Multiscale simulations of colloidal suspensions in external gradients
Creators: name: Olarte Plata, Juan David
Publisher: name: Imperial College London
Year: 2019

description: This thesis explores the link between microscopic properties of colloids and their macroscopic behaviour under external fields. In the first chapter, we present a historical perspective on colloidal science. We emphasise the response of colloids to temperature fields and velocity gradients, which are two types of external fields explored in this thesis. The second chapter describes the theoretical foundations in the fields of classical and statistical mechanics, as well as equilibrium and non-equilibrium thermodynamics. In Chapter 3 we present the computational approaches applied in this thesis, which includes atomistic and mesoscopic simulations. We introduce some key concepts of Molecular Dynamics, and describe in detail the Stochastic Rotation Dynamics (SRD) technique, a particle-based algorithm for a fluid environment that includes thermal fluctuations and hydrodynamic interactions. In Chapter 4, we study the thermal response of elongated colloids which can feature mass asymmetry by means of non-equilibrium computer simulations. We report a novel coupling between the thermal orientation effect and thermophoresis that results in significant differences in the Soret coefficient between symmetric and asymmetric colloids. We describe this coupling by means of non-equilibrium linear response theory. In Chapter 5, we present a theoretical framework for the thermal orientation effect. We show that the orientational probability distribution of colloids with mass asymmetry can be accurately described with the von Mises distribution. By means of atomistic and mesoscopic non-equilibrium simulations, the theoretical approach is shown to hold over a wide range of length scales. We study the main dependencies of the thermal orientation effect on the temperature, thermal gradient, and particle size and mass asymmetry. We derive explicit expressions for experimental observables, such as the average orientation of the colloids, or the thermophoretic torques induced on them. In Chapter 6, we describe the application of the von Mises probability distribution framework to characterise the coupling between the thermal orientation effect and thermophoresis found in Chapter 4. We derive theoretical predictions of the Soret coefficient of colloids with mass asymmetry, and validate them by means of atomistic non-equilibrium simulations. In Chapter 7, we deal with the thermal orientation effect on Janus particles featuring two hemispheres with different mass. By means of geometrical arguments, we derive expressions for the thermophoretic torques as a function of the particle orientation, which are validated by means of non-equilibrium atomistic simulations. We take advantage of the link between the orientational probability and the thermophoretic torques presented in Chapter 5, to arrive to the von Mises distribution from a different theoretical approach. In Chapter 8, we explore the shear viscosity of colloidal suspensions of spherical particles with varying degrees of softness. We use non-equilibrium SRD simulations to explore the dependence of the viscosity on the shear stress imposed. We describe the non-Newtonian behaviour by proposing a modification to the Krieger-Dougherty model, and we link this behaviour to the microscopic structure of the suspension. We show that the softness of colloidal interactions plays a key role in the rheology of the suspensions. In Chapter 9, we start from the inter-surface interactions of flat calcite surfaces to derive effective colloidal potentials. We take into consideration the colloid surface roughness by means of a convolution between the surface free energies and the probability distribution of surface heights. We show that surface roughness plays a major role in defining the attractive or repulsive nature of the colloid-colloid interactions. We use Langevin Dynamics to study the impact of surface roughness and concentration of colloids on the structure of the suspension.