@article{kammeraat2025correlated, title = {Correlated Cell Movements Drive Epithelial Finger Formation}, author = {Kammeraat, Sander C. and Keta, Yann-Edwin and Appleton, Paul and Newton, Ian P. and Liverpool, Tanniemola B. and Sknepnek, Rastko and N{\"a}thke, Inke and Henkes, Silke}, author+an = {2=highlight}, year = {2025}, month = aug, journal = {arXiv}, arxiv = {2508.01046}, doi = {10.48550/arXiv.2508.01046}, abstract = {Sheets of epithelial cells form protective barriers in multicellular organisms. When damaged, finger-like protrusions form at the advancing edge, closing the damaged area. Due to the resemblance to fluid spreading, existing models typically invoke instability mechanisms to explain the onset of fingers. Combining in vitro experiments on freely expanding MDCK cell monolayers with simulations of the self-propelled Voronoi model and an active viscoelastic theory, we show that instead fingers form spontaneously due to emergent, correlated cell motion within the cell layer and simply represent long-lived active fluctuations of the boundary. Simulations and theory both quantitatively match spatiotemporally correlated cell motion measured in the interior of the monolayer. To capture finger formation, we model the actomyosin cable at the advancing edge as a contractile semi-flexible polymer driven by correlated active noise representing the interior. The model not only exhibits spontaneous finger formation but also quantitatively predicts tangent-tangent and roughness correlation functions of the edge in space and time, as well as fluctuation spectra. Our results, therefore, indicate that correlated cell movements lead to robust finger formation, without the need for any feedback mechanism, suggesting that leader cells, cell-cell signalling, and division modulate an intrinsic process instead of causing it.} } @article{keta2025longrange, title = {Long-Range Order in Two-Dimensional Systems with Fluctuating Active Stresses}, author = {Keta, Yann-Edwin and Henkes, Silke}, author+an = {1=highlight}, year = {2025}, month = jul, journal = {Soft Matter}, arxiv = {2410.14840}, volume = {21}, number = {28}, pages = {5710--5719}, publisher = {The Royal Society of Chemistry}, issn = {1744-6848}, doi = {10.1039/D5SM00208G}, abstract = {In two-dimensional tissues, such as developing germ layers, pair-wise forces (or active stresses) arise from the contractile activity of the cytoskeleton, with dissipation provided by the three-dimensional surroundings. We show analytically how these pair-wise stochastic forces, unlike the particle-wise independent fluctuating forces usually considered in active matter systems, produce conserved centre-of-mass dynamics and so are able to damp large-wavelength displacement fluctuations in elastic systems. A consequence of this is the stabilisation of long-range translational order in two dimensions, in clear violation of the celebrated Mermin–Wagner theorem, and the emergence of hyperuniformity with a structure factor S(q) ∼ q2 in the q → 0 limit. We then introduce two numerical cell tissue models which feature these pair-wise active forces. First a vertex model, in which the cell tissue is represented by a tiling of polygons where the edges represent cell junctions and with activity provided by stochastic junctional contractions. Second an active disk model, derived from active Brownian particles, but with pairs of equal and opposite stochastic forces between particles. We study the melting transition of these models and find a first-order phase transition between an ordered and a disordered phase in the disk model with active stresses. We confirm our analytical prediction of long-range order in both numerical models and show that hyperuniformity survives in the disordered phase, thus constituting a hidden order in our model tissue. Owing to the generality of this mechanism, we expect our results to be testable in living organisms, and to also apply to artificial systems with the same symmetry.} } @article{naik2025keratins, title = {Keratins Coordinate Tissue Spreading by Balancing Spreading Forces with Tissue Material Properties}, author = {Naik, Suyash and Keta, Yann-Edwin and {Pranjic-Ferscha}, Kornelija and Hannezo, Edouard and Henkes, Silke and Heisenberg, Carl-Philipp}, author+an = {2=highlight}, year = {2025}, month = feb, journal = {bioRxiv}, biorxiv = {10.1101/2025.02.14.638262}, doi = {10.1101/2025.02.14.638262}, abstract = {For tissues to spread, they must be deformable while maintaining their structural integrity. How these opposing requirements are balanced within spreading tissues is not yet well understood. Here, we show that keratin intermediate filaments function in epithelial spreading by adapting tissue mechanical resilience to the stresses