Posters.docx

Ahmed Elhady (Max Planck Institute of Animal Behavior / University of Konstanz): Mechanistic theory of social foraging

Animals usually forage in groups in order to exploit resources. Using patch foraging as a quintessential foraging behavior, we develop an analytical framework to derive decision strategies underlying social foraging behavior and propose mechanisms by which groups of agents optimize their survival under different environmental conditions. This conceptual framework provides an analytical tractability allowing us to relate experimental behavioral data to underlying cognitive mechanisms.

Ana Novaes Dias (UFMG): Mean Field Theory for a Vicseklike Model on a Lattice

In this work, we study a model of active matter where particles on a triangular lattice can have only three velocity directions. Using a mean-field analysis, we derived motion equations for particle density in each direction, considering an alignment rule and a noise term. Aligned particles move through the lattice via a density transfer process constrained by excluded volume. The evolution of the system depends on its initial configuration, density, and noise intensity. Our focus is on understanding the conditions necessary to maintain and/or form ordered states. The results show that the model exhibits spatial patterns characteristic of active matter in discrete spaces, such as bands and traffic jams, which disappear in the presence of high noise, leading to a disordered state. The order-disorder phase transition can be either continuous or discontinuous for the same initial configuration, depending on the density.

Andrea Soledad Gotting (Physics Department, Institute of Sciences, National University of General Sarmiento, Argentina): Study of bacterial aggregation of Pseudomona Extremaustralis 2E-UNGS for bioremediation processes.

The process of bacterial aggregation that remains in suspension has already been described as a third microbial lifestyle, together with planktonic growth and the creation of biofilms. Pseudomonas extremaustralis 2E-UNGS is a microorganism native to the polluted Reconquista River in Argentina. This strain is able to aggregate, develop biofilms and biosorb metals, secreting biosurfactants and exopolymeric substances, properties that contribute to its application in the design of sustainable environmental biotechnologies, such as bioreactors. The aim of this work was to study the self-aggregation kinetics of P. extremaustralis 2E-UNGS using a combination of theoretical modelling and experimental approaches for its potential application in the design of improved biotreatments, especially of galvanic effluents. Kinetics was studied by means of brightfield microscopy imaging and the use of FIJI® software for subsequent digital analysis. A dispersion of the size and morphological characteristics of the cell aggregates as a function of time was obtained. In addition, a physical model based on self-assembled 2D micelle dispersions was used to describe the energetic cost of the aggregation process. The contrasted of numerical and experimental results shed light on some of the mechanisms underlying the bacterial aggregation phenomenon, useful for biotechnological applications.

Antonio Romaguera (Universidade Federal Rural de Pernambuco / Departamento de Física): Multifractal fluctuations in zebrafish (Danio rerio) polarization time series

In this work, we study the polarization time series obtained from experimental observation of a group of zebrafish (Danio rerio) confined in a circular tank. The complex dynamics of the individual tra- jectory evolution lead to the appearance of multiple characteristic scales. Employing the Multifractal Detrended Fluctuation Analysis (MF-DFA), we found distinct behaviors according to the parameters used. The polarization time series are multifractal at low fish densities and their average scales with ρ−1/4. On the other hand, they tend to be monofractal, and their average scales with ρ−1/2 for high fish densities. These two regimes overlap at critical density ρc, suggesting the existence of a phase transition separating them. We also observed that for low densities, the polarization velocity shows a non-Gaussian behavior with heavy tails associated with long-range correlation and becomes Gaussian for high densities, presenting an uncorrelated regime.

Bernardo Boatini (Institudo de Fisica da UFRGS): Wetting of Active Droplets

The wetting phenomenon is extensively explored in equilibrium physics, with applications spanning various natural and technological contexts. On the other hand, the field of active matter is a branch of physics that deals with systems composed of active particles, agents capable of extracting energy from the environment and converting it into persistent motion in a specific direction. Although the convergence of these research areas may seem unconventional, recent experimental works demonstrate the utility of this approach in elucidating the behavior of self-propelled droplets on substrates, and specify the meaning of what we called active wetting. This research aims at developing a computational model of a drop filled with self propelled agents, to bring together relevant insights towards both areas: wetting and active matter. To deal with the wetting properties, the selected model builds upon a well-established Potts model featuring three states: water, air, and a hydrophobic substrate. Additionally, two approaches to introduce activity to the droplet are proposed: an effective stochastic field acting on the droplet’s center of mass and an agent-based model simulating particles swimming within the droplet. To validate the computational simulations against real-world experiments, we propose an experimental setup employing droplets filled with micro swimmers (Paramecium Caudatum) under varying conditions to investigate the influence of activity on the wetting behavior over a super hydrophobic surface. On the computational front, we examine how the theoretical equilibrium wetting states of a droplet on a micro pillared super hydrophobic surface, changes across a spectrum of activity intensities. Our results show that the activity can drive the system to the dryer state, exploring the meta-stability toward the direction of the free energy minimum, which allows droplet movement with less attachment to the surface.

Constanza Rivas (Universidad de Concepción): Modeling early biofilm formation through multigenerational surface-sensing transmission on active particles

We study the formation of reversible and irreversible phases in the early stages of biofilm formation through multigenerational surface-sensing transmission. For this purpose, we present a quasi-1D lattice model in which bacteria are represented by active particles. We use a sensing parameter and a motility parameter to simulate bacteria swarming and swimming. The sensing parameter is the probability of staying on the surface, which increases as the particle spends more time in contact with the surface. The motility parameter is the probability of moving, which decreases as the particle spends more time on the surface. When a particle divides, it inherits half of the sensing its parent previously had. As time passes, evolutionary dynamics give birth to new generations of particles. Depending on the number of particles with high sensing, we can identify the formation of two separate phases: a reversible phase, when the biofilm is early formed but can disassemble, and the irreversible phase, when the biofilm reaches the conditions to initiate the maturation stage.

