Active & Biological Matter

Research

Nematic Activity in Tissues

Nematic activity - components pushing or pulling on their neighbours symmetrically along their long axis - seems to play an important role in tissue dynamics, both in chaotic and in ordered cell motion. I work on combining nematic activity with computational tissue models such as the vertex model, allowing us to better understand the physical underpinnings of, e.g., collective cell motion, morphogenesis, and cell sorting. A key result of this work was that adding internal dissipation to cell-resolution tissue models is crucial for capturing active-nematic-like behaviour and the emergence of spontaneous long-range correlations. More recently, I have also worked on using continuum lattice-Boltzmann simulations of active nematic tissues. In particular, we showed that a model that treats the axis of activity and the shape long axis as separate fields always appears extensile in confinement, regardless of the actual activity. This offers a plausible explanation for the long-standing contradictions that cells should be contractile, but tissues often behave in an extensile-like way.

Selected Publications

J. Rozman, S. P. Thampi, and J. M. Yeomans, Why Extensile and Contractile Tissues Could be Hard to Tell Apart, arXiv:2511.07012 (2025).
J. Rozman, Chaithanya K. V. S., J. M. Yeomans, and R. Sknepnek, Vertex Model with Internal Dissipation Enables Sustained Flows, Nat. Commun. 16, 530 (2025).
J. Rozman and J. M. Yeomans, Cell Sorting in an Active Nematic Vertex Model, Phys. Rev. Lett. 133, 248401 (2024).
J. Rozman, J. M. Yeomans, and R. Sknepnek, Shape-Tension Coupling Produces Nematic Order in an Epithelium Vertex Model, Phys. Rev. Lett. 131, 228301 (2023).

Morphogenesis & Tissue Mechanics

Physical rules - sometimes surprisingly simple - play a key role in a range of biological systems. Using vertex models, I seek to better understand the role of physics in the behaviour of tissues and the formation of shape. For example, we studied the possible shapes of organoid-like epithelial shells, finding that hexatic topological defects play a surprising role in that system. More recently, we analysed the usual area-and-perimeter vertex model extended to a fully 3D monolayer. We showed that apical-basal asymmetries prevent the famous rigidity transition, rendering the tissue permanently solid in the absence of active contributions.

Selected Publications

J. Rozman, M. Krajnc, and P. Ziherl, Basolateral Mechanics Prevents Rigidity Transition in Epithelial Monolayers, Phys. Rev. Lett. 133, 168401 (2024).
J. Rozman, M. Krajnc, and P. Ziherl, Collective Cell Mechanics of Epithelial Shells with Organoid-like Morphologies, Nat. Commun. 11, 3805 (2020).

Experimental Collaborations

I work closely with experimentalists to develop computational models of specific biological systems. By adapting the famous forest-fire model to growing graphs, we showed how clonal dominance in the Drosophila egg chamber can emerge through the coupling of divisions. In another collaboration, we showed that oriented cell elongation in the Xenopus neural plate can emerge spontaneously without cell polarisation from the asymmetric shape of the neural plate region.

Publications

M. Matsuda, J. Rozman, S. Ostvar, K. E. Kasza, and S. Y. Sokol, Mechanical Control of Neural Plate Folding by Apical Domain Alteration, Nat. Commun. 14, 8475 (2023).
J. Imran Alsous, J. Rozman, R. A. Marmion, A. Košmrlj, and S. Y. Shvartsman, Clonal Dominance in Excitable Cell Networks, Nat. Phys. 17, 1391 (2021).

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