Session P29: Active Matter and Liquid Crystals in Biological Systems I

Dipolar Extensile Dynamics in Microtubule-Based 2D Active Nematics

Theories describe 2D active nematics in terms of liquid crystals, hydrodynamics and chaotic fluids. However, the experimental system is inherently hierarchical and connecting the microscopic forces to the macroscopic flows remains an open challenge. Using an in vitro 2D active nematic composed of microtubules and kinesin motors, we probe the microscopic dynamics under varying active stresses. We find that locally, flows are dipolar extensile whose strain rate can be tuned by ATP concentration. Additionally, we uncover a velocity distribution of microtubules along the director which indicates the importance of many body interactions in bridging the length scales of the system.   

Design of active nematic systems with controllable defect dynamics and flows

Active matter encompasses a wide spectrum of non-equilibrium condensed matter systems, the constituents of which convert energy into mechanical work. They exhibit intriguing collective behaviors, such as flocking and activity-induced phase transitions. In the particular case of anisotropic particles or molecules, these materials can enter an active nematic state, which displays intricate motions of topological defects, characteristic of nematic liquid crystals, accompanied by chaotic-like flows. Active nematics are encountered in numerous biological systems, ranging from cytoskeletal polymer extracts, to tissue cells and dense bacterial suspensions. The dynamics of active nematics are not fully understood, and efforts to control and manipulate the structure and flow of this class of materials have been limited. In this talk, I will summarize our recent progress in active nematics research, with an emphasis on the interplay between molecular interactions, elasticity, and hydrodynamic forces. In particular, I will discuss an experimentally realizable approach, which relies on spatiotemporal patterning of activity, to manipulate defects and flows in active nematics. I will show how simulations can be harnessed to guide design of well-controlled dynamics, paving the way towards engineering active matter for practical applications.   

Control of bacterial dynamics by splay and bend in nematic vortices

Microswimmers exhibit collective behavior that can be controlled by an anisotropic environment such as a lyotropic chromonic liquid crystal. We explore the effect of splay and bend of the director field on the individual and collective behavior of motile Bacilli subtilis. The director field, imposed through photoalignment, is designed in the form of vortices of topological charge +1. Their geometry changes from pure radial to spiral and to the circular, representing thus deformations of a pure splay, splay-bend mix, and pure bend, respectively. In dilute dispersions, the bacteria follow the pre-imposed director field, but after their concentration reaches some threshold, they engage in a collective unipolar circulation. This collective behavior is controlled by the splay-to-bend ratio: vortices with dominating splay condense the bacterial swarms towards the center, while vortices with dominating bend push them away to the periphery. Vortices with splay-bend parity formed by 45-degree spiraling director produce the most stable swarming with a time-independent radius as long as the bacterial activity is constant. The change in swimming scenario as a function of splay-to-bend ratio is reminiscent of an unstable limit cycle.   

Spatiotemporal Optimal Control of an Extensile Active Nematic Suspension

Active nematic suspensions are self-driven fluids that exhibit rich spatiotemporal dynamics characterized by director field buckling, defect nucleation/annihilation and chaotic trajectories of those defects. Towards developing experimental methods for controlling these dynamics, we consider an optimal control problem which seeks to find the spatiotemporal pattern of active stress strength required to drive the system towards a desired director field configuration. As an exemplar, we consider an extensile active nematic fluid confined to a disk. In the absence of control, the system produces two topological defects that perpetually circulate. Optimal control identifies a time-varying active stress field that drives the defects to orbit in the opposite direction.   

Self propelling nematic microcapsules

The ability to produce controllable, self propelling microcapsules is of great interest to synthetic biology and the design of smart microreactors. Inactive fluid shells are already widely used as artificial cell models, micro-reactors, and in food and drug applications. However, combining activity, stability, and control remains a significant challenge. Building on an established active emulsion platform, we have developed a new approach to the problem of encapsulation by using nematic active double emulsions, where a solubilization mechanism induces activity and the molecular nematicity provides stability. We show that using a nematic liquid crystal as the shell material with homeotropic anchoring at both interfaces will result in a nematoelastic force on a displaced core droplet and act as a topological barrier against the coalescence of the core droplet with the outer phase. We further present a peculiar self-propulsion mode where the interplay of spontaneous symmetry breaking and autochemotaxis results in a "shark-fin meandering" motion of the shell in a 2D-confined geometry and helical swimming in 3D. This behavior can be controlled or switched off by introducing chemical gradients, topographical guidance or by changing the shell topology.   

The Dynamics of Active Nematic Defects on The Sphere Surface

A nematic liquid crystal confined to the surface of a sphere exhibits topological defects of total charge +2 due to the topological constraint. In equilibrium, the nematic field forms four +1/2 defects, located at the corners of a tetrahedron inscribed within the sphere, since this minimizes the Frank elastic energy. Here we study the active counterpart of such a system, in which a self-driven directional motion of the individual nematogens creates a large-scale flow that drives the system out of equilibrium. In this new state, defects exhibit complex dynamics which, depending on the strength of the active forcing, can be simple and periodic (for weak forcing) or chaotic (for strong forcing). We show that Onsager’s variational principle offers an exceptionally transparent way to derive the exact dynamical equations of the defects in such an active spherical nematic, and we explain its mobility at the hydrodynamics level.   

