Abstract:
Liquid Crystals (LCs) are soft materials with molecules possessing an orientational order similar to solids but with mobility similar to liquids. Addition of colloidal particles to LCs disturbs the molecular order and leads to the formation of topological defects. It has been shown that the particle - defect interactions lead to several types of self - assembled particle structures of 1 - D linear chains, 2 - D sheets, 3 - D cellular networks, which have not been observed in isotropic liquids. Moreover, the external field required to align LC molecules is also reported to be significantly reduced by addition of particles. This work aims to understand the factors influencing the particle self - assembly and the resulting viscoelastic properties of particles-in-LC composites. Two types of ordered matrices are explored in this work: 1) hexagonal (H1) LCs which are composed of cylinders of amphiphilic molecules arranged in a H1 lattice and 2) Nematic LCs (NLCs) which have rod like molecules oriented in a direction, without a positional order. We first present the self - assembly and rheology of colloidal particles in H1 LC phase constituted by mixing a non - ionic surfactant, nonaethylene glycol monodecyl ether (C12E9) and DI water in 1:1 proportion by weight. The size of colloidal particles is in the range of 100 nm - 500 nm which is substantially greater than the characteristic spacing between the surfactant cylinders (5.7 nm). We show that the colloidal particles assemble at the grain boundaries (GBs) of the H1 LC with particle structures dependent on the particle shape. The results reveal that while elongated and irregular particles form end to end interconnected particle networks, spherical particles form only aggregates at the H1 GBs. The difference in particle shape - driven self - assembly also manifests in the rheological response as the formation of interconnected network at the GBs increases the elasticity of the H1 LC phase more significantly than the presence of aggregates. In addition, the interplay of factors influencing the efficiency of the partitioning of the particle at the H1 GBs and their resultant effect on rheology is also investigated. Our results demonstrate that with increase in particle loading, density and fast phase transition kinetics, efficiency of partitioning of particles at the GBs decreases. We show that while complete partitioning of particles at the H1 GBs leads to a monotonic increase in the elastic moduli with particle loading, a decrease in partitioning efficiency leads to a non - monotonic trend in elastic moduli with particle loading. Changing the dispersing medium to NLCs presents a very different scenario. The rheological response resulting from the particle self - assembly of the hydrophilic Titania (TiO2) nanoparticles in a NLC, N-(4-Methoxybenzilidene)-4-butylaniline (MBBA) is investigated. Our results show that while particle aggregates are formed at low particle loadings, a sample spanning network of particle flocs is formed above a threshold particle concentration and the NLC is confined in the network. We show that as the concentration of particles increases, the nematic order is progressively disturbed and a metastable, structurally disordered composite is formed. Further extension of this work involves studying the anchoring transitions of NLC, 4-cyano-4?-pentylbiphenyl (5CB) at 5CB - aqueous and 5CB - solid interface. 5CB drop decorated glass surfaces are fabricated by flow coating, and the anchoring transition of 5CB is studied in response to varying surfactant concentrations and structures. The ordering transition of 5CB at 5CB - aqueous interface from tilted to perpendicular anchoring occurs at concentrations close to the critical micelle concentration (CMC) of surfactants, depending on the organization of the surfactant tails at the interface. The anchoring of 5CB molecules at 5CB - solid interface is also shown to correlate with the surface energy of solid surfaces (?sa), the sign of the anisotropic 5CB - surface interfacial energy (??sl), spreading coefficient (S) and 5CB surface tension (?5CB). We also demonstrate with an empirical model that the dipolar interactions dominate during the perpendicular anchoring of 5CB, whereas dispersion forces dominate during the parallel anchoring of 5CB at 5CB - solid interface. Overall, our work illustrates that controlling the partitioning efficiency of particles at the H1 GBs provides a valuable strategy for engineering the particle self - assembly which subsequently governs the rheological manifestation. These results can be extended to engineering polycrystalline materials wherein impurities are added to improve mechanical strength. We further demonstrate that confining the NLCs in a network of particle flocs paves a way to systematically introduce structural disorder to tune the rheology of particles-in-NLC composites. This presents a different view point of studying structurally disordered glasses as systems in which order is disturbed by introducing external random quench disorder. Our study also shows that the surfactant driven orientational transition of NLC molecules at the NLC - aqueous and NLC - solid interfaces is dictated by the surfactant concentration and configuration of the surfactant tails at the interfaces.