Table of contents

We review some of the theoretical results that we have obtained recently on the adsorption of polyelectrolytes on surfaces of opposite charge. We consider two problems, the formation of polyelectrolyte multilayers and the formation of complexes between rigid polyelectrolytes and small spheres.

For polyelectrolyte multilayers, the overcompensation of the adsorbing surface charge and the anchoring between consecutive layers are studied.

For polyelectrolyte-sphere complexes, the wrapping of the polymer on the sphere is shown to occur continuously at low ionic strength and discontinuously at high ionic strength.

The findings of some recent experiments are briefly compared with our results.

The electronic structure of liquid transition metals has been studied by time resolved photoelectron spectroscopy. Distinct changes in the valence band structure across the solid-liquid phase transitions have been observed for Pd, Mo and W. In the case of Pd the changes on melting are caused by a change in the photoemission process itself. For Nb, Mo, Ta and W we suggest a change in the atomic short range order from the bcc structure to an fcc-like short range order in the liquid state which causes a marked change in the valence band spectra of Mo and W due to the filling up of the pronounced gap in the density of states near the Fermi level. The measurements show that time resolved photoelectron spectroscopy can be used to study high temperature oxidation processes on the microsecond time scale.

The abrupt liquid-to-solid transition experienced by simple non-polar liquids with quasi-spherical molecules when compressed to a few molecular layers between smooth, solid surfaces can be drastically modified by the presence of polymeric chains end-attached to the surfaces (polymer brushes). The origin of this is the weak interpenetration of the brushes, which can be strongly compressed and yet have a very fluid interfacial region when sheared past each other, resulting in very low friction. The use of telechelic brushes, which are functionalized at both ends and could form loops rather than tails, should result in an even lower interpenetration, and thus in a better lubrication effect. A simple calculation, however, shows that brush-like dimer or multimer-structures might be more favourable energetically than simple loops in the case where the telechelic end-groups attract each other, leading to a very different form of interactions. Recent measurements of the normal and shear forces between surfaces bearing layers of telechelic brushes are in line with these expectations.

Membranes are two-dimensional sheets of molecules which are embedded and fluctuate in three-dimensional space. The shape and out-of-plane fluctuations of tensionless membranes are controlled by their bending rigidity. Due to their out-of-plane fluctuations, flexible membranes exhibit very different behaviour to flat two-dimensional systems. We discuss three properties of membranes: (i) the renormalization of the bending rigidity in fluid membranes due to undulations on short length scales; (ii) the suppression of the crystalline phase, and the hexatic-to-fluid transition; and (iii) the lamellar-to-sponge transition in systems with variable topology. We focus on simulation studies, which are based on the numerical analysis of dynamically triangulated surface models.

Concentration fluctuations in a homogeneous mixture are in general small and confined to the molecular lengthscale. It has been recently predicted that stressed fluids should exhibit anomalously large fluctuations. We will show here that anomalously large fluctuations also occur when macroscopic concentration gradients relax to the uniform state via diffusion. The measurements have been taken by low angle static light scattering and shadowgraphs. We find that at larger wavevectors the amplitude of the fluctuations diverges as q^-4 . It is also found that gravity stabilizes at a constant value the fluctuations below a critical wavevector. We will present data on a mixture close to a consolution critical point. Recent results will also be presented on ordinary liquid mixtures, a polymer and a protein solution. These new results have been obtained by means of a quantitative shadowgraph technique. They confirm that giant fluctuations are always associated with mass flow due to diffusion across a macroscopic gradient.

A system of smooth hard spheres with inelastic collisions is considered as an idealized model to describe rapid granular flow. A non-equilibrium statistical mechanics is formulated for this system, analogous to that for elastic collisions. The associated Liouville equation provides the basis for application of many-body methods such as linear response, kinetic theory, and the derivation of macroscopic hydrodynamic equations. These methods are illustrated for the simplest case of self-diffusion. A Green-Kubo expression for the self-diffusion coefficient is derived and evaluated using an approximate linear kinetic theory. Other recent applications of kinetic theory and kinetic models are reviewed briefly.

The propensity of liquid films to bead off poorly wettable substrates leads to a wide variety of liquid structures via mechanisms which are far from being fully understood. In particular, dewetting via unstable surface waves may be driven at least by dispersion forces, electrostatic forces, or by Marangoni-type transport. A hierarchy of dynamical instabilities finally transforms the initial homogeneous film into the final state, consisting of an ensemble of individual, isolated droplets. While these processes of self-organized structure formation are interesting in themselves, it may also be desirable to generate liquid structures in a more well-defined and predictable way. We have therefore investigated experimentally the behaviour of various liquids on substrates, the wettability of which has been laterally structured. The resulting artificial liquid objects display several remarkable properties, both statically and dynamically. Aside from potential applications as `liquid microchips', it is shown how fundamental quantities can be extracted from the shapes of the liquid surfaces, as determined by scanning force microscopy. The three-phase contact line tensions obtained in this way are in fair agreement with theoretical predictions and might help to resolve long-standing debates on the role of wetting forces on the nanometre scale.

We present some aspects of the rheology of field-responsive suspensions, a class of field-responsive liquid-matter systems possessing the ability to undergo significant changes in their strength upon application of an external field. Both the single-particle and the many-particle domains are discussed. In the former, consideration of the full non-linear dynamics of the particles leads to an anomalous behaviour of the viscosity whereas in the latter the most salient feature is the formation of chains and fractal structures. We indicate how to deal with the rheology at moderate concentrations, leaving open the problem at higher concentrations for which the complexity of the emergent structures strongly limits the knowledge of their dynamics.

