arXiv subjects

Claudio Maggi

Publications and source records attributed to Claudio Maggi.

At least 19 recordsLinked to original sources

arXiv2026

Microphase Separation in Quorum-Sensing Active Particles with Competing Interactions

Standard quorum-sensing models in active matter exhibit collective phenomena such as motility-induced phase separation. Here, we show that incorporating competing sensing ranges-- a minimal ingredient inspired by microbial communication --qualitatively changes this behavior, replacing macroscopic phase separation with self-organized microphases characterized by an emergent finite length scale. Starting from the microscopic dynamics, we derive a coarse-grained field theory whose coefficients are explicitly related to the moments of the microscopic sensing function. This mapping enables a direct comparison between particle-based simulations and continuum theory, allowing the characteristic modulation and correlation lengths to be predicted directly from the microscopic interaction parameters. Two-dimensional numerical simulations confirm these predictions and reveal a transition from macrophase separation to finite-wavelength density modulations as the competition between sensing scales increases. For stronger competing interactions, the system develops a peculiar cluster phase with an interstitial percolating network, which is captured by a higher-order gradient expansion. Our results identify competing quorum-sensing interactions as a simple microscopic mechanism for generating tunable active microphases.

cond-mat.soft↗

arXiv2024

Multiple temperatures and melting of a colloidal active crystal

Thermal fluctuations constantly and evenly excite all vibrational modes in an equilibrium crystal. As the temperature rises, these fluctuations promote the formation of defects and eventually melting. In active solids, the self-propulsion of "atomic" units provides another source of strong non-equilibrium fluctuations whose effect on the melting scenario is still largely unexplored. Here we show that when a colloidal crystal is activated by a bath of swimming bacteria, solvent temperature and active temperature cooperate to define dynamic and thermodynamic properties. Our system consists of repulsive paramagnetic particles confined in two dimensions and immersed in a bath of light-driven E. coli. The relative balance between fluctuations and interactions can be adjusted in two ways: by changing the strength of the magnetic field and by tuning activity with light. When the persistence time of active fluctuations is short, a single effective temperature controls both the amplitudes of vibrational modes and the melting transition. For more persistent active noise, energy equipartition is broken and multiple temperatures emerge, whereas melting occurs before the Lindemann parameter reaches its equilibrium critical value. We show that this phenomenology is fully confirmed by numerical simulations and can be framed within a minimal model of a single active particle in a periodic potential.

cond-mat.soft↗

arXiv2022

A microfluidic method for passive trapping of sperms in microstructures

Sperm motility is a prerequisite for male fertility. Enhancing the concentration of motile sperms in assisted reproductive technologies - for human and animal reproduction - is typically achieved through aggressive methods such as centrifugation. Here we propose a passive technique for the amplification of motile sperm concentration, with no externally imposed forces or flows. The technique is based upon the disparity between probability rates, for motile cells, of entering in and escaping from complex structures. The effectiveness of the technique is demonstrated in microfluidic experiments with microstructured devices, comparing the trapping power in different geometries. In these micro-traps we observe an enhancement of cells concentration close to 10, with a contrast between motile and non-motile increased by a similar factor. Simulations of suitable interacting model sperms in realistic geometries reproduce quantitatively the experimental results, extend the range of observations and highlight the ingredients that are key to optimal trap design.

cond-mat.soft↗

arXiv2022

Critical behavior of quorum-sensing active particles

It is still a debated issue whether all critical active particles belong to the same universality class. Here we numerically study the critical behavior of quorum sensing active particles that represents the archetypal model for interpreting motility-induced phase separation. Mean-field theory predicts that this model should undergo a full phase separation if particles slow-down enough when sensing the presence of their neighbours and that the coexistence line terminates in a critical point. By performing large-scale numerical simulations we confirm this scenario, locate the critical point and use finite-size scaling analysis to show that the static and dynamic critical exponents of this active system agree with the Ising universality class

