arXiv subjects

Nuno A. M. Araújo

Publications and source records attributed to Nuno A. M. Araújo.

At least 37 records · Page 2Linked to original sources

arXiv2022

Steering self-organisation through confinement

Self-organisation is the spontaneous emergence of spatio-temporal structures and patterns from the interaction of smaller individual units. Examples are found across many scales in very different systems and scientific disciplines, from physics, materials science and robotics to biology, geophysics and astronomy. Recent research has highlighted how self-organisation can be both mediated and controlled by confinement. Confinement occurs through interactions with boundaries, and can function as either a catalyst or inhibitor of self-organisation. It can then become a means to actively steer the emergence or suppression of collective phenomena in space and time. Here, to provide a common framework for future research, we examine the role of confinement in self-organisation and identify overarching scientific challenges across disciplines that need to be addressed to harness its full scientific and technological potential. This framework will not only accelerate the generation of a common deeper understanding of self-organisation but also trigger the development of innovative strategies to steer it through confinement, with impact, e.g., on the design of smarter materials, tissue engineering for biomedicine and crowd management.

nlin.AO↗

arXiv2021

The impact of one-way streets on the asymmetry of the shortest commuting routes

On a daily commute, the shortest route from home to work rarely overlaps completely the shortest way back. We analyze this asymmetry for several cities and show that it exists even without traffic, due to a non-negligible fraction of one-way streets. For different pairs of origin-destination ($\rm OD$), we compute the log-ratio $r=\ln(\ell_{\rm D}/\ell_{\rm O})$, where $\ell_{\rm O}$ and $\ell_{\rm D}$ are the lengths of the shortest routes from $\rm O$ to $\rm D$ and from $\rm D$ to $\rm O$, respectively. While its average is zero, the amplitude of the fluctuations decays as a power law of the $\rm OD$ shortest path length, $r\sim \ell_{\rm O}^{-β}$. Similarly, the fraction of one-way streets in a shortest route also decays as $\ell_{\rm O}^{-α}$. Based on semi-analytic arguments, we show that $β=(1+α)/2$. Thus, the value of the exponent $β$ is related to correlations in the structure of the underlying street network.

physics.soc-ph↗

arXiv2021

Inheritances, social classes, and wealth distribution

We consider a simple theoretical model to investigate the impact of inheritances on the wealth distribution. Wealth is described as a finite resource, which remains constant over different generations and is divided equally among offspring. All other sources of wealth are neglected. We consider different societies characterized by a different offspring probability distribution. We find that, if the population remains constant, the society reaches a stationary wealth distribution. We show that inequality emerges every time the number of children per family is not always the same. For realistic offspring distributions from developed countries, the model predicts a Gini coefficient of $G\approx 0.3$. If we divide the society into wealth classes and set the probability of getting married to depend on the distance between classes, the stationary wealth distribution crosses over from an exponential to a power-law regime as the number of wealth classes and the level of class distinction increase.

physics.soc-ph↗

arXiv2021

Flow through time-evolving porous media: swelling and erosion

The flow through a porous medium strongly depends on the boundary conditions, very often assumed to be static. Here, we consider changes in the medium due to swelling and erosion and extend existing Lattice-Boltzmann models to include both. We study two boundary conditions: a constant pressure drop and a constant flow rate. For a constant flow rate, the steady state depends solely on the erosion dynamics while for a constant pressure drop it depends also on the timescale of swelling. We analyze the competition between swelling and erosion and identify a transition between regimes where either swelling or erosion dominate.

physics.flu-dyn↗

arXiv2021

Director alignment at the nematic-isotropic interface: elastic anisotropy and active anchoring