Measured pair interactions among highly charged colloidal spheres dispersed in simple electrolytes yield several surprises. Isolated pairs of like-charged spheres are found to repel each other, much as predicted by conventional theory. The same spheres, however, can develop a strong and long-ranged attraction for each other when confined either by charged glass walls, or by neighbouring spheres. Such like-charge attractions are inconsistent with the mean-field theory for macroionic interactions. These and other experimental observations further constrain recently proposed extensions to the mean-field theory.

Many binary systems (and their mixtures) which might be expected to be `ionic', from electronegativity considerations, are found to exhibit pronounced `covalent effects' in their condensed-phase structure and dynamical properties. Recent work, involving both electronic structure calculations and computer simulation, has suggested that the interactions arise and are describable within the ionic model - provided that many-body effects, whose origin is the change in an ion's properties caused by interaction with its environment, are included. In systems where they are substantial, the many-body effects promote remarkably rich changes in the intermediate-range structure of an ionic liquid. AlCl₃becomes molecular, BeCl₂a `living polymer' of extended chains, and the distinctive intermediate-range order (IRO) of the three-dimensional-network, glass-forming systems ZnCl₂ , BeF₂and SiO₂is reproduced. The structural changes have considerable dynamical consequences: for ZnCl₂the slow structural relaxation, leading to the glass transition, may be traced back to the relaxation of the IRO. On shorter timescales (higher frequency) these liquids exhibit spectroscopic bands usually assigned to quasi-molecular units. The formation and dissociation of these units is crucial in ionic conduction, and other transport properties.

The phase behaviour of very asymmetric binary mixtures can be understood in terms of the depletion interaction. For hard particles this yields a narrow deep attractive well surrounding the hard core. Colloids with similar interaction potentials are known to destabilize the liquid, causing it to show a wide fluid-solid coexistence, and in extreme cases they exhibit an exotic solid-solid condensation. For a mixture this means that phase separation is not fluid-fluid, as previously thought, but normally fluid-solid, and if the asymmetry is very large, even solid-solid. We present in this work the result of devising a density functional theory for an infinitely asymmetric mixture of parallel hard cubes. This model is singular and undergoes a collapse in a close-packed solid (an extreme fluid-solid demixing). We avoid this collapse by introducing a small amount of polydispersity in the large particles; the resulting phase diagram shows the fluid-solid and solid-solid demixing scenarios described above.

Results of Monte Carlo simulations with various polarizable potential models and reverse Monte Carlo simulations of water are reported at different thermodynamic state points from ambient to supercritical conditions. It is shown that polarizable potential models can reproduce the change of the experimental partial pair correlation functions of water with the temperature and density considerably better than simple nonpolarizable models. Thus, for instance, only the polarizable models can reproduce the experimentally observed elongation of the hydrogen bonds with increasing temperature and decreasing density. On the other hand, the densities of the polarizable water models decrease unexpectedly fast with increasing temperature, which affects also the reproduction of other thermodynamic properties at states of high pressure and high temperature. In analysing the properties of the hydrogen bonded clusters it is found that the space-filling percolating network of the molecules breaks down around the critical point, although a large number of hydrogen bonds still remain in the system above the critical point.

Using neutron diffraction with isotopic substitution, the structures of aqueous solutions of tertiary butanol have been studied as a function of concentration. As the behaviour of this system is thought to be driven by hydrophobic interactions, particular attention was paid to the hydration of the non-polar headgroups and the nature of the intermolecular contacts. As concentration is increased from 0.06 to 0.16 mole fraction tertiary butanol, there is evidence for the growth of small clusters of the alcohol molecules, with butanol-butanol coordination of two to three even at the lowest concentration. Orientational pair correlation functions show that the dominant intermolecular contacts between alcohol molecules are between the non-polar head groups, as would be expected for a system driven by hydrophobic interactions. As concentration increases, however, there is evidence of mixed polar-non-polar contacts. The alcohol group's hydrogen bonding requirements appear to be fully met by polar contacts with water molecules: there is no evidence for significant butanol-butanol hydrogen bonding.

When average one-particle densities are spatially uniform at a microscopic level the state is considered to be a homogeneous liquid. A prescription is required for the basic dynamical elements involved in the averaging itself, and for hydrogen, at low densities and temperatures, these are the familiar hydrogen molecules. But, at compressions now achievable both by static and dynamic means, a more basic description in terms of protons and electrons incorporating residual pairing correlations is necessary. The latter are dependent in large part on the nature of effective state-dependent pair interactions between protons, and in a narrow band of densities near r_s= 1.33, these may be especially weak. The hydrogen liquidsrefers to the (quantum) diatomic liquid, the high density monatomic liquid, and the variably correlated transition phases between the two.

Single-particle response functions for liquid D₂and H₂ , obtained from previous inelastic neutron scattering measurements, are compared with an exact quantum calculation for D₂and a Wentzel-Kramers-Brillouin (WKB) model for D₂and H₂ . The exact and WKB calculations both provide satisfactory descriptions of the experimental response function of these fluids over a wide range of momentum and energy transfers, which spans from the roto-vibrational excitations up to the molecular dissociation regime.

Understanding rare transitions occurring in complex systems, for instance chemical reactions in solution, poses the problem of finding and analysing the trajectories that move from one basin of attraction to another on a complicated potential energy surface. We have developed a systematic approach for finding these trajectories using computer simulations without preconceived knowledge of transition states. The approach follows from a novel statistical mechanics and thermodynamics of trajectories and has been demonstrated with several applications.

