Topological Defects in a Living Nematic Ensnare Swimming Bacteria

Abstract

Active matter exemplified by suspensions of motile bacteria or synthetic self-propelled particles exhibits a remarkable propensity to self-organization and collective motion. The local input of energy and simple particle interactions often lead to complex emergent behavior manifested by the formation of macroscopic vortices and coherent structures with long-range order. A realization of an active system has been conceived by combining swimming bacteria and a lyotropic liquid crystal. Here, by coupling the well-established and validated model of nematic liquid crystals with the bacterial dynamics, we develop a computational model describing intricate properties of such a living nematic. In faithful agreement with the experiment, the model reproduces the onset of periodic undulation of the director and consequent proliferation of topological defects with the increase in bacterial concentration. It yields a testable prediction on the accumulation of bacteria in the cores of þ1=2 topological defects and depletion of bacteria in the cores of −1=2 defects. Our dedicated experiment on motile bacteria suspended in a freestanding liquid crystalline film fully confirms this prediction. Our findings suggest novel approaches for trapping and transport of bacteria and synthetic swimmers in anisotropic liquids and extend a scope of tools to control and manipulate microscopic objects in active matter.

Fine structure of the topological defect cores studied for disclinations in lyotropic chromonic liquid crystals

“…In particular, the 1/2 disclinations tend to attract the bacteria while the −1/2 lines repel them, as observed for both low27 and high28 concentrations of swimmers. Similar effects are seen in theoretical models2829. Since the spatial extent of disclination cores in LCLCs is either larger or comparable to the length of swimming bacteria, their fine structure might influence the dynamics of the latter.…”

“…Furthermore, disclinations in LCLCs have been demonstrated to control the dynamic behaviour of swimming bacteria, by influencing the spatial distribution of their concentration and even the geometry and polarity of bacterial flows27. In particular, the 1/2 disclinations tend to attract the bacteria while the −1/2 lines repel them, as observed for both low27 and high28 concentrations of swimmers. Similar effects are seen in theoretical models2829.…”

Low-noise phase of a two-dimensional active nematic system

“…Unlike its polar counterpart, where the appearance of macroscopic polar order results in collective directed motion or flocking [3,4], the active nematic involves driven apolar constituents, which means on average the system goes nowhere [5] making its properties far more subtle. Examples of active nematics include monolayers of melanocytes [6,7], fibroblasts [8], neural progenitors [9], myxobacteria [10,11], swimming filamentous bacteria [12][13][14], vibrated rods [15] and microtubule-kinesin suspensions [16].…”

Approximate time-dependent solution of a master equation with full linear birth-death rates

“…There are growing interests on dynamics of phase-singularities (PSs) in complex systems such as ventricular fibrillation [1,2,[4][5][6], defect dynamics in fluids [7,8] and liquid crystals [9], living creatures [10], quantum vortex dynamics [11] and so on. A master equation approach on the number n of PS for studying birth-death dynamics of PSs in a 2D Complex Ginzburg-Landau equation is invented first by Gil, Lega and Meunier [12].…”

“…Appendix A. Derivation of equation (11) The first and the second equation in equation (10) are written explicitly as…”