Activity in nematics drives interfacial flows that lead to preferential alignment that is tangential or planar for extensile systems (pushers) and perpendicular or homeotropic for contractile ones (pullers). This alignment is known as active anchoring and has been reported for a number of systems and described using active nematic hydrodynamic theories. The latter are based on the one-elastic constant approximation, i.e., they assume elastic isotropy of the underlying passive nematic. Real nematics, however, have different elastic constants, which lead to interfacial anchoring. In this paper, we consider elastic anisotropy in multiphase and multicomponent hydrodynamic models of active nematics and investigate the competition between the interfacial alignment driven by the elastic anisotropy of the passive nematic and the active anchoring. We start by considering systems with translational invariance to analyze the alignment at flat interfaces and, then, consider two-dimensional systems and active nematic droplets. We investigate the competition of the two types of anchoring over a wide range of the other parameters that characterize the system. The results of the simulations reveal that the active anchoring dominates except at very low activities, when the interfaces are static. In addition, we found that the elastic anisotropy does not affect the dynamics but changes the active length that becomes anisotropic.

cond-mat.soft↗

arXiv2021

Wetting boundary conditions for multicomponent pseudopotential lattice Boltzmann

The implementation of boundary conditions is among the most challenging parts of modeling fluid flow through channels and complex media. Here, we show that the existing methods to deal with liquid-wall interactions using multicomponent Lattice Boltzmann are accurate when the wall is aligned with the main axes of the lattice but fail otherwise. To solve this problem, we extend a strategy previously developed for multiphase models. As an example, we study the coalescence of two droplets on a curved surface in two dimensions. The strategy proposed here is of special relevance for binary flows in complex geometries.

physics.comp-ph↗

arXiv2021

Effect of particle deformability on shear thinning in a 3D channel

Soft particles suspended in fluids flowing through microchannels are often encountered in biological flows such as cells in vessels. They can deform under flow or when subject to mechanical stresses as they interact with themselves. Deformability adds distinct characteristics to the nature of the flow of these particles. We simulate deformable particles suspended in a fluid at a high volume fraction flowing through a 3D wetting channel. We find a discontinuous shear thinning transition, which depends on the particle deformability. The capillary number is the main parameter that controls the transition. We also notice significant differences in the velocity profiles when compared to their 2D counterpart. To obtain these results, we improved and extended to 3D a multicomponent Lattice Boltzmann method which prevents the coalescence between the droplets.

cond-mat.soft↗

arXiv2020

Invasion Sandpile Model

Motivated by multiphase flow in reservoirs, we propose and study a two-species sandpile model in two dimensions. A pile of particles becomes unstable and topples if, at least one of the following two conditions is fulfilled: 1) the number of particles of one species in the pile exceeds a given threshold or 2) the total number of particles in the pile exceeds a second threshold. The latter mechanism leads to the invasion of one species through regions dominated by the other species. We studied numerically the statistics of the avalanches and identified two different regimes. For large avalanches the statistics is consistent with ordinary Bak-Tang-Weisenfeld model. Whereas, for small avalanches, we find a regime with different exponents. In particular, the fractal dimension of the external perimeter of avalanches is $D_f=1.47\pm 0.02$ and the exponent of their size distribution exponent is $τ_s=0.95\pm 0.03$, which are significantly different from $D_f=1.25\pm 0.01$ and $τ_s=1.26\pm 0.04$, observed for large avalanches.

cond-mat.stat-mech↗

arXiv2019

Propagation of active nematic-isotropic interfaces on substrates

Motivated by results for the propagation of active-passive interfaces of bacterial Serratia marcescens swarms [Nat. Comm., 9, 5373 (2018)] we use a hydrodynamic multiphase model to investigate the propagation of interfaces of active nematics on substrates. We characterize the active nematic phase of the model and discuss its description of the statistical dynamics of the swarms. We show that the velocicity correlation functions and the energy spectrum of the active turbulent phase of the model scale with the active length, for a range of activities. In addition, the energy spectrum exhibits two power-law regimes with exponents close to those reported for other models and bacterial swarms. Although the exponent of the rising branch of the spectrum (small wavevector) appears to be independent of the activity, the exponent of the decay changes with activity systematically, albeit slowly. We characterize also the propagation of circular and flat active-passive interfaces. We find that the closing time of the circular passive domain decays quadratically with the activity and that the structure factor of the flat interface is similar to that reported for swarms, with an activity dependent exponent. Finally, the effect of the substrate friction was investigated. We found an activity dependent threshold, above which the turbulent active nematic forms isolated islands that shrink until the system becomes isotropic and below which the active nematic expands, with a well defined propagating interface. The interface may be stopped by a fricion gradient.

