Session W20: Active Matter and Liquid Crystals in Biological and Bio-Inspired Systems III

Understanding and designing the collective dynamics of droplets of active nematic fluids

Active nematic fluids consume fuel at the microscopic scale, converting this energy into forces that can drive macroscopic motions over scales far larger than their microscopic constituents. In some cases, the mechanisms that give rise to this phenomenon have been well characterized, and can explain experimentally observed behaviors in both bulk fluids and those confined in simple stationary geometries. More recently, active fluids have been encapsulated in viscous drops or elastic shells so as to interact with an outer environment or a deformable boundary. Such systems are not as well understood. In this work, we examine the behavior of droplets of an active nematic fluid. We study their linear stability about the isotropic equilibrium over a wide range of parameters, identifying regions in which different modes of instability dominate. Simulations of their full dynamics are used to identify their nonlinear behavior within each region. When a single mode dominates, the droplets behave simply: as rotors, swimmers, or extensors. When parameters are tuned so that multiple modes have nearly the same growth rate, a pantheon of modes appears, including zigzaggers, washing machines, wanderers, and pulsators. Based on these quantitative characterizations, we further examine the collective dynamics of active droplets as they interact with each other through the exterior fluid flow.   

Spontaneous flow transition in confined 3D Active nematic droplets

Active matter collectively organizes mesoscale active stresses that drive a variety of emergent phenomena at the macroscopic scale including spontaneous large-scale flows. In 3D active nematics, the characteristic length scale of these flows is set by the ratio of nematic elasticity and active stress. Continuum hydrodynamic theory predicts that confinement below the active length scale stabilizes the otherwise chaotic active dynamics of the active nematic. This transition between a quiescent and a flowing state is reminiscent of the Fréedericksz transition. In this work, we probe how the interplay between confinement, active stress, and nematic elasticity controls the flow to no-flow transition in 3D microtubule-based active nematics confined in oil-water emulsions. This study provides a first novel method to measure active stresses in biomimetic active matter, which is a requirement for a quantitative comparison between experiments and theory.   

Topological defect-propelled swimming of nematic colloids

Dynamics of far-from-equilibrium topological defects in nematic liquid crystals (NLCs) can be used as a fundamental propulsion mechanism in microscopic active matter. Here, we demonstrate swimming of topological defect-propelled disk colloids with hybrid anchoring in (passive) nematic fluids through experiments and numerical simulations. Upon rotation of the disk colloids by an external magnetic field, the defects periodically elongate along the disk’s edges and sweep along the disk’s face. This dynamic swim stroke generates stresses with broken symmetries that propel the colloid. Defects elongate significantly adjacent to the disk at higher rotation rates, altering swimming direction. In this regime, the swimming speed and direction are determined by the colloid’s angular velocity, sense of rotation and defect polarity; these effects allow trajectory planning. We study the effective pair interactions of two defect-propelled swimmers, which are highly anisotropic and depend on the microscopic structure of the defect stroke, including the local defect topology and polarity. More generally, this work aims to develop biomimetic active matter based on the underlying relevance of topology.   

Understanding Active Turbulence in Fluidized Dry Active Nematics

Active turbulence within assemblies of self-propelling rods (SPRs) is a fascinating phenomenon characterized by chaotic active flows. To elucidate the fundamental

generation mechanism of active turbulence within 2D dense, dry SPRs, we perform large-scale Brownian dynamics simulations and investigate the cyclic process by

which dry SPRs undergo persistent swarming motions. Moreover, we characterize the coherent system dynamics by tracking the statistical and morphological

properties of disclination defects. We find the natural aggregation of rods leads to the formation of smectic-like rod layers, the deformation of which creates unique polar

structures and impedes the forward motion of +1/2-order defects. In turn, we demonstrate how rod motion near defects gives rise to local-positional orders and the

formation of interlocking polar lanes. Furthermore, we perform instability analysis on a bottom-up active-liquid-crystal model to determine the underlying cause of polar

lane breakup, which effectively leads to the formation of defects. Thus, the perpetual cycle consisting of unstable, smectic-like layers generating topological defects,

which induce smectic-like layers, serves as the origin of chaotic active flows within dense, dry SPRs.   

