Articles publicats en revistes (Física de la Matèria Condensada)

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Emergence of social hierarchies in a society with two competitive groups
(Elsevier Ltd., 2025-06-18) Sadurní Parera, Marc; Perelló, Josep; Montero Matellanes, M. Mikel
Agent-based models describing social interactions among individuals can help to better understand emerging macroscopic patterns in societies. One of the topics which is worth tackling is the formation of different kinds of hierarchies that emerge in social spaces such as cities. Here we propose a Bonabeau-like model by adding a second group of agents. The fundamental particularity of our model is that only a pairwise interaction between agents of the opposite group is allowed. Agent fitness can thus only change by competition among the two groups, while the total fitness in the society remains constant. The main result is that for a broad range of values of the model parameters, the fitness of the agents of each group show a decay in time except for one or very few agents which capture almost all the fitness in the society. Numerical simulations also reveal a singular shift from egalitarian to hierarchical society for each group. This behaviour depends on the control parameter , playing the role of the inverse of the temperature of the system. Results are invariant with regard to the system size, contingent solely on the quantity of agents within each group. Finally, scaling laws are provided thus showing a data collapse from different model parameters and they follow a shape which can be related to the presence of a phase transition in the model.
Perspective on interdisciplinary approaches on Chemotaxis
(Wiley-VCH, 2025-10-28) Simmchen, Juliane; Gordon, Daniel; MacKenzie, John; Pagonabarraga Mora, Ignacio; Roggatz, C. Christina; Endres, Robert G.; Xiao, Zuyao; Friedrich, Benjamin; Qiu, Tian; Painter, Kevin J.; Golestanian, Ramin; Contini, C.; Ucar, Mehmet Can; Yossifon, G.; Sommer, J.U.; Rappel, W.-J.; Wan, K.; Armitage, J.; Install, R.
Most living things on Earth – from bacteria to humans – must migrate in some way to find favourable conditions. Therefore, they nearly all use chemotaxis, in which their movement is steered by a gradient of chemicals. Chemotaxis is fundamental to many processes that control our well-being, including inflammation, neuronal patterning, wound healing, tumour spread in cancer, even embryogenesis. Understanding it is a key goal for biologists. Despite the fact that many basic principles appear to have been conserved throughout evolution, most research has focused on understanding the molecular mechanisms that control signal processing and locomotion. Cell signaling – cells responding to time-varying external signals – underlies almost all biological processes at the cellular scale. Chemotaxis of single cells provides particularly amenable model systems for quantitative cell signaling studies, even in the presence of noise and fluctuations, because the output, the cell’s motility response, is directly observable. However, the different scientific disciplines involved in chemotaxis research rarely overlap, so biologists, physicists and mathematicians interact far too infrequently, methodologies and models differ and commonalities are often overlooked, such as the possible influence of physical or environmental conditions, which has been largely neglected.
Sorting of binary active-passive mixtures in designed microchannels
(Royal Society of Chemistry, 2025-10-14) Serna, Horacio; Barriuso Gutiérrez, C. Miguel; Pagonabarraga Mora, Ignacio; Polin, Marco; Valeriani, Chantal
Mixtures of active and passive particles are ubiquitous at the microscale. Many essential microbial processes involve interactions with dead or immotile cells or passive crowders. When passive objects are immersed in active baths, their transport properties are enhanced and can be tuned by controlling active agents’ spatial and orientational distribution. Active–passive mixtures provide a platform to explore fundamental questions about the emergent behaviour of passive objects under simultaneous thermal and active noise and a foundation for technological applications in cargo delivery and bioremediation. In this work, we use computational simulations to study an active–passive mixture confined in microchannels designed with funnel-like obstacles that selectively allow the passage of passive particles. Active particles follow overdamped Langevin translational dynamics and run-and-tumble rotational dynamics. We find that adjusting the tumbling rate of active agents and the microchannel geometry leads to a maximum enhancement of the transport properties of the passive particles (diffusion coefficient and advective velocity) that correlates with the highest mixture sorting efficiency and the shortest response time. We demonstrate that the active drift is the cause of the observed enhanced separation of the mixture in contrast with scenarios where only thermal or active diffusion are present.
