1. Introduction
2. Response To Time (Aging)
3. Response To Forces (Rheology)

By definition, foam is a nonequilibrium dispersion of gas bubbles in a relatively small volume of liquid containing surface-active macromolecules, such as surfactants. These preferentially adsorb at the gas/liquid interfaces and are responsible both for the tendency of a liquid to foam and the stability of the resulting dispersion; see "Foams" for a review of foam properties and applications, and a collected bibliography. The physical properties of bulk foams ultimately arise from the physical chemistry of bubble interfaces and the collective structure formed by the random packing of gas bubbles. As shown in the following schematic diagram, this involves phenomena on many length scales:

Hierarchy of structure and self-organization in an aqueous foam. Large gas bubbles are separated by thin liquid films which are stabilized against rupture by physical-chemical effects arising from the presence of adsorbed surfactants or other surface-active macromolecules. If the volume fraction of liquid is greater than about 5%, the bubbles are nearly spherical as shown; for drier foams, the bubbles are more nearly polyhedral as set by the competition between surface tension and interfacial forces. [from "Relaxation in Aqueous Foams".]

Here are some photographs of the bubble packing structure in real foams:

Note that both materials are disordered. Even if you made an ordered foam, the evolution processes discussed below would quickly introduce disorder. Also note that the structure does not seem to depend on either the chemical composition or average bubble size. The only parameter that dramatically affects bubble shape is liquid content (bubbles are nearly polyhdral for dry foams and nearly spherical for wet foams). This beautiful property means that we can focus on the idealized random geometrical structure common to all foams, rather than worry about details of chemistry and preparation that vary from sample to sample.

Much is known already about the behavior and interrelationships of surfactant monolayers and soap films, depicted at the two smallest scales above. Consequently our research interests are focussed at the largest two scales since, by contrast, relatively little is known about behavior at the bubble scale and its interrelationships with the macroscopic stability and rheology. This is in part because the structure, distribution, packing, and dynamics of gas bubbles are largely inaccessible to traditional experimental measurements such as surface observation, freeze fracture, electrical conductivity, or external pressure. These methods are all either indirect and open to ambiguity of interpretation, invasive and therefore not suitable for in-situ or time-evolution studies, and/or too slow for all but the most stable foams. The development of a superior means of quickly and noninvasively measuring the structure and dynamics of foams at the bubble scale is a problem of considerable basic and applied interest. Our efforts along these lines exploit novel multiple-light scattering techniques, and one of our general goals is to establish and extend the range of structural and dynamical information that can be extracted from such measurements. Another general goal is to use these diffusing-light techniques to establish a fundamental understanding of foam stability and rheology in terms of the underlying mesostructure. Special interest is in the collective behavior of the bubble-packing structure that cannot be deduced simply from knowledge of the pairwise interactions of isolated bubbles.Back to the top.

2. Response To Time (Aging)

Liquid-based foams are nonequilibrium systems. With time, they can evolve by a combination of three basic mechanisms: drainage, film rupture, and coarsening by diffusion of gas from smaller to larger bubbles. Since there is an unavoidably large density mismatch between the gas and liquid portions of a foam, there is no way to prevent drainage on earth. However, it can be slowed down to the point of insignificance for dry foams with small bubbles. The direct coalesence of neighboring bubbles by rupture of the intervening soap film can also be prevented by proper choice of surfactant and concentration. The third mechanism, coarsening, is present for all foams and cannot be eliminated. It is driven by surface tension and is a means for gradually decreasing the total interfacial area with time. For us, coarsening is a useful feature because it allows us to prepare foams with a reproducible distribution of sizes: from any initial state, the system will evolve to a stationary distribution where the mean size grows approximately as time raised to a power between 1/3 and 1/2 (depending on liquid content).Back to the top.

3. Response To Forces (Rheology)

As a macroscopic form of matter, liquid-based foams exhibit striking mechanical properties. If pushed gently, they resist deformation elastically like a solid. If pushed hard, they can flow and deform arbitrarily like a liquid. Furthermore, if their pressure, temperature or volume is changed, they respond roughly as PV/T=constant. Thus, they are neither solid, liqud, nor vapor, yet can exhibit the hallmark features of all three basic states of matter. Exactly how this occurs is not understood. Experimentally, our approach is to quantify this behavior with sophisticated commercial rheometer and to correlate it with details of the microscopic bubble-packing structure obtained via diffusing-light spectroscopies. Theoretically, we address the same issues using computer simulations.Back to the top.