Agent-based simulation studies of dry active nematics

We study an agent-based simulation model of active nematics, represented as flexible coarse-grained filaments, to explore the role of filament properties in governing collective behavior. We examine the relationship between filament flexibility and diffusion for a single isolated self-propelled filament, and the effects of particle flexibility on motility-induced phase separation in dense systems. Next, we study a system of dry extensile active filaments confined in a channel geometry with non-slip boundary conditions. We study interaction of topological defects with confining channel walls and the resulting defect density profiles across the channel width. Surprisingly, we observe that +½ defects concentrate and travel along channel walls, with defect density profiles that differ significantly from those observed in experimental and simulation studies of wet extensile systems [1]. We consider how the absence of hydrodynamic interactions may lead to these observed differences in defect dynamics and patterning. [1] Hardouin et al Comm Phys 2019 doi:10.1038/s42005-019-0221-x   

Physics of blood clotting in insects

Insects have an open circulatory system that makes any physical damage to their bodies extremely dangerous. Insects developed a fast and effective way of creating a primary seal to the wound halting the blood loss or invasion of microorganisms. The formed blood clot is also used as a scaffold for formation of the new tissue. We studied surface properties of different Lepidoptera species (Manduca sexta, Vanessa cardui and Enyo lugubris) to compare with aqueous Apolipophorin-III (ApoLp-III) protein  solutions from Manduca sexta and Bombyx mori. We showed that the surface tension of hemolymph can be explained by adsorption of ApoLp-III to the hemolymph-air interface. Applying silanization we modified glass surfaces and demonstrated that due to its low surface tension, hemolymph spreads and adheres to both hydrophilic and hydrophobic surfaces. We then studied formation of crust over the air-blood interface and related it to the kinetics of wound sealing. Microscopy observations revealed that the concentration of blood cells is small and could not influence the crust formation questioning the mechanism of blood clotting in insects.   

Colloidal inclusions in active nematics

Suspending micron-sized colloidal particles in equilibrium thermotropic nematic liquid crystals provides a versatile platform for self-assembly. Colloidal inclusions induce formation of elastic deformations and topological defects of the nematic director, leading to complex colloidal interactions that are challenging to reproduce by alternative methods. Motivated by these advances, we explore the behavior of colloidal particles immersed in microtubule-based active nematic liquid crystals. Analogous to equilibrium systems, colloidal particles can induce defects and elastic deformations of the active nematics, which in turn power complex colloidal dynamics. We describe experimental studies which explore the intricate relationship between the shape and dynamics of colloidal particles immersed in active nematic liquid crystals.   

Microscopic active forces have mesoscopic elastic consequences

Active materials are those in which individual components convert ambient free energy into mechanical work. Of particular interest because of their analytical tractability and potential for describing biological phenomena are active systems composed of cytoskeletal filaments and molecular motors. These assemblies in sufficiently dense two-dimensional suspensions form active nematic liquid crystals (LCs). These are materials with long range spatial ordering whose components can nonetheless interchange positions. Due to the large time and length scale separations between the action of molecular motors and the flows they generate in the LC, most modelling has focused on coarse graining microscopic details in favor of mesoscale phenomenological models. The logic of this coarse graining is that the microscopic details can be subsumed by hydrodynamic parameters at this longer scale. In this work, we use a combination of experimental perturbations, machine learning, and microscopic modelling to investigate how changes in specific molecular motor properties are manifested at the scale of the system. We find that increasing the availability of fuel for our specific motors leads to a non-monotonic trend in activity that is related to the microscopic crosslinking of the motor clusters on pairs of filaments. The consequence of these microscopic differences in crosslinking is that as ATP is increased, the mechanics of the material change non-trivially alongside the activity.   

Migration and division of 3T6 cells near topological defects with integer charge

In this work, we investigate the effects of topological defects in cell monolayers (3T6) with nematic order. We characterize the alignment, density, and dynamics of the cells near azimuthal +1 defects and hyperbolic -1 defects induced by micropatterned ridges. We observe increased density near +1 topological defects, while near -1 defects we observe a decreased density. This behavior depends on other parameters such as ridge height and cell density. For example, lower ridges result in a later onset of the nematic order and less density variation associated with the defects. We hypothesize that the mechanism for this behavior is different from that observed in other cell types, namely that it depends strongly on the cells' division rate. To test alternative hypotheses suggested by simulations, we perform immunostaining experiments of the proliferation marker Ki-67 and analyze cell dynamics.   

