Collaborators: Fridtjof Brauns, Yimin Luo, M. Cristina Marchetti

Apolar elongated cells growing on an aligned liquid crystal elastomer collectively sense the substrate orientation. As cells grow and divide along their long axis, they create locally aligned domains of high density separated by low density disordered regions. The ordered domains then grow in size and align with the substrate. At high densities cells jam and growth ceases. The jamming density depends on the order: Highly aligned cells jam at higher densities. The degree of order of the final state depends on the initial seeding density, with smaller seeding densities leading to more aligned monolayers. While the mechanism of sensing the substrate orientation is not known, adding a focal adhesion kinase inhibitor, which disrupts cell-substrate adhesion, leads to the loss of collective directional sensing. We model this system as an overdamped compressible active nematic with growing density. The initial cell aggregation is driven by a Cahn-Hilliard like free-energy and the coupling of density gradients to the orientational dynamics. We incorporate the interaction with the substrate via two possible mechanisms: as an external field biasing the direction of the nematic, and as anisotropic friction experienced by cells. We first show that arresting rearrangements and growth by making rotational viscosity and growth-rate density dependent reproduces the dependence of final order on seeding density. We next show that activity, anisotropic friction, flow-alignment, and alignment of Q-tensor to density gradients all act in concert to engender substrate-sensing at the collective level. We qualitatively reproduce the swirling and laning patterns seen in experiments and plan on exploring the relative importance of friction anisotropy, activity, and density-gradient alignment.

I presented the homogeneous theory in a poster at the 2024 Active Solids Conference at KITP.

I presented some early results in a poster at the 2024 Boulder School for Condensed Matter.

Phase separated cell colonies form elongated worms (googly eyes placed in post processing) due to nematic aligning to density gradients and activity stretching the aligned domains.