Sedimentation and structural features of suspensions of squirmer-like microswimmers under agravitational field
(Royal Society of Chemistry, 2025-01-15) Barriuso Gutiérrez, C. Miguel; Serna, Horacio; Pagonabarraga Mora, Ignacio; Valeriani, Chantal
The effect of gravity on the collective motion of living microswimmers, such as bacteria and micro-algae, is pivotal to unravel not only bio-convection patterns but also the settling of bacterial biofilms on solid surfaces. In this work, we investigate suspensions of microswimmers under the influence of a gravitational field and hydrodynamics, simulated via the dissipative particle dynamics (DPD) coarse-grained model. We first study the collective sedimentation of passive colloids and microswimmers of the puller and pusher types upon increasing the imposed gravitational field and compare them with previous results. Once sedimentation occurs, we observe that, as the gravitational field increases, the bottom layer undergoes a transition to an ordered state compatible with a hexagonal crystal. In comparison with passive colloids, both pullers and pushers easily rearrange at the bottom layer to anneal defects. Specifically, pullers are better than pushers in preserving the hexagonal order of the bottom mono-layer at high gravitational fields.
Block copolymer nanocomposites under soft confinement
(American Chemical Society, 2025-05-14) Diaz, Javier; Pinna, Marco; Zvelindovsky, Andrei; Pagonabarraga Mora, Ignacio
Block copolymer (BCP) melts can be blended with solvents to self-assemble into complex droplets with internal structures. Controlling the morphology of these softly confined structures is crucial for various applications, including drug delivery. The addition of nanoparticles (NPs) to BCP droplets produces hierarchical co-assembly with intricate structures, where BCPs act as scaffolds. However, incorporating NPs can significantly alter the BCP droplet structure, leading to emergent behavior. Computer simulations reveal that confinement-induced frustration leads to a Janus-like morphology, with spatially segregated hexagonal and lamellar structures within the droplet bulk. Systematic exploration of NP loading and chemical interactions demonstrates various phase transitions, which are rationalized based on changes in the effective composition and solubility of the BCP droplet. A time-dependent model enables the study of the kinetics of several NP-induced layered morphologies, indicating that changes in the effective solubility of the BCP droplet result in a slow progression toward an onion morphology.
Virtual magnetic hills to unlock the inner phases of hexagonal colloidal ice
(Royal Society of Chemistry, 2026-02-04) Baillou, Renaud.; Terkel, Matthew.; Tierno, Pietro
We study the low energy states in a hexagonal colloidal ice realized by using repulsive paramagnetic colloids confined by gravity within a honeycomb lattice of traps. In contrast to similar systems featuring optical or topographic double wells, here we introduce field tunable “virtual” magnetic hills. These hills are created by placing pairs of fixed paramagnetic particles close to the semi-cylindrical traps that contain the interacting, mobile colloids. With this strategy, a single magnetic field can be used to simultaneously tune the particle pair-interactions and the hill elevation, without losing the trap bistability at any field strength. We use numerical simulations to explore the rich low energy states of the system. By varying both the relative distance and the magnetic content of the fixed particles, not only the effects of the first but also of the second nearest neighbors can be accessed, allowing the inner charge-ordered ice-II phase to be reached. Our strategy of controlling the vertex energetics via fixed, field tunable interstitial units may be extended to other geometrically frustrated systems on different length scales, including nanoscale spin ice and macroscopic magnetic metamaterials.