The hydration of one excess proton in water under ambient conditions is investigated by means of atomistic computer simulations. The ab initiopath integral technique employed takes into account nuclear quantum effects such as tunnelling and zero-point motion at finite temperatures. In addition, the interactions are calculated by `on-the-fly' electronic structure calculations in the framework of density functional theory.

This contribution is a short overview of some of the highlights in the application of density functional based ab initiomolecular dynamics methods to aqueous liquids and their chemistry. Recent progress in the study of liquid water at neutral and finite pH is discussed in some detail, such as the first principles computation of the molecular dipole moment in the liquid and the equilibrium constant for auto-dissociation. Also mentioned are some recent simulations of chemical reactions in aqueous solutions. We conclude with an outlook for the near future.

The temporal evolution of the optical absorption of solvated electrons in a pure water jet between 5 and 70 °C has been investigated in two pulse femtosecond experiments. A 60 fs (FWHM) UV pulse at 270 nm directly ionized pure water and the subsequent absorption was probed by a white light continuum at 12 different wavelengths in the range between 450 and 1000 nm. Due to the thickness of the water jet the time resolution is limited to about 150 fs. The transient absorption contains not only information on the temporal evolution of the absorption spectrum but also data on the time dependence of the concentration of the solvated electrons. We have used the optical sum rules to separate the temporal evolution of the absorption spectrum from the concentration of the electrons in the time interval between 300 fs and 100 ps. At ultra-short times the absorption spectra are displaced to the red and undergo a substantial blue-shift during the first few picoseconds. After about 5 ps the absorption spectrum of thermally equilibrated solvated electrons is recovered. Within our time resolution the data show no evidence of transient electronically excited states of solvated electrons. We interpret the temporal evolution of the absorption spectrum using the optical sum rules and deduce the time dependent decrease of the mean squared dispersion in position, r² (t ) , of the electrons. Certainly, r² (t ) is related to the solvation process of electrons in polar fluids and therefore contains information on the solvation dynamics.

The level-crossing model is applied to pump-probe spectroscopy of the intramolecular vibration of hydrogen-bonded HDO molecules dissolved in H₂ O. The parameters of the model are extracted from experimental data. It is shown that within the linear approximation for the dependence of the vibrational transition frequency on the hydrogen-bridge length, the pumping light saturates not only the v= 0v= 1, but also the v= 1v= 2 transition, resulting in the suppression of superabsorption in transmission spectra. The non-linearity of the transition frequency may be introduced through either anharmonic corrections to the vibrational energy levels, or different curvatures of parabolic levels. As the result, the 1 2 transition shifts out of resonance with the pumping field, no population of the v= 2 level appears, and the transmission spectra fit better to experimental observations.

In high temperature liquid alloy semiconductors, the electronic conductivity ( ) is usually in the range 5 - 500 ^-1cm^-1 . Several workers have long argued that, since such values are lower than those predicted by the `Mooij' limit or the Ioffe-Regel criterion (k_Fl> 1), the carriers in such systems are either localized or on the threshold of localization, i.e. they are characterized by a low mobility. There are no direct ways to measure the mobility of current carriers in high temperature liquids but we shall show that, with reasonable assumptions, reliable estimates of the mobility can be made by combining electron transport and magnetic susceptibility data. Our conclusions will challenge the idea that the apparent metal to non-metal transition observed in liquid alloy semiconductors is related to disorder induced localization.

The structural and electronic properties of liquid Rb_x Te_1-xmixtures (x= 0.0, 0.2, and 0.5) are studied by ab initiomolecular-dynamics simulations. It is shown that the transition from the metallic to the semiconducting state induced by adding Rb atoms is reproduced, and that this transition is related to the structural change in the Te chain. It is also shown from the calculated electronic density of states that almost complete charge transfer from Rb to Te occurs in the mixtures. The correlation between the spatial distribution of the transferred charge in the Te chains and the positions of Rb⁺ions is investigated.

The three partial structural functions of molten copper halides CuX (X = Br, I) have been estimated from anomalous x-ray scattering (AXS) measurements coupled with the reverse Monte Carlo (RMC) simulation technique. For both cases of molten CuBr and CuI, the Cu-Cu partial structure factor of a_CuCu (Q ) was found to be rather structureless and the closest Cu-Cu distance was significantly smaller than that for X-X, indicating like-ion penetration into the first unlike-ion coordination shell, similar to the molten CuCl case. We then suggest a disordered close packing of anions where Cu ions meander so as to take a strongly disordered distribution by penetrating through the tetrahedral holes.

The method of anomalous x-ray diffraction (AXD) was applied to an aqueous solution of 6 molal RbBr in water. Data were gathered at two wavelengths below the absorption edge of Br¯. Corrections were made for attenuation, inelastic scattering and background and container scattering. The corrected data were renormalized and put on an absolute scale of electron units. Fourier transformation of these results provided information on the Br¯ hydration. It is found that the Br-O nearest neighbour distance is 3.4 Å and the first coordination shell contains between 6 and 7.5 water molecules. There is no evidence to suggest any longer range hydration structure.

Measurements of the surface x-ray scattering from several pure liquid metals (Hg, Ga, and In) and from three alloys (Ga-Bi, Bi-In, and K-Na) with different heteroatomic chemical interactions in the bulk phase are reviewed. Surface-induced layering is found for each elemental liquid metal. The surface structure of the K-Na alloy resembles that of an elemental liquid metal. Bi-In displays pair formation at the surface. Surface segregation and a wetting film are found for Ga-Bi.

