RESUMO
Saturated monoglycerides can form firm gels in water. These gels are networks of stiff plate-like beta-crystals of monoglycerides (a "cardhouse"), grown from a space-filling lamellar liquid-crystalline phase. The molecular mechanism of crystallization is discussed in the light of network formation. The concentration dependence of gel development of (shear-cooled) monoglyceride gels has been studied by rheology. A gelation mechanism has been proposed, consisting of two steps: (i) After formation of a nucleus, rapid crystallization in a lateral direction occurs (probably within one bilayer) by which the first space-filling network is formed. (ii) This is followed by reinforcement of the network by which stacks of crystalline bilayers are formed. The plate-like crystals are linked in connective domains or junction zones, probably containing all the material (cosurfactants, diglycerides, etc.) that does not fit in the crystalline array. Small deformation rheology shows that above about 2 wt% monoglyceride a percolating network is formed. The large deformation rheology is typical for a particle gel with a relatively small strain at failure (both in shear deformation and compression). The connective domains or junction zones already fail when relatively small deformations are put on the system.
RESUMO
The coalescence efficiency of two droplets in simple shear flow was experimentally investigated. The effects of the radius of the colliding droplets, the viscosity of the continuous medium, and the radii ratio on the coalescence efficiency were studied. The coalescence efficiency was determined from the critical angle, alpha(crit), above which the droplets coalesce and below which no coalescence takes place. It was found that the coalescence efficiency decreases and then increases with the flow number. The theories of the drainage models and the trajectory analysis were unable to predict the experimental results. This was attributed to the inadequacy of the existing expression used for the critical film thickness. On the basis of the experimental data obtained a new expression for the critical film thickness was proposed. The new expression predicts a decrease followed by an increase in the critical film thickness. The increase in the critical film thickness is, perhaps, due to instabilities that take place at the interfaces. Implementing the proposed expression for the critical film thickness, the experimentally measured coalescence efficiency, the capture angle and the contact time were successfully predicted. The experimental investigation revealed that the coalescence efficiency does not depend on the radii ratio in the range 0.7 to 1. The experimental investigation also showed that the coalescence efficiency increases as the viscosity of the continuous phase decreases. Measuring the capture angle showed that coalescence can take place either in the compression or in the extensional quadrant of the shear plane. A variation in the capture angle and the contact time was observed for a given fixed set of conditions. The change of the angle alpha with time was measured. The measured values were compared to those calculated from an equation derived by R. S. Allan and S. G. Mason (J. Colloid Interface Sci. 17, pp. 383, 1962). A good agreement was obtained when the droplets were of equal radii and small in size. When the droplets were large in size and of different radii, the agreement was not satisfactory. Measuring the distance between the centers of the colliding droplets showed that the two droplets vibrated when they came close to each other. Copyright 2001 Academic Press.
