Dynamical density functional theory for the segregation and drying of binary colloidal dispersions

In this thesis we give an introduction to the drying and assembly of colloidal films. Then, we describe our work to develop a dynamical density functional theory (DDFT) model for the drying of colloidal films on planar surfaces. This theory is able to describe the microstructure of the colloidal films, including packing effects at interfaces and the stratification that occurs during drying. We consider mixtures of two different sizes of colloids. Firstly, we treat the three dimensional (3D) mixture as being a mixture of hard-spheres with density profiles that vary only in the direction perpendicular to the substrate, reducing the system to a 1D model. We also neglect the influence of gravity. This model is based on a DDFT with an implementation of fundamental measure theory (FMT) which can describe accurately highly confined hard-sphere fluids. Depending on the solvent evaporation rate, size ratio, and the initial concentrations of the two species, we observe varying degrees of stratification in the final dried films. Our model predicts some of the structures described in the literature previously from experiments and computer simulations.

Secondly, we consider mixtures of hard disks (i.e. a 2D mixture) with a fixed 2:1 particle size ratio and with an additional attractive interaction between the particles, in order to explore the influence of phase-separation on the final film structure. In particular, this system exhibits variations in the density profiles both in the perpendicular and parallel directions to the substrate. We also replace FMT with a local density approximation (LDA) which enables us to obtain the 2D density profiles with limited computational resources. We explore the influence on the ordering of varying the initial concentrations and the evaporation rate. When the initial concentration is high and the evaporation rate is sufficiently high both phase separation and size segregation occur and our model shows how these influence the final dried film structures. We also explored a 2-step drying process, which makes phase separation occur at the region close to the substrate. Our observations of cases where we partially dry the dispersion and then stop evaporation to let the system relax, could be used to guide experimental work to obtain a specific film structures and may be useful in the coatings industry.