Confinement-driven emergence of hyperuniform fluids
(American Physical Society, 2025-12-15) Leoni, Fabio; Franzese, Giancarlo; Oguz, Erdal C.; Martelli, Fausto
Controlling emergent structural order in spatially constrained systems is a fundamental challenge. Using large-scale simulations of a model fluid at equilibrium conditions, we show that geometric confinement alone can stabilize fluid and hyperuniform labyrinthine phases. Moreover, confinement can induce self-assembly into distinct regimes—ranging from nonhyperuniform to antihyperuniform configurations—providing a robust mechanism for tuning spatial order. Our results identify confinement as a minimal design principle for engineering systems with target structural properties, including (anti)hyperuniformity, without relying on genetic or chemical specificity, and with broad applications in multiple disciplines and technologies.
Efficient parallel algorithms for free-energy calculation of millions of water molecules in the fluid phases
(Frontiers Media, 2025-09-16) Coronas, Luis Enrique; Vilanova, Oriol; Franzese, Giancarlo
Simulating water droplets made up of millions of molecules and on timescales as needed in biological and technological applications is challenging due to the difficulty of balancing accuracy with computational capabilities. Most detailed descriptions, such as ab initio, polarizable, or rigid models, are typically constrained to a few hundred (for ab initio) or thousands of molecules (for rigid models). Recent machine learning approaches allow for the simulation of up to 4 million molecules with ab initio accuracy but only for tens of nanoseconds, even if parallelized across hundreds of GPUs. In contrast, coarse-grained models permit simulations on a larger scale but at the expense of accuracy or transferability. Here, we consider the CVF molecular model of fluid water, which bridges the gap between accuracy and efficiency for free-energy and thermodynamic quantities due to i) a detailed calculation of the hydrogen bond contributions at the molecular level, including cooperative effects, and ii) coarse-graining of the translational and rotational degrees of freedom of the molecules. The CVF model can reproduce the experimental equation of state and fluctuations of fluid water across a temperature range of 60$\,^{\circ}$ around ambient temperature and from 0 to 50 MPa. In this work, we describe efficient parallel Monte Carlo algorithms executed on GPUs using CUDA, tailored explicitly for the CVF model. We benchmark accessible sizes of 17 million molecules with the Metropolis and 2 million with the Swendsen-Wang Monte Carlo algorithm.
Adsorption of wastewater pollutants on amorphous TiO2: an atomistic simulation study
(Royal Society of Chemistry, 2026-02-23) von Einem, Maria; Balzaretti, Filippo; Romero, Manuela; Colombi Ciacchi, Lucio; Franzese, Giancarlo; Köppen-Hannemann, Susan
The increasing presence of polluting chemicals in man-made wastewater poses significant environmental and health risks. Advanced oxidation processes, particularly those involving photocatalytic materials like titanium dioxide (TiO2), offer a promising solution for degrading these pollutants. This study employs force field molecular dynamics simulations to investigate the interactions between pollutants, the TiO2 surface, water and ions, aiming to elucidate their role in the adsorption process. The results reveal that the protonation state of pollutants significantly influences their contact with the TiO2 surface, with negatively charged species showing a higher affinity for the surface’s active sites, especially those containing carboxylate groups. The formation of hydrogen bond networks affects the stability of these contacts positively, while the tendency of some pollutants to aggregate hinders surface contacts. Furthermore, we observe cations (Na+) to alter the surface-near environment in a typical electrical double-layer manner, as well as to participate in pollutant adsorption and aggregation. These findings provide insights into the adsorption features triggering the initial pollutant degradation on amorphous TiO2, which could enhance the design of more efficient wastewater treatment technologies .
