Transport and assembly of colloids in liquid crystals

Abstract

[eng] Particles dispersed in nematic liquid crystals (NLCs), also called nematic colloids, are nowadays typical experimental systems studied in the field of Soft Matter. In this scenario, the interactions and forces between colloidal particles are in equilibrium, and thus, elasticity governs inter-particle interactions. Furthermore, the application of an AC electric external field leads to a totally different scenario where the system is out-of-equilibrium, and thus, new physical phenomena can emerge. Nematic colloidal particles can be propelled under the application of an electric field, which allows for driven particles scenarios. Briefly, motion of colloids arises from the appearance of electroosmotic flows surrounding the particles. The symmetry breaking of these flows at particles apexes induces net propulsion to the particles.

In this dissertation, we show that the transport modes of a single particle propelled in a NLC are different for the motion parallel and perpendicular to the averaged molecular orientation of the material, the director field. Here, the observed trends do not depend on the driving mechanism neither the propulsion speed. While motion is ballistic in the driving direction (parallel to the nematic director), our experiments show that transversal fluctuations can become superdiffusive depending on the configuration of the NLC surrounding the particle. For the superdiffusive behaviour, which is related to dipolar configurations of the nematic director, we have proposed a mechanism based on the geometry of the liquid crystal backflow to justify the persistence of thermal fluctuations affecting on the orientation of the dipolar configuration, and thus, the observed superdiffusive modes.

Going further, and by using hundreds of anisometric particles, we have induced colloidal clusters assembled above a topological defect. By tuning the elastic properties of the NLC material by in situ modification of the anchoring conditions, two different configurations can be induced, aster or rotating mills. This process is achieved thanks to a photosensitive surface which allows to switch from the trans (homeotropic) to the cis (planar) isomer (NLC) under UV-light irradiation. In the case of asters, we observe the formation of stationary clusters that display radially extended density gradients with three different states of aggregation, an innermost “jammed” part, followed by a liquid-like corona and ending in a diluted, gas-like, phase. Moreover, we can describe our system with a non-equilibrium equation of state and directly determine the effective pressure and temperature in the system. In contrast, for the rotating mills we obtained a dynamic assembly that has been analysed in terms of velocities and order parameters. To understand either the different assemblies obtained or the phases observed, we developed a theoretical model that combines different interactions resulting from phoretic propulsion, dipolar forces and hydrodynamics allowing to capture the basic physics of the process.

The last part of this thesis is based on steering and guiding the collective colloidal transport by means of both photo-patterning and confining devices. The first set of experiments of experiments is based on understanding the dynamics of an ensemble of flocking particles dispersed in a NLC. Furthermore, we show the implementation of obstacles for the flock to pass.

To conclude, this work not only increases our fundamental knowledge of micron-sized particles dispersed in anisotropic materials, such as nematic liquid crystals, but it serves as a starting platform to explore the motion of driven colloids inside them. Special emphasis will be put on the implementation of new techniques for guiding and steering the colloidal trajectories by means of both photo-patterning and microfluidic devices, as it has been demonstrated to be key towards the control of colloidal trajectories.

[cat] Les partícules dispersades en un cristall líquid nemàtic (CLN), també anomenats col·loides nemàtics, són avui dia sistemes experimentals estudiats en el camp de la “Soft Matter”. En aquest escenari, les interaccions i forces entre partícules col·loïdals es troben en equilibri, així, l’elasticitat governa les interaccions entre partícules. Més enllà, l’aplicació d’un camp elèctric altern dóna lloc a un escenari totalment diferent, on el sistema es troba fora d’equilibri, i així, nous conceptes físics poden sorgir. Els col·loides nemàtics es poden propulsar sota l’acció d’un camp elèctric, el qual permet escenaris on els col·loides són dirigits. Breument, el moviment de les partícules és degut a l’aparició de fluxos electroosmòtics al voltant de les partícules. El trencament de la simetria dels fluxos a banda i banda de la partícula indueix una propulsió neta.

En aquest manuscrit es mostra des de l’estudi del transport d’una partícula individual en termes de modes de transport fins a l’estudi de moviments col·loïdals col·lectius. L’estudi d’una partícula individual s’ha dut a terme sota l’acció de dos mecanismes diferents de propulsió, el de sedimentació-difusió i la “Liquid Crystal-Enabled Electrokinetics”. Per altra banda, prenent avantatge de la última hem pogut auto-assemblar centenars de partícules formant clústers amb una distribució radial inhomogènia de la densitat de partícules. Per entendre aquest tipus de clústers hem desenvolupat un model teòric i el Dr. Arthur Straube ha realitzat simulacions que concorden tant qualitativa, com quantitativament amb les tendències experimentals. Finalment, hem estudiat el fet de traslladar un eixam de partícules, però també el fet de posar-li obstacles per on hagi de passar.

Per concloure, aquest treball no només augmenta el coneixement general de partícules micro- mètriques dispersades en CLNs, però serveix com a eina per iniciar-se i explorar el moviment dirigit de col·loides dispersat en aquests.