Rheology Bilayer phases

The extent of the structural transition must depend on the amount of shear exerted on the system versus the deformability of the droplet and continuous lamellar phase. Furthermore, as mentioned above, the rheological (i.e., functional) properties of a system in the a-gel state depend on parameters such as the number of droplets, their size, and how many times bilayers have folded themselves around several droplets, thus forming entanglements. Among these parameters one can identify one key... 

The main topics to be covered build upon our knowledge of the mechanical and rheological properties of membranes and their response to perturbations. In the remaining parts of this introductory section, some of these properties are briefly described to set the basis for a discussion of the behavior and response of membranes to external forces. The following sections consider in detail the morphological changes and poration electric fields can induce in vesicles made of membranes in different phases, and the effects of media environment and various molecular inclusions in the lipid bilayer, that is, the specific membrane composition. Finally, some application aspects of the work are discussed. 

In the case of liquid detergents, surfactants are almost always present. At low to intermediate concentration, most neat surfactant solutions have low viscosity and are close to Newtonian in flow. Only at higher surfactant concentrations, when structured micellar bilayers and other complex phases are formed, do systems tend to differ greatly from Newtonian. This behavior also helps drive the viscosity of finished formulations. In the great majority of liquid detergent formulations, concentrations of surfactant are such that little structure is developed by the surfactants themselves, resulting in formulations of low viscosity. As such, thickeners and/or rheology modifiers are often required to obtain the desired viscosity and flow characteristics. 

Abstract The effect of lecithin (natural surfactant) addition to gelatin on the surface rheological properties of the water/ heptane interfacial layer and emulsion films formed by these liquids was studied. It was found that the gelatin/ledthin mixtures form complexes in the aqueous phase. Self-assembly of these complexes leads to the formation of viscoelastic interfacial adsorption layers characterized by a yield stress and elastic modules that provide stability of the emulsion films and emulsion systems. The above mentioned parameters evolve in time, though the formation of equilibrium interfacial layers proceeds during several hours emulsion bilayer films require only several minutes. 

The surface rheological proprieties can be controlled by using monolayers of different lipid compositions. We used saturated and unsaturated lipids and their mixtures with different amounts of cholesterol. At the experimental temperature and film pressure the saturated lipids were in the liquid condensed or solid phase whereas the unsaturated lipids in the liquid expanded phase [3]. The mechanical properties of the monolayers can be tuned with addition of different amounts of cholesterol. The results are used for the proof of bilayer or multilayer synthesis and the conditions of their occurrence. 

Van der Linden and co-workers have developed a theory for the rheological behaviour of multilamellar vesicles on the basis of their bending and dynamic properties, which could be suitable for an understanding of the phase transitions of lamellar phases under shear. These authors calculated the energy required for the deformation of the vesicles, which is determined both by the bending energy of the bilayer and by the interactions between such layers. With this model, they could obtain relaxation times and elastic frequencies for the different modes of deformation of the multilamellar vesicles that were of the same order of magnitude, like the shear rates at which the transformation of the lamellar phases into monodisperse multilamellar vesicles takes place. 

All the above disperse systems contain self-assembly structures (i) micelles (spherical, rod-shaped, lamellar) (ii) liquid crystalline phases (hexagonal, cubic or lamellar) (iii) liposomes (multilamellar bilayers) or vesicles (single bilayers). They also contain thickeners (polymers or particulate dispersions) to control their rheology. All these self-assembly systems involve an interface whose property determines the structures produced and their properties. 

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