Statistical and Biological Physics

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Hierarchical defect-induced condensation in active nematics

Timo Krüger‡, Ivan Maryshev‡, and Erwin Frey

One of the hallmark features of active nematics is the formation of oppositely charged, point-like, topological half-integer defects that exhibit creation/annihilation dynamics. So far, such ±1/2 defects have been found only in dense and spatially uniform active systems, and it has been argued that they are absent in dilute, phase-separated active systems in the nonequilibrium steady state. In our study, we show that, in addition to density de-mixed and ordered bands, dilute phase-separated active nematics can form condensed -1/2 defects of a new kind. In stark contrast to the well-studied homogeneous case, these new defects coincide with density peaks, lack a positively charged counterpart, condense nematic fluxes, and coexist with undulating bands from which they are formed by a new hierarchical condensation phenomenon. Moreover, we observe the formation of lateral arcuate structures that resemble +1/2 defects, a phenomenon for which we coin the term filamentous arc ejections.
Finally, we demonstrate how to artificially create defects to achieve a controllable concentration of material in localized domains