Colloidal Dispersions in Fluid Media: Electric, Magnetic and Light Control

Abstract

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.

Durant aquesta tesi, he treballat amb dispersions de partícules en l'aigua, així com també amb dispersions en cristall líquid nemàtic (NLC).

Com a primer estudi d'aquesta tesi, he investigat la influència de camps elèctrics en dispersions col·loïdals de partícules sòlides en un medi aquós. He estudiat l'agregació de partícules col·loïdals isotròpiques (esfèriques) i anisomètriques allargades (amb forma de pera) en un medi aquós confinat en dues dimensions, quan se sotmet a un camp elèctric de corrent alterna (AC) perpendicular a la superfície de confinament.

En un segon estudi he demostrat que es poden utilitzar inclusions anisomètriques paramagnètiques per controlar localment l'orientació d’un NLC, per mitjà de camps magnètics febles. Per entendre millor el fenomen també he desenvolupat un model teòric basat en la densitat d'energia lliure del NLC. A més, he estat capaç de generar patrons complexos, que també s’expliquen amb model proposat.

En la tercera fase d'aquest projecte, he investigat el moviment controlat d'inclusions micromètriques disperses en NLC, impulsades per un corrent altern (AC) a traves d’un mecanisme anomenat “electroforesi habilitada per cristall líquid” (LCEEP). He demostrat que microgotes aquoses es poden propulsar per LCEEP. Es pot fer que aquestes microgotes transportin micropartícules sòlides de poliestirè, o dur a terme una reacció química mitjançant la coalescència de dos microgotes que contenen reactius separats. A més, també he demostrat el control de l'activació o desactivació de la LCEEP mitjançant l'ús de partícules fotosensibles, en funció de la irradiació UV-visible.

En l'última part d’aquesta tesi he desenvolupat una nova tècnica per a controlar separadament la propulsió i la direcció de moviment de les partícules transportades per LCEEP. Mitjançant l’ús de patrons fotoinduïts, es poden formar i controlar dinàmicament conjunts de partícules en un medi de NLC. Aquests eixams es formen, es transporten i es dirigeixen dinàmicament per irradiació local amb llum UV. Amb aquesta tècnica he pogut demostrat diferents aplicacions potencials: des de la formació i reconfiguració de xarxes cristal·lines compostes d'eixams de partícules, a la segregació de partícules de diferents mides, així com l'emmagatzematge i posterior alliberament d'un eixam dins d’un canal micromètric, o la formació de jets de partícules. Tots aquests fenòmens revelen noves possibilitats en el camp del transport col·lectiu d'inclusions propulsades.