Table of contents

The structure and thermodynamic properties of the interface between a drop of liquid and its vapour have long been matters of controversy. The system can be described in terms of its macroscopic thermodynamic properties, in terms of local thermodynamic properties or in the language of statistical mechanics. It is shown that the second of these routes leads to difficulties but that there is good evidence that the first and third are concordant, at least as far as the term in R^-1 in an expansion in powers of the reciprocal of the drop radius, R. There are, however, serious problems if the expansion is taken to higher powers of R^-1.

A water film (thickness e) between two hydrophobic solids is metastable. If contact is established (over an initial disc of radius R(0)>R*, where R* is a certain nucleation size), the dry path expands (up to a size R(t) at time t), and the rejected water forms a rim (of width l(t)) around the patch. We predict R(t) approximately t³4 / and l(t) approximately t¹2 /. Typically, over 5*10^-3 s we expect R(t) approximately 1.6 mm.

Bond directionality and network formation from local structural units are the signature of covalent bonding. On melting a 3D network of covalent bonds tends to break into a metallic liquid (e.g. in Si, Ge and GaAs), unless a sufficiently large electronegativity difference between the components stabilizes the electronic structure through chemical short-range order. The melt may then be a semimetal (e.g. Li₄Pb and KPb), an ionic semiconductor (e.g. CsAu) or an insulator (e.g. ZnCl₂). Bonding appears to be more stable in networks of lower dimensionality (D=2 as in GeSe₂ and YCl₃, D=1 as in Se and BeCl₂, and D=0 as in P, SbCl₃ and AlBr₃). Melting from D=2 to D=0 occurs in AlCl₃. Intermediate-range order may be preserved in the melt through interatomic correlations over distances of order 5-10 AA. The experimental evidence on illustrative examples of these various trends is reviewed, with emphasis on the interconnection between stable local coordination and intermediate-range order. Parallel illustrations are given of results from simulations based on empirical potentials or fully quantal methods, from data analysis based on the Reverse Monte Carlo method and from primitive models amenable to integral-equations techniques.

We describe three sets of experiments on suspensions of 'hard-sphere' colloids. Suspensions of equal-sized particles exhibit the hard-sphere freezing and glass transitions. Mixtures of spheres of two different sizes can form the ordered binary crystals AB₂ and AB₁₃. The addition of non-adsorbing polymer to a one-component suspension leads, through the depletion mechanism, to a range of phase behaviour, which includes colloidal gas, liquid, crystal and gel.

Following a brief review of the physics by which X-ray scattering can be made surface sensitive, we will discuss some details of the effects that interfacial roughness and asymmetry of interfacial profiles have on measurements of X-ray specular reflectivity. Experiments related to these effects, including study of the width and interfacial roughness of the liquid/vapour interface for H₂O, superfluid ⁴He, thin wetting films of cyclohexane on Si, and metallic liquid Ga, have been reviewed elsewhere.

Supercooled liquids approaching the liquid-glass transition exhibit unusual properties associated with structural relaxation. The mode-coupling theory (MCT) of supercooled liquid dynamics shows that non-linear interactions among density fluctuation modes can produce effects resembling these observations. MCT produces detailed predictions for the dynamics of density fluctuations in supercooled liquids, including a crossover from liquid to glassy dynamics at a critical temperature T_c. Recent light scattering studies of supercooled CaKNO₃ are described that have provided detailed tests of MCT both above and below T_c, including a cusp in the scaling time at T_c=105+or-5 degrees C. These experiments are reviewed, and comparisons with both the idealized and extended versions of MCT are described.

In this paper, we describe recent neutron inelastic scattering studies of the elementary phonon-roton excitations of liquid ⁴He. The paper deals with a systematic investigation in both superfluid and normal phases of the temperature dependence of the excitation spectrum, covering a range in wavevector from 0.3 to 2.1 AA^-1. These measurements are discussed in the light of theoretical models of the excitation spectrum of liquid ⁴He.

This paper reviews recent (numerical) progress in the understanding of entropic phase transitions in complex fluids. In particular, I discuss (liquid-)crystal formation and demixing in binary mixtures. In some cases it appears that lessons learnt in the study of complex fluids may have an unexpected relevance for simple fluids.

