Rustum Roy recently observed that, operating from a finite number of elements, materials science faces an over-supply problem, too many scientists, and too few elements. Like all Malthusian dilemmas, relief is unlikely within the assumptions. Materials scientists, however, enjoy opportunities to develop new materials through morphological engineering of traditional substances. Carbon, after all, provided a century of progress for polymer chemists and a revolution in the manufacturing world. The trend from atomic- to molecular- to chain-level engineering can obviously be extended to mesostructure engineering and beyond. The tailoring of porosity is an example of such an extension.

This issue of the MRS Bulletin examines the concept of engineered porosity from three perspectives. Smith, Hua, and Earl discuss characterization based on classic gas adsorption and more recent NMR and scattering methods. Two articles, one by Harold and Lee, and the other by Fain, look at the pore structure requirements for important applications, reactive separations and gas separation. Finally, the materials science of engineered porosity is discussed in four articles. Even describes emulsion-derived foams in which surface-active species impose micron-scale structure that is retained during polymerization. Shapovalov broadly reviews methods to introduce porosity in metals. Durian discusses recent work on relaxation phenomena in aqueous foams. Finally, I review the factors that control structure in mesoporous aerogels.

Aqueous foams are macroscopically homogeneous complex fluids composed of co-existing gas and liquid phases. A hierarchy of structure and self-organization at progressively smaller length scales is ultimately responsible for the unique material properties of foams, and is shown schematically in Figure 1. A key feature of this microstructure is the continuous random network of liquid which typically occupies 1–10 vol% and separates the tightly packed gas bubbles. As shown by the photographs in Figure 2, the average bubble diameter can vary widely from about 10 μm to 1 cm, while the average bubble shape can vary from nearly spherical to nearly polyhedral, depending on the liquid volume fraction. At the smallest structural scale, surface active molecules are preferentially adsorbed at the gas/liquid interfaces and give rise to several physical-chemical effects that deter the coalescence of neighboring bubbles and thereby lend stability to the foam. However, the delicate structure formed by gas bubbles packed in a continuous liquid phase is not truly stable with either time or applied forces. This article reviews recent progress in understanding the structural response of aqueous foams to temporal and mechanical perturbations.

Porous metals are engineered materials; they are designed for special properties. Technological progress necessitates expanding the choices of such materials, making the development of new porous metals a relevant challenge for materials scientists.

Although a wealth of information has already been accumulated on these materials, new results are published every year, extending the engineer's capability to manufacture porous metals and revealing their unknown and often unusual properties. This survey describes the state of the art and some recent accomplishments in the field. This article discusses manufacturing practices, structure, properties, and applications of porous metals. Promising new research issues are also highlighted. Materials whose pores were not formed in situ, like honeycomb structures made by high-energy beams etc., are not covered.

Porous polymeric foams are well-known materials and have been produced by a wide variety of techniques ranging from leaching soluble fillers through gas-blowing to phase separation. Although these materials are widely used, their structure is often irregular and difficult to control. However, a novel method for producing porous materials with a more regular structure has been developed based on high internal phase emulsions (HIPE). These foams are called emulsion-derived foams and are commercially known as Polyhipe® (Unilever) polymers.

If one takes a liquid oil, introduces water and a suitable emulsifier with agitation, a dilute emulsion is produced with small water droplets dispersed within the oil phase, about the consistency of milk. If the amount of the water (i.e., the internal or pore forming) phase is increased, a concentrated emulsion will be formed and the consistency of the mixture changes into a more viscous fluid. As further water is introduced, a limit is reached where the droplets are so prevalent that they approximate a close-packed structure (see cover of this issue); beyond this point the droplets must distort to accommodate any additional internal phase. For a collection of monodispersed spheres this packing limit corresponds to 74% by volume and is the accepted definition of a high internal phase emulsion, i.e., an emulsion with an internal phase greater than 74%.4 The structure of the emulsion is now analogous to soap bubbles, with thin films surrounding and separating the drops. Note also that, compared to “standard” emulsion polymerization, this is the exact reverse or “inverse” of that process. In the standard route, the dispersed oil phase is polymerized to yield a collection of polymeric spheres; with inverse emulsion, the continuous phase is polymerized, yielding a rigid matrix.

