Session B05: Active Matter and Liquid Crystals in Biological and Bio-Inspired Systems II

Live

Topology and anchoring boundary conditions are effective tools to control static defect configurations in equilibrium liquid crystals. In contrast, in active liquid crystals, motile topological defects randomly nucleate and annihilate by local energy consumption, making their dynamics weakly impacted by the topological constraints. Here we show that geometrical confinement can be an effective means to achieve control on the dynamical defect configuration of an active system. In particular, we study a 2-dimensional active nematic, consisting of microtubule bundles sheared by kinesin dimers, in the vicinity of a lateral boundary. While positive and negative 1/2 defects equally populate the bulk, we find that the lateral boundary is exclusively populated by negative defects, which exhibit an exotic behavior : despite being like-charged, they attract each other and eventually fuse together. We show that geometrical patterning of the lateral boundary can allow for control over defect nucleation and induce directional flows. These results provide promising tools for designing autonomous microfluidic transport devices and micromachines.   

Live

We developed an experimental system for studying the fluctuations of a soft interface between an active and passive fluid. In our system, active microtubule bundles are entirely segregated to one phase of a separated liquid-liquid mixture. The spontaneous motion of the active phase causes large interfacial deformations, allowing us to quantify the roughening of the interface by active stresses and to study other intriguing phenomena associated with active interfaces.   

Live

Active nematic films are essentially two-dimensional suspensions of active rod-like particles, such as cytoskeleton filaments driven by motor proteins, that locally consume energy and align collectively to generate orientational order. Increasing activity promotes the spontaneous formation of topological defects in the nematic structure and large-scale structures such as kink walls concomitant with inducing self-sustained fluid flows. Using the hydrodynamic description of active nematics, we derive how the defect velocity and the orientational dynamics of the defects depend on both fluid flow and flow-induced alignment. The resultant non-local hydrodynamic interactions between defects are especially important for incompressible flows - the situation relevant to most active cytoskeletal suspensions. We give an analytic treatment of a pair of defects in the overdamped limit. Coarse-graining the defect equations lead to a hydrodynamic model of the binary defect gas, from which we derive the coupling of the fluid flow to both the defect and polarization density.   

Live

Active nematics are a class of material in which aligned components are driven out of equilibrium by extensile forces along the direction of their alignment. Such systems display remarkable phenomenology including defect pair unbinding, long range flows, and low Reynolds number turbulence. Building off of the theory of passive liquid crystals, continuum models of active nematics have been successful in capturing a good deal of the rich phenomenology present. However, such theories do not provide a link between the microscopic nature of the driving force and mesoscopic dynamics. To address this issue, we interrogate actin liquid crystals that are driven by synthetic myosin motors. By modulating specific motor properties and relating these changes to observable differences in nematic flow and motor dynamics we are able to start teasing apart how microscopic changes in driving contribute to the mesoscopic phenomena in active nematics.   

Live

Active nematics consist of energy-driven particles or subunits that carry out collective motion inherently in an out-of-equilibrium steady state. These active systems are capable of self-organization where energy is pulled from the environment and +1/2 and -1/2 disclinations are continuously created and annihilated in a quasi-2D configuration. The active material is confined between two immiscible fluids, an oil layer and an aqueous layer. In this work, we report phenomena where we introduce submerged retactangular structures, trenches and via microfabrication. The submerged trenches generate a virtual boundary, close to which -1/2 defects tend to stagnate. +1/2 defects tend to locate away from the boundary reminiscent to hard boundaries. When examining an array of submerged steps, we find similar defect distributions to the submerged trench. We also report that the thickness of the oil can change in defect concentration and morphology. We quantify these results by analyzing defect distributions with time and the impact of oil layer thickness on defect morphology.   

Live

In active matter systems, energy consumed at the small scale by individual agents gives rise to emergent flows at large scales. For 2D active nematic microtubule systems, these flows are characterized by the dynamics of mobile defects in the director field. As these defects wind about each other, their trajectories trace out braids, and the topological properties of these braids encode the most important global features of the flow. In particular, the topological entropy of a braid quantifies how chaotic the associated flow is. Since microtubule bundles, an extensile system, stretch out exponentially in time, the resultant defect movement must correspond to braids with positive topological entropy. Indeed, we conjecture that the emergent defect dynamics are optimal in that they give braids which maximize the topological entropy, suitably normalized. In this study, we consider four +1/2 defects on a sphere. We compare the defect motion predicted by this conjecture to the actual motion of defects observed in experiment.   

Live

Kinesin motors and microtubules are the biological building blocks that convert chemical energy into mechanical work. When suspended in water, they assemble in a 3D active isotropic liquid crystal. When suspended in a passive colloidal liquid crystal made of elongated fd viruses, they assemble in a 3D active nematic liquid crystal.
The strategy of building composite materials where only a fraction of the rods are active allow to decouple nematic elasticity and activity, which is challenging in a single component active material. Here, nematic elasticity is controlled by the density of the passive nematic background, while the activity is controlled by the kinesin/MT concentrations. We confined the 3D active nematic in droplets using an oil-water emulsion. Preliminary results show that confinement can stabilize the inherent chaotic dynamics of 3D active nematic droplets. We study how the critical radius depends on the activity and nematic elasticity. The existence of such transition between a quiescent state and a flowing state is reminiscent of the effect of confinement in the Fréedericksz transition for passive liquid crystals under an external magnetic field.   

