arXiv subjects

Luca Angelani

Publications and source records attributed to Luca Angelani.

At least 19 recordsLinked to original sources

arXiv2026

Internal stress drives ferromagnetic-like ordering in networks of proliferating bacteria

Proliferation is a defining feature of life. Through growth, division, and death, living systems consume energy and inject mass, breaking conservation laws and driving collective phenomena from biofilm formation to embryonic development. Yet, while active matter physics has advanced our understanding of self-propelled agents, quantitative frameworks for proliferating systems are still emerging, and most work focuses on simplified settings. Here, we study \textit{E.coli} bacteria growing inside a network of single-file microchannels as a minimal model of structured environments. Competition for free volume drives the spontaneous emergence of coherent growth patterns that persist across generations but vanish when the channel links exceed the typical cell size at birth. Despite the strongly out-of-equilibrium character of the dynamics, the observed phenomenology can be quantitatively captured by an effective equilibrium description in which the flow state at each node is represented by a spin variable with ferromagnetic interactions. Simulations of growing elastic cells show that this coupling arises from internal stress accumulated at network nodes due to dynamical constraints. Our results reveal a surprising correspondence between proliferating active matter and equilibrium statistical mechanics, highlighting open fundamental questions and offering a first step toward describing growth phenomena in real-world complex environments.

cond-mat.stat-mech↗

arXiv2025

Fluidization induced by Magnetic Interactions in Confined Active Matter

We investigate magnetic active matter in confined geometries using both experiments with magnetic toy robots Hexbugs and simulations of elongated magnetic active Brownian particles in circular domains. Standard active particles tend to accumulate at boundaries, forming clusters even at relatively low densities. In the presence of magnetic interactions, we provide evidence for a fluidization effect that inhibits clustering and shifts its onset to higher packing fractions. Moreover, magnetic dipolar interactions give rise to novel collective behaviors, such as train-like formations, rotating pairs, and particle vortices.

cond-mat.soft↗

arXiv2025

Local entropy production rate of run-and-tumble particles

We study the local entropy production rate and the local entropy flow in active systems composed of non-interacting run-and-tumble particles in a thermal bath. After providing generic time-dependend expressions, we focus on the stationary regime. Remarkably, in this regime the two entropies are equal and depend only on the distribution function and its spatial derivatives. We discuss in details two case studies, relevant to real situations. First, we analyze the case of space dependent speed,describing photokinetic bacteria, cosidering two different shapes of the speed, piecewise constant and sinusoidal. Finally, we investigate the case of external force fields, focusing on harmonic and linear potentials, which allow analytical treatment. In all investigated cases, we compare exact and approximated analytical results with numerical simulations.

cond-mat.stat-mech↗

arXiv2024

Generalized time-fractional kinetic-type equations with multiple parameters

In this paper we study a new generalization of the kinetic equation emerging in run-and-tumble models. We show that this generalization leads to a wide class of generalized fractional kinetic (GFK) and telegraph-type equations depending by two (or three) parameters. We provide an explicit expression of the solution in the Laplace domain and show that, for a particular choice of the parameters, the fundamental solution of the GFK equation can be interpreted as the probability density function of a stochastic process obtained by a suitable transformation of the inverse of a subordinator. Then, we discuss some particular interesting cases, such as generalized telegraph models, diffusion fractional equations involving higher order time derivatives and fractional integral equations.

cond-mat.stat-mech↗

arXiv2024

Entropy production of run-and-tumble particles

We analyze the entropy production in run-and-tumble models. After presenting the general formalism in the framework of the Fokker-Planck equations in one space dimension, we derive some known exact results in simple physical situations (free run-and-tumble particles and harmonic confinement). We then extend the calculation to the case of anisotropic motion (different speeds and tumbling rates for right and left oriented particles), obtaining exact expressions of the entropy production rate. We conclude by discussing the general case of heterogeneous run-and-tumble motion described by space-dependent parameters and extending the analysis to the case of d-dimensional motions.

cond-mat.stat-mech↗

arXiv2024

Anomalous random flights and time-fractional run-and-tumble equations

Random flights (also called run-and-tumble walks or transport processes) represent finite velocity random motions changing direction at any Poissonian time. These models in d-dimension, can be studied giving a general formulation of the problem valid at any spatial dimension. The aim of this paper is to extend this general analysis to time-fractional processes arising from a non-local generalization of the kinetic equations. The probabilistic interpretation of the solution of the time-fractional equations leads to a time-changed version of the original transport processes. The obtained results provides a clear picture of the role played by the time-fractional derivatives in this kind of random motions. They display an anomalous behavior and are useful to describe several complex systems arising in statistical physics and biology. In particular, we focus on the one-dimensional random flight, called telegraph process, studying the time-fractional version of the classical telegraph equation and providing a suitable interpretation of its stochastic solutions.

