Self-assembly of model isotropic and anisotropic colloids and proteins
: a real space analysis

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Self-assembly is a promising approach for generating complex functional microstructures with numerous technological applications. However, the understanding of the underlying mechanisms and the conditions required to achieve specific structures remain rudimentary, limiting its further utilisation. In this thesis, we investigated several self-building systems with varying interactions between constituent particles for a better knowledge of the self-assembly.
The first system involved using PMMA particles that are sterically stabilised to mimic hard spheres. Hard spheres exhibit a thermodynamic phase transition, from fluid to crystal phases, as a function of the packing fraction. However, a remarkable disagreement of experimental and simulation homogeneous nucleation rates by more than 10 orders of magnitude is observed in less supercooled region. We attempted to address this discrepancy by developing a novel confocal microscope image processing technique which allows crystallite identification of colloids with much smaller than typical size scale required for particle-resolved imaging, and thus enables the observation of much rare events, namely the formation of nuclei near the freezing volume fraction. We further examined our results by using larger PMMA with particle tracking methods locating the colloids. Unfortunately, we found no clear dependence of nucleation rate on the particle packing fraction, consistent with the previous experimental works. Hence this discrepancy remains a puzzle.
Bi-continous protein gels with depletion attractions induced by polymer addition is the main topic of the second section. The phase behaviour of individual protein species were firstly studied. Various strategies were then employed, including surface modification of protein molecules and adjustment of adding sequence, to achieve the target structures. Expected binary gel networks were only accessible with specific polymer-protein size ratio. Notably, this study represents the first realization of binary protein gels in which the proteins preserve their intrinsic properties through the depletion interaction.
The third system is composed of rod-like particles and polymers to investigate the possibility of recovering the higher-dimensional (d → ∞) hard sphere glass transition, based on the assumption that the behaviour of the system is represented by the numbers of interactions and the fact that both hard spheres in high d and hard rods with high aspect ratio interact with many neighbours. We developed a reproducible procedure for the dispersion of sepiolite clay particles, B20, and then prepared the mixture of rods and polymers at different volume fractions. With a small size ratio, q = 0.2 and the consequent short-range attraction, we tested a localised phase diagram at low rod volume fractions. During gelation, the dynamics, characterised by the structural relaxation time from the fitting of c(t), exhibited a remarkable increase, while the network structure, indicated by g(r), remained similar to its initial configuration. Our findings suggest that this loss of ergodicity in our rod-polymer mixture is similar to the dynamical arrest in the hard sphere high-dimensional case. Therefore, this simple experimental system provides a potential approach to interpreting higher-dimensional hard sphere virtification.
Date of Award20 Jun 2023
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorAdrian C Barnes (Supervisor) & J L R Anderson (Supervisor)

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