cond-mat.stat-mech↗

arXiv2021

Critical active dynamics is captured by a colored-noise driven field theory

We numerically investigate the correlation function, the response and the breakdown of the Fluctuation-Dissipation Theorem (FDT) in active particles close to the motility-induced critical point. We find a strong FDT violation in the short time and wavelength regime, where the response function has a larger amplitude than the fluctuation spectrum. Conversely, at larger spatiotemporal scales, the FDT is restored and the critical slowing-down is compatible with the Ising universality class. Building on these results, we develop a novel field-theoretical description employing a space-time correlated noise which qualitatively captures the numerical results already at the Gaussian level. By performing a one-loop renormalization group analysis we show that the correlated noise does not change the critical exponents with respect to the equilibrium. Our results demonstrate that a correlated noise field is a fundamental ingredient to capture the features of critical active matter at the coarse-grained level.

cond-mat.soft↗

arXiv2021

Collective effects in confined Active Brownian Particles

We investigate a two-dimensional system of active particles confined to a narrow annular domain. Despite the absence of explicit interactions among the velocities or the active forces of different particles, the system displays a transition from a disordered and stuck state to an ordered state of global collective motion where the particles rotate persistently clockwise or anticlockwise. We describe this behavior by introducing a suitable order parameter, the velocity polarization, measuring the global alignment of the particles' velocities along the tangential direction of the ring. We also measure the spatial velocity correlation function and its correlation length to characterize the two states. In the rotating phase, the velocity correlation displays an algebraic decay that is analytically predicted together with its correlation length while in the stuck regime the velocity correlation decays exponentially with a correlation length that increases with the persistence time. In the first case, the correlation (and, in particular, its correlation length) does not depend on the active force but the system size only. The global collective motion, an effect caused by the interplay between finite-size, periodicity, and persistent active forces, disappears as the size of the ring becomes infinite, suggesting that this phenomenon does not correspond to a phase transition in the usual thermodynamic sense.

cond-mat.stat-mech↗

arXiv2020

Universality class of the motility-induced critical point in large scale off-lattice simulations of active particles

We perform large-scale computer simulations of an off-lattice two-dimensional model of active particles undergoing a motility-induced phase separation (MIPS) to investigate the systems critical behaviour close to the critical point of the MIPS curve. By sampling steady-state configurations for large system sizes and performing finite size scaling analysis we provide exhaustive evidence that the critical behaviour of this active system belongs to the Ising universality class. In addition to the scaling observables that are also typical of passive systems, we study the critical behaviour of the kinetic temperature difference between the two active phases. This quantity, which is always zero in equilibrium, displays instead a critical behavior in the active system which is well described by the same exponent of the order parameter in agreement with mean-field theory.

cond-mat.stat-mech↗

arXiv2020

Activity-controlled clogging and unclogging of micro-channels

We propose a mechanism to control the formation of stable obstructions in two-dimensional micro-channels of variable sections taking advantage of the peculiar clustering property of active systems. Under the activation of the self-propulsion by external stimuli, the system behaves as a switch according to the following principle: by turning-on the self-propulsion the particles become active and even at very low densities stick to the walls and form growing layers eventually blocking the channel bottleneck, while the obstruction dissolves when the self-propulsion is turned off. We construct the phase diagram distinguishing clogged and open states in terms of density and bottleneck width. The study of the average clogging time, as a function of density and bottleneck width, reveals the marked efficiency of the active clogging that swiftly responds to the self-propulsion turning on. The resulting picture shows a profound difference with respect to the clogging obtained through the slow diffusive dynamics of attractive passive Brownian disks. This numerical work suggests a novel method to use particles with externally tunable self-propulsion to create or destroy plugs in micro-channels.

