arXiv subjects

Teresa Lopez-Leon

Publications and source records attributed to Teresa Lopez-Leon.

13 recordsLinked to original sources

arXiv2023

Spontaneous Self-Constraint in Active Nematic Flows

Active processes drive and guide biological dynamics across scales -- from subcellular cytoskeletal remodelling, through tissue development in embryogenesis, to population-level bacterial colonies expansion. In each of these, biological functionality requires collective flows to occur while self-organized structures are protected; however, the mechanisms by which active flows can spontaneously constrain their dynamics to preserve structure have not previously been explained. By studying collective flows and defect dynamics in active nematic films, we demonstrate the existence of a self-constraint -- a two-way, spontaneously arising relationship between activity-driven isosurfaces of flow boundaries and mesoscale nematic structures. Our results show that self-motile defects are tightly constrained to viscometric surfaces -- contours along which vorticity and strain-rate balance. This in turn reveals that self-motile defects break mirror symmetry when they move along a single viscometric surface, in contrast with expectations. This is explained by an interdependence between viscometric surfaces and bend walls -- elongated narrow kinks in the orientation field. Although we focus on extensile nematic films, numerical results show the constraint holds whenever activity leads to motile half-charge defects. This mesoscale cross-field self-constraint offers a new framework for tackling complex 3D active turbulence, designing dynamic control into biomimetic materials, and understanding how biological systems can employ active stress for dynamic self-organization.

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arXiv2023

Dynamics of active defects on the anisotropic surface of an ellipsoidal droplet

Cells are fundamental building blocks of living organisms displaying an array of shapes, morphologies, and textures that encode specific functions and physical behaviors. Elucidating the rules of this code remains a challenge. In this work, we create biomimetic structural building blocks by coating ellipsoidal droplets of a smectic liquid crystal with a protein-based active cytoskeletal gel, thus obtaining core-shell structures. By exploiting the patterned texture and anisotropic shape of the smectic core, we were able to mold the complex nematodynamics of the interfacial active material and identify new time-dependent states where topological defects periodically oscillate between rotational and translational regimes. Our nemato-hydrodynamic simulations of active nematics demonstrate that, beyond topology and activity, the dynamics of the active material are profoundly influenced by the local curvature and smectic texture of the droplet, as well as by external hydrodynamic forces. These results illustrate how the incorporation of these constraints into active nematic shells orchestrates remarkable spatio-temporal motifs, offering critical new insights into biological processes and providing compelling prospects for designing bio-inspired micro-machines.

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arXiv2022

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.

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arXiv2022

Frustrated run and tumble of swimming E-coli bacteria in nematic liquid crystals

In many situations bacteria move in complex environments, as for example in soils, oceans or the human gut-track microbiome. In these natural environments, carrier fluids such as mucus or reproductive fluids show complex structure associated with non-Newtonian rheology. Many fundamental questions concerning the the ability to navigate in such environments remain unsolved due to the inherent complexity of the natural surroundings. Recently, the interaction of swimming bacteria with nematic liquid crystals has attracted lot of attention. In these structured fluids, the kinetics of bacterial motion is constrained by the orientational molecular order of the liquid crystal (or director field) and novel spatio-temporal patterns arise from this orientational constraint, as well as from the interactions with topological defects. A question unaddressed so far is how bacteria are able to change swimming direction in such an environment. In this work, we study the swimming mechanism of a single bacterium, E. coli, constrained to move along the director field of a lyotropic chromonic liquid crystal (LCLC) that is confined to a planar cell. In such an environment, the spontaneous run and tumble motion of the bacterium gets frustrated: the elasticity of the liquid crystal prevents flagella from unbundling. Interestingly, in order to change direction, bacteria execute a reversal motion along the director field, driven by the relocation of a single flagellum to the other side of the bacterial body, coined as a frustrated tumble. We present a detailed experimental characterization of this phenomenon, exploiting exceptional spatial and temporal resolution of bacteria and flagella dynamics during swimming, obtained using a two color Lagrangian tracking technique. We suggest a possible mechanism behind the frustrated run and tumble motion, accounting for these observations.

