A class of lamellar biological hydrogels comprised of fluid membranes of lipids and surfactants with small amounts of low molecular weight poly(ethylene glycol)-derived polymer lipids (PEG-lipids) were studied by x-ray diffraction, polarized light microscopy, and rheometry. In contrast to isotropic hydrogels of polymer networks, these membrane-based birefringent liquid crystalline biogels, labeled L-alpha,g, form the gel phase when water is added to the liquid-like lamellar L-alpha phase, which reenters a liquid-like mixed phase upon further dilution. Furthermore, gels with larger water content require less PEG-lipid to remain stable. Although concentrated (approximately 50 weight percent) mixtures of free PEG (molecular weight, 5000) and water do not gel, gelation does occur in mixtures containing as little as 0.5 weight percent PEG-lipid. A defining signature of the L-alpha,g regime as it sets in from the fluid lamellar L-alpha phase is the proliferation of layer-dislocation-type defects, which are stabilized by the segregation of PEG-lipids to the defect regions of high membrane curvature that connect the membranes.

Crystalline orientation and phase transition acceleration induced by shear are demonstrated to occur in different edible fats using synchrotron X-ray diffraction. Cocoa butter, milk fat, stripped milk fat, and palm oil were crystallized statically and under shear (90 s -1 and 1400 s -1 ) from the melt (50 °C) to 18 °C at a constant rate of 3 °C min -1 . The 2D diffraction patterns clearly showed crystallite orientation imposed by the flow in the different phases. A dramatic reduction in phase transition times was also evident. These effects of shear on crystallizing fats are of great importance for the industrial processes carried out in chocolate, dairy, margarine, and shortening production, since temperature and shear treatments are the tools for tailoring the desired crystalline structures.

The crystallization of multicomponent systems involves several competing physicochemical processes that depend on composition, temperature profiles, and shear rates applied. Research on these mechanisms is necessary in order to understand how natural materials form crystalline structures. Palm oil was crystallized in a Couette cell at 17 and 22 degrees C under shear rates ranging from 0 to 2880 s(-1) at a synchrotron beamline. Two-dimensional x-ray diffraction patterns were captured at short time intervals during the crystallization process. Radial analysis of these patterns showed shear-induced acceleration of the phase transition from alpha to beta(') . This effect can be explained by a simple model where the alpha phase nucleates from the melt, a process which occurs independently of shear rate. The alpha phase grows according to an Avrami growth model. The beta(') phase nucleates on the alpha crystallites, with the amount of beta(') crystal formation dependent on the rate of transformation of alpha to beta(') as well as the growth rate of the beta(') phase from the melt. The shear induced alpha- beta(') phase transition acceleration occurs because under shear, the alpha nuclei form many distinct small crystallites which can easily transform to the beta(') form, while at lower shear rates, the alpha nuclei tend to aggregate, thus retarding the nucleation of the beta(') crystals. The displacement of the diffraction peak positions revealed that increased shear rate promotes the crystallization of the higher melting fraction, affecting the composition of the crystallites. Crystalline orientation was observed only at shear rates above 180 s(-1) at 17 degrees C and 720 s(-1) at 22 degrees C .