Hydrodynamic effects on phase transformations in colloidal suspensions

Abstract

This thesis deals with the study of colloidal systems. A colloidal system is made of an insoluble phase, for instance fluid droplets or particulate solids, suspended in a dispersing medium. The size of a colloidal particle ranges from a few tens of nanometers to a few microns, a length scale between atoms and molecules on the one side, and human cells or sand grains on the other side. By virtue of their dimensions, colloids are subjected to the thermal fluctuations transmitted by the surrounding fast-moving molecules that compose the dispersing fluid, and consequently, they display a peculiar dynamics known as Brownian motion. As a result of this erratic motion, colloids explore the configurational phase space, driving the system to its thermodynamic equilibrium by forming the most favorable equilibrium structure. The ability to self-assemble renders them an ideal model system to study the behavior of atomic and molecular systems on accessible time and length scales. Moreover, the rich variety of colloids that is synthesized nowadays enables the design of new materials with advanced properties. In this thesis we try to answer some of the most intriguing questions posed in Soft Matter, and we do this by performing numerical simulations. We provide a brief introduction on colloidal systems and their physical properties in Chapter 1. We describe, in particular, the colloidal systems that are the subject of our investigation and the simulation techniques employed to address the problems. In Chapter 2 we show that it is possible to exploit the self-orientation process of an asymmetric dumbbell particle in a shallow channel for particle sizes down to a few tens of nanometres. This result might find application to govern nano-sized particle position and orientation in a micron-sized channel. In Chapter 3 we show that hydrodynamic interactions, usually a missing ingredient in numerical studies, do not affect the nucleation rate of hard-sphere colloids. We draw this conclusion by computing the crystal nucleation rate with direct simulations and the seeding method, and by analysing the amount of fivefold symmetry clusters in the supersaturated fluid under gravity, by varying the softness of pair interactions and the solvent viscosity. None of these properties influences the local structure of the fluid and, indirectly, the nucleation rate. In Chapter 4 we remarkably obtain the formation of the thermodynamically unstable FCC crystal in a stable fluid of long-range interacting colloids after application of oscillatory shear. We also characterize the observed BCC-FCC Martensitic transformation during the oscillation cycle. We draw an out-of-equilibrium phase diagram by varying the frequency and strain amplitude of the oscillations. In Chapter 5 we transform layers of MgZn2 into MgCu2 Laves phase by applying oscillatory shearing with specific characteristics. We draw an out-of-equilibrium phase diagram by varying the frequency and strain amplitude of the oscillations and explain the transformation mechanism. In conclusion, in this thesis we elucidate the role of hydrodynamic effects in a variety of systems by employing different simulation methods. We hope that our results will inspire further work in this direction.