Density and shape govern the dynamical self-organization of active matter on a droplet
Abstract
Morphogenesis emerges from dynamic feedback among geometry, mechanics, and chemistry; however, disentangling these contributions in living systems remains challenging. Here, we focus on the interplay between geometry and mechanics by developing a minimal in vitro model in which purified microtubules and kinesin motor clusters self-organize into a two-dimensional active nematic cortex at the surface of spherical water-in-oil droplets. The spherical geometry enforces a total topological charge of +2, here realized by four +1/2 defects whose trajectories reveal robust, self-sustained oscillations. Using full-surface reconstructions, we show that the collective dynamics of the defects lead to a periodic switching between planar and tetrahedral arrangements through alternating coiling and hemisphere-crossing phases. By tuning microtubule density, the system spans a continuum from a classic defect-dominated active nematic to a regime resembling an extensile filament confined to a curved surface, where low density is associated with increased trajectory variability and direction reversals. Geometric perturbations introduced through controlled squeezing redistribute curvature and induce the nucleation of additional defects, thereby reorganizing the entire topological landscape while preserving total charge. Together, these results show that periodic morphogenetic-like cycles, defect topology, and material organization can arise solely from the interplay of activity, density, and curvature. This reconstituted system provides a versatile platform for elucidating the coupling between mechanics and geometry underlying shape formation in active biological matter.
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BibTeX
@misc{leroux2026density,
title = {{Density and shape govern the dynamical self-organization of active matter on a droplet}},
author = {Romain Leroux and Andre Estevez-Torres and Raphael Voituriez and Ananyo Maitra and Nicolas Lobato-Dauzier and Jean-Christophe Galas},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2603.23735},
url = {https://hal.sorbonne-universite.fr/hal-05761224v1},
note = {CC BY-SA 4.0},
}
RIS
TY - UNPB
TI - Density and shape govern the dynamical self-organization of active matter on a droplet
AU - Romain Leroux
AU - Andre Estevez-Torres
AU - Raphael Voituriez
AU - Ananyo Maitra
AU - Nicolas Lobato-Dauzier
AU - Jean-Christophe Galas
PY - 2026
DA - 2026/09/28
PB - HAL
DO - 10.48550/arxiv.2603.23735
UR - https://hal.sorbonne-universite.fr/hal-05761224v1
AB - Morphogenesis emerges from dynamic feedback among geometry, mechanics, and chemistry; however, disentangling these contributions in living systems remains challenging. Here, we focus on the interplay between geometry and mechanics by developing a minimal in vitro model in which purified microtubules and kinesin motor clusters self-organize into a two-dimensional active nematic cortex at the surface of spherical water-in-oil droplets. The spherical geometry enforces a total topological charge of +2, here realized by four +1/2 defects whose trajectories reveal robust, self-sustained oscillations. Using full-surface reconstructions, we show that the collective dynamics of the defects lead to a periodic switching between planar and tetrahedral arrangements through alternating coiling and hemisphere-crossing phases. By tuning microtubule density, the system spans a continuum from a classic defect-dominated active nematic to a regime resembling an extensile filament confined to a curved surface, where low density is associated with increased trajectory variability and direction reversals. Geometric perturbations introduced through controlled squeezing redistribute curvature and induce the nucleation of additional defects, thereby reorganizing the entire topological landscape while preserving total charge. Together, these results show that periodic morphogenetic-like cycles, defect topology, and material organization can arise solely from the interplay of activity, density, and curvature. This reconstituted system provides a versatile platform for elucidating the coupling between mechanics and geometry underlying shape formation in active biological matter.
ER -