ABSTRACT
Diffusion is a mass transport mechanism that has profound influence on our daily lives and the world that surrounds us. We often encounter the effects of diffusion with little regard for the technical details of the phenomenon. For example, we are all familiar with the confluence of aromas that emanate from the fragrance counter of a major department store, the dissolution of sugar in a cup of coffee, or the drying of paint on a wall. All of these and many other ordinary, common occurrences are influenced by diffusion. Moreover, the ubiquity of the phenomenon extends to the technical realms of science, medicine, and engineering. Diffusion contributes to the transport of oxygen through living tissues, the presence of sulfur and nitrous oxides in acid rain, the growth of stalactites and related speleothems, the corrosion and degradation of support structures in buildings and bridges, and the time-controlled release of pharmaceuticals in sophisticated drug-delivery schemes. One important feature shared by these various examples is the notion that an individual chemical species can move through a material or medium at a rate or in a direction that differs from that of the medium as a whole. For example, in the absence of circulation or ventilation, the scent from an open bottle of perfume will quickly fill a room. Even if left unstirred, sugar can still dissolve and mix through a cup of coffee. This motion or transport of individual chemical species is generically referred to as diffusion and the phenomenon is an observable consequence of unseen events on the molecular scale.
