Fluids with nematic orientational order emerge in various materials, from nematic liquid crystals to biological and synthesised active matter. I will show the role of orientation of defects (disclinations) within the nematic field on the generated textures, forces and torques. The relaxation of the defect directions has a direct effect on braiding and annihilation dynamics in 2D layers, while in quasi-2D layers, defect-defect interactions can lead to a universal logic response. In three dimensions, disclinations are in shape of lines and loops, which can have a rich non-equilibrium dynamics, particularly in active nematic systems. I will present the dynamic modes of single disclination loops, as well as topological reconfiguration dynamics of multiple loops. The transitional dynamics towards and between different dynamic steady states of three-dimensional active nematics is determined by the coarsening or refinement of the defect lines. The dynamic steady-state disclination density depends on activity and other viscous and elastic material parameters. We show that intrinsic chirality increases the defect density and can in principle even lead to a transition into an active blue phase, characterized by the suppression of the velocity field generated by the active stress and an effective jamming of the disclination network. The presented work uses mechanistic, topological, and numerical approaches to understand the non-equilibrium dynamics of disclinations in driven and active nematic soft matter.

Ziga Kos is an Assistant Professor at Faculty of Mathematics and Physics at University of Ljubljana. He obtained his PhD in Ljubljana in 2019 and then worked as a postdoctoral fellow at Massachusetts Institute of Technology until 2021, when he returned to University of Ljubljana and Institute Jozef Stefan. His research explores the role of orientational order in non-equilibrium soft matter physics, particularly in liquid crystal dynamics and active matter.

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