A brief review of recent work on bilayer bulk phases is given, ranging from vesicular through planar to connected (cubic and L₃) phases. The emphasis is on understanding the dilute phases. A new model is proposed for the transition from the L₃ or sponge phase to a vesicular phase.

Ab initio studies of H⁺ and OH^- in water reveal a variety of interesting structures. In the case of the OH^-, an H₉O₅^- structure is observed in which the four hydrogen bonds with the ligands lie in a plane that also contains the oxygen atom of the anion. In the case of H⁺, there is strong evidence for the existence of the H₉O₄⁺ structure originally postulated by Eigen and coworkers. Proton migration in this system is relatively fast and is observed to occur via the Grotthus mechanism.

A simple fluid may be composed of either a single element or molecules involving no more than two types of atom (e.g. NaCl, H₂O). This limitation means that their microscopic structure and dynamics may be studied more completely. Such fluids have been examined experimentally and theoretically for many years and their overall behaviour is fairly well understood. For this reason current work has focused on accurate structural and dynamic data, and its interpretation in terms of precise computer simulations or theoretical treatments. Some topical examples will be described in order to illustrate the character of the questions about simple fluids that may be investigated by radiation scattering techniques.

The onset of deviations from hydrodynamic behaviour in the dynamic structure factor S(k, omega ) of gaseous ³⁶Ar has been recently observed in small-angle inelastic neutron Brillouin scattering experiments. The interpretation of S(k, omega ) in terms of three extended hydrodynamic modes, already applied in other works to several experimental and simulation results obtained at much larger k-values and densities, was shown to be valid also for k approximately=1 nm^-1 at densities well below the critical one. In order to investigate in more detail the onset of the non-hydrodynamic regime we have calculated the intermediate scattering function F(k,t) of a Lennard-Jones system by means of molecular-dynamics simulations at the same densities as in the experiment and at wave vectors small enough for the Brillouin triplet to be visible. We present here some preliminary results where F(k,t) is analysed in terms of extended hydrodynamic modes.

Using a pulsed-source neutron spectrometer we have measured the quasi-elastic incoherent dynamic structure factor of pure water in the temperature range 256-293 K. The high energy resolution and the wide energy transfer window allowed us to separate the translational diffusive motions from the relational ones. The translational dynamics has been analysed on the basis of a random-jump model that describes quite well the average behaviour of the quasi-elastic broadenings as a function of momentum transfer.

We have investigated non-equilibrium fluctuations in liquids subjected to a stationary temperature gradient by performing small-angle Rayleigh scattering experiments. Experimental results have been obtained for liquid toluene, liquid n-hexane and for three mixtures of these liquids. The experiments confirm that the strength of the nonequilibrium fluctuations is proportional to the square of the temperature gradient and inversely proportional to the fourth power of the wave number of the fluctuations. In the case of the pure liquids excellent agreement is obtained with theoretical predictions based on the linearized equations of fluctuating hydrodynamics. In the case of the mixtures some questions remain about the magnitude of the Soret effect.

The density functional format for classical fluids in thermal equilibrium has the joint virtues of simplicity and reasonable accuracy, but the drawback that ignorance of its basic structure makes systematic improvement difficult. It seems clear that incorporation of auxiliary densities plays an important role, but their genesis is not well understood. A sequence of increasingly complex lattice models has been set up and solved in order to probe this question. The Bethe lattice translates to a modified mean field continuum free energy functional, while the introduction of rudimentary non-simple connectivity results in an effective but complex integral equation form. In either case, or their extensions, there is evidence that the underlying model represents no real system, even with multi-body interactions, and it is suggested that what is missing in the description is the effect of non-local collective amplitudes.

We describe a recent dielectric theory of solvation dynamics in which the solute and solvent molecules are represented as interaction site models (ISMs), while measurable properties are expressed as averages over the fluctuating local solvent polarization charge density. No mention is made of cavities in a dielectric. The theory is based on linear response to surrogate or renormalized interactions rather than on the factual (bare or exact) interactions that would give the exact response if the calculation were continued to infinite order. Another novel feature is the use of the reference memory function approximation (RMFA), to combine static structure functions with frequency-dependent global properties such as the dielectric function epsilon _omega to obtain functions of wavenumber and frequency such as the longitudinal dielectric function epsilon _L(k, omega ).