We report the existence of two new topologically ordered glass phases of smectics in strainedaerogel. In contrast to the case of unstrainedaerogel, we find compelling theoretical arguments that a smectic in uniaxially stretched aerogel exhibits, for homeotropic nematic-aerogel alignment, a `smectic Bragg glass' in the universality class of the `XYBragg glass'. On the other hand, a uniaxial compression, with homeotropic alignment, leads to an entirely novel type of anisotropic smectic elasticglass phase that we call the `m= 1 Bragg glass'. This latter phase exhibits anomalous elasticity, characterized by exponents that we calculate to high precision. We present a phase diagram for the system in the aerogel density-strain parameter space, which should be accessible experimentally. We also make numerous other scaling predictions for experimentally observable quantities.

We discuss the thermal Casimir effect in a thin hybrid nematic cell, and we analyse the enhancement of the fluctuation-mediated force due to the frustrating effect of competing surface interactions. The force exhibits a non-algebraic and non-monotonic dependence on the distance, and it dominates the total structural interaction in the system. Similar behaviour is found in the Fréedericksz cell, characterized by a bulk destabilizing external force.

We consider the depletion interaction between mesoscopic particles and non-adsorbing free polymer chains. For one and two spherical particles we investigate the depletion profiles of monomer and end densities of the embedding polymer solution. The profiles take a finite limit for large Flory radius and Edwards screening length. The influence of the excluded volume interaction between chain monomers is studied for the case of one particle. We demonstrate how the polymer-induced force between two particles emerges from the monomer density profile near the surface of one of the particles. The evaluation of the polymer densities follows from a new type of `fusion procedure' in the equivalent field theory.

In this paper, we explore the possibility of using low-frequency dielectric dispersion (LFDD) measurements in suspensions of non-spherical particles. We present results on the LFDD of clay suspensions, with laminar, non-homogeneously charged particles, and of monodisperse spheroids of haematite (iron III oxide). The heterogeneous surface of the clay (montmorillonite) is manifested in dielectric measurements, since two relaxation times (one associated with faces and the other with edges) can be separated. Furthermore, it is found that the latter is absent at pH 7, thereby directly confirming that the edges of the clay platelets are uncharged at such pH. Experiments conducted with monodisperse haematite spheroids allow a preliminary check of existing theories for the LFDD in such geometries. Comparison of the zeta potentials deduced from dielectric and electrophoresis measurements confirm the validity of the models, and suggest that haematite could be a model spheroidal colloid, to which standard electrokinetic theories are applicable.

We study the dynamics of contact of a soft object (rubber bead, soft shell, vesicles, living cells) on a wet substrate by removal of the intercalated liquid film. The profiles of the contact zone are observed by reflection interference contrast microscopy. The adhesion forces (either hydrophobic, electrostatic or specific) are measured by micropipettes, flow cells or `microkarcher' techniques. For vesicles, the adhesion induces a tension of the membrane, which relaxes by the formation of transient macroscopic pores. We study the dynamics of opening and closing of pores.

We present a study of the clustering counterion of iodine counterions around a cationic rodlike polyelectrolyte in aqueous solution. The novel synthetic polyelectrolyte used here has a poly( p -phenylene) backbone and can be considered as rodlike in good approximation. The correlation of the counterions with the macroion in aqueous solution is determined by small-angle x-ray scattering which is mainly sensitive to the I¯ counterions. The measured intensities agree with the prediction of the Poisson-Boltzmann cell model if radius of the macroion is treated as an adjustable parameter. The small value of radius of the macroion found here shows that the cell model underestimates the correlation of the counterions to the macroion.

We predict theoretically the gradual formation of fluctuating, connected microemulsion networks from disconnected cylinders as the spontaneous curvature and the radius are varied, in agreement with recent direct measurements of these topological transitions. We discuss the role of the topological defects, the network junction and the end-cap of the disconnected cylinders, in the connectivity transition. The optimal shapes and curvature energies of the junctions and end-caps are calculated numerically and compared with analytic approximations.

We discuss the phase behaviour of suspensions of charged colloidal particles, specifically at low salt concentrations. The total free energy Fof a three-component system of charged colloids and positive and negative salt ions can be expressed as F= F_DLVO +F₀ , where F_DLVOis the free energy of a one-component system of colloidal particles interacting via the DLVO pair potential, and F₀consists of density-dependent contributions from the salt ions. We argue that F₀drives a gas-liquid spinodal at low salt concentrations, in accordance with several experimental observations. The underlying mechanism is equivalent to that of the well-established gas-liquid transition in simple electrolytes. The present theory connects directly the classic linearized Poisson-Boltzmann theories for simple electrolytes (Debye-Hückel) and colloidal suspensions (DLVO).

We show how the free-energy landscape of a system, normally used only for calculating its equilibriumphase diagram, can be used to predict the kinetic pathways that are permitted in the course of phase separation. Applications to one particular soft condensed matter system, a colloid-polymer mixture, are briefly described.

The phase behaviour of a polydisperse mixture of hard spheres is examined within a moment-density approximation. The role of size fractionation and the difference between quenched and annealed phase behaviour is outlined. The quenched phase diagram shows a terminal level of polydispersity above which no fluid-crystal transition occurs. It is demonstrated that this singularity arises from the re-entrant nature of the freezing transition at high polydispersities. In the annealed situation by contrast, the polydisperse crystal is spinodally unstable with respect to fluctuations in polydispersity. The direction of the instability suggests that the equilibrium annealed state is a fractionated crystal.

