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

We investigate the rheological implications of partitioning and self-assembly of colloidal particles at the grain boundaries (GBs) of hexagonal (H<inf>1</inf>) liquid crystal (LC) phase as a function of particle loading, shape and phase transition kinetics. The rheology of spherical silica particles (SiO<inf>2</inf>, diameter = 140 nm)/H<inf>1</inf> and irregular hematite particles (Fe<inf>2</inf>O<inf>3</inf>, size = 110 nm)/H<inf>1</inf> composites is measured as the samples are cooled from an isotropic to H<inf>1</inf> phase at 2 and 0.2 °C/min. At 2 °C/min, SiO<inf>2</inf>/H<inf>1</inf> composites show a consistent increase in G′ as the particle loading increases from 0.5 to 7.5 wt. % while Fe<inf>2</inf>O<inf>3</inf>/H<inf>1</inf> composites exhibit a small drop in G′ above 2.5 wt. % particle loading. On the other hand, SiO<inf>2</inf>/H<inf>1</inf> and Fe<inf>2</inf>O<inf>3</inf>/H<inf>1</inf> 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 SiO<inf>2</inf> and Fe<inf>2</inf>O<inf>3</inf> particles aggregate at the H<inf>1</inf> GBs. The different rheological responses of SiO<inf>2</inf>/H<inf>1</inf> and Fe<inf>2</inf>O<inf>3</inf>/H<inf>1</inf> composites at 2 °C/min are due to the segregation of Fe<inf>2</inf>O<inf>3</inf> particles inside the H<inf>1</inf> domains. We further show that the moving H<inf>1</inf> front cannot accommodate the larger sized Fe<inf>2</inf>O<inf>3</inf> particle aggregates during phase transition, leading to a reduction in the particle partitioning efficiency (f<inf>p</inf>) at the H<inf>1</inf> GBs. Our results indicate that f<inf>p</inf> of particles of different shapes and sizes are determined only by the average area of the H<inf>1</inf> domains.