Recent neutron-diffraction studies on dilute solutions have shown that a change in coordination numbers from nine to eight water molecules in the first hydration sphere occurs when going from the light to the heavy lanthanide (III) ions. To acquire a deeper insight into the structure of the first coordination sphere of these ions Monte Carlo and molecular-dynamics computer simulations were performed. The initial results will be compared with neutron-diffraction data and solid-state structures. Recently there has been increasing interest in Ln³⁺ complexes as relaxation agents in magnetic-resonance imaging. We have therefore studied water exchange on a series of gadolinium (III) complexes by oxygen-17 NMR. Results will be given and compared with octaaqua Gd³⁺.

The equilibrium structure of aqueous Na-Cl electrolytes in the concentration regime between 1 and 5 mol l^-1 is studied by means of a molecular dynamics computer simulation using interaction-site descriptions of water and the ionic components. The electrostatic interactions are treated both with the newly developed charged-clouds scheme and with Ewald summation. In the case of a 5 mol l^-1 electrolyte, the results for pair correlations obtained by the two methods are in excellent agreement. However, the charged-clouds technique is much faster than Ewald summation and makes simulations at lower salt concentrations feasible. It is found that both ion-water and ion-ion correlation functions depend only weakly on the ionic concentration. Sodium and chloride ions exhibit only a negligible tendency to form contact pairs. In particular, no chloride ion pairs in contact are observed.

Electroneutrality and chemical potential balance in conventional models for ions in charged capillaries are discussed. The Poisson-Boltzmann (PB) equation and molecular dynamics (MD) simulation of ions inside an infinitely long charged pore show that inconsistencies exist. The PB solution shows that chemical potentials of individual ions are the same and electroneutrality is obeyed provided that the correct boundary condition is used at the wall. The MD simulations show that for a neutral system, the chemical potentials of the ions are different and hence they cannot be in equilibrium with a bulk solution. Modifications of existing models and set-ups of simulations are necessary to model the behaviour of ions in capillaries in equilibrium with a bulk solution.

The effect of induced dipoles on the structure and dynamics of melts of stoichiometry MX₂ is discussed. Interest centres on the case of M being a small, highly polarizing cation and X a highly polarizable anion. It is shown that 'covalent' effects on the short range structure and the appearance of a pre-peak in the structure factor may be explained within a purely ionic model. A connection between these structural features and the dynamical properties of 'strong' liquids is suggested.

Near-critical fluctuations of ionic solutions with a liquid-liquid phase transition are investigated by measurements of turbidity and static and dynamic light scattering. Phase diagrams are also presented. The systems are substituted ammonium salts (picrate, nitrate, iodide) dissolved in higher alcohols and water. The correlation lengths, determined from the time correlation functions using the Kawasaki mode coupling theory, also describe the temperature dependence of scattering intensity and turbidity. Depending on the specific properties of the system mean-field or Ising behaviour is observed.

The phase equilibria of symmetric and asymmetric non-additive hard-sphere mixtures are studied via Monte Carlo simulation with the Gibbs ensemble technique. The phase diagram is also determined by a simple first-order perturbative theory (MIX1). The theory gives results for the coexistence curve in reasonable agreement with the simulation. The asymmetry of the curve increases and the position of the critical point shifts to higher concentration and lower density monotonically as function of the size ratio. The inadequacy of the usual integral equation approach for giving quantitative information about the location of the critical point is also confirmed.

A microscopic approach for the description of critical phenomena in mixtures is presented. Near the critical point, the theory develops a renormalization group structure which allows for the identification of several possible critical behaviours corresponding to different fixed points. For a generic neutral binary mixture, the most stable fixed point gives rise to the same critical exponents as predicted by the well known phenomenological approach, providing a microscopic mechanism for such a behaviour. The order parameter psi is identified and two exact relations expressing psi in terms of experimentally accessible quantities are given. As an example, some results for the neon-krypton mixture are presented.

We determine the liquid-vapour coexistence curve of the restricted primitive model of electrolytes by means of Monte Carlo simulations performed in the Gibbs ensemble and in a hyperspherical geometry. It is found that the Stillinger-Lovett condition is satisfied in the liquid phase but not in the vapour phase. This means that the latter could be dielectric. Whether this unexpected result is due to finite-size effects or not remains an open question.