arising in the tissue during the spreading process. By analysing the expansion of the enveloping cell layer (EVL) over the large yolk cell in early zebrafish embryos in vivo, we found that keratin network maturation in EVL cells is promoted by stresses building up within the spreading tissue. Through genetic interference and tissue rheology experiments, complemented by a vertex model with mechanochemical feedback, we demonstrate that stress-induced keratin network maturation in the EVL increases tissue viscosity, which is essential for preventing tissue rupture. Interestingly, keratins are also required in the yolk cell for mechanosensitive actomyosin network contraction and flow, the force-generating processes pulling the EVL. These dual mechanosensitive functions of keratins enable a balance between pulling force production in the yolk cell and the mechanical resilience of the EVL against stresses generated by these pulling forces, thereby ensuring uniform and robust tissue spreading.} } @article{keta2024emerging, title = {Emerging {{Mesoscale Flows}} and {{Chaotic Advection}} in {{Dense Active Matter}}}, author = {Keta, Yann-Edwin and Klamser, Juliane U. and Jack, Robert L. and Berthier, Ludovic}, author+an = {1=equalhighlight;2=equal}, year = {2024}, month = may, journal = {Physical Review Letters}, volume = {132}, number = {21}, pages = {218301}, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.132.218301}, abstract = {We study two models of overdamped self-propelled disks in two dimensions, with and without aligning interactions. Both models support active mesoscale flows, leading to chaotic advection and transport over large length scales in their homogeneous dense fluid states, away from dynamical arrest. They form streams and vortices reminiscent of multiscale flow patterns in turbulence. We show that the characteristics of these flows do not depend on the specific details of the active fluids, and result from the competition between crowding effects and persistent propulsions. This observation suggests that dense active suspensions of self-propelled particles present a type of “active turbulence” distinct from collective flows reported in other types of active systems.}, arxiv = {2306.07172} } @article{ADD, title = {Intermittent Relaxation and Avalanches in Extremely Persistent Active Matter}, author = {Keta, Yann-Edwin and Mandal, Rituparno and Sollich, Peter and Jack, Robert L. and Berthier, Ludovic}, author+an = {1=highlight}, year = {2023}, journal = {Soft Matter}, volume = {19}, number = {21}, pages = {3871--3883}, issn = {1744-683X, 1744-6848}, doi = {10.1039/D3SM00034F}, abstract = {We use numerical simulations to study the dynamics of dense assemblies of self-propelled particles in the limit of extremely large, but finite, persistence times. In this limit, the system evolves intermittently between mechanical equilibria where active forces balance interparticle interactions. We develop an efficient numerical strategy allowing us to resolve the statistical properties of elastic and plastic relaxation events caused by activity-driven fluctuations. The system relaxes via a succession of scale-free elastic events and broadly distributed plastic events that both depend on the system size. Correlations between plastic events lead to emergent dynamic facilitation and heterogeneous relaxation dynamics. Our results show that dynamical behaviour in extremely persistent active systems is qualitatively similar to that of sheared amorphous solids, yet with some important differences.}, arxiv = {2212.09836} } @article{dis_cm, title = {Disordered collective motion in dense assemblies of persistent particles}, author = {Keta, Yann-Edwin and Jack, Robert L. and Berthier, Ludovic}, author+an = {1=highlight}, journal = {Physical Review Letters}, shortjournal = {PRL}, volume = {129}, issue = {4}, pages = {048002}, year = {2022}, month = {07}, doi = {10.1103/PhysRevLett.129.048002}, abstract = {We explore the emergence of nonequilibrium collective motion in disordered nonthermal active matter when persistent motion and crowding effects compete, using simulations of a two-dimensional model of size polydisperse self-propelled particles. In stark contrast with monodisperse systems, we find that polydispersity stabilizes a homogeneous active liquid at arbitrary large persistence times, characterized by remarkable velocity correlations and irregular turbulent flows. For all persistence values, the active fluid undergoes a nonequilibrium glass transition at large density. This is accompanied by collective motion, whose nature evolves from near-equilibrium spatially heterogeneous dynamics at small persistence, to a qualitatively different intermittent dynamics when persistence is large. This latter regime involves a complex time evolution of the correlated displacement field.