Applications of physical techniques to studies of the structure and phase behaviour of self-organized liquids consisting of a mixture of amphiphilic molecules and their compatible solvents began around 1980. After two decades of intense experimental and theoretical activity, the field of `complex fluids' shows clear signs of maturity. Besides a visibly increasing number of publications in this area in recent years, one can begin to identify both theories and experiments on this subject, gravitating along certain well defined lines of thought and aspects of the problems. This collection of ten feature articles reflects my comprehension of some aspects of currently active areas of research in complex fluids and their future trends. It is no small task for an active researcher to take time out of his busy schedule to comprehensively summarize his current accomplishments. I used both friendly persuasion and the fact that I have reached my sixtieth birthday to urge these authors to finish their respective tasks. I would like to thank them individually for their cooperation in bringing out this special issue on time. I am especially grateful to the editor of Journal of Physics: Condensed Matter for not imposing any restriction on the length of each contribution.

In this issue, the ten articles are grouped topically into pairs starting from a continuum theory of percolation and its experimental realization in water-in-oil microemulsion systems and goes on to experimental evidence of the validity of the corresponding states in phase behaviour and interfacial tension scaling in nonionic microemulsion systems and measurements of bending constants and spontaneous curvature of surfactant monolayers, a theory of phase separation dynamics and morphology of copolymer mixtures and an experimental study of spinodal decomposition in a micellar solution and a microemulsion, a neutron scattering study of micellar formation and the mesophase structures of triblock copolymer-water systems and newly discovered Asphaltine micelle and microemulsion systems, and a liquid theory of a sticky charged hard-sphere system and a combined neutron and x-ray study of charge condensation phenomenon in ionic micellar systems.

One of the characteristics of the ubiquitous phase behaviour shown by water-in-oil microemulsions is the existence of a liquid - liquid phase separation boundary or the so-called cloud-point curve and the associated lower consolute critical point. It has been known for some time that, starting within the vicinity of the critical point, there is a percolation line cutting across the entire one-phase region (temperature - volume fraction plane) showing progressively lower percolation temperature as the volume fraction increases. The percolation locus can easily be detected by electrical conductivity or viscosity measurements. This percolation phenomenon is the result of clustering of water-in-oil microemulsion droplets at certain temperatures depending on the volume fraction. Because of the formation of transient polydispersed clusters, counterions find pathways to migrate across the bulk sample. The continuum percolation theory based on either a sticky-sphere model (S H Chen, J Rouch, F Sciortino and P Tartaglia 1994 J. Phys.: Condens. Matter 6 10855) or a distance-dependent probability of percolation (J Xu and G Stell 1988 J. Chem. Phys. 89 1101, C Cametti, P Codastefano, P Tartaglia, J Rouch and S H Chen 1990 Phys. Rev. Lett. 64 1461) can satisfactorily explain this phase diagram with a percolation line. Professor George Stell has done pioneering work on the continuum theory of percolation, which is an extension of earlier work by A Coniglio and coworkers (1977 J. Phys. A: Math. Gen. A 10 1123). In the first article of this issue he summarizes a general theory of clustering with comments on its potential application to problems in colloids. In the second article, Professor P Tartaglia's group in Rome and Professor J Rouch present a new theory of electrical conductivity and dielectric relaxation of water-in-oil droplet microemulsions based on the idea of transient polydispersed fractal cluster formation.

A major impetus of recent progress in the physics of microemulsions, in my opinion, comes from a systematic determination and interpretation of the phase behaviour of a series of model ternary nonionic microemulsion systems, C_iE_j/water/n-alkane, mainly by the group of Professor M Kahlweit at the Max-Planck Institut für Biophysikalische Chemie in Göttingen, Germany. One of the prominent members of the group, Dr R Strey, and his student T Sottmann have found evidence of the principle of corresponding states in these phase diagrams. In the third article they summarize their results, together with a proposed new universal scaling relation on interfacial tensions in 17 nonionic microemulsion systems. One of the popular phenomenological models of microemulsions considers the effective Hamiltonian of the system to be a sum of two terms expressed as a surface integral of two curvature energies of the surfactant monolayer partitioning the water and the oil domains. The two curvature terms are respectively the square of the deviation of the mean curvature from the spontaneous curvature, C₀, and the Gaussian curvature. The two phenomenological constants of the two terms are respectively the bending elastic modulus, K, and the saddle splay modulus, . The determination of these three parameters at different phase points are the primary experimental input to the theory. In the fourth article, Professor J Meunier and his former student Dr H Kellay review the results of their measurements of these constants for microemulsions with different surfactants and relate them to observed phase behaviours.