Unveiling the entropic role of hydration water in SOD1 partitioning within FUS condensate
(American Institute of Physics (AIP), 2026-03-07) Coronas, Luis Enrique; Timr, Stepan; Sterpone, Fabio; Franzese, Giancarlo
Biological processes such as the sequestration of Superoxide Dismutase 1 (SOD1) into biomolecular condensates, including FUS and stress granules, are vital for understanding disease mechanisms, including amyotrophic lateral sclerosis (ALS). Moreover, protein-crowder interactions within these condensates are recognized as fundamental to cellular phase separation and disease-related processes. However, the specific role of the hydration environment in governing SOD1’s behavior and transition dynamics within these condensates remains poorly understood, limiting our ability to accurately model these critical biological systems. Therefore, we incorporate explicit water into an implicit solvent model (OPEP) to investigate how water influences SOD1’s behavior, residence times, and transition rates among associative states. We employ the advanced CVF water model, which accurately captures hydrogen- bond networks at the molecular level. While the OPEP model indicates that Bovine Serum Albumin (BSA) crowders reduce SOD1’s partition coefficient (PC) primarily through nonspecific interactions, our explicit-water approach points to hydration entropy in BSA as a key contributor to the observed PC reduction. This result offers a new perspective on the system’s free-energy landscape, complementing those obtained from OPEP alone. Our research supports the notion that explicitly modeling water can enhance our understanding of protein-crowder interactions and their biological implications, further emphasizing the potential role of water in cellular phase separation and disease-related processes.
Small-angle X-ray scattering unveils the internal structure of lipid nanoparticles.
(Elsevier, 2024-02-12) Spinozzi, Francesco; Moretti, Paolo; Romano Perinelli, Diego; Corucci, Giacomo; Piergiovanni, Paolo; Amenitsch, Heinz; Alfredo Sancin, Giulio; Franzese, Giancarlo; Blasi, Paolo
Lipid nanoparticles own a remarkable potential in nanomedicine, only partially disclosed. While the clinical use of liposomes and cationic lipid-nucleic acid complexes is well-established, liquid lipid nanoparticles (nanoemulsions), solid lipid nanoparticles, and nanostructured lipid carriers have even greater possibilities. However, they face obstacles in being used in clinics due to a lack of understanding about the molecular mechanisms controlling their drug loading and release, interactions with the biological environment (such as the protein corona), and shelf-life stability. To create effective drug delivery carriers and successfully translate bench research to clinical settings, it is crucial to have a thorough understanding of the internal structure of lipid nanoparticles. Through synchrotron small-angle X-ray scattering experiments, we determined the spatial distribution and internal structure of the nanoparticles’ lipid, surfactant, and the bound water in them. The nanoparticles themselves have a barrel-like shape that consists of coplanar lipid platelets (specifically cetyl palmitate) that are covered by loosely spaced polysorbate 80 surfactant molecules, whose polar heads retain a large amount of bound water. To reduce the interface cost of bound water with unbound water without stacking, the platelets collapse onto each other. This internal structure challenges the classical core-shell model typically used to describe solid lipid nanoparticles and could play a significant role in drug loading and release, biological fluid interaction, and nanoparticle stability, making our findings valuable for the rational design of lipid-based nanoparticles.
Non-linear inhibitory responses enhance performance in collective decision-making
(Springer Nature, 2025-03-27) March-Pons, David; Pastor Satorras, Romualdo; Miguel López, María del Carmen
The precise modulation of activity through inhibitory signals ensures that both insect colonies and neural circuits operate efficiently and adaptively, highlighting the fundamental importance of inhibition in biological systems. Modulatory signals are produced in various contexts and are known for subtly shifting the probability of receiver behaviors based on response thresholds. Here we propose a non-linear function to introduce inhibitory responsiveness in collective decision-making inspired by honeybee house-hunting. We show that, compared with usual linear functions, non-linear responses enhance final consensus and reduce deliberation time. This improvement comes at the cost of reduced accuracy in identifying the best option. Nonetheless, for value-based tasks, the benefits of faster consensus and enhanced decision-making might outweigh this drawback.
Decoding the Conformation of Polylactic Acid in Block Copolymer Micelles
(American Chemical Society, 2026-01-28) Muñoz López, José María; Tuveri, Gian Marco; Barbieri, Valentino; Basile, Marco; Cosenza, V.; Lorenz, Christian D.; Ruiz-Perez, Lorena; Battaglia, Giuseppe
Understanding how molecular features dictate the self-assembly of amphiphilic block copolymers into well-defined nanostructures is essential for the rational design of advanced soft materials. However, the large number of interdependent parameters involved, such as particle size, aggregation number, interfacial curvature, and molecular weight, makes it challenging to establish general design principles. Here we establish a scaling-based framework for PEG-b-PLA micelles with a fixed hydrophilic–hydrophobic ratio. Systematic variation of molecular weights enables precise control of micelle size and aggregation number, quantified by DLS, cryo-TEM, and MALS.
