Cristián Huepe

We study the emerging collective states in a simple mechanical model of a dense group of self-propelled polar disks with off-centered rotation, confined within a circular arena. Each disk presents self-alignment towards the sum of contact forces acting on it, resulting from disk-substrate interactions, while also displaying mutual alignment with neighbors due to having its center of rotation located a distance R R behind its centroid, so that central contact forces can also introduce torques. The effect of both alignment mechanisms produces a variety of collective states that combine high-frequency localized circular oscillations with low-frequency milling around the center of the arena, in fluid or solid regimes. We consider cases with small/large R R values, isotropic/anisotropic disk-substrate damping, smooth/rough arena boundaries, different densities, and multiple systems sizes, showing that the emergent collective states that we identify are robust, generic, and potentially observable in real-world natural or artificial systems.

We study a model of self-propelled particles interacting with their k nearest neighbors through polar alignment. By exploring its phase space as a function of two nondimensional parameters (alignment strength g and Péclet number Pe), we identify two distinct order-disorder transitions. One occurs at a low critical g value independent of Pe, has no significant density-order coupling, and is consistent with the transition previously predicted by the mean-field approach. Up to the system sizes studied, it appears continuous. The other is discontinuous, depends on a combined control parameter involving Pe and g that controls the alignment strength, and results from the formation of small, dense, highly persistent clusters of particles that follow metric-like dynamics. These dense clusters form at a critical value of the combined control parameter Pe / g α , with α ≈ 1.5 , which appears to be valid for different alignment-based models. Our study shows that models of active particles with metric-free interactions can produce characteristic length scales and self-organize into metric-like collective states that undergo metric-like transitions. Published by the American Physical Society 2025

Skepticism and resistance towards vaccines have been reported worldwide in the coronavirus disease 2019 (COVID-19) pandemic. During the restrictions on public gatherings, these attitudes were mostly voiced on social media, providing a vast digital record for examining their motivations. This paper systematizes the antivaccine arguments in Chilean Twitter (now called X) interactions over six months in 2021, in which the country achieved its highest COVID-19 vaccination rates, analyzing 72,441 tweets from 20,293 different accounts. We connect these arguments to recent work in political theory that categorizes the populist criticism of science into three types of objections: a moral, a democratic, and an epistemic objection. We find that all three are clearly identifiable in the data, in somewhat similar proportion: some denounced the vaccination scheme as a conspiracy led by selfish global elites (moral); others complained that the authorities, following scientific advice but with no democratic warrant, were taking away their freedoms (democratic); and yet others pointed to a broad distrust of the scientific procedure in which the vaccine was developed, trusting instead their personal opinions and anecdotal evidence (epistemic). We also characterize the posting and interaction rates of the accounts that use these objections, and whether they switch between them.

We present the linear response theory for an elastic solid composed of active Brownian particles with intrinsic individual chirality, deriving both a normal mode formulation and a continuum elastic formulation.

We present the linear response theory for an elastic solid composed of active Brownian particles with intrinsic individual chirality, deriving both a normal mode formulation and a continuum elastic formulation. Using this theory, we compute analytically the velocity correlations and energy spectra under different conditions, showing an excellent agreement with simulations. We generate the corresponding phase diagram, identifying chiral and achiral disordered regimes (for high chirality or noise levels), as well as chiral and achiral states with mesoscopic-range order (for low chirality and noise). The chiral ordered states display mesoscopic spatial correlations and oscillating time correlations, but no wave propagation. In the high chirality regime, we find a peak in the elastic energy spectrum that leads to a non-monotonic behavior with increasing noise strength that is consistent with the emergence of the `hammering' state recently identified in chiral glasses. Finally, we show numerically that our theory, despite its linear response nature, can be applied beyond the idealized homogeneous solid assumed in our derivations. Indeed, by increasing the level of activity, we show that it remains a good approximation of the system dynamics until just below the melting transition. In addition, we show that there is still an excellent agreement between our analytical results and simulations when we extend our results to heterogeneous solids composed of mixtures of active particles with different intrinsic chirality and noise levels. Our work provides a thorough analytical and numerical description of the emergent states in a densely packed system of chiral self-propelled Brownian disks, thus allowing a detailed understanding of the phases and dynamics identified in a minimal chiral active systems.

We report and characterize the emergence of a noise-induced state of quenched disorder in a generic model describing a dense sheet of active polar disks. In this state, self-propelled disks become jammed with random orientations, only displaying small fluctuations about their mean positions and headings. The quenched disorder phase appears at intermediate noise levels, between moving polar order and standard dynamic disorder. We show that it results from retrograde forces produced by angular fluctuations with Ornstein-Uhlenbeck dynamics, compute its critical noise, and argue that it could emerge in a variety of systems.

We report and characterize the emergence of a noise-induced state of quenched disorder in a generic model describing a dense sheet of active polar disks with non-isotropic rotational and translational dynamics. In this state, randomly oriented self-propelled disks become jammed, only displaying small fluctuations about their mean positions and headings. The quenched disorder phase appears at intermediate noise levels, between the two states that typically define the flocking transition (a standard disordered state that displays continuously changing headings due to rotational diffusion and a polar order state of collective motion). We find that the angular fluctuations in this dense system follow an Ornstein-Uhlenbeck process leading to retrograde forces that oppose self-propulsion, and determine its properties. Using this result, we explain the mechanism behind the emergence of the quenched state and compute analytically its critical noise, showing that it matches our numerical simulations. We argue that this novel type of state could be observed in a broad range of natural and artificial dense active systems with repulsive interactions.

We analyze 6 months of Twitter conversations related to the Chilean Covid-19 vaccination process, in order to understand the online forces that argue for or against it and suggest effective digital communication strategies. Using AI, we classify accounts into four categories that emerge from the data as a result of the type of language used. This classification naturally distinguishes pro- and anti-vaccine activists from moderates that promote or inhibit vaccination in discussions, which also play a key role that should be addressed by public policies. We find that all categories display relatively constant opinions, but that the number of tweeting accounts grows in each category during controversial periods. We also find that accounts disfavoring vaccination tend to appear in the periphery of the interaction network, which is consistent with Chile’s high immunization levels. However, these are more active in addressing those favoring vaccination than vice-versa, revealing a potential communication problem even in a society where the antivaccine movement has no central role. Our results highlight the importance of social network analysis to understand public discussions and suggest online interventions that can help achieve successful immunization campaigns.