T1 transitions as active hexatic defect annihilations

We show, analytically and numerically, that T1 transitions in epithelial tissue

monolayers can be accurately and predicatively described by the annihilation

dynamics of an active, hexatic defect quadrupole. In this context, we find that

the change in morphology (exchange of neighbors) of the T1 network, corresponds

to a rotation of the hexatic director, naturally occurring in the defect

annihilation. Furthermore, we discover that the flow field generated during the

annihilation, coincides with that experimentally observed during T1 remodeling

events [1]. Finally, we demonstrate that the explicit dependence of our analytic

expressions in activity, allows for experimental measurements of the magnitude

and, more importantly, the sign of the activity in these systems. Our work constitutes

an essential step towards a deeper understanding of cell intercalation

and other remodeling events in epithelial layers, where small-scale hexatic order

has been recently discovered [2][3].

[1] S. J. Streichan, M. F. Lefebvre, N. Noll, E. F Wieschaus, Boris I Shraiman,

eLife 7:e27454 (2018)

[2] J.-M. Armengol-Collado, L. N.Carenza, and L. Giomi, arXiv:2202.00651

(2022).

[3] J.-M. Armengol-Collado, L. N. Carenza, J. Eckert, D. Krommydas, and L.

Giomi, arXiv:2202.00668 (2022).