In this work, the aggregation behaviour of surface-modified colloidal particles was studied. In view of this, a method was developed which allows the dynamic scaling functions s (t ) and (x ) to be calculated from single-cluster light scattering data. The method was employed to investigate the aggregation of colloidal particles covered with different amounts of bovine serum albumin (BSA). It was found that BSA molecules do not alter the aggregation regime when the samples aggregate at the protein's isoelectric point. Far from this condition, however, the adsorbed protein molecules increase the residual electrostatic interaction and the aggregation regime changes from diffusion- to reaction-controlled aggregation.

Colloidal systems with a Yukawa interaction potential have been investigated by static and dynamic light scattering. By simultaneously measuring the dynamic and static properties of the colloidal systems we obtained the hydrodynamic function H (Q ) at different concentrations. Further the self-diffusion coefficient of highly ordered colloids was determined by tracer and FRAP measurements and compared with BD simulation and theoretical predictions.

The effect of polyethyleneglycol (PEG) on the phase behaviour of oil-rich sodium dodecyl sulphate, hexanol, water and dodecane mixtures has been investigated. PEG causes the disappearance of the `sponge' phase and induces the formation of a new isotropic phase, labelled L₅ , located between the microemulsion, L₂ , and the lamellar, L , phases. Small angle neutron scattering and electrical conductivity results show that at local scales the L₅phase consists of inverted bilayers, connected at larger scales. These features suggest a sponge-like structure for the L₅phase, a hypothesis which is further supported by a static and dynamic light scattering study.

The strength of depletion interactions has been characterized by measuring the second virial coefficient, B₂ , of suspensions of lysozyme molecules in the presence of non-adsorbing polymer poly(ethylene glycol) (PEG). The solution properties of the PEG are characterized for three molecular weights (1000, 6000 and 12 000) providing an opportunity for quantitative comparisons of measurements and theoretical predictions of B₂ . We report non-monotonic changes in B₂as the concentration and molecular weight of PEG are increased. The observed minimum in B₂cannot be predicted by the standard depletion model of Asakura and Oosawa, and is closely associated with the proximity of the polymer solution lower critical solution temperature. The location and depth of the minima in B₂are well captured by the thermal polymer reference interaction site model theory where the polymer mesh size is treated as a function of temperature.

The lipid-bilayer component of biological membranes is a two-dimensional liquid under physiological conditions. Computer-simulation calculations based on statistical mechanical models predict this liquid to exhibit structure in the form of fluctuating lipid domains in the nanometre range. Indirect and direct experimental evidence for the small-scale structure is provided by fluorescence energy-transfer techniques and atomic-force microscopy, respectively.

Building on Kramers' theory for reaction kinetics in liquids and using laboratory experiments, we show how strengths of molecular anchoring and material cohesion in fluid-lipid membranes increase with rate of force and tension loading. Expressed on a scale of log(loading rate), the dynamic spectra of pull-out forces and rupture tensions image the microscopic and mesoscopic energy barriers traversed in molecular extraction and membrane failure. To capture such images, we have pulled single molecules from membranes with force rates from 1 to 10⁴pN s^-1and ruptured giant membrane vesicles with tension rates from 10^-2to 10²mN m^-1s^-1 .

A temperature-dependent irreversible variation of the average aggregation number of GM1 micelles, with no change in the chemical structure of the molecule, has been observed by light and x-ray scattering. GM1 is an amphiphilic molecule of biological origin, similar to phospholipids but with an extended headgroup, made up of many sugar units. A simple model has been developed to describe the experimental results. It assumes that the polar headgroups of GM1 monomers may exist in two different stable conformations, each of them with a very similar energy, dependent on its own internal structure and displaying preferential interactions with the surrounding heads once inserted in the micelle. The interconversion between the conformational minima is then described as a cooperative transition occurring at the micelle surface, overcoming a naturally emerging barrier due to collective effects. To assess the extent of interactions among the GM1 conformer headgroups different amounts of an amphiphilic spacer have been inserted in GM1 micelles. The thermal hysteresis phenomenon was still observed on mixed micelles until a molar ratio GM1/spacer ~1/3 was reached, corresponding to the critical concentration calculated from the mean-field theory of dilute magnetic alloys in a 2D lattice. The observed behaviour, then, appears as a critical phenomenon of topological nature happening in a confined two-dimensional system, that is the micelle surface.

The aim of the present work is to characterize by computer simulation the free-energy difference between B- and Z-DNA in saline solutions. We use a new scheme to rigorously calculate the electrostatic contribution and we use it to test the theoretical predictions for a so-called `empty' DNA model (a set of charged hard spheres placed at the phosphate positions). The ions are considered as charged spheres in a continuum medium of dielectric constant equal to that of water. Also, we investigate the total free-energy difference for three different models differing exclusively in the degree of definition of the molecular shape. The comparison against experimental data shows that a precise shape is not required to give acceptable results. Finally, we use a simple grooved primitive model to study the effect of the ionic size and the ionic charges.

The phase behaviour of a fluid confined between two plane-parallel solid substrates is investigated within the framework of lattice-gas calculations where the mean-field intrinsic free energy is employed. By minimizing the grand potential numerically, phase diagrams are constructed. Substrates are composed of alternating strips of weakly and strongly adsorbing material. The lattice gas may consist of a high-density region stabilized by the strongly adsorbing portion of the substrate while a low-density region exists over the weakly adsorbing ones (the `bridge' phase). The `bridge' phases coexist with either a liquidlike or a gaslike phase occupying the entire space between the substrates. All three phases join at a triple point, and two critical points exist at which the `bridge' and gaslike phases or `bridge' and liquidlike phases become (separately) indistinguishable. By misaligning the substrates in the x -direction, the lattice gas can be exposed to a shear strain which causes the width of the one-phase region for the `bridge' phases to vary and the triple-point location to alter.

