Light control in active nematics

Abstract

[eng] Living systems are composed of large number of active units that convert local chemical energy into motion. These active systems can be observed across scales, ranging from human crowds to cell cytoskeleton. The continuous energy input sustains these systems out of equilibrium, resulting in the emergence of complex hierarchical structures and large-scale collective motion, which cannot be fully comprehended through equilibrium statistical physics. Moreover, these systems can exhibit spatiotemporally chaotic flows in a state known as active turbulence. One of the significant challenges in the field of active matter is to control these chaotic flows and harness their potential for practical applications. The experimental system used in this thesis is an aqueous-based active gel based on a mixture of microtubules and ATP-fuelled kinesin motor proteins developed by Z. Dogic (Brandais University). In the presence of soft interfaces, this active gel has the ability to self-organize and form a 2D active nematic liquid crystal, commonly known as active nematic. This thesis aims to develop innovative strategies for controlling the active flows of the material using light, while also expanding our knowledge and understanding of the material. A new method for in situ generating hydrogel objects within the active nematic layer using UV light patterns has been developed by incorporating the precursors of the hydrogel to the standard formulation of the material. The properties of these objects can be modified by altering the conditions of the photo-polymerization process. First, this method has been used to imprint flexible cantilevers capable of probing the forces inside the material. As a result, the activity parameter and the shear viscosity of the material have been measured for the first time. Furthermore, this method has been employed to study the influence of substrate interaction. Upon polymerization of the underlying aqueous phase, the material experiences a two-stage transition to a biphasic active fluid. Experiments have been combined with numerical simulations conducted by the group of Dr. A. Doostmohammadi (University of Copenhaguen) to elucidate the mechanisms that govern the transition, revealing the significant role of higher-order activity terms in these scenarios. Lastly, this method has also been used to devise a novel approach for tailoring active flows based on square lattices of triangular objects. These arrays produce the rectification of the active flows to a predetermined direction while allow active mixing. Experimental outcomes are combined with numerical simulations conducted by Dr. R.C.V. Coelho (Universidade de Lisboa) to optimize the performance of the system and to demonstrate its versatility. Additionally, the development of a photosensitive active nematic has been achieved by fusing the motor proteins with optically dimerizable proteins, thereby enabling the spatiotemporal control of the active material's dynamics through light. Its light response has been characterized, revealing a FAST-SLOW behaviour instead of the expected ON-OFF response. The material has been tested in various scenarios which include aligned flows, and the impact of distributing the activity input has been assessed as well.

[cat] Els sistemes vius estan formats per un gran nombre d’unitats actives que transformen l’energia química local en moviment. La entrada continuada d’energia manté aquests sistemes fora de l’equilibri, produint moviments col·lectius a gran escala. A més, aquests sistemes poden exhibir fluxos caòtics en l’espai-temps, en un estat conegut com a turbulència activa. Un dels reptes més grans en el camp de la matèria activa és controlar aquests fluxos caòtics per aprofitar el seu potencial per aplicacions pràctiques. En aquesta tesi s’utilitza un gel actiu format per microtúbuls i kinesines que en contacte amb una interfase tova, s’autoorganitza formant un cristall líquid nemàtic actiu 2D, conegut com a nemàtic actiu. L’objectiu d’aquesta tesi és desenvolupar noves estratègies per controlar els fluxos actius del material utilitzant la llum, i alhora expandir el nostre coneixement del material. S’ha desenvolupat un nou mètode que permet generar in situ objectes d’hidrogel dintre del nemàtic actiu utilitzant patrons de llum UV, les propietats dels quals poden ser modificades canviant les condicions de la fotopolimerització. Aquest mètode s’ha emprat per imprimir voladissos capaços de mesurar les forces a dins del material. Com a resultat, el paràmetre d’activitat així com la seva viscositat han sigut mesurades per primera vegada. A més, aquest mètode s’ha emprat per estudiar la influència de la interacció amb el substrat. Al polimeritzar la fase aquosa subjacent, el material experimenta una transició en dues etapes cap a un fluid actiu bifàsic. Els experiments s’han combinat amb simulacions per dilucidar els mecanismes que governen aquesta transició. Finalment, s’ha ideat un nou mètode per controlar els fluxos actius basat en xarxes quadrades de objectes triangulars. Aquestes distribucions produeixen la rectificació dels fluxos en un sentit predeterminat alhora que permeten la mescla activa. Els resultats experimentals es combinen amb simulacions per optimitzar el seu rendiment i demostrar la seva versatilitat. Addicionalment, s’ha aconseguit desenvolupar un nemàtic actiu fotosensible, permetent així controlar la seva dinàmica en l’espai-temps utilitzant la llum. S’ha caracteritzat la resposta a la llum del material, i s’ha testat el material en diversos escenaris incloent fluxos alineats, així com estudiat l’efecte de distribuir l’entrada d’activitat.