Structure-rheology relationship in monoolein liquid crystals.

HYPOTHESES: Monoolein liquid crystals find use in foods and pharmaceuticals. Our hypotheses were: (a) liquid crystal symmetry dominates yielding and large deformation, and (b) strain rate frequency superposition (SRFS) may be used to determine mesophase long and short relaxation times. EXPERIMENTS: Liquid crystal microstructure and rheology were characterised as a function of temperature and composition. Their structure was assessed using small angle X-ray scattering (SAXS) and polarised light microscopy. Small and large deformation rheology was characterised using frequency and amplitude sweeps, large amplitude oscillatory shear tests, and SRFS. FINDINGS: We have contributed to the structure-rheology relationship governing the properties of the lamellar, cubic, and hexagonal mesophases. Initially, we characterised a number of monoolein-water binary phase transitions, which showed similar behaviour with earlier efforts. Frequency sweeps revealed that the cubic phases had the highest elasticity followed by the lamellar and hexagonal phases. The stiffening and thickening ratios, extracted from the Lissajous-Bowditch plots, were used to quantify intra-cycle non-linearities. The cubic phases displayed abrupt yielding with more pronounced stiffening and thinning behaviour compared to the others. Each liquid crystal phase displayed unique rheological behaviour upon large deformation and, by linking rheology with SAXS and composition, we show that their symmetry defined their rheology.

Rheological behaviour of attractive emulsions differing in droplet-droplet interaction strength.

HYPOTHESIS: We hypothesise that interaction strength between oil droplets determine the rheological properties of oil-in-water (O/W) emulsions by simultaneous formation and break-up of bonds between droplets. Using small (SAOS) and large (LAOS) amplitude oscillatory shear measurements, we aim to distinguish different classes of emulsions based on the specific microstructural evolution of the emulsions. EXPERIMENTS: Concentrated O/W emulsions differing in droplet-droplet interaction strength were obtained. Different interaction strength was obtained using different types of interactions; (a) electrostatic attraction, (b) salt bridging, or (c) crosslinking. FINDINGS: In line with our hypothesis, different rheological events in emulsions depend on the droplet-droplet interaction strength. Strong interactions lead to monotonous yielding, and droplets undergo jamming or densification to provide strain hardening and gel-like behaviour. Emulsions with weak interactions exhibit two-step yielding (SAOS) and continuous yielding in LAOS; indicating a soft-glassy material. In emulsions above maximum packing, and with weak interactions the rheology is controlled by cluster/cage breaking, and transient formation of new clusters. For medium-strength interactions, two-step yielding was reduced, and apparent stain-hardening occurred. The probability of two distinct time scales of yielding is hindered by stronger interactions and jamming. Overall, in concentrated emulsions, yielding is determined by network rupture and reformation, cluster rearrangement and -breaking, which in turn is influenced by interaction type and strength. We present a more differentiated categorisation of emulsions based on interaction strength.

Clustering of oil droplets in o/w emulsions: Controlling cluster size and interaction strength.

Clustering of oil droplets changes the rheological properties of oil-in-water (o/w) emulsions and can be used as a tool to structure foods. The aim of this study was to manipulate both oil droplet cluster size and cluster strength in liquid o/w emulsions, and to investigate the effect of these parameters on the rheological properties. Clustered emulsions were prepared using three different methods: (i) clustering by protein-proanthocyanidin interactions, (ii) clustering by hetero-aggregation of oppositely-charged emulsion droplets, and (iii) enzymatic clustering of protein-stabilised droplets using transglutaminase. Clustering by protein-proanthocyanidin interactions allowed to control oil droplet cluster size from 1 to 140 µm. Clusters decreased in size upon both an increase and decrease in pH, but were stable against changes in ionic strength. Hetero-aggregation of oppositely-charged oil droplets (gelatine/whey protein and gelatine/DATEM) allowed to control cluster size from 1 to 40 µm. Clusters showed a strong decrease in size in response to changes in pH and a small decrease in size with increasing ionic strength. Enzymatic clustering did not allow to control cluster size. Cluster strength of proanthocyanidin-stabilised clusters was found to be higher than that of hetero-aggregated clusters. Stabilisation of clusters was likely induced by different protein-proanthocyanidin interactions such as H-bridges, π-π stacking, and hydrophobic interactions, whereas hetero-aggregation is based on electrostatic interactions. Upon clustering, emulsion viscosity increased by up to three orders of magnitude. We conclude that protein-proanthocyanidin interactions and hetero-aggregation are effective methods to tune droplet cluster size and strength in o/w emulsions, and that cluster size and interaction strength control the rheological properties of o/w emulsions with clustered oil droplets.