Davi Lazzari (Instituto de Física – UFRGS): Tuning collective actuation of active solids by optimizing activity localization

Active solids, more specifically elastic lattices embedded with polar active units, exhibit collective actuation when the elasto-active feedback, generically present in such systems, exceeds some critical value. The dynamics then condensates on a small fraction of the vibrational modes, the selection of which obeys non trivial rules rooted in the nonlinear part of the dynamics. So far the complexity of the selection mechanism has limited the design of specific actuation. Here we investigate numerically how, localizing the activity on a fraction of modes, one can select non-trivial collective actuation. We perform numerical simulations of an agent based model on triangular and disordered lattices and vary the concentration and the localization of the active agents on the lattices nodes. Both contribute to the distribution of the elastic energy across the modes. We then introduce an algorithm, which, for a given fraction of active nodes, evolves the localization of the activity in such a way that the energy distribution on a few targeted modes is maximized -- or minimized. We illustrate on a specific targeted actuation, how the algorithm performs as compared to manually chosen localization of the activity. While, in the case of the ordered lattice, a well educated guess performs better than the algorithm, the latter outperform the manual trials in the case of the disordered lattice. Finally, the analysis of the results in the case of the ordered lattice leads us to introduce a design principle based on a measure of the susceptibility of the modes to be activated along certain activation paths.

Emanuel Fortes Teixeira (Instituto de Física da Universidade Federal do Rio grande do Sul (UFRGS)): Segregation in binary mixture with differential contraction among active rings

Cells of different types spontaneously separate, forming distinct tissues, a process known as cell segregation. Understanding the mechanisms underlying cell segregation is crucial for understanding the formation and function of biological tissues. Concepts explaining such processes, such as Differential Adhesion and Velocity Difference, have been simulated based on models of point-like active particles. However, hypotheses that take into account contractions of the cell membrane, such as Differential Surface Contraction, cannot be explored using these models. To verify the Surface Contraction hypotheses for cell segregation proposed by Harris in 1976, we introduced an extended cell model capable of incorporating many features of other models while maintaining simplicity and physical appeal. A ring formed by active particles connected by springs and subjected to an area conservation potential represents an individual cell. We identified spontaneous mixed ordered states (checkerboard pattern) and segregation of a binary mixture of rings when using a differential contraction term. From this, we showed that the segregation parameter quantifying this process exhibits a monotonous relationship with this contraction term, thus demonstrating Harris's Surface Contraction hypothesis. Finally, we observed that the process occurring in segregation is cluster coalescence. However, we found that the decay exponent of the segregation parameter and the growth of cluster size is consistent with $\lambda \sim 1/3$ which differs from the prediction of previous theoretical approaches that predict $\lambda \sim 1/4$.

Fernando Francisco Silva Filho (IF USP): Stochastic Thermodynamics and collective effects on 4-state molecular motors

In the last 30 decades, stochastic thermodynamics has been one of current subjects in non-equilibrium statistical mechanics (and contemporary physics in general). One important topic on this subject is the study of nanoengines and its performance in artificial and natural (biological) frameworks, dealing with the inescapable random fluctuation due to small size of such setups in a thermal environment. In this poster, we aimed to study molecular motors: transport proteins that are capable to transform chemical energy (in general from hydrolysis reaction) into mechanical work to walk against an external load. Such small scale engines are responsible for several functions that are vital for life itself, as hormone transportation, signal delivery along axons of neurons and cellular reproduction. In general, these proteins are diffusive in colloidal means with several obstructions and interaction with other motors. Besides the thermodynamics behind molecular motors are well known, the effects of these collective interaction and the presence of obstruction on the performance of this small engine is not well known. In this work, we modelled such complexes interaction considering a set of N 4-state molecular motor in a filament. Starting from one single engine using a Markov jump process, we introduce the volume exclusion, which permits to study the effect of a traffic jam. We also introduce the intra-molecular chemical interaction that is responsible to create clusters and chemical bounds. Our goal is to compute the thermodynamics quantities (such as entropy production, work output and efficiency) under such a complex environment, studying the effect of the collective effects and the volume exclusion on the general performance, finding paths and strategies to improve it.

Fidel Álvarez (Pontificia Universidad Católica de Chile): Growth Dynamics in a Mechanical Model of Cellular Colonies

Cellular colonies are structures of microorganisms that remain attached to each other and/or to a surface. To study the effects of mechanical stress on the dynamics of a growing colony, a minimal discrete physical model of these cellu- lar systems is proposed considering only microscopic quantities from mechanical forces, and a cell growth and division process. Using simulations that model the discrete model evolution, macroscopic dynamics of contact and growth within non-motile circular-shaped cell colonies are successfully reproducible. To find a link between the microscopic quantities involved in the dynamics and the macro- scopic observables, an out-of-equilibrium continuum theory is developed. The observed dynamics in the discrete model are accurately described by the contin- uum theory at the mesoscopic limit, describing along the colony the existence of maximum inner pressure and velocity as a function of microscopic quantities. Particularly, a constitutive relation between velocity and inter-particle overlap is found, describing that the growth dynamics of a colony are equivalent in two spatial configurations: a free and a channel-limited expansion. Finally, evidence of a regime of stability has been found on the expansion front of the collective. As a second part of this work, given the dynamics of the system, a competitive genetic surfing dynamic is studied considering two different cell strains in the channel-limited configuration. The observed genetic surf shows a frequency distribution of dominance between strains that transits from an exponential law with exponent 3/2 to a log-normal distribution depending on the initial strain relative proportion and the channel width, suggesting that this system’s compe- titive dynamics can be described by mean-field theories that describe growth processes.

