Spontaneous topological charging of tactoids in a living nematic

Abstract

Living nematic is a realization of an active matter combining a nematic liquid crystal with swimming bacteria. The material exhibits a remarkable tendency towards spatio-temporal self-organization manifested in formation of dynamic textures of self-propelled half-integer topological defects (disclinations). Here we report on the study of such living nematic near normal inclusions, or tactoids, naturally realized in liquid crystals close to the isotropic-nematic (I-N) phase transition. On the basis of the computational analysis, we have established that tactoid's I-N interface spontaneously acquire negative topological charge which is proportional to the tactoid's size and depends on the concentration of bacteria. The observed negative charging is attributed to the drastic difference in the mobilities of +1/2 and −1/2 topological defects in active systems. The effect is described in the framework of a kinetic theory for point-like weakly-interacting defects with different mobilities. Our dedicated experiment fully confirmed the theoretical prediction. The results hint into new strategies for control of active matter.

Surface anchoring controls orientation of a microswimmer in nematic liquid crystal

“…7f, g, one can see a chain of defects with non-integer topological degrees inside these zones. Defects significantly affect other microswimmers' trajectories 9,10 and thus affect interactions between microswimmers in LC.…”

“…It was shown that bacteria in a LC demonstrate a wealth of intriguing phenomena, among which is the emergence of large-scale collective behavior. The injection of energy by microswimmers was also shown to result in the creation of moving topological defects (local singularities of molecular orientation) in the LC director field [9][10][11] . Manipulation of bacterial trajectories by the prescribed director patterning suggests novel approaches to control the transport of cargo 8 .…”

Emergence of Radial Tree of Bend Stripes in Active Nematics

“…1(d)], we start tracking the bacterial velocities and orientations, as shown in Supplemental Movies 2 and 3 [62]. For relatively low bacterial concentrations (less than 5 × 10 9 cells per cm 3 ), the orientations of fluorescent bacterial bodies are reliably detectable, and the local director field of the LC may be reconstructed from the local bacterial orientation (see Supplemental Movie 4 [62]), since bacterial bodies tend to align parallel to the LC director [27,29,30] (see Appendix A for details).…”

“…There, attractive depletion interactions result in a bundling of the filaments and the subsequent formation of a remarkable active gel comprised of microtubular bundles that are continuously sheared and remodeled by molecular motors. Current examples of active nematics include microtubule-motor protein mixtures [12][13][14][15], actin-motor protein systems [16][17][18], bacterial suspensions [19,20], cellular nematics [21][22][23][24][25][26], and suspensions of swimming bacteria in liquid crystals (termed living liquid crystals) [27][28][29][30].…”

“…In contrast to passive nematics, active nematics continuously create and annihilate pairs of AE1=2 topological defects, giving rise to nontrivial defect dynamics [29,30,[35][36][37][38][39][40][41][42]. For a given level of activity, an active nematic exhibits a well-defined concentration of defects, and the effective elastic constants of the material are reduced relative to those of an equivalent passive system [17,43].…”