Activity-driven emulsification of phase-separating binary mixture
(American Physical Society, 2025-03-07) Diaz, Javier; Pagonabarraga Mora, Ignacio
Active particles self-assemble into emergent structures that respond sensitively to external constraints. Consequently, their behavior under confinement is complex, especially in soft confined media, leading to diverse emergent morphologies. Through computer simulations, we investigate the dynamical interplay between active Brownian particles and a binary mixture. Our results show that active particles stabilize nonequilibrium morphologies, arresting coarsening by exerting active pressure that competes with surface tension. For moderate activities, particles stabilize an active emulsion with a well-defined droplet size. At higher activities, when particles can cross the liquid domains, a dynamic emulsion with large droplet dispersion is sustained. Furthermore, active particles drive phase-separated mixtures away from equilibrium configurations, demonstrating a rich coassembly behavior due to competing energy scales in the system.
Non-Reciprocal interactions reshape topological defect annihilation
(American Physical Society, 2025-04-23) Rouzaire, Ylann; Pearce, Daniel J. G.; Pagonabarraga Mora, Ignacio; Levis, Demian
We show how nonreciprocal ferromagnetic interactions between neighboring planar spins in two dimensions, affect the behavior of topological defects. Nonreciprocity is introduced by weighting the coupling strength of the two-dimensional XY model by an anisotropic kernel. As a consequence, in addition to the topological charge, the actual shape of the defects becomes crucial to faithfully describe their dynamics. Nonreciprocal coupling twists the spin field, selecting specific defect shapes, dramatically altering the pair annihilation process. Defect annihilation can either be enhanced or hindered, depending on the shape of the defects concerned and the degree of nonreciprocity in the system. We introduce a continuous description—for which the phenomenological coefficients can be explicitly written in terms of the microscopic ones—that captures the behavior of the lattice model.
Activity leads to topological phase transition in 2D populations of heterogeneous oscillators
(American Physical Society, 2025-05-06) Rouzaire, Ylann; Rahmani, Parisa; Pagonabarraga Mora, Ignacio; Peruani, Fernando; Levis, Demian
Populations of heterogeneous, noisy oscillators on a two-dimensional lattice display short-range order. Here, we show that if the oscillators are allowed to actively move in space, the system undergoes instead a Berezenskii-Kosterlitz-Thouless transition and exhibits quasi-long-range order. This fundamental result connects two paradigmatic models—the XY and Kuramoto models—and provides insight into the emergence of order in active systems.
Resolving the different bulk moduli within individual soft nanogels using small-angle neutron scattering
(American Association for the Advancement of Science, 2022-01-01) Houston, Judith; Fruhner, Lisa Sarah; Cotte, Alexis de la; Rojo-González, Javier; Petrunin, Alexander V.; Gasser, Urs; Schweins, Ralf; Allgaier, Jürgen; Richtering, Walter; Fernández-Nieves, Alberto; Scotti, Andrea
The bulk modulus, K, quantifies the elastic response of an object to an isotropic compression. For soft compressible colloids, knowing K is essential to accurately predict the suspension response to crowding. Most colloids have complex architectures characterized by different softness, which additionally depends on compression. Here, we determine the different values of K for the various morphological parts of individual nanogels and probe the changes of K with compression. Our method uses a partially deuterated polymer, which exerts the required isotropic stress, and small-angle neutron scattering with contrast matching to determine the form factor of the particles without any scattering contribution from the polymer. We show a clear difference in softness, compressibility, and evolution of K between the shell of the nanogel and the rest of the particle, depending on the amount of cross-linker used in their synthesis.