Long-time self-diffusion and sedimentation of tracer spheres in dispersions of rigid colloidal host rods has been measured in situas a function of rod volume fraction for various sphere and rod dimensions. The sphere friction factor, which was always the same for diffusion and sedimentation, is determined only by the macroscopic rod viscosity when the rods are relatively mobile (dilute regime). However, when the host-rod dynamics is slow (semi-dilute regime) the tracer friction is much smaller than expected from the host viscosity, and markedly dependent on the sphere/rod size ratio. These experiments on well defined rod-sphere mixtures, supported by low-shear rheology and birefringence measurements, confirm that current models for hindered tracer dynamics do not (adequately) incorporate host mobility.

This paper presents the state-of-the-art in studies of structure and dynamics of water confined in a prototype hydrophilic substrate: porous Vycor glass. Experimental data and molecular dynamics simulations are compared.

We report here some of the results obtained through a small-angle neutron scattering experiment performed on a binary mixture absorbed inside Vycor porous glass. The aim of the experiment was to study the way the critical behaviour is modified when the mixture is spatially constrained to the random pore network of almost regular size (75 Å) characterizing the Vycor. The presence of concentration fluctuations inside the pores, the extent of which comes to exceed the mean pore size and increases more and more even below the bulk critical temperature, is observed.

Surface crystallization in alcohol melts results in the formation of a crystalline bilayer at the melt's surface abovethe freezing temperature. We demonstrate here that the structure of this surface-frozen bilayer can be varied using specific bulk additives, water and diols. Surface-specific x-ray methods show that water swells the bilayer by intercalating into its centre at a molecular water:alcohol ratio of ~1:2. It also induces surface freezing at chain lengths not showing the effect when dry. Diols are found to induce a reversible monolayer-bilayer surface phase transition. The surface phase diagram is determined and found to depend on the diol's chain length and concentration, in addition to temperature. The physics underlying the phase behaviour is discussed.

We present a study of a simple model of a nematic liquid crystal confined between two walls in slab geometry (slit pore). A rich phenomenology, associated with capillary effects as well as orientational transitions, arises. In a previous paper (1999 Phys. Rev. Lett.822697) on the semi-infinite problem (a single wall) we found anchoring transitionswhose existence depends on the values of the surface parameters and temperature. We discuss the persistence of this transition in the confined problem and the effects of the confinement on the nematic-isotropic transition.

We report results of wetting on non-planar and heterogeneous surfaces calculated from an effective interfacial Hamiltonian model. The lack of translational invariance along the substrate induces a series of structural changes of the interface such as unbending and a number of non-thermodynamic singularities and can modify the location of the wetting transition. We show that the order of the wetting transition in the planar homogeneous system strongly affects the behaviour of the non-planar and heterogeneous surfaces.

We have carried out simultaneous measurements of the optical reflectivity and the thermal radiation for the mercury-sapphire system. We have found that the optical emissivity shows remarkable changes not only at the liquid-gas transition but also at the prewetting transition of mercury on the sapphire substrate. Furthermore, in the prewetting supercritical phase, a sharp dip in the emissivity is observed, and this anomaly is much stronger than that in the reflectivity. We conclude that this strong anomaly in the emissivity is a clear indication of permittivity fluctuations in the wetting layer, because the emissivity is equal to the `absorbing power' of the interface (Kirchhoff's law) and it should be sensitive to the diffuse scattering caused by the interfacial fluctuations.

²H NMR experiments in deuterated supercooled liquids are discussed in relation with α- and β-relaxation heterogeneity with particular emphasis on the latter. The results are exemplified using data from deuterated ortho-terphenyl, propylene carbonate and toluene where partial deuteration provides additional information on intramolecular motion.

The dynamics of the rotational freezing transition of the rotator phase (RP) crystal of ethanol into its orientational glass (OG) phase is monitored by measurements of molecular rotational components in the quasielastic neutron scattering spectrum. Such a transition is known to share several features in common with the canonical liquid glass transition (both taking place within the same temperature interval), and therefore some of the results from such a study should be of relevance for our understanding of the latter. We show that the basic features of the RP OG transition can be understood in terms of a hard-needle model by means of a mapping of the aspect ratio onto a temperature scale.

On cooling, many supercooled liquids arrive at a rigid disordered state, metastable to the crystal. In this paper, we characterize such amorphous ground states in a binary mixture of soft discs in 2D using molecular dynamics simulations. We find these ground states exhibit very strong correlations between local fivefold and sevenfold sites and an inherent stress heterogeneity. The heat capacity C_pis found to undergo an abrupt increase on heating through a glass transition temperature. We show that this increase is largely due to volume fluctuations.

Although H₂ O has been the focus of a considerable amount of research since the beginning of the century, its peculiar physical properties are still not well understood. First we discuss some of the anomalies of this `complex fluid'. Then we describe a qualitative interpretation in terms of percolation concepts. Finally, we discuss recent experiments and simulations relating to the liquid-liquid phase transition hypothesis that, in addition to the known critical point in water, there exists a `second' critical point at low temperatures. In particular, we discuss very recent measurements at Tsukuba of the compression-induced melting and decompression-induced melting lines of high-pressure forms of ice. We show how knowledge of these lines enables one to obtain an approximation for the Gibbs potential G (P ,T) and the equation of state V (P ,T) for water, both of which are consistent with the possible continuity of liquid water and the amorphous forms of solid water.