cond-mat.soft↗

arXiv2020

Hidden velocity ordering in dense suspensions of self-propelled disks

Recent studies of the phase diagram for spherical, purely repulsive, active particles established the existence of a transition from a liquid-like to a solid-like phase analogous to the one observed in colloidal systems at thermal equilibrium, in particular in two dimensions an intermediate hexatic phase is observed. Here, we present evidence that the active dense phases (solid, hexatic and liquid) exhibit interesting dynamical anomalies. First, we unveil the growth - with density and activity - of ordered domains where the particles' velocities align in parallel or vortex-like patches. Second, when activity is strong, the spatial distribution of kinetic energy becomes heterogeneous with high energy regions correlated to defects of the crystalline structure. This spatial heterogeneity is accompanied by temporal intermittency, with sudden peaks in the time-series of kinetic energy. The observed dynamical anomalies are not present in a dense equilibrium system and cannot be detected by considering only the structural properties of the system.

cond-mat.stat-mech↗

arXiv2019

Statistical Field Theory and Effective Action Method for scalar Active Matter

We employ Statistical Field Theory techniques for coarse-graining the steady-state properties of Active Ornstein-Uhlenbeck particles. The computation is carried on in the framework of the Unified Colored Noise approximation that allows an effective equilibrium picture. We thus develop a mean-field theory that allows to describe in a unified framework the phenomenology of scalar Active Matter. In particular, we are able to describe through spontaneous symmetry breaking mechanism two peculiar features of Active Systems that are (i) The accumulation of active particles at the boundaries of a confining container, and (ii) Motility-Induced Phase Separation (MIPS). \textcolor{black}{We develop a mean-field theory for steric interacting active particles undergoing to MIPS and for Active Lennard-Jones (ALJ) fluids.} \textcolor{black}{Within this framework}, we discuss the universality class of MIPS and ALJ \textcolor{black}{showing that it falls into Ising universality class.} We \textcolor{black}{thus} compute analytically the critical line $T_c(τ)$ for both models. In the case of MIPS, $T_c(τ)$ gives rise to a reentrant phase diagram compatible with an inverse transition from liquid to gas as the strength of the noise decreases. \textcolor{black}{However, in the case of particles interacting through anisotropic potentials, } the field theory acquires a $φ^3$ term that, \textcolor{black}{in general, cannot be canceled performing the expansion around the critical point.} In this case, the \textcolor{black}{Ising} critical point might \textcolor{black}{be replaced} by a first-order phase transition \textcolor{black}{region}.

cond-mat.stat-mech↗

arXiv2019

The effect of time-correlated noise on the Kuramoto model studied via the unified colored noise approximation

Many natural and social phenomena are characterized by synchronization. The Kuramoto model, taking into account the basic ingredients for observing synchronized states, allows to study mathematically synchronization in a simplified but nontrivial picture. Here we study how a noise that is correlated on a finite time-scale $τ$ impacts the ability of the Kuramoto model to achieve synchronization. We develop an approximated theory that allows to compute the critical coupling constant $k_c$ as a function of the correlation time $τ$. We obtain that that $k_c(τ)$ decreases as $τ$ increases indicating that time-correlated noise promotes synchronization. Moreover, we show that theory describes qualitatively well the degree of synchronization near $k_c$ obtained numerically. Finally, we show that, independently on the value of $τ$, the curves of the order parameter versus $k$ scale on the same master curve even at values of $k$ very far from $k_c$.

cond-mat.stat-mech↗

arXiv2019

Invariance properties of bacterial random walks in complex structures

Motile cells often explore natural environments characterized by a high degree of structural complexity. Moreover cell motility is also intrinsically noisy due to spontaneous random reorientation and speed fluctuations. This interplay of internal and external noise sources gives rise to a complex dynamical behavior that can be strongly sensitive to details and hard to model quantitatively. In striking contrast to this general picture we show that the mean residence time of swimming bacteria inside artificial complex microstructures, can be quantitatively predicted by a generalization of a recently discovered invariance property of random walks. We find that variations in geometry and structural disorder have a dramatic effect on the distributions of path length while mean values are strictly constrained by the sole free volume to surface ratio. Biological implications include the possibility of predicting and controlling the colonization of complex natural environments using only geometric informations.