FRANCISCO CAROL BONFIM LEAL (Universidade Federal Rural do Pernambuco): Avalanche dynamics of zebrafish schools: unveiling self-organization and phase transitions

Collective behavior in animal groups exhibits intriguing dynamics that can be analyzed through the lens of self-organized criticality. In this context, we analyze behavioral cascades in zebrafish (Danio rerio) groups of varying sizes within controlled tank environments. Through experimental observations and data analysis, we unveil scale-free signatures reminiscent of self-organized critical processes in the collective movement of zebrafish. Notably, as fish density varies, we observe a dynamic phase transition: at low densities, coordinated and highly polarized movement dominates, while at high densities, the group fractures into uncorrelated domains. These findings shed light on the complex dynamics of collective behavior in fish groups and provide valuable insights into the responses of individuals to environmental stimuli. Moreover, the observed phase transition highlights the sensitivity of zebrafish behavior to changes in population density, which has implications for understanding collective behavior in various contexts, from ecological systems to preclinical studies. Finally, we compare our findings with the known results of avalanche analyses of collective motion and neuronal activity. All follow the same power law, indicating a possible universality in one parameter of avalanche processes.

Guillermo Fadic(Universidad de Chile): Phase diagram of bioconvection patterns formed by magnetotactic bacteria

Systems composed of self-propelled individuals present a striking property seen all around in nature: the ability to generate collective motion, transforming the single agents into a group with its own attributes and behaviors. Magnetotactic bacteria constitute a convenient setup to study these systems given the ease of aligning their swimming direction with an external magnetic field. We work with dense suspensions of the strain Magnetospirillum gryphiswaldense, which we confine between two plane walls and expose to a homogeneous magnetic field that directs the bacteria towards the walls. In this configuration, clusters are generated on the walls in an orderly manner due to hydrodynamic interactions, forming patterns. A recirculating flow is produced by the bacteria that results in bioconvection. In this work we characterize these patterns by measuring a characteristic wavelength and estimating a critical value of the applied magnetic field intensity which mandates the transition from a uniform cover to a stationary pattern. We found that there is a strong dependency between these parameters and the density of the bacterial suspension.

Ignacio Bordeu (Universidad de Chile): Modelling branching morphogenesis

The formation and development of organs is a multi-scale process that involves physical, cellular and chemical signals. The formation of branched organs, such as the lungs or vasculature, seems to be driven by simple growth rules that can be inferred from the spatial and topological organisation of the developing ramified structure. Here we show some universal properties and tissue-specific rules that govern a variety of mammalian organs, from mouse salivary gland, to liver organoids and to human lungs.

Ignacio Peña Orellana (Pontificia Universidad Catolica de Valparaiso.): Characterization and control of the motility dynamics of MG1655 Escherichia Coli

Characterization and control of the motility dynamics of MG1655 Escherichia Coli The bacterium Escherichia coli (E. coli) is arguably the most extensively studied microorganism, known for its characteristic “run-and-tumble” (RT) motion. E. coli propels by rotating their helical flagella, each driven by a rotary motor that indices rotation by a proton motive force across the cytoplasmic membrane driven by a transmembrane channel formed by the proteins motA and motB 1. Deleting the motA and/or motB genes results in non-functional flagella. The motility of E. coli near surfaces plays a crucial role in the early stages of biofilm formation and pathogenic infection. It is well-established that E. coli's run-and-tumble (RT) patterns differ near surfaces compared to bulk liquid, where bacteria can exhibit circular motion due to hydrodynamic interactions 2, potentially trapping them until a tumbling event allows escape. These dynamics depend on various parameters, including bacterial morphology and flagellar activity 3. In this study, we aim to investigate the motility patterns of E. coli on surfaces, focusing on the smooth transition from non-motile to motile states, by utilizing a self-engineered non-motile MG1655 mutant with deletions in motAB, modified to express motAB under the control of an arabinose-inducible pBAD promoter. This modification allows us to restore motility by adjusting the arabinose concentration in the culture medium. We will present a detailed statistical analysis of various motility dynamics by tracking individual bacterial movements in a diluted suspension within a non-confined 2D environment 4. These tracks were obtained using fluorescence inverted microscopy. Our findings provide valuable insights, revealing for the first time the transitional effects of transmembrane channel dynamics on E. coli surface behavior. References 1. Hosking, E. R., Vogt, C., Bakker, E. P. & Manson, M. D. The Escherichia coli MotAB Proton Channel Unplugged. J. Mol. Biol. 364, 921–937 (2006). 2. Lauga, E., DiLuzio, W. R., Whitesides, G. M. & Stone, H. A. Swimming in Circles: Motion of Bacteria near Solid Boundaries. Biophys. J. 3. Benyoussef, W., Deforet, M., Monmeyran, A. & Henry, N. Flagellar Motility During E. coli Biofilm Formation Provides a Competitive Disadvantage Which Recedes in the Presence of Co-Colonizers. Front. Cell. Infect. Microbiol. 12, 896898 (2022). 4. Taute, K. M. High-throughput 3D tracking of bacteria on a standard phase contrast microscope. Nat. Commun. (2015).

Italo Ignacio Salas (Universidad de Chile): Emerging double power-law through dynamical complex networks in motility-induced phase separation in Active Brownian Particles

We investigate the behavior of active Brownian particles (ABP) within a temporal complex network framework approach. We focused on the node degree distribution, average path length, and average clustering coefficient across the P\'eclet number and packing fraction region. In the single phase or gas region, particle interactions mirror a random graph, and the average number of unique interactions display a non-monotonic behavior with the packing fraction. As we ventured toward the Motility-Phase Separation (MIPS) frontier, a hybrid distribution with a time-evolving pattern emerged, combining Gaussian and power law components. Moreover, we discovered a double power-law distribution in the phase-separated region, with two characteristic slopes symbolizing the emerging gas and solid regions. Our approach involved various numerical and theoretical analyses to capture the role of the packing fraction and Péclet number in the topological properties of the dynamical complex network that arises during the ABP dynamics. These findings provide a robust understanding of a phase transition between two states and how this transition manifests as a random to scale-free behavior.

