Foam Structure and Rheology

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w:\awards\awards96\*.doc 9623567 Durian This project seeks to elucidate the nature of the random packing structures of bubbles in liquid-based foams and the consequences of such structures, their dynamics, and their time evolution, for the macroscopic rheological behavior. This included efforts in two main areas: (a) The propagation of light through foams by multiple-scattering and the use of this generic phenomenon as a structural probe, and (b) The influence of evolution and shear-induced structural rearrangements on linear shear rheology and time evolution. Special interest is in determining if a linear regime exists, if structural rearrangements can play a role equivalent to thermal fluctuations and hence be assigned a "temperature", and if flow can affect time evolution. Rheology measurements will be accomplished with a commercial device and also with a custom Couette-cell which allows simultaneous access for measurement of the bubble-packing structure and dynamics by multiple light scattering techniques. The fundamental understanding thus achieved may provide a framework for more complex systems where dense random packing and irreversible structural rearrangements dictate macroscopic behavior, such as dense emulsions, colloidal suspensions, and granular media. %%% Liquid-based foams consist of a large number of gas bubbles very closely packed together in a small amount of liquid, as familiar from everyday life. The proposed experiments are designed to help achieve a fundamental understanding of the important properties of these materials, namely their stability and flow behavior, in terms of their microscopic structure and composition. Little is yet known due to the fact that light is strongly scattered by the bubble interfaces, thus restricting visual observation of the bubble-packing structure to the surface bubbles. However, we have re cently developed new experimental techniques which take advantage of multiple light scattering and which reveal for the first time details of the packing structure and its behavior. The fundamental understanding achieved through use of these techniques may enable rational design of foaming-liquid formulations, foaming techniques, and anti-foaming agents. This could benefit diverse industries where foams are currently dealt with by trial-and-error: food, cosmetic, detergent, mining, petroleum, pulp & paper, paint & coating, textile, leather, polymer, and waste and water remediation. ***