1,721,079 research outputs found
How non-equilibrium correlations in active matter reveal the topological crossover in glasses
No full textAs shown by early studies on mean-field models of the glass transition, the geometrical features of the energy landscape provide fundamental information on the crossover from high-temperature simple relaxational dynamics to low-temperature activated relaxation. In particular, the critical slowing down of dynamics typical of glass formers has been related to a crossover from a saddle-dominated energy landscape (at high temperatures) to a minima-dominated landscape (at low temperatures). We show that active particles can serve as a useful tool to gain insight into this topological crossover. Once configurations equilibrated down in the glassy phase are provided, we show how features of the landscape are revealed by switching on some activity in particle dynamics. In particular we explain here the mechanism, taking as a reference point the pure p-spin model, by which the presence of self-propulsion is expected to induce critical stationary non-equilibrium correlations in correspondence to the minima-to saddles crossover
Statistical field theory and effective action method for scalar active matter
Full text linkWe employ statistical field theory techniques for coarse graining the steady-state properties of active Ornstein-Uhlenbeck particles. The computation is carried out in the framework of the unified colored noise approximation that allows an effective equilibrium picture. We thus develop a mean-field theory that allows us to describe in a unified framework the phenomenology of scalar active matter. In particular, we are able to describe througha spontaneous symmetry-breaking mechanism two peculiar features of active systems: (i) the accumulation of active particles at the boundaries of a confining container and (ii) motility-induced phase separation (MIPS). We develop a mean-field theory for steric interacting active particles undergoing MIPS and for active Lennard-Jones(ALJ) fluids. Within this framework, we discuss the universality class of MIPS and A LJ fluids, showing that it falls into the Ising universality class. We thus compute analytically the critical lineTc(τ) for both models. In the case of MIPS,Tc(τ) gives rise to a reentrant phase diagram compatible with an inverse transition from liquid togas as the strength of the noise decreases. However, in the case of particles interacting through anisotropic po-tentials, the field theory acquires aφ3term that, in general, cannot be canceled performing the expansion around the critical point. In this case, the Ising critical point might be replaced by a first-order phase-transition region
Fractal aggregation of active particles
No full textWe study active run-and-tumble particles in two dimensions with an additional two-state internal variable characterizing their motile or nonmotile state. Motile particles change irreversibly into nonmotile ones upon collision with a nonmotile particle. The system evolves towards an absorbing state where all particles are nonmotile. We initialize the system with one nonmotile particle in a bath of motile ones and study numerically the kinetics of relaxation to the absorbing state and its structure as a function of the density of the initial bath of motile particles and of their tumbling rate. We find a crossover from fractal aggregates at low density to homogeneous ones at high density. The persistence of single-particle dynamics as quantified by the tumbling rate pushes this crossover to a higher density and can be used to tune the porosity of the aggregate. At the lowest density the fractal dimension of the aggregate approaches that obtained in single-particle diffusion-limited aggregation. Our results could be exploited for the design of structures of desired porosity. The model is a first step towards the study of the collective dynamics of active particles that can exchange biological information
Self-Sustained Density Oscillations of Swimming Bacteria Confined in Microchambers
No full textWe numerically study the dynamics of run-and-tumble particles confined in two chambers connected by thin channels. Two dominant dynamical behaviors emerge: (i) an oscillatory pumping state, in which particles periodically fill the two vessels, and (ii) a circulating flow state, dynamically maintaining a near constant population level in the containers when connected by two channels. We demonstrate that the oscillatory behavior arises from the combination of a narrow channel, preventing bacteria reorientation, and a density-dependent motility inside the chambers
First-passage time of run-and-tumble particles
No full textWe solve the problem of first-passage time for run-and-tumble particles in one dimension. Exact expression is derived for the mean first-passage time in the general case, considering external force fields and chemotactic fields, giving rise to space-dependent swim speed and tumble rate. Agreement between theoretical formulae and numerical simulations is obtained in the analyzed case studies --constant and sinusoidal force fields, constant gradient chemotactic field. Reported findings can be useful to get insights into very different phenomena involving active particles, such as bacterial motion in external fields, intracellular transport, cell migration, animal foraging
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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