Javiera Navarro Pérez(Pontificia Universidad Catolica de Valparaiso): Characterization of MG1655 E. coli bacterial chemotaxis navigation in a complex media XX Javiera Navarro, Viviana Clavería* Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile *viviana.claveria@pucv.cl Understanding the fundamental principles of bacterial navigation is essential for grasping critical phenomena, such as biofilm formation, soil transport, or the development of bacteriobots for disease treatment. For swimming bacteria, navigating efficiently through complex environments such as porous media is a significant challenge[1-3]. In this study, we explored the intricate relationship between the micro-scale navigation of bacteria and their large-scale movement in heterogeneous environments. We conducted experiments using the E. coli MG1655 mutant with deletions in motAB, which has been genetically modified to express motAB under the control of an arabinose-inducible pBAD promoter. Specifically, we investigated how a disordered environment influences the migration patterns of these bacteria from non-motile to motile colonies with step-increased motility activation that depends on the arabinose concentration of their cultured media, resulting in distinct motility characteristics. We used common agar plates in reduced media, media enriched by nutrients, with and without chemoattractant. Our findings demonstrate how the stages at different motilities of E. coli MG1655 exhibit different chemotactic behavior where the free path length is significantly restricted. References: [1]Dehkharghani, A., Waisbord, N., & Guasto, J. S. (2023). Self-transport of swimming bacteria is impaired by porous microstructure. Communications Physics, 6(18). https://doi.org/10.1038/s42005-023-01136-w [2]de Anna, P., Pahlavan, A. A., Yawata, Y., et al. (2021). Chemotaxis under flow disorder shapes microbial dispersion in porous media. Nature Physics, 17(1), 68–73. https://doi.org/10.1038/s41567-020-1002-x [3]Bhattacharjee, T., & Datta, S. S. (2019). Bacterial hopping and trapping in porous media. Nature Communications, 10, 2075. https://doi.org/10.1038/s41467-019-10115-1

Joaquin Morales (Universidad de Chile): Collective fractional Brownian motion driven by spatiotemporal light gradients

In this research, a stochastic microscopic model is proposed to describe the collective Brownian motion driven by spatiotemporal gradients of light performed by a genus of zooplankton called Daphnia. The model was constructed considering an elongated water column and N self-propelled particles governed by a set of stochastic Langevin differential equations considering factors such as phototaxis and gyrotaxis. The constructed agent-based model contrasts the local and global conditions of microswimmers, so it aims to answer the following question: what is the relation between the swimming persistence, given by the Péclet number, and the intensity of the light forcing, to achieve this behavior? A sweep of these parameters shows that it is possible to distinguish three regimes: one in which the particles remain at their equilibrium level (isolume) without performing DVM, a second regime in which the particles perform the migration successfully, and a third one in which the light forcing is not sufficient to return the swimmers to the isolume causing them to descend indefinitely in the water column. Finally, the impact on these three regimes is explored by considering subdiffusion and superdiffusion in the system due to the introduction of fractional Brownian motion.

Jorge Mario Escobar Agudelo (UNESP-IFT): Correlated Disorder and Viscoelasticity of Soft Colloidal Gels

In our work we seek to develop a complete quantitative theory for the universal mechanical properties of disordered elastic systems near the onset of rigidity, focusing on the role played by correlated disorder in elastic systems with relevance to soft colloidal gels. The most important expected result is a new effective medium theory that incorporates correlations between random links of dilute networks, which could be extended to active matter systems in which the dynamics of objects in a network in which the links are removed as they pass through it can be studied.

Kelly Aparecida Molica (Universidade Federal de Viçosa): Cell self-organization in culture: from chance to motility cell-induced gradients

Cell aggregation involves the processes of movement, shedding, adhesion, and collisions essential for tissue regeneration, morphogenesis, and cancer progression. In this sense, our work proposes a hybrid agent-based model for the cell aggregation observed in monolayer culture. Thus, we consider the simplest candidate for this system to be CCA (Cluster Cluster Aggregation) and include the actions of death, replication, and shedding. The effects of each action in the CCA model were investigated, focusing on the pattern of structures generated in the simulation and the evolution of aggregates over time. The results show that in the original model, the shapes of the aggregates are branched and the size distribution functions (number of aggregates of size s at time t) exhibit power law behavior for some regimes. The addition of replication in the original model changes the pattern of the aggregate structures, they quickly become larger and have more compact shapes. In turn, death in the CCA model changes the behavior of the scaling law to an exponential decay, which indicates the presence of a characteristic size. The shedding of particles also changes the pattern of the structures: the aggregates are immersed in a dust of particles that have detached and the distribution function changes their behavior. Furthermore, we consider all actions simultaneously in the CCA model and change the brownian motion of the aggregates to a motion guided by chemical signals (chemotaxis) via the diffusion equation. In this case, the aggregates form more quickly and are similar in size. The size distribution function presents a regime that scales with the gaussian function. Finally, we consider that particles can transform. Once transformed, they can detach from the aggregate and move against the signal gradient. The distribution function presents a regime that is a power law combination with exponential.