A model molecular fluid with two-dimensional classical degrees of freedom and two internal quantum states is studied by path integral Monte Carlo simulations. The temperature-density phase diagram is obtained to a high degree of precision using finite-size scaling techniques and local structure analysis methods. We locate the phase boundaries of gas, liquid, square and hexagonal lattice solid phases together with a preferred internal quantum state resulting in tricritical and triple points. The simulation results for the liquid-solid transitions are compared with recent density functional theory calculations.

Dynamic light-scattering measurements on suspensions of hard colloidal spheres, in their metastable amorphous state prior to crystallization, identify the kinetic glass transition by the arrest of particle-concentration fluctuations on the experimental time-scale. This kinetic glass transition coincides with a spectacular change in the mechanism of crystallization. The measured intermediate scattering functions are quantitatively interpreted by mode-coupling theory. We find that both alpha and beta processes are necessary to describe the slow structural relaxation in the fluid near the glass transition.

The experimental phase behaviour of binary mixtures of nitrogen with three very different components is discussed. The phase diagrams have also been determined theoretically but with modest success. It will be shown that our knowledge of the unlike potential parameters is insufficient.

We discuss spinodal decomposition and nucleation in near-critical fluids under shear flow. Recent experiments have shown that these phenomena are unexpectedly complex. We also show that phase-separating fluids have anomalous rheological and acoustic properties.

The phase separation in fluids close to a critical point can be observed in the form of either an interconnected pattern (critical case) or a disconnected pattern (off-critical case). These two regimes have been investigated in different ways. First, a sharp change in pattern is shown to occur very close to the critical point when the composition is varied. No crossover has been observed between the t¹ behaviour (interconnected) and a t^1/3 behaviour (disconnected), where t is time. This latter growth law, which occurs in the case of compact droplets, will be discussed. Second, it has been observed that a growing interconnected pattern leaves a signature in the form of small droplets. The origin of such a distribution will be discussed in terms of coalescence of domains. No distribution of this kind is observed in the off-critical case.

Critical and off-critical quenches of two-dimensional binary mixtures have been studied by a computer simulation approach. Several measures of the size and morphology of the growing structure were investigated, e.g. temporal development of correlations and cluster distributions. The morphology of the structure plays a determining role in the growth. For critical quenches, the structure grows with an exponent of 2/3, in agreement with recent theoretical work. In contrast, for off-critical quenches closed domains coarsen according to a 1/3 law, which could be explained by an evaporation-condensation mechanism.

A structural study of liquid sulphur has been performed around the polymer transition as a function of temperature. Below this transition, the liquid is shown to mainly consist of S₈ molecular units. Above the transition, small-angle scattering data can be used to evidence the presence of large polymerized units.

The effect of ultrafast interconversion of geometrical isomers on the infrared spectrum was obtained through molecular dynamics simulation. Arresting the interconversion by introduction of a very high barrier allowed exchange broadening in the absence of that barrier to be convincingly demonstrated. The spectra obtained could all be fitted very well with Bloch lineshapes and the lifetimes extracted in this way agreed well with those obtained directly from the simulation except in those cases where the interconversion rate was too slow or too fast.

The shadow representation of the wave function of superfluid ⁴He is reviewed. Analogies with and differences from a classical fluid are discussed and the role of quantum delocalization is pointed out. Results for g(r), S(q) and the momentum distribution of superfluid ⁴He in the ground state and at finite T are presented.

Inelastic X-ray scattering with an energy resolution of the order of meV is a young method that is proving to be an excellent tool for investigating collective excitations in condensed matter. The high energy resolution can be achieved by extreme backreflection (Bragg angle close to 90 degrees ) from perfect crystals. This technique is used on the instrument INELAX for inelastic scattering experiments at DORIS, DESY Hamburg. Energy transfers from a few meV up to 5 eV for any wavevector between 0.3 AA^-1 and 14 AA^-1 are accessible. One of the successful applications of this method is the investigation of the dynamical structure factor of liquids. Results on liquid lithium are presented and compared with neutron data and molecular dynamics.