cond-mat.soft↗

arXiv2018

Dynamic density shaping of light driven bacteria

Many motile microorganisms react to environmental light cues with a variety of motility responses guiding cells towards better conditions for survival and growth. The use of spatial light modulators could help to elucidate the mechanisms of photo-movements while, at the same time, providing an efficient strategy to achieve spatial and temporal control of cell concentration. Here we demonstrate that millions of bacteria, genetically modified to swim smoothly with a light controllable speed, can be arranged into complex and reconfigurable density patterns using a digital light projector. We show that a homogeneous sea of freely swimming bacteria can be made to morph between complex shapes. We model non-local effects arising from memory in light response and show how these can be mitigated by a feedback control strategy resulting in the detailed reproduction of grayscale density images.

physics.bio-ph↗

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

Effective equilibrium picture in $xy-$model with exponentially correlated noise

We study the effect of exponentially correlated noise on $xy-$model in the limit of small correlation time discussing the order-disorder transition in mean-field and the topological transition in two dimensions. We map the steady states of the non-equilibrium dynamics into an effective equilibrium theory. In mean-field, the critical temperature increases with the noise correlation time $τ$ indicating that memory effects promote ordering. This finding is confirmed by numerical simulations. The topological transition temperature in two dimensions remains untouched. However, finite size effects induce a crossover in the vortices proliferation that is confirmed by numerical simulations.

cond-mat.stat-mech↗

arXiv2017

Effective equilibrium states in the colored-noise model for active matter I. Pairwise forces in the Fox and unified colored noise approximations

The equations of motion of active systems can be modeled in terms of Ornstein-Uhlenbeck processes (OUPs) with appropriate correlators. For further theoretical studies, these should be approximated to yield a Markovian picture for the dynamics and a simplified steady-state condition. We perform a comparative study of the Unified Colored Noise Approximation (UCNA) and the approximation scheme by Fox recently employed within this context. We review the approximations necessary to define effective interaction potentials in the low-density limit and study the conditions for which these represent the behavior observed in two-body simulations for the OUPs model and Active Brownian particles. The demonstrated limitations of the theory for potentials with a negative slope or curvature can be qualitatively corrected by a new empirical modification. In general, we find that in the presence of translational white noise the Fox approach is more accurate. Finally, we examine an alternative way to define a force-balance condition in the limit of small activity.

cond-mat.soft↗

arXiv2017

Effective equilibrium states in the colored-noise model for active matter II. A unified framework for phase equilibria, structure and mechanical properties

Active particles driven by colored noise can be approximately mapped onto a system that obeys detailed balance. The effective interactions which can be derived for such a system allow to describe the structure and phase behavior of the active fluid by means of an effective free energy. In this paper we explain why the related thermodynamic results for pressure and interfacial tension do not represent the results one would measure mechanically. We derive a dynamical density functional theory, which in the steady state simultaneously validates the use of effective interactions and provides access to mechanical quantities. Our calculations suggest that in the colored-noise model the mechanical pressure in coexisting phases might be unequal and the interfacial tension can become negative.

cond-mat.soft↗

arXiv2017

Heat, temperature and Clausius inequality in a model for active brownian particles

Methods of stochastic thermodynamics and hydrodynamics are applied to the a recently introduced model of active particles. The model consists of an overdamped particle subject to Gaussian coloured noise. Inspired by stochastic thermodynamics, we derive from the system's Fokker-Planck equation the average exchanges of heat and work with the active bath and the associated entropy production. We show that a Clausius inequality holds, with the local (non-uniform) temperature of the active bath replacing the uniform temperature usually encountered in equilibrium systems. Furthermore, by restricting the dynamical space to the first velocity moments of the local distribution function we derive a hydrodynamic description where local pressure, kinetic temperature and internal heat fluxes appear and are consistent with the previous thermodynamic analysis. The procedure also shows under which conditions one obtains the unified coloured noise approximation (UCNA): such an approximation neglects the fast relaxation to the active bath and therefore yields detailed balance and zero entropy production. In the last part, by using multiple time-scale analysis, we provide a constructive method (alternative to UCNA) to determine the solution of the Kramers equation and go beyond the detailed balance condition determining negative entropy production.

cond-mat.stat-mech↗