}, arxiv = {2201.04902} } @article{DAMTP2020, title = {Collective motion in large deviations of active particles}, author = {Keta, Yann-Edwin and Fodor, Étienne and van Wijland, Frédéric and Cates, Michael E. and Jack, Robert L.}, author+an = {1=highlight}, journal = {Physical Review E}, shortjournal = {PRE}, volume = {103}, issue = {2}, pages = {022603}, year = {2021}, month = {02}, doi = {10.1103/PhysRevE.103.022603}, abstract = {We analyze collective motion that occurs during rare (large deviation) events in systems of active particles, both numerically and analytically. We discuss the associated dynamical phase transition to collective motion, which occurs when the active work is biased towards larger values, and is associated with alignment of particles’ orientations. A finite biasing field is needed to induce spontaneous symmetry breaking, even in large systems. Particle alignment is computed exactly for a system of two particles. For many-particle systems, we analyze the symmetry breaking by an optimal-control representation of the biased dynamics, and we propose a fluctuating hydrodynamic theory that captures the emergence of polar order in the biased state.}, arxiv = {2009.07112} } @article{Umea2020, title = {Translational and rotational velocities in shear-driven jamming of ellipsoidal particles}, author = {Keta, Yann-Edwin and Olsson, Peter}, author+an = {1=highlight}, journal = {Physical Review E}, shortjournal = {PRE}, volume = {102}, issue = {5}, pages = {052905}, year = {2020}, month = {11}, doi = {10.1103/PhysRevE.102.052905}, abstract = {We study shear-driven jamming of ellipsoidal particles at zero temperature with a focus on the microscopic dynamics. We find that a change from spherical particles to ellipsoids with aspect ratio \(\alpha=1.02\) gives dramatic changes of the microscopic dynamics with much lower translational velocities and a new role for the rotations. Whereas the velocity difference at contacts -- and thereby the dissipation -- in collections of spheres is dominated by the translational velocities and reduced by the rotations, the same quantity is in collections of ellipsoids instead totally dominated by the rotational velocities. By also examining the effect of different aspect ratios we find that the examined quantities show either a peak or a change in slope at \(\alpha\approx1.2\), which thus gives evidence for a crossover between different regions of low and high aspect ratio.}, arxiv = {2006.05305} } @article{Umea2019, title = {Orientational {O}rdering in {A}thermally {S}heared, {A}spherical, {F}rictionless {P}articles}, author = {Marschall, Theodore and Keta, Yann-Edwin and Olsson, Peter and Teitel, S.}, author+an = {2=highlight}, journal = {Physical Review Letters}, shortjournal = {PRL}, volume = {122}, issue = {18}, pages = {188002}, year = {2019}, month = {05}, doi = {10.1103/PhysRevLett.122.188002}, abstract = {We numerically simulate the uniform athermal shearing of bidisperse, frictionless, two-dimensional spherocylinders and three-dimensional prolate ellipsoids. We focus on the orientational ordering of particles as an asphericity parameter \(\alpha \to 0\) and particles approach spherical. We find that the nematic order parameter \(S_2\) is nonmonotonic in the packing fraction \(\phi\) and that, as \(\alpha \to 0\), \(S_2\) stays finite at jamming and above. The approach to spherical particles thus appears to be singular. We also find that sheared particles continue to rotate above jamming and that particle contacts preferentially lie along the narrowest width of the particles, even as \(\alpha \to 0\).}, arxiv = {1806.01739} } @article{UBC2019, title = {Cooperative motion and shear strain correlations in dense 2{D} systems of self-propelled soft disks}, author = {Keta, Yann-Edwin and Rottler, Jörg}, author+an = {1=highlight}, journal = {EPL}, volume = {125}, issue = {5}, pages = {58004}, year = {2019}, month = {04}, doi = {10.1209/0295-5075/125/58004}, abstract = {We present a detailed study of cooperative and correlated dynamics in a 2{D} model system for soft active matter that undergoes motility-induced phase separation ({M}{I}{P}{S}). Particle displacement correlation functions reveal increasing dynamical heterogeneity with increasing activity. In the phase-separated regime as well as in the truly glassy phase, transverse mean squared displacements as well as shear strain correlations exhibit a regime of long-range behaviour with strain correlations decaying as \(r^{−2}\) on time scales short compared to the rotational diffusion time. We attribute the origin of the \(r^{−2}\) decay to elastic interactions that lead to similar behaviour in passive glass formers.} }