Leonardo Leiva (Pontificia Universidad Católica): Yielding Transition in Active Systems

Active matter is a growing area of recent interest due to the wide range of fascinating systems and phenomena that can be studied. In particular, high-density active systems have yet to be studied intensely despite having many questions and no answers. This work is focused on a dense active system to understand its dynamic controlled by avalanches, unifying quasistatic and dynamic regimes. Previous works found significant differences between passive and active systems in the study of amorphous materials at close-to-yielding transition. This discrepancy may arise from the dynamic. We present the Controlled Relaxation Time Model (CRTM) to solve this. It allows the system to relax until a fixed time. Using CRTM, we modeled an active system composed of soft disks. We characterized the dynamic through avalanche size and its correlation length to establish a scale relation.

Luiz Menon Junior (Pontifícia Universidade Católica do Rio de Janeiro): Random search with stochastic resetting in heterogeneous environments

We investigate random searches under stochastic resetting at a rate r within a one-dimensional heterogeneous bounded domain,described by a position-dependent diffusion coefficient D(x). Our analysis takes into account all interpretations of stochastic integration. We characterized the efficiency of the search by the mean first passage time (MFPT) to reach a target. Our goal is to identify regions in the parameter space where resetting is beneficial, with a particular focus on determining the optimal resetting rate r that minimizes the MFPT .

Javiera Catalina Navarro Perez (Pontificia Universidad Catolica de Valparaiso): Characterization of MG1655 E. coli bacterial chemotaxis navigation in a complex media

Javiera Navarro, Viviana Clavería* Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile *viviana.claveria@pucv.cl Understanding the fundamental principles of bacterial navigation is essential for grasping critical phenomena, such as biofilm formation, soil transport, or the development of bacteriobots for disease treatment. For swimming bacteria, navigating efficiently through complex environments such as porous media is a significant challenge[1-3]. In this study, we explored the intricate relationship between the micro-scale navigation of bacteria and their large-scale movement in heterogeneous environments. We conducted experiments using the E. coli MG1655 mutant with deletions in motAB, which has been genetically modified to express motAB under the control of an arabinose-inducible pBAD promoter. Specifically, we investigated how a disordered environment influences the migration patterns of these bacteria from non-motile to motile colonies with step-increased motility activation that depends on the arabinose concentration of their cultured media, resulting in distinct motility characteristics. We used common agar plates in reduced media, media enriched by nutrients, with and without chemoattractant. Our findings demonstrate how the stages at different motilities of E. coli MG1655 exhibit different chemotactic behavior where the free path length is significantly restricted. References: [1]Dehkharghani, A., Waisbord, N., & Guasto, J. S. (2023). Self-transport of swimming bacteria is impaired by porous microstructure. Communications Physics, 6(18). https://doi.org/10.1038/s42005-023-01136-w [2]de Anna, P., Pahlavan, A. A., Yawata, Y., et al. (2021). Chemotaxis under flow disorder shapes microbial dispersion in porous media. Nature Physics, 17(1), 68–73. https://doi.org/10.1038/s41567-020-1002-x [3]Bhattacharjee, T., & Datta, S. S. (2019). Bacterial hopping and trapping in porous media. Nature Communications, 10, 2075. https://doi.org/10.1038/s41467-019-10115-1

Marcos Pasa (Instituto de Física – UFRGS): Stokes Flow With Active Rings

This study presents the development of a computational model for simulating multiple cells represented by rings composed of various active particles, within a Stokes flow geometry. The research focuses on out-of-equilibrium systems that exhibit remarkable collective behaviors due to the intrinsic activity of the particles. The proposed model extends previous work that addressed the dynamics of a single active ring, enabling a quantitative analysis of the tissue formed by multiple rings. Critical issues related to the numerical stability of the model are addressed, enhancing its robustness and allowing for simulations involving tens of thousands of particles. Statistical tools are employed to analyze the generated data, characterizing the patterns of the resulting tissue. Additionally, as a byproduct of this research, an open-source library has been developed to facilitate the construction of generic computational models.

Marina Palacio Fornero (Instituto de Física Enrique Gaviola - Facultad de Matematica, Astronomia, Física y Computación - Universidad Nacional de Córdoba): Human sperm dynamics under confinement

Current methods used in the fertilization techniques are expensive and can be improved by being miniaturized. Our team has contributed to the development of microdevices specialized in human reproduction[1,2]. To improve the Assisted Reproductive Technology (ART), we propose incorporating the use of microfluidic devices to enhance the sperm quality used. These devices consist of geometries that ultraconfine the microswimmers. The dynamics inside such microdevices differ from the one observed far from surfaces because wall interactions dominate[2]. This phenomenon can induce a sperm transport with different properties to be studied carefully, which is the aim of my thesis and preliminary results will be presented in this School. [2] Geometrical guidance and trapping transition of human sperm cells, A. Guidobaldi, Y. Jeyaram, I. Berdakin, V. V. Moshchalkov, C. A. Condat, V. I. Marconi, L. Giojalas, and A. V. Silhanek Phys. Rev. E 89, 032720 (2014). Doi: 10.1103/PhysRevE.89.032720 [2] Hitting the wall: Human sperm velocity recovery under ultra-confined conditions, M. A. Bettera Marcat, M. N. Gallea, G. L. Miño, M. A. Cubilla, A. J. Banchio, L. C. Giojalas, V. I. Marconi, and H. A. Guidobaldi, Biomicrofluidics 14, 024108 (2020). Doi: 10.1063/1.5143194. Sci.Light 2020. Doi: 10.1063/10.0001078

Matheus Gomes (UFRPE): Exploring Dynamics of BEAM Robots in Complex Potentials Using 3D Printed Models

Self-propelled particles are pivotal in understanding complex behaviors in biological systems, active colloids, vibrational grains, and autonomous robotic devices. This work introduces a comprehensive experimental methodology for fabricating 3D printed parts to study the dynamics of BEAM robots. Utilizing freely accessible methods and open-source programs, we explore the dynamics of BEAM robots within parabolic potentials, both isotropic and anisotropic, as well as other complex potentials. Our experimental results reveal significant behavioral changes contingent on the control parameters associated with the anisotropy of the potential geometry. Numerically, it is possible to change a phenomenological parameter continuously, even infinitesimally. However, experimentally, this presents a challenge. Adjusting a control parameter in a 3D printed trapping potential is complex and requires precise manipulation. Despite these challenges, our approach demonstrates the feasibility of studying such dynamic systems experimentally, providing valuable insights and advancing our understanding of self-propelled particle dynamics. These findings have significant implications for future research in related fields.