The structure of the liquid semiconductor thallium selenide (Tl₂Se) has been determined to the partial structure factor level using the technique of neutron scattering and isotopic substitution. The results show strong hetero-coordination with a Tl-Se closest distance of 3.2 AA, a broad Tl-Tl distance centred at 4.5 AA and the Se-Se distance at 4.8 AA, The results agree with a structure that is essentially determined by ionic interactions.

First results from the most recent investigation of the temperature dependence of the dynamic structure factor of Rb₈₀(RbBr)₂₀ using cold-neutron time-of-flight spectroscopy are compared with the dispersion relations of collective excitations in Rb(RbCl) and Rb(RbBr) mixtures. Dispersion curves are obtained from the maxima in the longitudinal current correlation function that extend the information on the collective atomic dynamics in this mixture to considerably larger momentum transfers than were covered by the dispersions determined from the fits to S(Q, omega ) in the previous investigations of the same mixture. A small minimum is found in the dispersion near 22 nm^-1, which can be related to a maximum in the partial static structure factor S_MX(Q) and which could be caused by an interaction of optic and acoustic modes in this region of momentum transfers, where S_NN(Q) and S_CC(Q) have comparable weights, demonstrating a change in the interatomic interaction due to the addition of 20 mol% salt to the metal.

The energy dispersive X-ray diffraction measurements for expanded fluid Hg have been carried out up to the sub- and supercritical regions. The dense vapour near the critical region has also been studied. We have obtained the structure factors and the pair distribution functions as functions of temperature (RT-1530 degrees C), pressure (5-1970 bar) and density (13.55-1.86 g cm^-3).

This paper reviews neutron scattering experiments on the static and dynamic structure factor of expanded liquid alkali metals from the melting point up to their critical point. Near the critical point, the change from a liquid metal to a non-metal takes place, which implies that the interatomic forces must exhibit drastic changes when the critical point is approached. Characteristic changes of the microscopic structure of the expanded liquid alkali metals have been observed. They expand non-uniformly and the dominant effect on the properties of the expanded liquids derives from a reduced mean coordination number and not from a drastic change of the interparticle distance. Measurements of the coherent dynamic structure factor S(Q, omega ) of expanded liquid rubidium reveal that up to about three times the critical density, the collective ion-ion dynamics is still controlled by the 'electron sea' of the expanded liquid metal. At lower densities of about twice the critical density, however, a drastic change of shape of S(Q, omega ) is observed, which might be due to the presence of paired units in the expanded liquid.

The results of sound velocity measurements in mercury, performed at temperatures from 300 up to 2050 K and pressures from 30 up to 1900 bar by a precise pulsed phase-sensitive technique for frequency 10 MHz, are presented. The data obtained contain direct evidence of a first-order plasma phase transition in mercury that takes place in the vapour near saturation and in the supercritical fluid.

The structure factors of several liquid K-Tl and Cs-Tl alloys have been measured by means of neutron time-of-flight diffraction at the spallation neutron source ISIS. A large first sharp diffraction peak (FSDP) or prepeak is observed in all measurements at relatively short wave vector, indicating long-range correlations or larger molecular units. So far, we have analysed the data for equiatomic KTl. We arrive at the conclusion that the liquid contains large clusters of thallium atoms, possibly bearing some resemblance to those found in crystalline KTl or K₈Tl₁₁.

We describe preliminary results of realistic (atomic-detail) simulation studies of tilted smectic liquid crystals. Results obtained for a one-layer freely suspended thin film of DOBAMBC in the smectic C* phase are described in detail. Our studies show unambiguously that electrostatic interactions are not required to produce tilted smectic phases, and suggest that molecular tilt in DOBAMBC is driven by the anisotropic molecular shape. We also find evidence for tilt-induced bond orientational order in our simulations.

The orientation taken by a bulk nematic liquid crystal in contact with a substrate can vary with certain parameters, leading to anchoring transitions. Such bulk reorientations are the result of structural transitions taking place in the region of the liquid crystal close to the substrate-the interfacial region-which are driven by the molecules located at the substrate surface. Starting from the experimentally measured orientational distribution of these molecules, it is possible to predict correctly the orientational behaviour of the rest of the liquid crystal and in particular the anchoring transitions.