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arXiv2021

Helfrich-Hurault elastic instabilities driven by geometrical frustration

The Helfrich-Hurault (HH) elastic instability is a well-known mechanism behind patterns that form as a result of strain upon liquid crystal systems with periodic ground states. In the HH model, layered structures undulate and buckle in response to local, geometric incompatibilities, in order to maintain the preferred layer spacing. Classic HH systems include cholesteric liquid crystals under electromagnetic field distortions and smectic liquid crystals under mechanical strains, where both materials are confined between rigid substrates. However, richer phenomena are observed when undulation instabilities occur in the presence of deformable interfaces and variable boundary conditions. Understanding how the HH instability is affected by deformable surfaces is imperative for applying the instability to a broader range of materials. In this review, we re-examine the HH instability and give special focus to how the boundary conditions influence the mechanical response of lamellar systems to geometrical frustration. We use lamellar liquid crystals confined within a spherical shell geometry as our model system. Made possible by the relatively recent advances in microfluidics within the past 15 years, liquid crystal shells are composed entirely of fluid interfaces and have boundary conditions that can be dynamically controlled at will. We examine past and recent work that exemplifies how topological constraints, molecular anchoring conditions, and boundary curvature can trigger the HH instability in liquid crystals with periodic ground states. We then end by identifying similar phenomena across a wide variety of materials, both biological and synthetic. With this review, we aim to highlight that the HH instability is a generic and often overlooked response of periodic materials to geometrical frustration.

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arXiv2020

Temperature-driven anchoring transitions at liquid crystal / water interfaces

Controlling the anchoring of liquid crystal molecules at an interface with a water solution influences the entire organization of the underlying liquid crystal phase, which is crucial for many applications. The simplest way to stabilize such interfaces is by fabricating droplets of liquid crystal in water; however, a greater sensitivity to interfacial effects can be achieved using liquid crystal shells, i.e. spherical films of liquid crystal suspended in water. Anchoring transitions on those systems are traditionally triggered by the adsorption of surfactant molecules onto the interface, which is neither an instantaneous nor a reversible process. In this study, we report the ability to change the anchoring of 4-cyano-4'-pentylbiphenyl (5CB), one of the most widely used liquid crystals, at the interface with dilute water solutions of polyvinyl alcohol (PVA), a polymer commonly used for stabilizing liquid crystal shells, simply by controlling the temperature in the close vicinity of the liquid crystal clearing point. A quasi-static increase in temperature triggers an instantaneous reorientation of the molecules from parallel to perpendicular to the interfaces, owing to the local disordering effect of PVA on 5CB, prior to the phase transition of the bulk 5CB. We study this anchoring transition on both flat suspended films and spherical shells of liquid crystals. Switching anchoring entails a series of structural transformations involving the formation of transient structures in which topological defects are stabilized. The type of defect structure depends on the topology of the film. This method has the ability to influence both interfaces of the film nearly at the same time, and can be applied to transform an initially polydisperse group of nematic shells into a monodisperse population of bivalent shells.

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arXiv2020

Structural transformations in tetravalent nematic shells induced by a magnetic field

The role of applied fields on the structure of liquid crystals confined to shell geometries has been studied in past theoretical work, providing strategies to produce liquid crystal shells with controlled defect structure or valence. However, the predictions of such studies have not been experimentally explored yet. In this work, we study the structural transformations undergone by tetravalent nematic liquid crystal shells under a strong uniform magnetic field, using both experiments and simulations. We consider two different cases in terms of shell geometry and initial defect symmetry: i) homogeneous shells with four s = +1/2 defects in a tetrahedral arrangement, and ii) inhomogeneous shells with four s = +1/2 defects localized in their thinner parts. Consistently with previous theoretical results, we observe that the initial defect structure evolves into a bipolar one, in a process where the defects migrate towards the poles. Interestingly, we find that the defect trajectories and dynamics are controlled by curvature walls that connect the defects by pairs. Based on the angle between Bs, the local projection of the magnetic field on the shell surface, and n+ 1/2 , a vector describing the defect orientations, we are able to predict the nature and shape of those inversion walls, and therefore, the trajectory and dynamics of the defects. This rule, based on symmetry arguments, is consistent with both experiments and simulations and applies for shells that are either homogeneous or inhomogeneous in thickness. By modifying the angle between Bs and n+1/2, we are able to induce, in controlled way, complex routes towards the final bipolar state. In the case of inhomogeneous shells, the specific symmetry of the shell allowed us to observe a hybrid splay-bend Helfrich wall for the first time.