Domain separation in bacterial monolayers on curved surfaces

Bacterial monolayers are ubiquitous in nature as the first step in a bacterial colony’s growth on a 2D substrate. These are most commonly studied on flat surfaces, but curved substrates are important in cases such as the growth of oil-degrading bacteria on the surface of oil droplets. A monolayer of rod-shaped bacteria behaves as an extensile active nematic. However, due to the particulate nature of the colonies at small length scales, they also display characteristics of granular materials such as pseudo-crystalline microdomains and grain boundaries. We study the interplay of nematic and granular behavior through hard-rod simulations of growing bacterial colonies confined to the surface of spherical droplets, varying the aspect ratio of the bacteria and the curvature of the surface. The size of the emergent microdomains sets a length scale for the system’s activity, matching the spatial correlation lengths of both pressure and velocity. This connection of length scales implies that the monolayer’s combined nematic and granular nature has direct consequences for its emergent behavior at larger scales and especially for the forces it exerts on its environment.   

Flows and forces in the collective motion of the nematode Turbatrix Aceti

We experimentally study the collective motion of the nematode Turbatrix Aceti. We have previously shown that these nematodes, which self-propel and undulate their bodies, constitute a new kind of active matter where synchronization of the motion and the synchronization of oscillations happen at the same time. We have discovered that under favorable conditions they will produce a collectively beating and moving metachronal wave, which is capable of driving fluid flows. In this presentation, we will discuss both these flows and the forces that they generate.   

Fuel-free light-driven colloidal swimmers

Conventional artificial active swimmers use chemical fuel to propel themselves. We present swimmers made by an oil droplet partially wetting a black dynabead that use no chemical fuel and are driven solely by light. The black dynabead absorbs light and provides a temperature gradient responsible for the motion of the swimmer. A thin-film resistive heater which acts as its own thermometer is used as a local heat source in order to characterize the motion and probe the mechanism, Marangoni or thermophoresis, for propulsion. As expected we observe motility-induced phase separation in dense colonies of these swimmers. We show that interactions between individual swimmers are non-reciprocal and orientation dependent.   

Flocking Of Actin Propelled Beads

Polymerizing actin filaments convert chemical energy into mechanical work, giving eukaryotic cells the ability to move and exert forces. This motility is controlled by a large family of actin associated regulatory proteins. In this project, we created dense suspensions of micron sized beads propelled by polymerizing actin filaments. We bound Arp2/3 activating proteins to micron sized polystyrene bead, creating a thick branched network of polymerizing actin at the bead surface that spontaneously break symmetry. This anisotropic polymerization generates a polar force that propels the bead forward while leaving behind an actin 'comet tail'. Confining the beads in quasi-2D geometries leads to a transition from a active gas to an active polar phase, where actin-propelled beads form finite-sized flocks. We show that confinement controls the steric interactions between colliding beads. We are investigating what other interactions between beads are required to generate such polar flocks.   

Strong coupling between the director field and fluid flow in microtubule-based active nematics

Active nematics are bioinspired fluids with local orientational order that exhibit chaotic dynamics and self-mixing.  We consider here a well-studied laboratory example consisting of a two-dimensional (2D) layer of microtubule bundles crosslinked by kinesin molecular motors and driven by adenosine triphosphate (ATP).  In the usual nematohydrodynamic model of this system, the rotation of the director field need not perfectly follow the fluid, even in highly ordered regions away from topological defects.  Similarly, the velocity of the topological defects need not match the local fluid velocity; defects can move either more slowly or quickly than the surrounding fluid.  However, recent work showing that +1/2  defects "stir" microtubule-based active nematics suggests that there is a strong correlation between the defect velocity and the local fluid velocity.  Furthermore, the relatively long length of the microtubule bundles within the active nematic suggests that steric interactions prevent the director field from rotating without a corresponding deformation of the surrounding patch of material.  We thus explore here the limit of strong coupling between the fluid flow and the director field, whereby the directors evolve as though they are passive rods within the material and topological defects move as though they are passive tracers within the fluid.  We present direct experimental evidence on the correlation between these velocities and we derive a nematic transport equation for this system.