Structure, mechanics, and rheology of concentrated emulsions and fluid foams.

Abstract

Whether enjoying the luxury of a bubble bath or enduring the drudgery of washing dishes, one is likely to be struck by the beauty and intricate structure of foams, froths, or ‘‘suds.’’ Keen observers may even notice the unusual elastic and yield properties, not seen in the constituent aqueous and gaseous phases. Scientifically, the interest in and the study of foams have been truly multidisciplinary and have not been confined to chemists, engineers, and physicists. Foams have traditionally inspired mathematicians for their geometric properties and as equilibrium structures in which the surface area is minimized (1). Metallurgists (2) have realized the similarity between foams and polycrystalline metals, both in their structure and coarsening behavior (grain growth). Similarly, botanists and life scientists in general have noticed strong structural parallels between foams and living tissues (3). Gas–liquid foams are abundant in nature and their technological applications are numerous. They are used to advantage in fire fighting, enhanced oil recovery, foods (e.g., whipped cream), cosmetics (e.g., shaving cream), and in many other ways. The ‘‘world of foams’’ may be considerably expanded by the realization that concentrated liquid–liquid emulsions, although generally characterized by a much smaller mean size of the dispersed units, are structurally identical to gas–liquid foams, which is readily revealed under the microscope. Macroscopically, they behave like