Mayron González (Universidad de Chile): Dynamics of Microswimmers Under Quorum Sensing

Active Brownian Particles (ABP) models provide a valuable framework for un- derstanding the dynamics of different systems in active matter. In this work, we investigate the dynamics of ABPs that interact through a ”cognitive-based interaction.” This interaction is modeled as a vision cone, where each agent possesses a cone that allows them to align with neighbors at a given distance and within the cone, as described in [1]. Additionally, the dynamics of the system are constrained by circular confinement. To study the behavior of this system, we performed simulations varying the active velocity, opening angle, and packing fraction to compare differences in mean vorticity, mean velocity, and radius of gyration. This system exhibits three distinct regimes related to the opening of the vision cone. In the presence of confinement, we observed differences in the system’s behavior compared to [1]. The first regime is characterized by a dilute phase at low angles close to θ = π/10 , the second regime at θ = π/4, presents a series of ABP ”trains” that form clusters which are not persistent over time. The third regime,at θ = π/2, is characterized by large clusters that are persistent over time. [1] Negi, R. S., Winkler, R. G., & Gompper, G. (2022). Emergent collective behavior of active Brownian particles with visual perception. Soft Matter, 18(33), 6167–6178. https://doi.org/10.1039/d2sm00736c.

Michael Torres Ramirez (Universidade Federal do Ceará): Plateau-Rayleigh-like instability in interacting binary mixtures of active particles

We investigate a novel non-equilibrium analogue of the Plateau-Rayleigh instability using a two-dimensional binary mixture of interacting particles with different diffusivities. Our computational simulations reveal an instability phenomenon characterized by the activity differences of the active particles. This results in rupture dynamics that cannot be described by the conventional surface tension approach. Unlike traditional systems where surface tension governs stability, our findings suggest that the activity differences between the particles introduce a new form of instability, offering insights into the behavior of active matter systems.

Nathan de Oliveira Silvano (Universidade do Estado do Rio de Janeiro): Emergent Gauge Symmetry in Active Brownian Particles

We investigate a two-dimensional system of interacting Active Brownian Particles. Using the Martin-Siggia-Rose-Janssen-de Dominicis formalism, we built up the generating functional for correlation functions. We study in detail the hydrodynamic regime with a constant density stationary state. Our findings reveal that, within a small density fluctuations regime, an emergent U(1) gauge symmetry arises, originated from the conservation of fluid vorticity. Consequently, the interaction between the orientational order parameter and density fluctuations can be cast into a gauge theory, where the concept of ``electric charge density" aligns with the local vorticity of the original fluid. We study in detail the case of a microscopic local two-body interaction. We show that, upon integrating out the gauge fields, the stationary states of the rotational degrees of freedom satisfy a non-local Frank free energy for a nematic fluid. We give explicit expressions for the splay and bend elastic constants as a function of the Péclet number (Pe) and the diffusion interaction constant (kd).

Nycholas Guedes Rufini (Universidade de São Paulo): Dynamics of the Vallis model for El Niño phenomenon with periodic perturbation

In this study we consider a periodic perturbation in the Vallis model for the El Niño Southern Oscillation (ENSO), a nonlinear dynamical system that describes the event in a simplified manner. The proposed perturbation simulates the variation of the mean effect of equatorial winds, adding to the model a mechanism of energy exchange, thus allowing us to simulate regions of the ocean-atmosphere framework locally unbalancing the energy distribution, similarly to active matter systems. We analyzed the system's dynamics for long intervals of time with focus on parametrical configurations where the original Vallis model presented chaotic behavior, following the literature. Utilizing the Lyapunov Spectrum, we identified periodic structures (typically called shrimps) immersed in chaotic regions in the parameter-space relating quantities of the original model with themselves and with the perturbation amplitude. We show that the inclusion of the perturbation causes originally periodic domains to exhibit chaotic behavior, as well as it leads to the genesis of new periodic structures. Notably, we have also found multistability in the new system, with periodic and chaotic attractors for the same point in the parameter-space.

Opiyo Alphonce Ndolo (Pontificia Universidad Católica de Valparaiso, Chile): Quantitative analysis of Bacterial navigation and diffusion under hemodynamic conditions for bacteriotherapy application

