On the effect of capillarity and polarity in colloidal suspensions of nanoparticles

Abstract

This thesis deals with a series of phenomena occurring in systems composed of particles with dimensions ranging from a few nanometers up to several microns. The structural properties of these so-called colloidal systems can easily be altered by mechanical or thermal stress of the order of thermal fluctuations. In this sense they belong to the broad branch of physics known as soft matter. Out of the endless possibilities of colloidal systems, in this thesis we focus on colloids trapped at a fluid-fluid interface, and on liquid crystalline colloidal systems. In Chapter 1 we present the phenomenological background that motivated our study. Furthermore, we briefly review the main theoretical tools needed to deal with the introduced phenomenological backgound. In Chapters 2 and 3 we deal with colloids at a fluid-fluid interface. Colloidal particles strongly adsorb at fluid-fluid interfaces in order to reduce the interfacial energy cost. Once adsorbed they can generate deformations in the fluid-fluid interface, which depend on the particle shape and surface chemistry. Such deformations in turn induce capillary interactions between the adsorbed particles, that can be exploited to regulate the self-assembly of the particles while confined to the fluid-fluid interface. In Chapter 2 we consider a variety of Janus particles, i.e. particles having two “faces” with different chemical surface properties, which includes dumbbells, elongated dumbbells and spherocylinders. In Chapter 3 we consider cubic colloidal particles adsorbed at a fluid-fluid interface. By using a numerical method that takes into account the interfacial deformations, we study under what conditions capillarity is able to drive either the Janus particles and the cubes to self-assemble into stable chainlike structures at a fluid-fluid interface. Our numerical findings are in agreement with recent experimental observations. In Chapters 4 and 5 we focus on lyotropic suspensions of curved or bent rods. In Chapter 4 we develop a phenomenological Landau-de Gennes theory able to describe these systems, by using a Q-tensor expansion of the chemical-potential dependent grand potential and introducing a bend flexoelectric term which couples the polarization and the divergence of the Q-tensor. We employ our theory to investigate the stability of the so-called twist-bend and splay-bend nematic phases, the former being a nematic phase characterized by a heliconical variation with bend and twist deformations in the molecular orientation, the latter being a nematic phase characterized by alternating domains of splay and bend. In Chapter 5 we consider an extension of the theory introduced in Chapter 4 that enables us to investigate the effect of spatial distortions in the nematic director field on the density of the twist-bend and splay-bend nematic phases. We find a one-dimensional density modulation in the splay-bend phase. As a consequence the splay-bend phase has the key characteristics of a smectic rather than a nematic phase. By contrast, we find that the twist-bend phase has a homogeneous density and hence is a proper nematic phase. Our results confirm recent simulation findings.