arXiv subjects

Mathieu Dedenon

Publications and source records attributed to Mathieu Dedenon.

6 recordsLinked to original sources

arXiv2025

Spatiotemporal Control of Charge +1 Topological Defects in Polar Active Matter

Topological defects are a conspicuous feature of active liquid crystals that have been associated with important morphogenetic transitions in organismal development. Robust development thus requires a tight control of the motion and placement of topological defects. In this manuscript, we study a mechanism to control +1 topological defects in an active polar fluid confined to a disk. If activity is localized in an annulus within the disk, the defect moves on a circular trajectory around the center of the disk. Using an ansatz for the polar field, we determine the dependence of the angular speed and the circle radius on the boundary orientation of the polar field and the active annulus. Using a proportional integral controller, we guide the defect along complex trajectories by changing the active annulus size and the boundary orientation.

cond-mat.soft↗

arXiv2025

Three-dimensional formulation of curved nematic shells

In soft matter, the phase of nematic liquid crystals can be made from anisotropic molecules in single component materials, or as a suspension of mesoscopic nematogens. The later offers more versatility in the experimental design of complex shapes, in particular thin curved shells, and is often found in biological systems at multiple scales from cells to tissues. Here, we investigate theoretically the transition from three-dimensional nematics to a two-dimensional description restricted to a tangent plane, using a mean-field approach. We identify a transition from first to second order isotropic-nematic transition in presence of weak tangential anchoring. Then, we clarify the conditions under which a two-dimensional description of thin nematic shells is relevant. Nonetheless, using the example of active nematic stress, we identify physical differences between two- and three-dimensional descriptions in curved geometry. Finally, we construct a thin film approximation of nematohydrodynamics for a nematic shell in contact with a curved substrate. All together, those results show that a tangential restriction of nematic orientation must be used with care in presence of curvature, activity, or weak anchoring boundary conditions.

cond-mat.soft↗

arXiv2025

The importance of being discrete -- An agent-based model for active nematics and more

We propose an agent-based model of active flexible rods. Inspired by cytoskeletal flows, we introduce activity by an internal flow that contributes to the dissipative forces. The active force between our agents is central and reciprocal, ensuring linear and angular momentum conservation. For nematic activity, we find spontaneous, thresholdless flows and stochastic flow-reorientation, which is accompanied by the formation of topological defects. Defects appear and vanish with activity-dependent rates, and $+1/2$-defects self-propel. These hallmarks of active turbulence are present even on the scale of individual agents. The particle-based interactions lead to an emergent coupling between density and orientation that generates density dipoles around $+1/2$-defects. Finally, we highlight the versatility of our agent-based model by presenting spontaneous flows in three dimensions and tissue growth. Our framework opens the way for an integrated description of living materials, including several forms of activity in the same system.

cond-mat.soft↗

arXiv2024

Noise-induced transitions from contractile to extensile active stress in isotropic fluids

Tissues of living cells are a prime example of active fluids. There is experimental evidence that tissues generate extensile active stress even though their constituting cells are contractile. Fluctuating forces that could result from cell-substrate interactions have been proposed to be able to induce a transition from contractile to extensile active stress. Through analytic calculations and numerical computations, we show that in isotropic active fluids, nonlinearities and a coupling between fluctuating forces and fluid density are necessary for such a transition to occur. Here, both transitions from extensile to contractile and vice versa are possible.

cond-mat.soft↗

arXiv2023

Density-polarity coupling in confined active polar films: asters, spirals, and biphasic orientational phases

Topological defects in active polar fluids can organise spontaneous flows and influence macroscopic density patterns. Both of them play, for example, an important role during animal development. Yet the influence of density on active flows is poorly understood. Motivated by experiments on cell monolayers confined to discs, we study the coupling between density and polar order for a compressible active polar fluid in presence of a +1 topological defect. As in the experiments, we find a density-controlled spiral-to-aster transition. In addition, biphasic orientational phases emerge as a generic outcome of such coupling. Our results highlight the importance of density gradients as a potential mechanism for controlling flow and orientational patterns in biological systems.

cond-mat.soft↗

arXiv2019

Model of lamellipodium initiation during cell spreading

Cell spreading requires a major reorganisation of the actin cytoskeleton, from a cortical structure to a lamellipodium where filaments are mostly parallel to the substrate. We propose a model inspired by the physics of nematic liquid crystals and fluid membranes, in which the coupling between actin mechanics, filaments orientation, and the local curvature of the cell membrane naturally yields the collective reorientation of actin filaments at the highly curved edge of a spreading cell. Filament orientation increases the traction force exerted by the frictional flow of polymerising actin on the substrate, creating a positive feedback loop between edge curvature, filament alignment, and traction force that promotes cell spreading. We establish the condition under which this feedback leads to a full wetting transition, which we interpret as the initiation of a lamellipodium, and we uncover the existence of bi-stability between partial and full spreading which could trigger spontaneous cell polarization and lead to migration.

physics.bio-ph↗