We observe with an atomic force microscope (AFM) the surface of a rough substrate that gives bistable anchoring for typical nematic liquid crystals. This surface is obtained by oblique evaporation of a thin film of SiO (average thickness approximately 150 AA) on a smooth glass plate. It appears highly disordered and exhibits very small structures with a typical horizontal size of 500 AA and a typical vertical size of 200 AA. The fractal properties of the AFM data acquired indicate that the growth process of the substrate SiO structures is governed by a diffusion law. It is reasonable to expect a decrease of the surface order parameter for the interface layer of a liquid crystal close to this kind of substrate.

The assumption of a timescale separation between conformational kinetics and torsional modes, implicit in the rotational isomeric state approximation, is analysed on the basis of asymptotic solutions of a multidimensional diffusion equation.

The effect of bidispersity on the isotropic-nematic transition is discussed within the framework of Onsager's second-virial theory. The behaviour of bidisperse hard rods beyond the nematic phase is investigated by simulating binary mixtures of hard parallel spherocylinders with different lengths. The behaviour of these mixtures differs markedly, quantitatively as well as qualitatively, from the phase behaviour of the separate monodisperse components. In particular, it is observed that bidispersity favours columnar order over smectic order.

An ab initio approach to simulate the interactions and correlations of charged rodlike colloidal particles is proposed, which combines molecular dynamics for the mesoscopic rods and classical density-functional theory for the microscopic counterions. Many-body inter-rod interactions induced by non-linear counterion screening are systematically included in this approach. It is found that these forces can satisfactorily be described by an effective Yukawa-segment model which, however, differs in general from the traditional Derjaguin-Landau-Verwey-Overbeek theory.

Experimental results on critical static and dynamic light scattering in C₆E₃-water micellar solutions are very well explained by a modified version of the dynamical droplet model, which assumes that the supramolecular solutions are made of polydispersed fractal clusters with a fractal dimension d_f=2.5 and a polydispersity exponent tau =2.2. The model takes into account the finite size of the monomers constituting the clusters, at the origin of dynamical background effects.

Liquid crystalline polymers in nematic solvents are studied theoretically. Three different scenarios are discussed: (i) confined polymers; (ii) grafted polymers and (iii) double brushes. The competition between nematic distortion, chain elasticity, excluded volume and nematic anchoring enables these systems to exhibit a number of remarkable properties. Notably, they can undergo tilting and anchoring transitions and show a quasi-piezoelectric effect. These phenomena may have applications in the design of liquid crystal displays and in making smart electro-optic, mechano-optic and electromechanic materials.

Direct measurements of the velocity of a high-molecular-weight polydimethylsiloxane melt in the immediate vicinity of a solid wall (100 nm from the interface) have been conducted, using a novel optical technique based on evanescent wave induced fluorescence and pattern photobleaching. On silica, quite distinct behaviours are observed: (i) for weak polymer-surface interactions (silica modified by grafting of C₁₈ hydrocarbon chains), a sharp transition between almost no and strong slippage at the wall appears for high enough shear rate, in good agreement with a recent model that ascribes the onset of slippage to a stretching transition of a few chains attached to the surface; (ii) for strong polymer-surface interactions (bare silica) chains irreversibly adsorbed build a pseudo-brush; only weak slippage develops with b (the length under the surface where the velocity extrapolates to zero) independent of the shear rate. No sharp transition appears within the explored shear rate range.

Polymeric liquids often exhibit non-linear responses under flow conditions in spite of the fact that inertia effects are usually negligible. Non-linearities arise when, as a consequence of flow, the distribution of molecular conformations departs significantly from equilibrium. In turn, this will occur whenever the strength of the velocity gradient becomes larger than the inverse of the longest relaxation time of the polymer molecule. Based on this general concept, various detailed interpretations are given to the observed non-linearities (the non-Newtonian viscosity, for example) depending on the specific relaxation mechanism which prevails under different conditions (e.g. dilute solutions or concentrated systems, and branched or linear chains). Here, cases where new results were recently obtained are briefly discussed, including a very curious non-linear effect which is encountered in liquid crystalline polymers.

Pulsed-field gradient NMR has been used to study the self-diffusion of polyions and counterions in polyelectrolyte solutions. The concentration dependence indicates a transition between the concentrated and semidilute regimes. In both regimes the molecular weight dependence is approximately M^-1 and mixtures of different molecular weights show an interesting behaviour. Mixtures with M_w/M_n<1.3 behave as a single fraction.