Live

Active matter consumes local fuels to self-propel. When confined in a closed circular boundary, they can self-organize into a circulatory flow. Such coherence originates from the interactions between the active matter and boundaries, and boundary conditions play an important role on self-organization of active fluid. Herein, we probed how fluid boundaries influenced the self-organization of active fluid. The fluid boundaries were created by confining the active fluid in a compressed water-in-oil droplet. Due to surface tension, the droplet shaped into a cylinder-like geometry. Since water and oil were both fluids, their interface was fluid. We systematically probed how droplet shapes and the amount of oil surrounding the droplet influenced the development of circulation. We found that the formation of circulatory flows depended on the thickness of the oil layer surrounding the droplet, implying that the fluid dynamics between the active fluid within the droplet and the oil outside the droplet were coupled. We used a 3D COMSOL-based simulation successfully reproduced such oil-layer dependence. Finally, we developed two milli-fluidic devices to deform the droplet and alter the oil layer thickness manually to trigger and suppress the intra-droplet circulatory flow in real time.   

Live

Living organisms are built from cells displaying all varieties of morphologies and textures that encode for specific functions and physical behaviors. In the current work, we build biomimmetic structural units by coating ellipsoidal droplets of smectic liquid crystal with an active nematic obtained from a cytoskeletal gel. As an extension to recent research on active spherical systems, we exploit the patterned structure and the anisotropic shape of the shells core to mold the complex nematodynamics of the interfacial active material. We show the existence of novel time-dependent states in which topological defects periodically oscillate between a rotational and a translational configuration. Continuum hydrodynamic simulation of active nematics further support that, beyond topology and activity, these behaviors are profoundly influenced by the geometric properties and the texture of the droplet, as well as by external hydrodynamic forces. Our results illustrate how incorporating new constrains to conventional nematic shells orchestrates remarkable spatiotemporal motifs, paving the way for the design of the next generation of bioinspired micro-machines.   

Live

Coarse-grained simulations have proved to be an invaluable tool in the study of soft condensed matter due to their computational efficiency and their ability to model systems in which the background solvent plays a significant role but is not of principle interest. One successful method is Multi-Particle Collision Dynamics (MPCD), a mesoscopic particle-based algorithm that replaces pair interactions between fluid molecules with a many-particle stochastic collision operator, reproducing flows in long length-scales. MPCD algorithms have been extended to simulate complex fluids including nematic liquid crystals, and more. In this work, we present collision operators which extend MPCD to active systems. By including a novel activity term we obtain local dipole forces between fluid particles. Comparing to experimental flows in active nematic films, we conclude that our algorithm successfully captures key characteristics such as the continuous creation and annihilation of defect pairs and active turbulence. We expect that our extended MPCD will shed new light in the study of composite active-passive systems.   

Live

Motivated by experimental studies of the dynamics of active interfaces in microtubule-based active matter, we formulate a hydrodynamic model that describes the non-equilibrium fluctuations of soft interfaces observed in liquid-liquid phase separation. Numerical solution of the continuum equations reproduces different features of the interfacial dynamics observed in the experiments. Using this model ,we characterize the activity-induced roughening of the active-passive interface, highlighting the role of nonequilibrium active fluctuations.   

Live

Topological defects are signatures of active nematics, that are systems intrinsically out of equilibrium. We study defects in flat and curved space, and how their dynamics is influenced by other defects and the underlying director field. In our experiments, we look at turbulent motion of +1/2 and -1/2 defects in a microtubule-kinesin active nematic. In this talk, we explain how we confine this system to flat 2D cells and shapes of varying Gaussian curvature, K, like ellipsoids and tori. We identify the defects using image analysis, and calculate the defect orientations from the divergence of the nematic tensor order parameter. Using defect positions and orientations, we compute spatial and orientational correlations to investigate defect-ordering. In flat space, we observe short-range ordering of defects, and absence of any global orientational order. The qualitative features of the correlations persist independent of the defect density. We confirm these observations with simulation results. In curved geometries, we study the evolution of defect distributions and how the dynamics couple to the underlying K. We analyze the fluctuations about the steady state of the nematic to extract sound modes, in an attempt to test recent theoretical predictions.   

Live

Liquid crystals (LCs) represent a state of matter that are intermediate between simple liquids and crystalline solids. Its orientational ordering in the nematic phase gives rise to structural anisotropy, the ability to form topological defects, and extraordinary sensitivity to external stimuli, making it an attractive class of materials for applications including display and biosensing. Active LCs, which convert other forms of energy into motion, can therefore offer a new platform to study non-equilibrium matter and to design biomimetic applications. Examples of active LCs include bacteria-LC composites, certain tissues, and dense biopolymer suspensions. When driven internally or externally out of equilibrium, the induced defect dynamics and hydrodynamic flows in these systems are often-times difficult to control, limiting their further applications. In this talk, I will discuss several recent experimental and simulation efforts which demonstrate that nucleation and self-propulsion of defects can be manipulated through, for example, spatiotemporal patterning of activity. I demonstrate that hydrodynamic simulations are particularly successful in elucidating the regulated dynamics and flows observed in cytoskeletal polymer-based active LC experiments performed by our collaborators. I will further show how the highly controlled defect motion and spontaneous flows can be used to transmit force and even information, paving the way towards the design of LC-based autonomous materials systems.