cond-mat.soft↗

arXiv2019

Enhanced propagation of motile bacteria on surfaces due to forward scattering

How motile bacteria move near a surface is a problem of fundamental biophysical interest and is key to the emergence of several phenomena of biological, ecological and medical relevance, including biofilm formation. Solid boundaries can strongly influence a cell's propulsion mechanism, thus leading many flagellated bacteria to describe long circular trajectories stably entrapped by the surface. Experimental studies on near-surface bacterial motility have, however, neglected the fact that real environments have typical microstructures varying on the scale of the cells' motion. Here, we show that micro-obstacles influence the propagation of peritrichously flagellated bacteria on a flat surface in a non-monotonic way. Instead of hindering it, an optimal, relatively low obstacle density can significantly enhance cells' propagation on surfaces due to individual forward-scattering events. This finding provides insight on the emerging dynamics of chiral active matter in complex environments and inspires possible routes to control microbial ecology in natural habitats.

physics.bio-ph↗

arXiv2019

Lattice-Boltzmann simulation of free nematic-isotropic interfaces

We use a hybrid method of lattice Boltzmann and finite differences to simulate flat and curved interfaces between the nematic and isotropic phases of a liquid crystal described by the Landau-de Gennes theory. For the flat interface, we measure the interfacial velocity at different temperatures around the coexistence. We show that the interface is completely static at the coexistence temperature and that the profile width is in line with the theoretical predictions. The interface is stable in a range of temperatures around coexistence and disappears when one of the two phases becomes mechanically unstable. We stabilize circular nematic domains by a shift in temperature, related to the Laplace pressure, and estimate the spurious velocities of these lattice Boltzmann simulations.

cond-mat.soft↗

arXiv2019

Active nematic-isotropic interfaces in channels

We use numerical simulations to investigate the hydrodynamic behavior of the interface between nematic (N) and isotropic (I) phases of a confined active liquid crystal. At low activities, a stable interface with constant shape and velocity is observed separating the two phases. For nematics in homeotropic channels, the velocity of the interface at the NI transition increases from zero (i) linearly with the activity for contractile systems and (ii) quadratically for extensile ones. Interestingly, the nematic phase expands for contractile systems while it contracts for extensile ones, as a result of the active forces at the interface. Since both activity and temperature affect the stability of the nematic, for active nematics in the stable regime the temperature can be tuned to observe static interfaces, providing an operational definition for the coexistence of active nematic and isotropic phases. At higher activities, beyond the stable regime, an interfacial instability is observed for extensile nematics. In this regime defects are nucleated at the interface and move away from it. The dynamics of these defects is regular and persists asymptotically for a finite range of activities. We used an improved hybrid model of finite differences and lattice Boltzmann method with multi-relaxation-time collision operator, the accuracy of which allowed us to characterize the dynamics of the distinct interfacial regimes.

cond-mat.soft↗

arXiv2019

Ordering of Binary Colloidal Crystals by Random Potentials

Structural defects are ubiquitous in condensed matter, and not always a nuisance. For example, they underlie phenomena such as Anderson localization and hyperuniformity, and they are now being exploited to engineer novel materials. Here, we show experimentally that the density of structural defects in a 2D binary colloidal crystal can be engineered with a random potential. We generate the random potential using an optical speckle pattern, whose induced forces act strongly on one species of particles (strong particles) and weakly on the other (weak particles). Thus, the strong particles are more attracted to the randomly distributed local minima of the optical potential, leaving a trail of defects in the crystalline structure of the colloidal crystal. While, as expected, the crystalline ordering initially decreases with increasing fraction of strong particles, the crystalline order is surprisingly recovered for sufficiently large fractions. We confirm our experimental results with particle-based simulations, which permit us to elucidate how this non-monotonic behavior results from the competition between the particle-potential and particle-particle interactions.

cond-mat.soft↗

arXiv2018

Jamming and percolation properties of random sequential adsorption with relaxation

