Active boundary layers

The role of boundary layers in conventional liquid crystals is commonly subsumed in their anchoring on confining walls. In the classical view, anchoring enslaves the orientational field of the passive material under equilibrium conditions. In this work, we experimentally explore the role of confining walls in the behavior of an active nematic. We find that, under slip boundary conditions, the wall induces the accumulation of negatively charged topological defects in its vicinity, resulting in the formation of a topological boundary layer that polarizes the wall. While the dynamics of wall and bulk defects are decoupled, we find that the active boundary layer influences the overall dynamics of the system, to the point of fully controlling the behavior of the active nematic in situations of strong confinement. Finally, we show that wall defects exhibit behaviors that are essentially different from those of their bulk counterparts, such as high motility or the ability to recombine with another defect of like-sign topological charge. These exotic behaviors result from a change of symmetry induced by the wall in the director field around the defect. Finally, we show that the collective dynamics of wall defects can be described in terms of a one-dimensional Kuramoto-Sivashinsky -like description of spatio-temporal chaos.Comment: 10 pages, 6 figures in main text, 5 figures in S

Dynamics of ring disclinations driven by active nematic shells

When dispersed in thermotropic nematic liquid crystal oils, surfactant-ladden aqueous droplets often lead to the formation of a equatorial ring disclination in the nearby nematic matrix as a result of a balance between elasticity and interfacial energy. In this experimental work, the aqueous phase contains an extract of cytoskeletal proteins that self-assemble into an active quasi-two-dimensional shell featuring self-sustained periodic flows. The ensuing hydrodynamic coupling drives the surrounding liquid crystal and triggers oscillations in the disclinations. We describe the dynamic modes of the disclinations under different driving conditions, and explore their pathway to collapse under flow conditions

Reconfigurable Flows and Defect Landscape of Confined Active Nematics

Using novel micro-printing techniques, we develop a versatile experimental setup that allows us to study how lateral confinement tames the active flows and defect properties of the microtubule/kinesin active nematic system. We demonstrate that the active length scale that determines the self-organization of this system in unconstrained geometries loses its relevance under strong lateral confinement. Dramatic transitions are observed from chaotic to vortex lattices and defect-free unidirectional flows. Defects, which determine the active flow behavior, are created and annihilated on the channel walls rather than in the bulk, and acquire a strong orientational order in narrow channels. Their nucleation is governed by an instability whose wavelength is effectively screened by the channel width. All these results are recovered in simulations, and the comparison highlights the role of boundary conditions

Active nematic emulsions

The formation of emulsions from multiple immiscible fluids is governed by classical concepts such as surface tension, differential chemical affinity and viscosity, and the action of surface-active agents. Much less is known about emulsification when one of the components is active and thus inherently not constrained by the laws of thermodynamic equilibrium. We demonstrate one such realization consisting in the encapsulation of an active liquid crystal (LC)-like gel, based on microtubules and kinesin molecular motors, into a thermotropic LC. These active nematic emulsions exhibit a variety of dynamic behaviors that arise from the cross-talk between topological defects separately residing in the active and passive components. Using numerical simulations, we show a feedback mechanism by which active flows continuously drive the passive defects that, in response, resolve the otherwise degenerated trajectories of the active defects. Our experiments show that the choice of surfactant, which stabilizes the active/passive interface, allows tuning the regularity of the self-sustained dynamic events. The hybrid active-passive system demonstrated here provides new perspectives for dynamic self-assembly driven by an active material but regulated by the equilibrium properties of the passive component