cond-mat.stat-mech↗

arXiv2023

Optimal escapes in active matter

The out-of-equilibrium character of active particles, responsible for accumulation at boundaries in confining domains, determines not-trivial effects when considering escape processes. Non-monotonous behavior of exit times with respect to tumbling rate (inverse of mean persistent time) appears, as a consequence of the competing processes of exploring the bulk and accumulate at boundaries. By using both 1D analytical results and 2D numerical simulations of run-and-tumble particles with different behaviours at boundaries, we scrutinize this very general phenomenon of active matter, evidencing the role of accumulation at walls for the existence of optimal tumbling rates for fast escapes.

cond-mat.soft↗

arXiv2023

Run-and-tumble motion in trapping environments

Complex or hostile environments can sometimes inhibit the movement capabilities of diffusive particles or active swimmers, who may thus become stuck in fixed positions. This occurs, for example, in the adhesion of bacteria to surfaces at the initial stage of biofilm formation. Here we analyze the dynamics of active particles in the presence of trapping regions, where irreversible particle immobilization occurs at a fixed rate. By solving the kinetic equations for run-and tumble motion in one space dimension, we give expressions for probability distribution functions, focusing on stationary distributions of blocked particles, and mean trapping times in terms of physical and geometrical parameters. Different extensions of the trapping region are considered, from infinite to cases of semi infinite and finite intervals. The mean trapping time turns out to be simply the inverse of the trapping rate for infinitely extended trapping zones, while it has a nontrivial form in the semi-infinite case and is undefined for finite domains, due to the appearance of long tails in the trapping time distribution. Finally, to account for the subdiffusive behavior observed in the adhesion processes of bacteria to surfaces, we extend the model to include anomalous diffusive motion in the trapping region, reporting the exact expression of the mean-square displacement.

cond-mat.stat-mech↗

arXiv2022

Orthogonal run-and-tumble walks

Planar run-and-tumble walks with orthogonal directions of motion are considered. After formulating the problem with generic transition probabilities among the orientational states, we focus on the symmetric case, giving general expressions of the probability distribution function (in the Laplace-Fourier domain), the mean square displacement and the effective diffusion constant in terms of transition rate parameters. As case studies we treat and discuss two classes of motion, alternate/forward and isotropic/backward, obtaining, when possible, analytic expressions of probability distribution functions in the space-time domain. We discuss at the end also the case of cyclic motion. Reduced (enhanced) effective diffusivity, with respect to the standard 2D active motion, is observed in the cyclic and backward (forward) cases.

cond-mat.stat-mech↗

arXiv2022

Confined run-and-tumble swimmers in one dimension

The persistent character of the motion of active particles gives rise to accumulation at boundaries. I investigate the problem of run-and-tumble swimmers confined in a 1D box with hard walls, reporting expressions for the particles probability distribution and wall pressure. A crossover box length value is found below which the initial value of the pressure turns out to be higher than the asymptotic one, indicating a bounce effect of the active "wave" of swimmers. The case of attracting and repelling boundaries are also investigated using two different tumble rates for particles in the bulk and at walls. Escape problems are finally analyzed by considering partially permeable walls through which particles can leave the box.

cond-mat.stat-mech↗

arXiv2021

Alignment interactions drive structural transitions in biological tissues

Experimental evidence shows that there is a feedback between cell shape and cell motion. How this feedback impacts the collective behavior of dense cell monolayers remains an open question. We investigate the effect of a feedback that tends to align the cell crawling direction with cell elongation in a biological tissue model. We find that the alignment interaction promotes nematic patterns in the fluid phase that eventually undergo a non-equilibrium phase transition into a quasi-hexagonal solid. Meanwhile, highly asymmetric cells do not undergo the liquid-to-solid transition for any value of the alignment coupling. In this regime, the dynamics of cell centers and shape fluctuation show features typical of glassy systems.

cond-mat.soft↗

arXiv2020

Narrow-escape time and sorting of active particles in circular domains

It is now well established that microswimmers can be sorted or segregated fabricating suitable microfluidic devices or using external fields. A natural question is how these techniques can be employed for dividing swimmers of different motility. In this paper, using numerical simulations in the dilute limit, we investigate how motility parameters (time of persistence and velocity) impacts the narrow-escape time of active particles from circular domains. We show that the escape time undergoes a crossover between two asymptotic regimes. The control parameters of the crossover is the ratio between persistence length of the active motion and the typical length scale of the circular domain. We explore the possibility of taking advantage of this finding for sorting active particles by motility parameters.

cond-mat.stat-mech↗

arXiv2020

Probability distributions for the run-and-tumble models with variable speed and tumbling rate