The random sequential adsorption (RSA) model is a classical model in Statistical Physics for adsorption on two-dimensional surfaces. Objects are deposited sequentially at random and adsorb irreversibly on the landing site, provided that they do not overlap any previously adsorbed object. The kinetics of adsorption ceases when no more objects can be adsorbed (jamming state). Here, we investigate the role of post-relaxation on the jamming state and percolation properties of RSA of dimers on a two-dimensional lattice. We consider that, if the deposited dimer partially overlaps with a previously adsorbed one, a sequence of dimer displacements may occur to accommodate the new dimer. The introduction of this simple relaxation dynamics leads to a more dense jamming state than the one obtained with RSA without relaxation. We also consider the anisotropic case, where one dimer orientation is favored over the other, finding a non-monotonic dependence of the jamming coverage on the strength of anisotropy. We find that the density of adsorbed dimers at which percolation occurs is reduced with relaxation, but the value depends on the strength of anisotropy.

cond-mat.stat-mech↗

arXiv2018

Percolation of functionalized colloids on patterned substrates

We study the percolation properties for a system of functionalized colloids on patterned substrates via Monte Carlo simulations. The colloidal particles are modeled as hard disks with three equally-distributed attractive patches on their perimeter. We describe the patterns on the substrate as circular potential wells of radius $R_p$ arranged in a regular square or hexagonal lattice. We find a nonmonotonic behavior of the percolation threshold (packing fraction) as a function of $R_p$. For attractive wells, the percolation threshold is higher than the one for clean (non-patterned) substrates if the circular wells are non-overlapping and can only be lower if the wells overlap. For repulsive wells we find the opposite behavior. In addition, at high packing fractions the formation of both structural and bond defects suppress percolation. As a result, the percolation diagram is reentrant with the non-percolated state occurring at very low and intermediate densities.

cond-mat.soft↗

arXiv2018

Field-driven dynamical demixing of binary mixtures

We consider mixtures of two species of spherical colloidal particles that differ in their hydrodynamic radii, but are otherwise identical, in the presence of an external field. Since the particle-particle and particle-field interactions are the same for both species, they are completely mixed in the thermodynamic limit in the presence of any static field. Here, we combine Brownian Dynamics and Dynamic Density Functional theory of fluids to show that for sufficiently large differences in the hydrodynamic radius of the particles (and corresponding differences in their electrophoretic mobilities) dynamical demixing is observed. These demixed states are transient but, under certain conditions, packing effects compromise the relaxation towards the thermodynamic states and the lifetime of the demixed phases increases significantly.

cond-mat.soft↗

arXiv2017

Controlling percolation with limited resources

Connectivity - or the lack thereof - is crucial for the function of many man-made systems, from financial and economic networks over epidemic spreading in social networks to technical infrastructure. Often, connections are deliberately established or removed to induce, maintain, or destroy global connectivity. Thus, there has been a great interest in understanding how to control percolation, the transition to large-scale connectivity. Previous work, however, studied control strategies assuming unlimited resources. Here, we depart from this unrealistic assumption and consider the effect of limited resources on the effectiveness of control. We show that, even for scarce resources, percolation can be controlled with an efficient intervention strategy. We derive this strategy and study its implications, revealing a discontinuous transition as an unintended side-effect of optimal control.

cond-mat.dis-nn↗

arXiv2017

Annealing cycles and the self-organization of functionalized colloids

The self-assembly of functionalized (patchy) particles with directional interactions into target structures is still a challenge, despite the significant experimental advances on their synthesis. The self-assembly pathways are typically characterized by high energy barriers that hinder the access to stable (equilibrium) structures. A possible strategy to tackle this challenge is to perform annealing cycles. By periodically switching on and off the inter-particle bonds, one expects to smooth-out the kinetic pathways and favor the assembly of targeted structures. Preliminary results have shown that the efficiency of annealing cycles depends strongly on their frequency. Here, we study numerically how this frequency-dependence scales with the strength of the directional interactions (size of the patch $σ$). We use analytical arguments to show that the scaling results from the statistics of a random walk in configurational space.

cond-mat.soft↗