arXiv subjects

Lukas Hupe

Publications and source records attributed to Lukas Hupe.

8 recordsLinked to original sources

arXiv2025

Order and shape dependence of mechanical relaxation in proliferating active matter

Collective dynamics in proliferating anisotropic particle systems arise from an interplay between growth, division, and mechanical interactions, often mediated by particle shape. In classical models of prolate, rod-like growth, flow-induced alignment and division geometry reinforce one another, leading to robust nematic order under confinement. Here we introduce a complementary regime by considering smooth convex particles whose geometry can be oblate for part or all of their growth cycle, creating a tunable competition between these two alignment mechanisms. Using agent-based simulations of elliptical and rounded-rectangular particles in both channel and open-domain geometries, we systematically vary the division aspect ratio to span regimes of cooperation and competition between ordering cues. We find that oblate growth can reverse classical flow-alignment, destabilize microdomain formation in intermediate regimes, and open up new regimes with modified microdomain dynamics in free expansion and sustained orientation dynamics in channel geometry. These findings are explained by an order- and shape-dependent mechanical relaxation interpretation that is supported by explicit measurements. This sheds new light on the available relaxation pathways and therefore provides key ingredients for effective descriptions of collective anisotropic proliferation dynamics.

cond-mat.soft↗

arXiv2025

Prediction and control of geometry-induced nematic order in growing multicellular systems

In densely-packed two-dimensional systems of growing cells, such as rod-shaped bacteria, a number of experimental and numerical studies report distinct patterns of nematic orientational order in the presence of confinement. So far, these effects have been explained using variations of growing active nematic continuum theories, which incorporate feedback between growth-induced active stresses, the resulting material flow and nematic orientation, and were adapted to the specific geometry under investigation. Here, we first show that a direct, analytical prediction of orientation patterns based on a simple isotropic-growth assumption and the shear rate tensor of the expansion flow already covers previously observed cases. We use this method to tune orientation patterns and net topological defect charge in a systematic way using domain geometry, confirmed by agent-based simulations. We then show how this framework can be extended to quantitatively capture alignment strength, and explore its potential for cross-prediction across different geometries. Our simplified and unifying theoretical framework highlights the role of domain geometry in shaping nematic order of growing systems, and thereby provides a way to forward-engineer desired orientation patterns.

cond-mat.soft↗

arXiv2025

Phase separation in a mixture of proliferating and motile active matter

Proliferation and motility are ubiquitous drivers of activity in biological systems. Here, we study a dense binary mixture of motile and proliferating particles with exclusively repulsive interactions, where homeostasis in the proliferating subpopulation is maintained by pressure-induced removal. Using computer simulations, we show that phase separation emerges naturally in this system at high density and weak enough self-propulsion. We show that condensation is caused by interactions between motile particles induced by the growing phase, and recapitulate this behavior in an effective model of only motile particles with attractive interactions. Our results establish a new type of phase transition and pave a way to reinterpret the physics of dense cellular populations, such as bacterial colonies or tumors, as systems of mixed active matter.

cond-mat.soft↗

arXiv2024

Isovolumetric dividing active matter

We introduce and theoretically investigate a minimal particle-based model for a new class of active matter where particles exhibit directional, volume-conserving division in confinement while interacting sterically, mimicking cells in early embryogenesis. We find that complex motion, synchronized within division cycles, displays strong collective effects and becomes self-similar in the long-time limit. Introducing the method of normalized retraced trajectories, we show that the transgenerational motion caused by cell division can be mapped to a time-inhomogenous random walk with an exponentially decreasing length scale. Analytical predictions for this stochastic process allow us to extract effective parameters, indicating unusual effects of crowding and absence of jamming. Robustness of our findings against desynchronized divisions, cell size dispersity, and variations in confinement hints at universal behavior. Our results establish an understanding of complex dynamics exhibited by isovolumentric division over long timescales, paving the way for new bioengineering strategies and perspectives on living matter.