Active Liquid Crystals in Confinement

[eng] Living systems flow. What appears obvious from our daily observation of people, birds or insects remains surprisingly true at the smallest scale of life. Even at the earliest stages of embryonic development, the most elementary units of living systems, cell tissues, exhibit sustained currents. This perpetual movement is a signature of one of the fundamental properties of living systems - their ability to consume energy and transform it into directed motion. Living systems also cooperate. In the same way as fish swimming collectively form large scale structures to fool their predators, cells self-organize in tissues of increasingly complex shapes. Pattern formation in biology involves many processes from chemical signalling to hydrodynamics. Yet, the striking similarity between the flows and shapes adopted by collective systems at all scales of life motivated the development of a unifying theory, containing the minimal physical processes involved. This framework is called active soft matter. It refers to any system composed of self-driven units that consume and convert energy into directed motion. In some cases, the particles are so densely packed that they can be described as a continuous phase with long-range orientational order. This particular class has been termed active liquid crystals, of which cell tissues are the flagship illustration. These systems are characterized by a peculiar interplay between order and flows. The constant energy consumption drives them out of thermodynamic equilibrium. As a consequence, they are constantly deforming by sustained - and typically chaotic - flows. Reciprocally, the flow pattern directly depends on the local ordering of the particles. Beyond the apparent chaos, this interplay between activity and order also confers to active liquid crystals a fascinating ability to adapt to the environments where they reside. In this work, we investigate the interplay between the geometry, the order and the flows of an active liquid crystal. Using novel micro-printing techniques, we develop versatile experimental setups that allow us to study how geometrical confinement tames the active flows and defect properties. We specifically investigate the effect of lateral confinement, topology, boundary roughness and Gaussian curvature. We report dramatic transformations of the spatio-temporal dynamics of an in vitro microtuble-based active nematic system. The so-called active turbulence reorganizes into vortex lattices, directed, or defect-free unidirectional flows. Topological defects, which determine the active flow behavior, are created and annihilated on the boundaries rather than in the bulk, and acquire a strong orientational order in narrow channels. Their nucleation is governed by an instability whose wavelength is effectively screened by the lateral confinement. Their density, spatial distribution, orientation, and velocities evade most of the laws derived for unconfined active nematics. The careful description of the co-evolving order and flow patterns away from active turbulence enables us, to some extent, to disentangle the way they interact. In addition, we relate the transition to ordered regimes to generic descriptions of spatio- temporal chaos in out-of-equilibrium fluids, in an effort to understand the physics of these complex systems through universal laws. Dramatic transitions also occur in the case of closed interfaces i.e surfaces with no boundaries. In the last part of the manuscript, we report an original example of spinning active nematic droplets. We condense an active nematic layer on the outer surface of oil droplets with an ellipsoidal shape. In this configuration, topology and Gaussian curvature contribute to the emergence of a chiral symmetry breaking in the active deformations. This chirality is transferred to the solid-body dynamics of the ellipsoids, which rotate with a surprisingly constant pulsation. These results demonstrate how the non- equilibrium dynamics of active materials could be converted into macroscopic engines. Our result not only improve the theoretical understanding of active liquid crystals. We also demonstrate promising strategies to control the spatial organization and the active flows through geometrical confinement, which could contribute to the design of autonomous microfluidic systems performing complex tasks without any external input.[cat] Els sistemes vius flueixen. El que sembla evident a partir de la nostra observació diària de persones, aus o insectes segueix sent sorprenentment cert a la menor escala de la vida. Aquest moviment perpetu és una signatura d’una de les propietats fonamentals dels sistemes vius: la seva capacitat de consumir energia i transformar-la en moviment dirigit. Els sistemes de vida també cooperen. La cridanera similitud entre els fluxos i les formes adoptades pels sistemes col·lectius a totes les escales de la vida va motivar el desenvolupament d’una teoria unificadora, que contenia els processos físics mínims implicats. Aquest marc s’anomena matèria tova activa. Es refereix a qualsevol sistema compost per unitats impulsades per si mateixes que consumeixen i converteixen l’energia en moviment dirigit. En aquest treball s’investiga la interacció entre la geometria, l’ordre i els fluxos d’un cristall líquid actiu. Amb noves tècniques de microimpressió, desenvolupem configuracions experimentals versàtils que ens permeten estudiar com la confinament geomètrica doma els fluxos actius i les propietats dels defectes. Investiguem específicament l'efecte del confinament lateral, la topologia, la rugositat del límit i la curvatura gaussiana. Es reporten transformacions dramàtiques de la dinàmica espaciotemporal d’un sistema nemàtic actiu basat en microtubs. Una acurada descripció de l'ordre i dels patrons de flux que evolucionen lluny de les turbulències actives ens permet, fins a cert punt, desvincular la forma en què interactuen. A més, relacionem la transició a règims ordenats a descripcions genèriques del caos espaciotemporal en fluids fora d'equilibri, en un esforç per comprendre la física d'aquests sistemes complexos mitjançant lleis universals. A la darrera part del manuscrit, es presenta un exemple original de gota de gotes nemàtiques actives. Aquests resultats demostren com la dinàmica de no equilibri dels materials actius es podia convertir en motors macroscòpics. El nostre resultat no només millora la comprensió teòrica dels cristalls líquids actius. També demostrem estratègies prometedores per controlar l’organització espacial i els fluxos actius mitjançant confinament geomètric, que podrien contribuir al disseny de sistemes microfluídics autònoms que realitzen tasques complexes sense cap entrada externa