Catalytic inorganic membranes are among the most challenging and intriguing porous materials. Consisting of a thin film of mesoporous or microporous inorganic material deposited on a macroporous material, catalytic membranes are multifunctional materials that must be engineered for both chemical and physical properties. New approaches to carrying out chemical reactions are possible by tailoring the membrane catalytic activity and selectivity, permselectivity, and other thin film properties. Readers are referred to several recent reviews of inorganic membranes, in particular, Zaspalis and Burggraaf, Armor, Gellings and Bouwmeister, Hsieh, Stoukides, and Tsotsis et al.

Inorganic membranes are most conveniently classified according to pore size (see introductory article). Of particular importance is the ratio of the pore size to the molecular mean free path (MFP). Decreasing pore dimensions lead to increased selectivity with corresponding loss of permeability. Macroporous membranes have a pore size much larger than the MFP, leading to molecular (bulk) diffusion or viscous flow. Knudsen diffusion dominates in the mesoporous regime, where the pore size is comparable to the MFP. In addition, surface diffusion of the molecules along the pore walls may contribute, leading to an enhanced flux of the adsorbed species along the walls. The microporous regime is encountered when the pore size is comparable to the molecules. This regime makes possible much higher permselectivities, which depend on both molecular size and specific interactions with the solid. Finally, in dense membranes, molecular transport occurs through a solution-diffusion mechanism, which also involves specific interactions between the solute and membrane.

Some industrial processes require the separation of gas or vapor mixtures. Methods for separating the mixtures vary from separation by diffusion to separation by distillation. Many of the methods, such as distillation, are energy intensive. Membranes can reduce the energy required to produce a desired separation. Because of their corrosion resistance and high temperature applications, engineered inorganic membranes can significantly increase the efficiency of many of these processes. The magnitude of the separation factor, available operating conditions, enrichment, yield, and cost of the membranes play a large role in determining whether membranes can be more economical than other methods of separation. These factors have to be evaluated on a case-by-case basis.

Martin Marietta Energy Systems' Office of Technology Transfer conducted a preliminary market survey with the assistance of the University of Tennessee and commercial marketing experts in inorganic membranes. The survey assumed that membranes could be made with permeabilities a factor of 3 larger and with cost per unit area a factor of 3 smaller than is currently available. The results indicated that active implementation of such technology could expect to achieve the following results:

• $2 billion dollar per year sales market,

• $16.6 billion increase in the national GDP,

• $2 billion improvement in the balance of trade, and

• 6 quads per year decrease in energy use.

Progress in the synthesis and engineering of advanced porous materials demands better pore structure characterization. The analysis of pore structure is complicated by (1) the wide range in pore sizes observed, from molecular (<1 nm) to macroscopic (>1 mm) dimensions, (2) complex pore shapes and connectivities, (3) chemical and physical heterogeneities, and (4) pore structure changes that can occur during characterization.

The required pore structure information varies with application. Bulk density and the pore-size distribution are needed for thermal insulation. In this case, the dimension of interest is the so-called hydraulic radius since, for small pores, the gas-phase conductivity is proportional to the mean hydraulic radius to the mean free path. A few large but isolated pores will significantly affect conductivity but will go undetected in typical gas-absorption methods. In contrast, for separations, bottlenecks control performance. For transport, such as migration through geologic formations, both the pore-size distribution and pore connectivity are important. For adsorption, surface area and pore size are the relevant factors. Finally, the conventional concepts of pore structure lose meaning as the pore size approaches molecular dimensions, typical of adsorbents and gas-separation membranes.

Several emerging technologies depend on the development of porous materials with pore dimensions in the nanometer range (1 nm = 10 Å). Based on the canonical classification scheme, such materials are defined as “mesoporous materials.” Specialty uses, such as separation applications, require porosities engineered to meet stringent pore-size requirements, particularly for separating high-boiling-temperature or-ganics. In addition, commodity products, such as high-performance thermal insulation, await credible manufacturing methods to produce fine-scale pores. Finally, information discovered during the development of mesoporous materials could be helpful in finding new strategies to generate void-free ceramic materials by reverse engineering.

Although several methods exist to generate nanometer porosity, our ability to tailor porosity for specific applications is still primitive. Because we lack adequate structural data and models for pore formation, the relationship between synthetic protocol and pore structure remains enigmatic.