Quantitative analysis of bacterial navigation and diffusion under hemodynamic conditions for bacteriotherapy applications Alphonce Opiyo, Viviana Clavería* Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile *viviana.claveria@pucv.cl Cancer is a major cause of mortality, with almost 20 million new cases and about 10 million deaths in 20221. It is expected that by 2050, there will be a rise in the annual incidence of new cancer cases at 35 million2. These numbers indicate the urgent need for more innovative and efficient alternatives for early detection and treatment. Bacteria has been proposed to be used as therapeutic agents and gene delivery systems3 as a part of bacteriotherapy application. The ability of bacteria to navigate spatially within hemodynamic conditions remains poorly understood despite the progress made in this area. With the aim to fill up the knowledge gap associated with bacterial movement within blood-like situations characterized by stationary or dynamic state, we have focused on quantitative analysis of the swimming behavior of the engineered Escherichia coli strain (MG1655 ∆motAB pBAD-motAB) non-motile mutant ∆motAB with motility restored through arabinose induction, through the bloodstream under in vivo system using microfluidic chips that imitates arterioles and venules in the microcirculatory system. In this work, I will show how a given concentration of arabinose affects motility activation genes and swimming characteristics for bacteria to later, show the diffusive motion of motile and non-motile bacteria into a Y-shaped microchannel when the flow condition was constant, along with a fixed hematocrit value. I will conclude by sharing our perspectives and subsequent phases. References 1. Bray, F. et al. Global cancer statistics 2022: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries. CA. Cancer J. Clin. 74, 229–263 (2024). 2. Kraus, T., Lee, I. & Moniuszko, S. New cancer cases to increase 77% by 2050, WHO estimates - CBS News. HealthWatch https://www.cbsnews.com/news/who-predicts-35-million-cancer-cases-2050-77-percent-increase/ (2024). 3. Huovinen, P. Bacteriotherapy: the time has come, BMJ (Clinical research ed.) DOI: 10.1136/bmj.323.7309.353 (2001).

Oscar Sebastian Garrido (Universidad de Chile): Collective Organization of Magnetotactic bacteria in a Droplet under External Magnetic Field

Magnetotactic bacteria in a droplet in presence of a magnetic field self-assemble into a rotary motor. Gravity plays an important role in this dynamic locking the position of the motor, so using Helmholtz coils we generate a magnetic field in the direction opposite to gravity, countering its contribution and studying the movement of the droplet. Additionally we take a look closer inside the droplet and study the collective movement of the confined bacteria under different circumstances for the main paramenters as concentration, intensity of the magnetic field and size of the droplet.

Pablo Pérez (Universidad de Chile): Coupled dynamics of density and polarization fields of active Brownian particles.

When active Brownian particles are persistent, they tend to form clusters, a phenomenon known as motility-induced phase separation. Simulations of these particles without aligning interactions in a quasi-one-dimensional domain revealed that at cluster interfaces, particles align themselves pointing inwards into the cluster. We propose a set of continuous field equations in the coarse-grained approximation, based on a free energy minimization approach, to describe the dynamics of the density and polarization fields. We study the impact of coupling both fields, deriving the stress tensor and steady-state solutions for the new equations.

Paulo Casagrande Godolphim (Departamento de Fisica - Universidad de Chile): Mechanical Heterogeneity in Fish Embryos

Tissue mechanics is essential for animal development, yet the precise manner in which mechanics determine the fate of embryonic tissue is not fully understood. Our system of interest is the enveloping cell layer (EVL) of an Annual Killifish embryo during the epiboly stage (11-22 hpf), at which the EVL exhibits a wide and heterogeneous apical cell area size distribution. In this study, we employ a technique called micromechanics to infer mechanical fields, explore tissue heterogeneity, and construct quantitative-based models directly from experimental images. Applying micromechanics to a vertex model, we demonstrate that the experimental area distribution can only be reproduced with a tissue exhibiting heterogeneous mechanical properties. The heterogeneity in the model tissue is characterized by larger cells being softer than smaller ones. This correlation allows us to write functional relations between the model parameters and the experimental tissue geometry, which were used to construct new energy functional candidates for the tissue. Simulations suggest that promoting cell intrusion via chemical activation is a viable experimental assay to confirm the heterogeneous properties of embryos. Here, the area strain versus cellular area is the critical observable quantity, where, counterintuitively, a heterogeneous tissue should exhibit a flat response while in a homogeneous tissue, larger cells should deform more. We discuss the potential implications of these heterogeneities on the fate of the tissue and how they could act to reduce stress on the cell layer, potentially serving as an important evolutionary mechanism for the survival of the animal. Additionally, micromechanics can be applied to a wide range of experiments and models, making it a valuable technique for future studies.

Peter Marinko (University of Ljubljana, Ljubljana, Slovenia): Helically confined liquid crystal simulation

The poster presents the numerical modelling of a doubly helical structure in a helical confinement. Recently, such a novel cholesteric structure has been experimentally observed. Helical symmetry reduces the simulation problem to two dimensions. Cholesteric is modelled within the Landau-de Gennes formalism using one-constant approximation. For the shape and parameter values that correspond to the experimental data we succeed in obtaining a meta-stable state with a double twist director structure with the same sign and two localized defects in the two cusps of the twisted structure. In three dimensions that corresponds to two twist disclinations.

Renne Rodrigues Rosinelli Junior (Institute of Physics/ Univerty of Sao Paulo): Computational simulation of microscopic models for dilute biaxial nematic liquid crystals

I work with computational simulation of biaxial nematic liquid crystal. Consider a lattice system with non-spherical objects. Each lattice site can be either empty or occupied. Using Monte Carlo simulations for nearest-neighbor, the goal is to study thermal phase transitions for different asymmetries or biaxiality degree of the objects. We employ Zwanzig aproximation, which restricts the possible orientations of a nematogen to the coordinate axis. I would like to present a poster with some preliminary results. For now, we studied cases of fully occupied sites corresponding to rodlike and bricklike objects shapes.