In this paper we consider a telegraph equation with time-dependent coefficients, governing the persistent random walk of a particle moving on the line with a time-varying velocity $c(t)$ and changing direction at instants distributed according to a non-stationary Poisson distribution with rate $λ(t)$. We show that, under suitable assumptions, we are able to find the exact form of the probability distribution. We also consider the space-fractional counterpart of this model, finding the characteristic function of the related process. A conclusive discussion is devoted to the potential applications to run-and-tumble models.

math.PR↗

arXiv2019

Relation between heterogeneous frozen regions in supercooled liquids and non-Debye spectrum in the corresponding glasses

Recent numerical studies on glassy systems provide evidences for a population of non-Goldstone modes (NGMs) in the low-frequency spectrum of the vibrational density of states $D(ω)$. Similarly to Goldstone modes (GMs), i. e., phonons in solids, NGMs are soft low-energy excitations. However, differently from GMs, NGMs are localized excitations. Here we first show that the parental temperature $T^*$ modifies the GM/NGM ratio in $D(ω)$. In particular, the phonon attenuation is reflected in a parental temperature dependency of the exponent $s(T^*)$ in the low-frequency power law $D(ω) \sim ω^{s(T^*)}$, with $2 \leq s(T^*) \leq 4 $. Secondly, by comparing $s(T^*)$ with $s(p)$, i. e., the same quantity obtained by pinning \mttp{a} $p$ particle fraction, we suggest that $s(T^*)$ reflects the presence of dynamical heterogeneous regions of size $ξ^3 \propto p$. Finally, we provide an estimate of $ξ$ as a function of $T^*$, finding a mild power law divergence, $ξ\sim (T^* - T_d)^{-α/3}$, with $T_d$ the dynamical crossover temperature and $α$ falling in the range $α\in [0.8,1.0]$.

cond-mat.dis-nn↗

arXiv2018

Spontaneous assembly of colloidal vesicles driven by active swimmers

We explore the self-assembly process of colloidal structures immersed in active baths. By considering low-valence particles we numerically investigate the irreversible aggregation dynamics originated by the presence of run-and-tumble swimmers. We observe the formation of long closed chains -- vesicles -- densely filled by active swimmers. On the one hand the active bath drives the self-assembly of closed colloidal structures, and on the other hand the vesicles formation fosters the self-trapping of swimmers, suggesting new ways both to build structured nanomaterials and to trap microorganisms.

cond-mat.soft↗

arXiv2018

Currents and flux-inversion in photokinetic active particles

Many active particles, both of biological and synthetic origin, can have a light controllable propulsion speed, a property that in biology is commonly referred to as photokinesis. Here we investigate directed transport of photokinetic particles by traveling light patterns. We find general expressions for the current in the cases where the motility wave, induced by light, shifts very slow or very fast. These asymptotic formulas are independent on the shape of the wave and are valid for a wide class of active particle models. Moreover we derive an exact solution for the one-dimensional "run and tumble" model. Our results could be used to design time-varying illumination patterns for fast and efficient spatial reconfiguration of photokinetic colloids or bacteria.

cond-mat.soft↗

arXiv2018

Probing the non-Debye low frequency excitations in glasses through random pinning

We investigate the properties of the low-frequency spectrum in the density of states $D(ω)$ of a three-dimensional model glass former. To magnify the Non-Debye sector of the spectrum, we introduce a random pinning field that freezes a finite particle fraction in order to break the translational invariance and shifts all the vibrational frequencies of the extended modes towards higher frequencies. We show that Non-Debye soft localized modes progressively emerge as the fraction $p$ of pinned particles increases. Moreover, the low-frequency tail of $D(ω)$ goes to zero as a power law $ω^{δ(p)}$, with $2 \!\leq \! δ(p) \!\leq\!4$ and $δ\!=\!4$ above a threshold fraction $p_{th}$.

cond-mat.dis-nn↗

arXiv2016

Shape and Displacement Fluctuations in Soft Vesicles Filled by Active Particles

We investigate numerically the dynamics of shape and displacement fluctuations of two-dimensional flexible vesicles filled with active particles. At low concentration most of the active particles accumulate at the boundary of the vesicle where positive particle number fluctuations are amplified by trapping, leading to the formation of pinched spots of high density, curvature and pressure. At high concentration the active particles cover the vesicle boundary almost uniformly, resulting in fairly homogeneous pressure and curvature, and nearly circular vesicle shape. The change between polarized and spherical shapes is driven by the number of active particles. The center-of-mass of the vesicle performs a persistent random walk with a long time diffusivity that is strongly enhanced for elongated active particles due to orientational correlations in their direction of propulsive motion. In our model shape-shifting induces directional sensing and the cell spontaneously migrate along the polarization direction.

cond-mat.stat-mech↗