Colloidal rods in confinement

• Main Author: Klop, Kira
• Other Authors: Yeomans, Julia ; Aarts, Dirk
• Published: University of Oxford 2016
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.730427

We study colloidal rods in confinement and under flow on the single particle level using optical microscopy and laser scanning confocal microscopy. First, we investigate liquid crystals in confinement. A mathematical method for calculating distortion energies is developed and illustrated by applying it to both artificial data and experimental results. Next, we study an isotropic phase in strong confinement, and observe a capillary nematisation transition. This is the first observation of capillary nematisation in a colloidal liquid crystal. The results are compared to the Zwanzig model and a DFT model for semi-flexible rods, and we find good agreement with both. The DFT model, in which rod orientations are unrestricted and rod flexibility is taken into account, was found to have a better quantitative agreement with the experiments. In the second half of this thesis, we study single colloidal rods flowing in a plane Poiseuille flow. For the most simple case of non-magnetic Brownian rods in water, we observe aperiodic kayaking and xy-tumbling behaviour. The results are compared to Jeffery theory for non-Brownian rods in a simple shear flow and to multi-particle collision dynamics simulations. The experimental results were found to undergo the same type of behaviour as predicted by Jeffery's theory, but without periodicity, and particles were seen to switch from one type of behaviour to another. We find qualitative agreement between the experiment and simulations, which indicates that the presence of Brownian motion can explain the differences between theory and experiment. Following on, we investigate further deviations from Jeffery theory by studying the effect of an external magnetic field and of a non-Newtonian fluid on the orientational behaviour of rods. We find that a magnetic field shows somewhat similar behaviour as the kayaking and tumbling found in the non-magnetic system, but suppressed towards a plane perpendicular to the magnetic field. Rods flowing through a non-Newtonian fluid are found to be aligned in the direction of flow, and do not undergo any tumbling behaviour. The absence of tumbling can be explained by the depletion interaction between rods and the wall. It is unclear why the preferred orientation for rods is along the direction of flow.