Copolymers consisting of two or more incompatible monomer blocks often have amphiphilic character. One of the fascinating phenomena of copolymers is the spontaneous formation of ordered structures upon lowering the temperature. This is a result of a microphase separation not unlike that which occurs in bicontinuous microemulsions. Professor T Ohta and his group have been very active in developing model theories and computer simulations of these models in the area of non-equilibrium pattern formation. In the fifth article he and his collaborators describe their recent interesting results on the kinetic and morphology of phase separating copolymer--homopolymer and copolymer--copolymer mixtures. In the sixth article, Professor F Mallamace and his coworkers present new work on a comparative study of the kinetics of spinoidal decompositions in a ternary ionic microemulsion system, AOT/water/n-decane, and a micellar system, Butoxyethanol/water . This is done with a time-resolved (30 ms) ultra-low-angle light scattering set-up.

In the last few years, interest in theory and experiment of micellar aggregates formed by amphiphilic copolymer systems has come into sharp focus. In particular, the commercially available tri-block copolymers, (PEO)_n - (PPO)_m - (PEO)_n, or so called Pluronics, have drawn most attention. The physics is controlled by the fact that although both PEO and PPO are hydrophilic at low temperatures, the hydrophilicity of PPO decreases much more rapidly than that of the PEO as temperature rises. Thus at high temperatures micellar formation is free-energetically favourable in water. Since these are polymer surfactants the resultant micelles are larger than those from normal surfactants. Hence both light scattering and neutron scattering have been used for the study. Dr K Mortensen has done more small-angle neutron scattering studies on this type of micellar system and their meso-phases than anybody I know. In article seven he kindly summarizes these results. One can expect that microemulsion systems made of these copolymers and their compatible solvents will soon attract much attention. The most industrially relevant micelle and microemulsion systems one can think of are those made of petroleum residue called Asphaltene. Disposal and reuse of this byproduct of an oil refinery is currently one of the major concerns of all oil companies. Dr E Sheu of Texaco Inc is the acknowledged expert in research on emulsification of Asphaltene. In article eight he makes a timely summary of the present status of this research.

In a liquid theoretical treatment of a colloid, the reference system is taken to be a hard-sphere system establishing the basic dimensions of the constituent particle and its excluded volume effect. To be more realistic one needs to add perturbations due to a short-range attractive interaction and also, in the case of a charged system, an additional long-range electrostatic repulsion or attraction. So far, one has analytical models such as the one-component macroion theory and its extension for the case of the electrostatic repulsion. One also has the Baxter sticky-sphere model for the short-range attraction. In the most general case one may need both the attractive and repulsive inteactions. From a practical point of view of analysis of experimental data, one would like to have an analytical model. Recently, Professor Blum and his coworkers provided just such a model. In article nine, he and his coworkers have given a detailed exposition of an analytical solution of a sticky charged hard-sphere model. Although the analysis has not quite reached a practical level usable by experimentalists, the model solution holds great promise for future applications. For charged colloidal solutions, the traditional approach is to focus attention only on the macroions and their correlations. This is adequate for small-angle neutron scattering but is insufficient for small-angle x-ray data analysis. In fact for SAXS data analysis, the counterion distribution around a macroion becomes relevant. Thus, instead of the well known one-component macroion theory, one needs a multi-component ionic solution theory. Article ten, from my own group, presents a practical theory of this kind. It is based on an existing theory of Ronis and Khan (S Khan, T L Morton and D Ronis 1987 Phys. Rev. A 35 4295, S Khan and D Ronis 1987 Mol. Phys. 60 637) which is a multi-component ionic solution theory in the mean spherical approxiation assuming point counterions and salt ions. An analytical solution is achieved with the condition of non-additive small ion-macroion diameters. This latter condition is vital for the applicability of a rescaling procedure (J P Hansen and J B Hayter 1982 Mol. Phys. 46 651) which is used to correct an unphysical solution of MSA in the case of a strong macroion--macroion interaction at high charge. The theory is applied to analysis of SANS and SAXS data of ionic micellar solutions without added salt. The counterion condensation phenomenon is explained.

I hope this collection of articles will provide a comprehensive overview of, admittedly, only a small portion of the currently active area of complex fluids research. I would like to thank Miss Sue Counsell, the Assistant Editor for Journal of Physics: Condensed Matter, for handling the the refereeing process of the manuscripts efficiently and for her speedy communications.

Sow-Hsin Chen
Liquids Editorial Board
Cambridge, MA, USA