Modelling the behaviour of dense colloidal suspensions of cuboids. Effect of ordered crowding on dynamics and microrheology.

Abstract

The shape of colloidal particles drives their self-organisation in ordered structures, that in turn confer peculiar characteristics to the suspension. In the design of new smart materials, we first need to understand how particle diffusion and self-assembly relate to each other and to the macroscopic properties of the entire system. To this end, this thesis focused on the dynamics and microrheology of crowded suspensions with different degrees of long-range ordering. We modelled different systems using hard spheres and hard board-like particles, i.e., cuboids: the latter can self-assemble in many liquid crystalline phases depending on their geometry. All the systems have been studied by means of classic Monte Carlo and dynamic Monte Carlo simulations, which proved to be effective to model hard-core particles of any shape and to mimic Brownian motion. The first part of the thesis is dedicated to the derivation and benchmarking of our collision detection algorithm between one sphere and one cuboid. Thanks to the use of OpenMP directives, we managed to make the algorithm both time efficient and stable in different scenarios and user-friendly. Then, we investigated the diffusion of globular macromolecules in isotropic and uniaxial nematic phases of hard board-like particles. Macromolecules, modelled as hard spheres, showed anisotropic diffusion in nematic phases, with preferential displacement in the direction parallel or perpendicular to the director. Non-Gaussian distribution of particle displacements has been observed for both hard spheres and cuboids, in particular for prolate cuboids in isotropic phases. We performed cluster analysis in the isotropic phase, and we observed the formation of nematic-like cluster of hard board-like particles, which may induce local temporary non-isotropic diffusion. The third part of the thesis focuses on electrorheological fluids of nanocubes. At low dilution and specific field strength, we observed the formation of string-like clusters, with an equilibrium distribution of chain lengths and fixed response times. Performing passive microrheology simulations in the field-on and field-off states, we observed anisotropic viscoelasticity of concentrated string-like fluids, depending on the direction of the external field. We then investigated the rotational dynamics of cuboids in the induced transition from a uniaxial to a biaxial nematic phase. The response time is geometry-dependent, with remarkable slow dynamics for cuboids close to self-dual shape, despite such geometry is known to favour biaxiality. In the last part of the thesis, we applied active microrheology simulation techniques in isotropic phases of cuboids with prolate, self-dual shaped and oblate geometries. Local friction coefficient at different Péclet numbers shows typical force-thinning behaviour, both density and geometry dependent. Differences in the effective friction for systems with different particle geometry may be explained by the presence of nematic-like clusters and the ratio in size between the bath and the tracer particles.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabian Garcia Daza (Supervisor), Carlos Avendano (Supervisor) & Alessandro Patti (Supervisor)