Phase separation and collapse in almost density matched depletion induced colloidal gels in presence and absence of air bubbles: An MRI imaging study.

HYPOTHESIS: Vesicle-polymer dispersions are found in drug-delivery systems and consumer products but undergo phase separation. Previous studies of phase separation have focussed on systems with high density differences between continuous and vesicular phases. In this study, we investigate phase separation in vesicle-polymer mixtures with very small density differences, in the presence and absence of air bubbles.

EXPERIMENTS: Magnetic resonance (MR) imaging, X-ray Computed Tomography and rheological measurements are reported which characterise the properties and stability of vesicle suspensions composed of the cationic surfactant, diethylesterdimethyl ammonium chloride, mixed with non-adsorbing polymer. 1H T2 MR relaxation images are employed to observe phase separation, for a range of vesicle-polymer mixtures, which are analysed using Moran's I spatial autocorrelation to quantify the extent and rate of phase separation.

FINDINGS: It was found that in presence of air bubbles, phase separation follows a compression/collapse mechanism, typical of colloidal gels with large density differences between the phases. Without air bubbles, phase separation develops through the formation of tiny cracks and fractures in the samples. MRI enabled visualisation of the evolution of phase separation inside highly turbid samples. The rate of phase separation was found to generally increase with increasing polymer concentration and decrease with increasing vesicle volume fraction.