Polydispersity effects on colloidal phase transitions and kinetic arrest

Abstract

I have studied the effects of polydispersity in systems of hard-sphere, colloidal PMMA particles with and without short-range attraction. In hard-sphere, colloidal systems, the parameter controlling phase behaviour is Ø , the volume fraction of colloids in the solvent. As Ø increases in polydisperse systems, theory predicts a transition from a single phase fluid to a fluid coexisting with a solid (crystal), to a fluid coexisting with multiple solid phases. By considering a volume fraction series of particles with 12% polydispersity and comparing the results with previous experimental results and predictions of the volume fractions within the coexistence regions, we concluded that this system may be exhibiting both fluid-solid and fluid-solid-solid behaviour within the experimental coexistence region. Theory also predicts that coexisting phases in polydisperse hard-sphere systems will fractionate: they will contain different particle size distributions (psds). This was investigated by directly measuring psds for one sample within the coexistence region at different time points. The results show that no statistically significant size fractionation was present after 28 days but by 120 days the solid phase contained a slightly narrower distribution of larger particles than the coexisting fluid phase. At higher than the coexistence region in this polydisperse system, the expected coexisting solids are not observed. Instead, a novel, non-equilibrium phase is present. The dynamics were probed using 3-dimensional dynamic light scattering, which confirmed the non-equilibrium nature of the phase: significant dynamical heterogeneities and anomalous ageing behaviour were present. These experimental dynamics are compared with dynamics obtained from simulations of different hard-sphere psds, including the experimental particle size distribution. The effect of adding a short-range, depletion attraction to a polydisperse colloidal system was systematically explored. Phase boundaries and the position of the metastable gas-liquid binodal were determined experimentally. The resultant phase diagram topology is qualitatively different to a system of monodisperse particles with the same attraction range. Furthermore, within the metastable binodal region, three-phase gas-liquid-solid samples were observed, which is neither an equilibrium or metastable state in monodisperse systems. The coexisting samples were again characterised using electron microscopy and also small-angle x-ray scattering, which revealed significant size fractionation in the gas-liquid separated samples but not in the samples which eventually crystallised.