cond-mat.soft↗

arXiv2024

A minimal model of smoothly dividing disk-shaped cells

Replication through cell division is one of the most fundamental processes of life and a major driver of dynamics in systems ranging from bacterial colonies to embryogenesis, tissues and tumors. While regulation often plays a role in shaping self-organization, mounting evidence suggests that many biologically relevant behaviors exploit principles based on a limited number of physical ingredients, and particle-based models have become a popular platform to reconstitute and investigate these emergent dynamics. However, incorporating division into such models often leads to aberrant mechanical fluctuations that hamper physically meaningful analysis. Here, we present a minimal model focusing on mechanical consistency during division. Cells are comprised of two nodes, overlapping disks which separate from each other during cell division, resulting in transient dumbbell shapes. Internal degrees of freedom, cell-cell interactions and equations of motion are designed to ensure force continuity at all times, including through division, both for the dividing cell itself as well as interaction partners, while retaining the freedom to define arbitrary anisotropic mobilities. As a benchmark, we also translate an established model of proliferating spherocylinders with similar dynamics into our theoretical framework. Numerical simulations of both models demonstrate force continuity of the new disk cell model and quantify our improvements. We also investigate some basic collective behaviors related to alignment and orientational order and find consistency both between the models and with the literature. A reference implementation of the model is freely available as a package in the Julia programming language based on $\mathit{InPartS}$. Our model is ideally suited for the investigation of mechanical observables such as velocities and stresses, and is easily extensible with additional features.

cond-mat.soft↗

arXiv2024

Motility-induced mixing transition in exponentially growing multicellular spheroids

Growth drives cellular dynamics in dense aggregates including bacterial colonies, developing tissues, and tumors. We investigate the underlying physical principles emerging from the interplay of growth, steric repulsion, and motility in a minimal agent-based model of exponentially growing, three-dimensional spheroids. Our results reveal a motility-induced mixing transition: Without motility, deterministic radial motion from volume expansion dominates, while growth and division cause tangential, cellular-scale diffusion, largely independent of expansion velocity. Despite this small-scale diffusion, cell lineages remain confined to their local environment. This confinement persists at weak motility and is overcome only above a threshold, leading to tangential superdiffusivity and global cell mixing with a diverging timescale near the transition, reminiscent of glassy dynamics. Using a phenomenological model, we identify two effects governing this transition: Steric interactions that suppress motility-induced velocity below a threshold, and the expanding nature of the system which inhibits complete mixing. Our study highlights the complex interaction of local cell division and motility with global expansion, mediated exclusively by mechanics. The results provide a baseline for identifying additional biological mechanisms in experiments, for example in tissue spheroids. The mixing dynamics might also be relevant for competition or tumor progression by interacting with genetic heterogeneity.

cond-mat.soft↗

arXiv2022

Stress anisotropy in confined populations of growing rods

Order and alignment are ubiquitous in growing colonies of rod-shaped bacteria due to the nematic properties of the constituent particles. These effects are the result of the active stresses generated by growth, passive mechanical interactions between cells, and flow-induced effects due to the shape of the confining container. However, how these contributing factors interact to give rise to the observed global alignment patterns remains elusive. Here, we study, in-silico, colonies of growing rod-shaped particles of different aspect ratios confined in channel-like geometries. A spatially resolved analysis of the stress tensor reveals a strong relationship between near-perfect alignment and an inversion of stress anisotropy for particles with large length-to-width ratios. We show that, in quantitative agreement with an asymptotic theory, strong alignment can lead to a decoupling of active and passive stresses parallel and perpendicular to the direction of growth, respectively. We demonstrate the robustness of these effects in a geometry that provides less restrictive confinement and introduces natural perturbations in alignment. Our results illustrate the complexity arising from the inherent coupling between nematic order and active stresses in growing active matter which is modulated by geometric and configurational constraints due to confinement.

cond-mat.soft↗

arXiv2019

Estimating Lyapunov exponents in billiards

Dynamical billiards are paradigmatic examples of chaotic Hamiltonian dynamical systems with widespread applications in physics. We study how well their Lyapunov exponent, characterizing the chaotic dynamics, and its dependence on external parameters can be estimated from phase space volume arguments, with emphasis on billiards with mixed regular and chaotic phase spaces. We show that in the very diverse billiards considered here the leading contribution to the Lyapunov exponent is inversely proportional to the chaotic phase space volume, and subsequently discuss the generality of this relationship. We also extend the well established formalism by Dellago, Posch, and Hoover to calculate the Lyapunov exponents of billiards to include external magnetic fields and provide a software implementation of it.

nlin.CD↗