Resumen de Colloidal Dispersions in Fluid Media: Electric, Magnetic and Light Control
Sergi Hernández Navarro
In the present thesis I have worked with particle dispersion in water as well as in liquid crystal. As the first study of this thesis, I have studied the aggregation of isotropic (spherical) and elongated anisometric (pear-shaped) colloidal particles in aqueous medium, confined in two dimensions when subjected to perpendicular external alternating current (AC) electric fields. For low frequencies (f < 2.5kHz) the electrohydrodynamic flow is predominant, and particles tend to aggregate in clusters. On the contrary, for higher frequencies the repulsive dipolar interaction dominates, and particles disperse. Although both types of particles feature a similar behavior under AC field, pear-shaped particles present a richer phase diagram, that is, they have more phases than the spherical ones. I have also found that pear-shaped particles tend to form smaller and more elongated aggregates, with faster aggregation kinetics. I have also tested different ways to measure the strength of the colloidal aggregates using magnetic probes. The following studies of this thesis focus on colloidal dispersions in liquid crystals, which are widely used nowadays to clarify new fundamental concepts and original applications.(1–5) Nematic liquid crystals (NLC) are anisotropic organic fluids whose molecules exhibit the positional disorder of a liquid, but are aligned in a certain direction (called the director of the NLC) (6,7). The director field is usually controlled by certain boundary conditions imposed on the plates of the experimental cell. As a novel way to determine the director orientation, I have demonstrated that paramagnetic anisometric inclusions can be used to locally control the in-plane orientation of the director field by means of external weak magnetic fields. To better understand the phenomenon I have also developed a theoretical model based on the free energy density of the NLC. Additionally, I have found that, by rotating the paramagnetic inclusions more than 100º from their initial orientation, a target pattern of dark and light alternated circles appear. This phenomenon is also captured by the model proposed. In the third phase of this project, I have investigated the controlled motion of micrometer inclusions dispersed in a nematic liquid crystal, propelled by an alternating current (AC) electric field. Recently it has been reported in the literature that micrometric particles can be propelled in NLC by using AC fields, provided that these particles break the symmetry of the NLC director around them. The mechanism explaining this propulsion is called Liquid Crystal-Enabled Electrophoresis (LCEEP) (3). By taking advantage of this mechanism, I have demonstrated that aqueous microdroplets are also propelled by LCEEP. One can make these droplets transport solid polystyrene microparticles, or perform a chemical reaction by coalescing two microdroplets containing separate reactants. In addition, I have also demonstrated the control of the activation or deactivation of LCEEP by using photosensitive particles, which change the NLC director symmetry around them upon UV-visible irradiation. In the last part of this thesis I have developed a novel technique to separately control particle driving from steering under LCEEP. Using photo-induced patterns, I assemble and dynamically control ensembles of particles in a NLC medium. These swarms are assembled, transported and dynamically addressed by local irradiation of the photosensitive cell plate with UV light. With this technique I have demonstrated different potential applications: from the formation and reconfiguration of lattices composed of particle swarms, to segregation of particles with different sizes, as well as the storage and subsequent release of a swarm inside physical constraints, or the formation of particle jets. All these phenomena unveil novel possibilities in the field of collective transport of driven inclusions. References: (1) Koenig, G. M.; Lin, I.-H.; Abbott, N. L. Chemoresponsive Assemblies of Microparticles at Liquid Crystalline Interfaces. Proc. Natl. Acad. Sci. 2010, 107, 3998–4003. (2) Lintuvuori, J. S.; Stratford, K.; Cates, M. E.; Marenduzzo, D. Colloids in Cholesterics: Size-Dependent Defects and Non-Stokesian Microrheology. Phys. Rev. Lett. 2010, 105, 178302. (3) Lavrentovich, O. D.; Lazo, I.; Pishnyak, O. P. Nonlinear Electrophoresis of Dielectric and Metal Spheres in a Nematic Liquid Crystal. Nature 2010, 467, 947–950. (4) Pishnyak, O. P.; Tang, S.; Kelly, J. R.; Shiyanovskii, S.; Lavrentovich, O. D. Levitation, Lift, and Bidirectional Motion of Colloidal Particles in an Electrically Driven Nematic Liquid Crystal. Phys. Rev. Lett. 2007, 99, 127802. (5) Tasinkevych, M.; Mondiot, F.; Mondain-Monval, O.; Loudet, J.-C. Dispersions of Ellipsoidal Particles in a Nematic Liquid Crystal. Soft Matter 2014, 10, 2047–2058. (6) Oswald, P.; Pieranski, P. Nematic and Cholesteric Liquid Crystals: Concepts and Physical Properties Illustrated by Experiments; Taylor& Francis: Boca Raton, 2005. (7) Kleman, M.; Lavrentovich, O. D. Soft Matter Physics - An Introduction; Springer, 2003.