Emergent dynamics of large scale collective rotations in 2D active nematic

Motivated by recent work on active nematics we employ here a microtubule and kinesin motor gliding assay system to examine dynamic collective order in 2D. Without the need for a depletion agent, our system exhibits the classic nematic order transition that depends on microtubule and active motor density. However in the regime of very flexible microtubules nematic order is lost, suggesting that this ordering is a consequence of both microtubule stiffness and density. We demonstrate that the ordered state exhibts large scale coherent rotations and that the introduction of a time varying director is necessary for proper characterization of the active nematic system. We show that the rate of this collective rotation of the nematic order is strongly dependent on activity induced microtubule fluctuations.   

Defect dynamics of 3D active nematic turbulence

Three-dimensional active nematics are exciting new materials characterized by a network of continuously evolving defect lines and loops [1]. We report on detailed numerical investigation of confined 3D active nematics [2]. At sufficient activity, such system transitions into active turbulence with irregular dynamics of multiple defect loops with distinct topological events of loop crossover, annihilation, splitting and merging occurring in time. The dynamics of a single active defect loop depends on the local crosssection of the director profile that can span from +1/2 or -1/2 winding numbers to twist profiles. Depending on the loop orientational profile, we observe spontaneous shrinking, growing, and bending of a single zero-charge loop [3]. Our work aims to provide insight into 3D active turbulence from the perspective of the topology of the emergent 3D defects and their self-induced dynamics.

[1] G Duclos et al, Topological structure and dynamics of three dimensional active nematics, arXiv:1909.01381, 2019.
[2] S Copar, J Aplinc, Z Kos, S Zumer and M Ravnik, Topology of Three-Dimensional Active Nematic Turbulence Confined to Droplets, Phys. Rev. X 9, 031051, 2019.
[3] J Binysh, Z Kos, S Copar, M Ravnik, GP. Alexander, Three-Dimensional Active Defect Loops, arXiv:1909.07109, 2019.   

Topological Defects in Cell Monolayers Guided by Topography

Many types of cells display long-range alignment like nematic liquid crystals (LCs), and have topological defects. These defects are characterized by their topological charge, the number of rotations of the alignment around the defect. In 2D cell culturing, only +/- 1/2 defects are typically observed, although other defects exist in living systems. Topological defects in cell layers have been associated with increase in cell apoptosis [1] and clustering [2]. We induce alignment of cells via contact guidance into formations that impose arbitrary topological charges. We survey multiple cell types (EpH-4 and 3T6) to characterize their alignment, density, and dynamics near defects of various topological charges, and define LC parameters such as defect core sizes and elastic constants. We observe a large change in density of fibroblasts (3T6) near the topological defects , where our tunable confinement allows us to identify the isotropic defect cores.
[1] TB Saw et al. Nature 544, 212 (2017)
[2] K Kawaguchi et al. Nature 545, 327 (2017)   

Clustering, jamming, and topological defects in growing bacterial colonies at liquid interfaces

Active nematic matter encompasses a broad variety of systems including confined vibrating rods, flocks of birds, and colonies of bacteria. Here, we present experimental results on growing colonies of rod-shaped bacteria confined in two dimensions by adsorption at an oil-water interface. Using microfluidics and time-lapse microscopy, we investigate the roles of motility and growth on cluster formation, topological defects, and jamming. As colonies grow on finite liquid interfaces, we observe cluster formation where regions of tightly packed bacteria that display notably low swimming velocities increase in number and size over time. As the surface coverage increases, a densely packed monolayer of bacteria with long-range orientation order is observed and topological defects are quantified. This setup functions as a useful model system for low-friction confinement of a growing active nematic comprised of discrete particles as well as carrying important implications for the study of biofilm formation at liquid interfaces.   

Topological defects drive layer formation in bacteria colonies

The starvation-induced development of macroscopic fruiting bodies in Myxococcus xanthus begins with the formation of layered cell structures. Cells in these layers are densely packed, aligned via their rod shape, and retain their motility so that the population forms an active nematic liquid crystal. We investigate the origin of layering by looking at the formation of second layers and holes from cellular monolayers. These events occur at discontinuities in the cell direction field known as topological defects. Layers form at positive defects while holes open at negative defects. By measuring cell flows, we find an influx of cells towards positive defects and an outflux away from negative defects. We find that a model of monolayers as an active, dry extensile nematic with anisotropic friction can reproduce the measured flow fields and change in cell density at defects. Overall, we conclude that the conversion from a 2D cell layer to a 3D droplet is triggered by the formation of layers of cells at topological defects.   

Pulsating air bubbles can swim in anisotropic fluids

Small-scale locomotion in fluids has been of great interest, from microorganisms to active matter and spermbots. Here we explore the swimming of a spherical bubble with a periodic change in its radius. Its spatial symmetry and the scallop theorem [1] tell us that the bubble with the reciprocal motion cannot achieve propulsion. In anisotropic fluids, however, the bubbles can swim. The spherical bubble dispersed in homogeneously aligned nematic liquid crystals (LCs) accompanies either a hyperbolic point defect or a disclination ring called a Saturn-ring. The pulsating bubble generates LC flow of which spatial and time-reversal symmetry are broken because of LC’s director configuration and viscoelastic response, respectively. The bubble with the point defect exhibits the net propulsion, while the swimming of the other one depends on the shape of the ring defect. We also introduce our theoretical understanding of this propulsion mechanism, with ideas to maximize swimming efficiency.

[1] E. M. Percell, Am. J. Phys. 45, 3-11 (1977)