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arXiv2019

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.

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arXiv2017

A change in stripes for cholesteric shells via anchoring in moderation

Chirality, ubiquitous in complex biological systems, can be controlled and quantified in synthetic materials such as cholesteric liquid crystal (CLC) systems. In this work, we study spherical shells of CLC under weak anchoring conditions. We induce anchoring transitions at the inner and outer boundaries using two independent methods: by changing the surfactant concentration or by raising the temperature close to the clearing point. The shell confinement leads to new states and associated surface structures: a state where large stripes on the shell can be filled with smaller, perpendicular sub-stripes, and a focal conic domain (FCD) state, where thin stripes wrap into at least two, topologically required, double spirals. Focusing on the latter state, we use a Landau-de Gennes model of the CLC to simulate its detailed configurations as a function of anchoring strength. By abruptly changing the topological constraints on the shell, we are able to study the interconversion between director defects and pitch defects, a phenomenon usually restricted by the complexity of the cholesteric phase. This work extends the knowledge of cholesteric patterns, structures that not only have potential for use as intricate, self-assembly blueprints but are pervasive in biological systems.

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arXiv2016

Elastic interactions between topological defects in chiral nematic shells

We present a novel, self-consistent and robust theoretical model to investigate elastic interactions between topological defects in liquid crystal shells. Accounting for the non-concentric nature of the shell in a simple manner, we are able to successfully and accurately explain and predict the positions of the defects, most relevant in the context of colloidal self-assembly. We calibrate and test our model on existing experimental data, and extend it to all newly observed defects configurations in chiral nematic shells. We perform new experiments to check further and confirm the validity of the present model. Moreover, we are able to obtain quantitative estimates of the energies of $+1$ or $+3/2$ disclination lines in cholesterics, whose intricate nature was only reported recently.

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arXiv2016

Topological defects in cholesteric liquid crystal shells

We investigate experimentally and numerically the defect configurations emerging when a cholesteric liquid crystal is confined to a spherical shell. We uncover a rich scenario of defect configurations, some of them non-existent in nematic shells, where new types of defects are stabilized by the helical ordering of the liquid crystal. In contrast to nematic shells, here defects are not simple singular points or lines, but have a large structured core. Specifically, we observe five different types of cholesteric shells. We study the statistical distribution of the different types of shells as a function of the two relevant geometrical dimensionless parameters of the system. By playing with these parameters, we are able to induce transitions between different types of shells. These transitions involve interesting topological transformations in which the defects recombine to form new structures. Surprisingly, the defects do not approach each other by taking the shorter distance route (geodesic), but by following intricate paths.

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arXiv2015

Waltzing route towards double-helix formation in cholesteric shells

We study cholesteric order in liquid crystal shells with planar degenerate anchoring. We observe that the bipolar and radial configurations intensively reported for bulk droplets have a higher degree of complexity when the liquid crystal is confined to a spherical shell. The bipolar configuration is replaced by a structure where the boojums are linked to a stack of disclination rings that spans the shell, while the radial configuration exhibits a double helix structure where two disclinations wind around each other. Our results confirm recent numerical simulations and highlight the complexity of the defect structures arising when cholesteric liquid crystals are confined to spherical geometries. We also show that the position of the boojums is only ruled by the shell geometry, independently of the cholesteric pitch. To understand quantitatively this behavior, we develop a simple yet insightful theoretical framework which captures the essence of the observed phenomenology. We also show that the transition between the two configurations is solely governed by the confinement ratio c = h/p, where h is the average shell thickness and p is the cholesteric pitch. Finally, we perform a dynamical study of this transition, and report a fascinating defect waltz due to a chemical Lehmann effect.

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arXiv2015

Spherical nematics with a threefold valence

We present a theoretical study of the energetics of thin nematic shells with two charge one-half defects and one charge-one defect. We determine the optimal arrangement: the defects are located on a great circle at the vertices of an isosceles triangle with angles of 66 degrees at the charge one-half defects and a distinct angle of 48 degrees, consistent with experimental findings. We also analyse thermal fluctuations around this ground state and estimate the energy as a function of thickness. We find that the energy of the three-defect shell is close to the energy of other known configurations having two charge-one and four charge one-half defects. This finding, together with the large energy barriers separating one configuration from the others, explains their observation in experiments as well as their long-time stability.

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