We present a new time-dependent density functional approach for studying the relaxational dynamics of an assembly of interacting particles, subject to thermal noise. Starting from the Langevin stochastic equations of motion for the velocities of the particles, we are able by means of an approximate closure to derive a self-consistent deterministic equation for the temporal evolution of the average particle density. The closure is equivalent to assuming that the equal-time two-point correlation function out of equilibrium has the same properties as its equilibrium version. The changes over time of the density depend on the functional derivatives of the grand canonical free-energy functional F [ ] of the system. In order to assess the validity of our approach, we performed a comparison between the Langevin dynamics and the dynamic density functional method for a one-dimensional hard-rod system in three relevant cases and found remarkable agreement. In addition, we consider the case where one is forced to use an approximate form of F [ ].

This paper is devoted to the thermally activated dynamics of capillary condensation. We present a simple model which enables us to identify the critical nucleus involved in the transition mechanism. This simple model is then applied to calculate the nucleation barrier from which we can obtain information on the nucleation time. We present a simple estimation of the nucleation barrier in slab geometry both in the two-dimensional case and in the three-dimensional case. We extend the model to the case of rough surfaces which is closer to the experimental case and allows comparison with experimental data.

When a two-dimensional colloidal suspension of highly charged particles is subjected to a periodic one-dimensional (1D) light field, a liquid-solid transition can be induced. This phase transition is well known as light-induced freezing. However, upon further increase of the intensity, the crystal is found to remelt (laser-induced melting) to a 1D liquid. This reentrance behaviour is in good agreement with predictions based on Monte Carlo simulations. We suggest explaining this intriguing phenomenon in terms of particle fluctuations which tend to stabilize the crystalline phase.

We performed an x-ray scattering study of the smectic A (SmA) ordering of the liquid crystal 8CB (octylcyanobiphenyl) confined to a control porous glass with a typical void radius R= 0.2 µm. The voids were either nontreated or covered with silane. The results reveal a strong influence of spatial restriction and surface treatment on the temperature evolution of smectic ordering. The observations are qualitatively reproduced using the Landau-Ginsburg approach.

Thermodynamically self-consistent integral equation theories (TC-IETs) supplemented by a one-phase freezing criterion, and Monte Carlo simulations, are used to investigate the thermodynamic and structural properties, as well as the phase diagram, of the hard-core Yukawa fluid (HCYF). The attention is focused on rapidly decaying Yukawa tails, a potential regime suited to model in an approximate manner the interaction between globular proteins in protein solutions. TC-IETs are found to give a reasonably accurate description of the physical properties of the HCYF in this limit. The position of the sublimation line relative to the liquid-vapour binodal line, known to play a crucial role in the onset of crystallization in protein solutions, seems qualitatively reproducible. We suggest on this basis the possibility of extending the use of TC-IET to more realistic models of protein solutions, so as to take into account the true multicomponent nature of these fluids, a physical situation whose description still challenges the currently available computer simulation capabilities.

The salting-out effect of simple electrolytes on lysozyme has been studied by measuring the second virial coefficient B₂of the osmotic pressure as a function of salt concentration, and for different salts. The aim of this work has been to find a microscopic counterpart of the empirical Hofmeister series for the efficiency of cations and anions in inducing protein crystallization. The experimental results show that, for large enough ionic strengths, B₂scales linearly with the salt concentration. This trend is common to a number of different monovalent salts, however with efficiency strongly dependent on the specific anion. Conversely, changing the cation does not appreciably affect B₂ . The significance of these findings for the investigation of protein interactions near crystallization is discussed.

If the structure of a complex fluid is characterized by a nanoscopic or mesoscopic length scale comparable with the correlation length of critical fluctuations, a specific sharp crossover from classical mean-field behaviour to Ising asymptotic behaviour is observed. In the region far away from the critical point where the correlation length is still smaller than this structural length scale, one can observe mean-field behaviour. Ultimately, in the nearest vicinity of the critical point, the correlation length becomes dominant and one should expect Ising singular behaviour. Such a crossover is observed in polymer solutions, where the structural length scale is controlled by the molecular weight of polymer, and in aqueous salt solutions, where the range of Ising behaviour can be tuned by the salt concentration. The structural length diverges at a tricritical point. Crossover to mean-field tricriticality can be naturally incorporated into a universal scaling description of polymer solutions.

An overview of recent results of the critical behaviour of micellar systems is given. It is found that it is possible to define a master curve for the effective critical exponent _efffor systems that show a monotonic crossover. It is also shown that the critical behaviour of the osmotic susceptibility of a metastable critical system is the same as that of stable systems. Finally the two-exponential decay of the fluctuations of the order parameter is discussed.

Possibilities of fluid-solid and solid-solid phase transformations in colloidal suspensions and star polymer solutions are reviewed. We start from given interparticle pair potentials and predict the corresponding phase diagrams using computer simulations and density functional theory. When possible, the results are compared with experimental data. In particular, we discuss a cascade of freezing transitions for confined colloids and re-entrant melting and anisotropic solid phases for star polymer solutions.

We propose a simple statistical mechanical theory for a strongly dipolar fluid at low densities, based on the analogy between a system of polydisperse linear chains and the equilibrium structure of these fluids as revealed by computer simulations. At low densities, both steric and dipolar interactions between long chains are weak and thus the dipolar fluid is well described as an ideal gas of polydisperse chains. We have further investigated how the residual dipolar interaction between monomers and/or additional isotropic attractions between the spheres cause the dissociation of the chains and/or the condensation of the dipolar fluid.

Viscous fingers form when a less viscous fluid pushes a more viscous fluid in a linear channel. The instability of the interface results from a competition between viscous and capillary forces. We show here that by using complex fluids such as polymer or surfactant solutions one can act on the viscosity or the surface tension and modify the instability drastically. Two different polymer solutions, that exhibit either shear thinning or normal stress effects, are used. For the first fluid the viscous forces are altered leading to finger narrowing, whereas for the second fluid the viscous forces are supplemented by normal stresses, which leads to finger widening. For the surfactant solutions the modification of the capillary forces leads to finger widening.