Active microfluidic transport in two-dimensional handlebodies

Unlike traditional nematic liquid crystals, which adopt ordered equilibrium configurations compatible with the topological constraints imposed by the boundaries, active nematics are intrinsically disordered because of their self- sustained internal flows. Controlling the flow patterns of active nematics remains a limiting step towards their use as functional materials. Here we show that confining a tubulin-kinesin active nematic to a network of connected annular microfluidic channels enables controlled directional flows and autonomous transport. In single annular channels, for narrow widths, the typically chaotic streams transform into well-defined circulating flows, whose direction or handedness can be controlled by introducing asymmetric corrugations on the channel walls. The dynamics is altered when two or three annular channels are interconnected. These more complex topologies lead to scenarios of synchronization, anti-correlation, and frustration of the active flows, and to the stabilisation of high topological singularities in both the flow field and the orientational field of the material. Controlling textures and flows in these microfluidic platforms opens unexplored perspectives towards their application in biotechnology and material science

Active nematic emulsions

The formation of emulsions from multiple immiscible fluids is governed by classical concepts such as surface tension, differential chemical affinity and viscosity, and the action of surface-active agents. Much less is known about emulsification when one of the components is active and thus inherently not constrained by the laws of thermodynamic equilibrium. We demonstrate one such realization consisting in the encapsulation of an active liquid crystal (LC)–like gel, based on microtubules and kinesin molecular motors, into a thermotropic LC. These active nematic emulsions exhibit a variety of dynamic behaviors that arise from the cross-talk between topological defects separately residing in the active and passive components. Using numerical simulations, we show a feedback mechanism by which active flows continuously drive the passive defects that, in response, resolve the otherwise degenerated trajectories of the active defects. Our experiments show that the choice of surfactant, which stabilizes the active/passive interface, allows tuning the regularity of the self-sustained dynamic events. The hybrid active-passive system demonstrated here provides new perspectives for dynamic self-assembly driven by an active material but regulated by the equilibrium properties of the passive component

Active nematic emulsions

The formation of emulsions from multiple immiscible fluids is governed by classical concepts such as surface tension, differential chemical affinity and viscosity, and the action of surface-active agents. Much less is known about emulsification when one of the components is active and thus inherently not constrained by the laws of thermodynamic equilibrium. We demonstrate one such realization consisting in the encapsulation of an active liquid crystal (LC)-like gel, based on microtubules and kinesin molecular motors, into a thermotropic LC. These active nematic emulsions exhibit a variety of dynamic behaviors that arise from the cross-talk between topological defects separately residing in the active and passive components. Using numerical simulations, we show a feedback mechanism by which active flows continuously drive the passive defects that, in response, resolve the otherwise degenerated trajectories of the active defects. Our experiments show that the choice of surfactant, which stabilizes the active/passive interface, allows tuning the regularity of the self-sustained dynamic events. The hybrid active-passive system demonstrated here provides new perspectives for dynamic self-assembly driven by an active material but regulated by the equilibrium properties of the passive component

Reconfigurable flows and defect landscape of confined active nematics

The physics of active liquid crystals is mostly governed by the interplay between elastic forces that align their constituents, and active stresses that destabilize the order with constant nucleation of topological defects and chaotic flows. The average distance between defects, also called active length scale, depends on the competition between these forces. Here, in experiments with the microtubule/kinesin active nematic system, we show that the intrinsic active length scale loses its relevance under strong lateral confinement. Transitions are observed from chaotic to vortex lattices and defect-free unidirectional flows. Defects, which determine the active flow behaviour, are created and annihilated on the channel walls rather than in the bulk, and acquire a strong orientational order in narrow channels. Their nucleation is governed by an instability whose wavelength is effectively screened by the channel width. These results are recovered in simulations, and the comparison highlights the role of boundary conditions