Stabilization of active matter by flow-vortex lattices and defect ordering

Abstract

Active systems, from bacterial suspensions to cellular monolayers, are continuously driven out of equilibrium by local injection of energy from their constituent elements and exhibit turbulent-like and chaotic patterns. Here we demonstrate both theoretically and through numerical simulations, that the crossover between wet active systems, whose behaviour is dominated by hydrodynamics, and dry active matter where any flow is screened, can be achieved by using friction as a control parameter. Moreover, we discover unexpected vortex ordering at this wet–dry crossover. We show that the self organization of vortices into lattices is accompanied by the spatial ordering of topological defects leading to active crystal-like structures. The emergence of vortex lattices, which leads to the positional ordering of topological defects, suggests potential applications in the design and control of active materials.

Negative stiffness and modulated states in active nematics

“…Defect ordering: As discussed in the Introduction, experiments have reported nematic order of the orientation of +1/2 defects in microtubule bundle suspensions 27 . Previous numerical work has observed both nematic 23 and polar 21,27 order of the defects in dry systems and a lattice of flow vortices with rows of nematically ordered defects at the wet-to-dry crossover 26 . In our model we have observed two types of defect-ordered structures.…”

“…Systems where viscous dissipation dominates so that frictional damping is negligible and momentum is conserved are referred to as 'wet', while those where dissipation is mainly controlled by friction are referred to as 'dry'. Previous work has examined both wet 5,7,12,20 and dry [21][22][23][24][25] systems, as well as the crossover between the two 26 , revealing a rich dynamics driven by orientational instabilities and by the unbinding and proliferation of topological defects. A detailed summary of recent work most closely related to ours is given in Section 4.…”

“…In experiments in suspensions of microtubule bundles the +1/2 defects were observed to organize in nematically ordered states 27 . Numerical studies of dry systems have reported both nematic 23 and polar 21,27 order of the defect orientations and a remarkable defect-ordered state accompanied by a flow-vortex lattice at the crossover between wet and dry regimes 26 . This range of results indicates that more work is needed to understand what controls the nature of defect ordering in these systems.…”

Emergence of Lévy walks in systems of interacting individuals

“…However, it is still an open question how Lévy walks emerge in systems of interacting self-propelled particles. The current theory of Lévy walk [8] assumes noninteracting particles and power-law distribution of traveled distances from the inception.The collective behavior of large groups of interacting individuals such as bird flocks, fish schools, and the collective migration of cells or bacteria is another rapidly growing area of active matter research [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. There exist two main types of models used for a collective behavior: (1) Lagrangian models describing the movements of self-propelled particles in terms of nonlinear equations for the positions and velocities of all particles [20][21][22][23], and (2) kinetic models involving partial differential equations for the population densities [24][25][26][27][28][29][30].…”

Role of interfacial friction for flow instabilities in a thin polar-ordered active fluid layer

“…Friction has been considered in various active fluid flow problems as well; see, e.g., Refs. [33][34][35][36][37] for various theoretical and expreimental studies. While no systematic measurements of slip at interfaces involving active fluids are known, the above existing results suggest that considering the complex internal structure of the active fluid (e.g., the presence of actin filaments), a partial slip at the interfaces between the active fluid layer and the 3D embedding fluid cannot be ruled out.…”