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Control of active flows through soft interfaces

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Universitat de Barcelona. Departament de Ciència dels Materials i Química Física

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[eng]  Groups of animals, bacterial colonies, cellular tissues and assemblies of subcellular extracts are some examples of experimental systems studied in the field of Active Soft Matter. All of them are composed of autonomous self-propelled units that consume and transform energy to generate mechanical work. The interaction between these motile units lead to the emergence of cooperative spatiotemporal patterns, not observed in complex fluids in equilibrium. Despite the morphology and dynamics of these systems are being studied in detail, there is still absence of true control capabilities, which could bring new possibilities in the use or application of active flows. To this end, this thesis aims at the development of strategies to condition active flows by means of non-invasive bounds, namely rheological patterning and confinement, as a tool towards the control of the intrinsic unpredictable chaotic behaviour of active matter systems.
The experimental system used here is an active gel based on a mixture of cytoskeletal proteins, created in the laboratory of Z. Dogic from Brandeis University (MA, USA) in 2012. In brief, ATP-fuelled kinesin motor clusters crosslink and drive bundled microtubules, giving rise to an active network of biofilaments that develops far from thermodynamic equilibrium. The active gel can also self-organize at soft interfaces, where it forms a quasi-2d active nematic liquid crystal, which features spontaneous turbulent-like flows. In this thesis, first, we report experimental evidence of the existence of strong hydrodynamic coupling at the oil/water interface, where the active nematic resides, and the influence of the rheological properties of the oil phase. By changing the viscosity of the contacting oily fluid, we alter the morphology and dynamics of the active nematic, which we have characterized. In addition, in collaboration with M. C. Marchetti and S. Shankar from Syracuse University (NY, USA), we have fitted specific data to a hydrodynamical model in order to extract an estimate value for the viscosity of the active material. Second, based on these observations, and with the objective of steering the active flows, we impose viscosity patterns at the interface. For this purpose, we use a thermotropic liquid crystal, which self-assemble in well-known structures with marked anisotropic viscosity, externally- and in situ-tuneable by means of temperature and/or external fields. Under such rheological constraints, the active nematic flows are commanded at will, rapidly organizing either in localized rotating swirls or parallel stripes of aligned microtubule bundles. Through this process, we have also had the opportunity to study the interaction between active and passive nematic liquid crystals, which in this case, serve as reporters of the active flows. Finally, we prepare active emulsions by dispersing droplets of active gel in different fluids. Inside droplets, the active gel condenses at the inner surfaces to create an active nematic spherical shell, which develops in geometrically and topologically constrained conditions. Due to the confinement restrictions, the active nematic develops strikingly periodic dynamics that transmit coherent flows into the confining phase. Here, with experiments and simulations performed by M. Ravnik and Ž. Kos from the University of Ljubljana (Slovenia), we study emulsions of droplets with an active nematic shell dispersed in thermotropic nematic liquid crystals. In particular, we focus on the interaction between active flows and the usually static topological defects induced around inclusions in liquid crystals.
To conclude, this work not only increases our fundamental knowledge of both thermotropic (passive) and active nematic liquid crystals but it serves as a starting platform to explore the interaction between these two fluid ordered analogues at the interface. Special emphasis will be put on the implementation of anisotropic patterns at interfaces as it has demonstrated to be key towards controlling active flows.

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[cat]  Sistemes compostos per grups d’animals, colònies de bacteris, teixits de cèl·lules o assemblatges d’extractes cel·lulars, mostren comportaments dinàmics complexos significativament similars tot i que, evidentment, es desenvolupen a escales espai-temps molt diverses. Aquests sistemes, anomenats sistemes actius, estan generalment formats per unitats individuals auto-propulsades que consumeixen energia de l’ambient, a partir de la qual generen forces i treball mecànic. La interacció entre els constituents d’aquests sistemes propicia moviments col·lectius i cooperatius, així com patrons de flux que no s’observen en sistemes similars en equilibri termodinàmic.

Tot i que les característiques morfològiques i dinàmiques d’aquests sistemes s’estan estudiant amb detall, manquen encara estratègies per controlar els fluxos actius que se’n deriven. L’habilitat de controlar sistemes actius, no només en facilita la seva caracterització sinó que possibilita l’aplicació dels fluxos que se’n deriven, per exemple, en dispositius. Amb aquest objectiu, aquesta tesi se centra en el desenvolupament d’estratègies per al condicionament i control de fluxos actius mitjançant constriccions que procuren ser no invasives per als materials implicats. El material estudiat consisteix en un gel actiu aquós format per agregats de microtúbuls, reticulats per complexos de motors moleculars. En presència d’Adenosina trifosfat (ATP), els complexos motors exerceixen forces de cisalla locals entre els microtúbuls que, globalment, provoquen contínuament l’extensió, flexió i trencament dels agregats filamentosos. La interacció entre els constituents actius genera fluxos turbulents a escales molt més grans que les pròpies de les unitats constitutives del material. D’altra banda, en presència d’una interfície aigua/oli correctament funcionalitzada, el gel es pot densificar, desenvolupant els seus fluxos en contacte amb la fase oliosa. D’aquesta manera, s’obté un material actiu quasi-bidimensional molt dens, en el qual els filaments interaccionen entre si i s’organitzen en el pla donant lloc a un gel actiu amb ordre orientational.

En particular, en aquesta tesi, s’estudiarà l’efecte de l’acoblament hidrodinàmic d’aquest material amb fluids viscosos isotròpics, amb patrons reològics imposats per cristalls líquids i en confinament, com a eines per al control dels fluxos, fins ara, aparentment caòtics i impredictibles d’aquests sistemes actius.

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Tesis Doctorals - Departament - Ciència dels Materials i Química Física

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Control of active flows through soft interfaces