RUAN VICTOR ALMEIDA QUIRINO (universidade federal rural de pernambuco): Calculation of the Lyapunov exponent of the dynamics of robots in non-integrable domains

In this study, we experimentally investigate the robot-environment interaction. To accomplish this, we developed a robot using Arduino that interacts with a billiard table known as the lemon-shaped billiard. The lemon-shaped billiard was first introduced by Heller and Tomsovic and studied by Ree and Reichl. The shape of the billiard depends on a parameter value. As the parameter varies, the system continuously changes from mixed phase space to fully chaotic phase space. To understand the dynamics generated by the robot-environment interaction, we analyze the robot's sensitivity to initial conditions using the Lyapunov exponent as a quantitative measure of chaos. The Lyapunov exponent is widely used to evaluate the stability of a system. It measures the rate at which nearby points diverge during the robot's dynamic evolution. Analyzing this exponent allows us to determine whether a system is sensitive to initial conditions. If the distance between two distinct initial conditions remains constant (λ = 0), the system is in a state of stability. If the distance between two distinct initial conditions diverges exponentially (λ > 0), the system is considered sensitive, i.e., chaotic. We start by exploring the Lyapunov exponent in the context of the circular billiard. In this case, the dynamics of the robot-environment interaction are insensitive to initial conditions due to constant reflection angles. Thus, the Lyapunov exponent obtained from the experiment is close to zero, indicating the absence of chaos in the dynamics. However, considering future experiments with the contour of the lemon-shaped billiard, we hope to observe the mixed phase space for a set of parameters. Theoretical studies will guide us in choosing this set. Additionally, we highlight the use of Arduino. Arduino offers a programmable microcontroller board in C/C++, enabling the integration of sensors, actuators, and peripherals, making it a valuable tool for computer-controlled interactive projects. These approaches allow us to understand the sensitivity to initial conditions in dynamic systems and explore the applications of Arduino in robotics.

Sebastián de la Maza Rodríguez (Universidad de Chile): Aggregation processes in layered environments

Some microorganisms such as bacteria or microalgae are capable of self-organizing and accumulating on surfaces in quasi-two dimensional layers of great length compared to their own size, thus forming an active carpet. By disturbing the environment in which they are found, these active carpet generate hydrodynamic fluctuations that favor emerging biological functions. On this occasion we study a system composed of an active carpet between two fluids of different viscosities, where we explore both the correlation and the pairwise diffusion (horizontal and vertical) for different values of λ defined as the ratio between the viscosities and H, which is the height of the fluid where the active carpet is located. The above is crucial to be able to explore systems where aggregation processes are possible and the estimated time these processes take.

Vinícius Bayne Müller (UFRGS): Phase transitions of epidemic spreading in complex networks

A great number of relevant problems to science arise from systems of interacting components. Such systems are called networks, and they are used to describe from food webs to social networks. In particular, the use of networks is highly suitable in the study of epidemic spreading, an important topic for public health. In this context, the population and the connections between individuals are mathematically modelled by a random graph, where particular connections are less important than the network structure as a whole. Considering this, we intend to study the dynamics and the phase transitions of an epidemic, relating the system’s behavior with the network properties. We describe the epidemic using the susceptible-infected-susceptible (SIS) model and the network with a directed Erdös-Rènyi graph, from which we obtain the adjacency matrix containing the information about connections. By numerically solving the model’s differential equations and by iteration of the fixed-point equations, we obtain that the critical value of the infection rate for which there exists and endemic state is given by the inverse of the largest eigenvalue of the adjacency matrix, in accordance with the literature. We also show a great correspondence between the state of the system and the eigenvector associated with the largest eigenvalue of the matrix, in agreement with recent results for spectral properties of directed random graphs. To study the system’s stability, we give each interaction a weight drawn from a nonnegative probability distribution. We then use the numerical solutions of the model to create a phase diagram for the stability as a function of network properties (namely, the mean degree and the variance of the couplings). Besides the absence of a transition to time-dependent solutions, we observe an unpredicted stability transition, which data suggest being due to particular aspects of the chosen distribution.

Viviana Clavería (Pontificia Universidad Católica de Valparaíso): Quantitative analysis of Bacterial navigation and diffusion under hemodynamic conditions for bacteriotherapy application

Yago Pontual (Universidade Federal Rural de Pernambuco): Topological arrangement of self-propelled particles in confined potentials

Active matter, also known as self-propelled particles, exhibits intricate behavior when interacting with the environment. The Active Brownian Particles (ABP) model, with suitable modifications, serves as a valuable tool for studying biological and synthetic systems (Marchetti et al., 2013). In this study, we employ the ABP model to simulate the behavior of hexbugs that are confined within a potential. Hexbugs are autonomous, self-propelled devices capable of forward motion, while their orientation is subject to noise. This inherent noise leads to a mismatch between the velocity and orientation of the hexbugs. Motivated by the theoretical work of Rubens et al. (2022), we conduct an experimental study on the behavior of hexbugs within a harmonic potential defined as V (x, y) = x²+ √(1 − ϵ²)y² , where ϵ ∈ [0, 1] represents the controlling parameter for the eccentricity of the potential. By varying the control parameter ϵ, we observed a topological phase transition. As ϵ approaches 0, the shape of the orbits changes from ovals to lemniscates as ϵ approaches 1 (Rubens et al. (2022)) and (D’Cruz et al., 2021).

Rafael Dias Vilela (UFABC): Dynamics and sorting of run-and-tumble particles in fluid flows with transport barriers

We investigate the dynamics of individual run-and-tumble particles in a convective flow which is a prototype of fluid flows with transport barriers. We consider the most prevalent case of swimmers denser than the background fluid. As a result of gravity and the effects of the carrying flow, in the absence of swimming the particles either sediment or remain in a convective cell. When run-and-tumble also takes place, the particles may move to upper convective cells. We derive analytically the probability of uprise. Since that probability in a given fluid flow can vary strongly across species, our findings inspire a purely dynamical mechanism for species extraction in the dilute regime. Numerical simulations support our analytical predictions and demonstrate that a judicious choice of the fluid flow’s parameters can lead to particle sorting with an arbitrary degree of purity. Reference: Rafael Dias Vilela et al 2024 J. Phys. Complex. 5 035003