Editorial: Nonequilibrium multiphase and reactive flows in porous and granular materials
(Frontiers Media, 2023-12-01) Holtzman, Ran; Sandnes, Bjornar; Moura, Marcel; Icardi, Matteo; Planet Latorre, Ramon
Porous systems that involve the flow of multiple fluids, particles, or solutes, capable of undergoing reactions with each other or with the solid porous matrix, often exist in an out-of-equilibrium state. These systems are driven away from equilibrium by various underlying mechanisms. These mechanisms include interfacial instabilities caused by capillary or viscous forces, as well as physical alteration of the pore space through mechanical or chemical processes like fracturing, compaction, precipitation, and dissolution. An inherent feature of many porous and granular systems is their multiscale heterogeneity. An extreme example is in geosciences, where heterogeneity and mechanisms at the microscopic scales (e.g., in nanometer-sized pores) could strongly affect the behavior at the field scale (km-sized reservoirs). The multiscale, nonequilibrium nature of these systems is manifested by the emergence of complex, preferential flow patterns and dependencies on the path (hysteresis) and rate of external driving forces. Modeling, understanding, predicting, and even controlling the evolution of the flow and deformation in these systems is a substantial scientific challenge across disciplines including engineering, physics, geosciences and mathematics and plays a crucial role in multiple practical applications.
Reservoir computing in simulated neuronal cultures: Effectof network structure
(American Institute of Physics (AIP), 2026-02-17) Mats Houben, Akke; Haeb, Anna-Christina; García Ojalvo, Jordi; Soriano i Fradera, Jordi
Biological neurons are emerging as attractive candidates for artificial intelligence and machine learning applications given their natural energy efficiency and self-repair capacity. However, they differ from their idealized artificial counterparts. Biological neurons have highly variable and noisy dynamics and display intrinsic spontaneous activity instead of purely input-driven dynamics. Moreover, biological neuronal networks have physically constrained and highly plastic connections, leading to a complex and ever evolving connectivity structure. Here, we investigate (numerically and with preliminary experimental data) the stability of the input responses of neuronal cultures using a reservoir computing framework. Utilizing a numerical model for the growth and activity of neuronal cultures, previously used to model experimental data, we investigate the effect of large-scale network topology, specifically homogeneous vs modular architectures, on fading memory, reservoir performance under increasingly noisy dynamics, and robustness to network rewiring. We find that modular networks exhibit longer fading memory time, sustain higher performance under noisy conditions, and are more robust to connectivity rewiring than homogeneous networks. Finally, we observe no relationship between some characteristics of the network adjacency matrix (specifically its spectral properties) and reservoir computing performance.
Unravel the rotational and translational behavior of a single squirmer in flexible polymer solutionsat diﬀerent Reynolds numbers
(Springer Nature, 2025-12-01) Qi, Kai; Zhou, H.Y.; Corato, Marco de; Stratford, Kevin; Pagonabarraga Mora, Ignacio
Microorganisms such as bacteria and algae navigate complex fluids, where their dynamics are vital for medical and industrial applications. However, the influence of the Reynolds number (Re) on the transport and rotational behavior of microswimmers in viscoelastic media remains poorly understood. Here, we investigate these effects for a model squirmer in flexible polymer solutions across a range of Re using Lattice Boltzmann simulations. The interaction between swimmer activity and polymer heterogeneity strongly affects behavior, with rotational enhancement up to 1400-fold and reduced self-propulsion and diffusivity for squirmers. These effects result from hydrodynamic and mechanical interactions: polymers wrap ahead of pushers and accumulate behind pullers, enhancing rotation while hindering translation through forces and torques from direct contacts or asymmetric flows. The influence of Re and squirmer-polymer boundary conditions (no-slip vs. repulsive) is also examined. Notably, no-slip conditions intensify effects above a critical Reynolds number (). Below this value, stronger viscous drag minimizes differences. Our findings emphasize the crucial role of polymer-swimmer interactions in shaping microswimmer behavior in viscoelastic media, informing microrobotic design in complex environments.

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