Partitioning and self assembly of silica and hematite particles at grain boundaries of hexagonal liquid crystals: implications on rheology

Abstract

We investigate the rheological implications of partitioning and self-assembly of colloidal particles at the grain boundaries (GBs) of hexagonal (H1) liquid crystal (LC) phase as a function of particle loading, shape and phase transition kinetics. The rheology of spherical silica particles (SiO2, diameter = 140 nm)/H1 and irregular hematite particles (Fe2O3, size = 110 nm)/H1 composites is measured as the samples are cooled from an isotropic to H1 phase at 2 and 0.2 °C/min. At 2 °C/min, SiO2/H1 composites show a consistent increase in G′ as the particle loading increases from 0.5 to 7.5 wt. % while Fe2O3/H1 composites exhibit a small drop in G′ above 2.5 wt. % particle loading. On the other hand, SiO2/H1 and Fe2O3/H1 composites show a monotonic increase in G′ with particle loading at a cooling rate of 0.2 °C/min. Microscopy observations reveal that at 0.2 °C/min, both SiO2 and Fe2O3 particles aggregate at the H1 GBs. The different rheological responses of SiO2/H1 and Fe2O3/H1 composites at 2 °C/min are due to the segregation of Fe2O3 particles inside the H1 domains. We further show that the moving H1 front cannot accommodate the larger sized Fe2O3 particle aggregates during phase transition, leading to a reduction in the particle partitioning efficiency (fp) at the H1 GBs. Our results indicate that fp of particles of different shapes and sizes are determined only by the average area of the H1 domains.