hand, in the Italian and Spanish leagues only a few teams in
recent history have won their league tournaments. These leagues are based on
more robust and hierarchical structures established even before the beginning
of the tournament. For the sake of completeness, we also elaborate a totally
Gaussian model (which equalizes the winning, drawing, and losing probabilities)
and we show that the scores of the "Brasileir\~{a}o" cannot be reproduced. Such
aspects stress that evolutionary aspects are not superfluous in our modeling.
Finally, we analyse the distortions of our model in situations where a large
number of teams is considered, showing the existence of a transition from a
single to a double peaked histogram of the final classification scores. An
interesting scaling is presented for different sized tournaments.Comment: 18 pages, 9 figure