It is shown that the transformation of an L₃ -phase to an ionically charged L -phase with stacked bilayers can be induced by the hydrolysis reaction of an ester. This process takes place on a time scale of minutes with ethyloxalate as an ester and occurs spontaneously. If this low-viscous L -phase is subjected to shear it is transformed into a highly viscous vesicle phase (onion phase). The structural changes during this transformation process have been monitored by conductivity, rheology and small angle neutron scattering (SANS) experiments. We observed that the transformation occurs in several steps. For low shear rates the transformation time decreases linearly with the shear rate ( = constant 2000) while for higher shear rates a different mechanisms occurs.

We study the structure and dynamics of a transient network composed of droplets of microemulsion connected by telechelic polymers. The polymer induces a bridging attraction between droplets without changing their shape. A viscoelastic behaviour is induced in the initially liquid solution, characterized in the linear regime by a stretched-exponential stress relaxation. We analyse this relaxation in the light of classical theories of transient networks. The role of the elastic reorganizations in the deformed network is emphasized. In the non-linear regime, a fast relaxation dynamics is followed by a second one having the same rate as that in the linear regime. This behaviour, in step strain experiments, should induce a non-monotonic behaviour in the elastic component of the stress for a constant shear rate. However, we obtain in this case a singularity in the flow curve very different from the one observed in other systems, that we interpret in terms of fracture behaviour.

We consider a basin made of an impermeable soil, eroded by water flow (not by weathering). Our approach is based on standard deterministic models, within a `streamlet' picture where the flow is always directed downhill. The central assumption is that erosion occurs only when the surface shear stress (due to water flow) is above a certain threshold <sub>c</sub> . Some features of the landscape are simplified by the existence of <sub>c</sub> , and do not depend on detailed assumptions on the behaviour above <sub>c</sub> . In particular, if the initial landscape is a U-shaped valley, we can construct the `line of attack' - i.e. the border of the eroded regions - by a simple prescription.

We review recent and ongoing experiments in which grain dynamics are probed by correlations in intensity fluctuations for light multiply scattered from bulk samples under flow. This includes application of standard diffusing-wave spectroscopy (DWS), when the driving force is strong and the resulting flow is smooth and continuous, as well as use of higher-order temporal correlation functions, when the driving force is gentle and the resulting flow is an intermittent series of avalanches.

This special issue of Journal of Physics: Condensed Mattercontains the Proceedings of the Fourth Liquid Matter Conference held in Granada, Spain, 3 - 7 July 1999. Like the previous conferences held in Lyon (1990), Firenze (1993) and Norwich (1996), this event was organized under the auspices of the Liquids Section of the Condensed Matter Division of the European Physical Society and was co-sponsored by the University of Granada. The common aim of the series of Liquid Matter Conferences is to bring together every three years scientists from varied disciplines - physicists, chemists and biologists - who are working in the different areas of liquid matter science and thus to provide a forum for the exchange of the most recent ideas, technical developments and results. This special issue contains many of the papers presented orally at the conference as plenary or symposium lectures. Several papers based on poster contributions will appear in a forthcoming issue of the journal.

The Granada conference was attended by 669 delegates from 46 countries: 529 from Europe, 74 from America, 55 from Asia, 9 from Australia and 2 from Africa. The scientific program consisted of 12 plenary lectures, 81 symposium talks and four poster sessions in which 582 posters were presented. The 12 plenary lectures were chosen to cover a broad spectrum of activities and to follow the most recent developments in this scientific area. Thus, in addition to topics common to the previous conferences such as simple liquids, phase transitions or colloidal systems, the International Program Committee chose some new topics; for example, membranes and biological liquids, and the rheological properties of liquids.

Many new and important results were presented both in the lectures and in the poster sessions. Rather than simply highlighting these, we would like to convey some of the other positive aspects of the conference. The presence of a large number of young delegates; the increasing participation of scientists from different backgrounds to the traditional ones of physics and chemistry and of delegates coming from outside academia; the lively discussions, in particular those at the poster sessions, gave an optimistic view of a field that is growing and changing in many and different ways. Experimental, theoretical and computational techniques first developed for simpler fluids are becoming increasingly relevant for the more complex fluids while at the same time different areas of soft matter offer new and challenging problems to the community at large.

Thus the location of the conference in the beautiful town of Granada cannot be the only explanation for the very large attendance at this conference and this is a good omen for the Fifth Conference which is to be held in Konstanz, Germany, in September 2002. It is a great pleasure to acknowledge the local organizers for the wonderful work they did to make the conference run very smoothly and which made it an enjoyable experience. The generous support of the sponsors listed overleaf was essential for the running of the conference and is gratefully acknowledged.

Pedro Tarazona, Conference Chairman Luciano Reatto, Chairman of the International Program Committee Roque Hidalgo-Alvarez, Chairman of the Local Organizing Committee

Sponsors:

Ayuntamiento de Granada Caja General de Ahorros de Granada Cámara de Comercio Industria y Navegación Cervezas Alhambra Comisión Interministerial de Ciencia y Tecnología EDP Sciences European Physical Society Genesys Iberia Lineas Aéreas de España Institute of Physics (IOP) Publishing Ltd Junta de Andalucía Ministerio de Educación y Cultura Optilas Ibérica OWIS GMBH Repsol Royal Society of Chemistry Unicaja Universidad de Granada