The self-consistent-field analytical theory of polymer brushes is utilized to consider the equilibrium behaviour of a polyelectrolyte brush immersed in a solution of mobile polyelectrolyte. It is shown that under the conditions of the solution-dominance regime, when the mobile chains penetrate into the brush, the characteristics of the system are determined by the effective second virial coefficient upsilon _eff, dependent on the degrees of charging of grafted and mobile chains, the length of mobile chains and their concentration in the bulk of the solution. In concentrated solutions and melts of mobile polyelectrolytes the brush thickness exhibits non-monotonic behaviour upon increasing the difference between the degrees of charging of grafted and mobile chains. At equal degrees of charging the system exhibits quasineutral behaviour.

The reorientational motion of probe molecules in polymeric mesophases is studied by linear and non-linear electron spin resonance. A non-exponential decay of the orientational correlations, which is homogeneous for observation times longer than 10 mu s, is found. In the undercooled phase of the polymer the apparent distribution of correlation times tau is extended over two decades. The temperature dependence of tau is well represented by a Vogel-Fulcher law and is consistent with the frequency-temperature superposition principle.

We present low-angle light scattering data on aggregation runs of dense colloidal systems. We show that the scattered pattern exhibits a peak at finite wavevectors if the reaction occurs in a diffusion limited regime, and universal scaling similar to that of spinodal decomposition applies. If the runs are made under conditions closer to reaction limited behaviour, however, the peak appearance is delayed until it is eventually lost altogether.

There is need to understand the structure of hard-sphere dispersions under flow in order to develop models for colloid rheology since changes in structure are related to non-Newtonian behaviour. Small-angle neutron scattering (SANS) data for polymer lattices, behaving as nearly hard spheres, at shear rates through the region of the shear thinning are presented. Intensity distributions from these new and detailed low-angle scattering experiments are compared with the intensities calculated directly from computer simulations employing a Rouse model.

We study orientational relaxation in a colloidal suspension of spherical particles whose dynamics is described by a generalized Smoluchowski equation in position and orientation space. We generalize the first cumulant relation for translational diffusion to the case of orientational diffusion. By specializing to a model of dipoles interacting with a Heisenberg pair interaction we obtain the spectrum of the memory function for orientational relaxation in a semi-dilute suspension. Four characteristic times are needed to describe the memory function accurately.

Preliminary results are reported for the density and charge density profiles of a charged colloidal suspension in sedimentation equilibrium. Calculations are based on a simple free energy functional, which includes excluded volume and Coulomb contributions. The suspension is shown to behave like a condenser.

This paper briefly reviews simulations of a simple model oil/water/surfactant system. The model describes only the most characteristic elements of oil/water/surfactant mixtures: oil and water do not mix, and a surfactant is an amphiphilic molecule; one side of the molecule likes oil more than water, and the other side likes water more than oil. Molecular dynamics simulations have been performed on this simple model to study the effects of surfactant chain length on the oil/water interfacial tension, the relation between micellar shape and surfactant structure, and the solubilization of an oil droplet in an aqueous micellar solution.

Sponge-like phases of amphiphilic mixtures are characterized by mesoscopic domains of water and/or oil, separated by surfactant films. These systems are analysed using a mean-spherical model to approximate the free energy of an interfacial system with a bending Hamiltonian for the case of zero spontaneous curvature. Both the scattering structure factor and the phase diagram are determined as functions of concentrations and bending moduli. The scaling properties of the structure factor distinguish a typical microemulsion regime with a well defined structural length scale from a regime unstable with respect to phase separation. The phase diagram shows the multi-phase sequence observed in experiment.

Micellar solutions of the ganglioside GM1 are studied as a function of temperature in the concentration range 0.003-0.30 g cm^-3 in water with light and X-ray scattering. In a narrow temperature range around 50 degrees C, the micelles decrease their aggregation number from 300 to 235. The solution structuring, which is due to the electrostatic interactions among micelles, is found to be strongly affected by the micellar change. For instance, at high concentration the isotropic micellar solution at 25 degrees C transforms into a primitive cubic phase at 60 degrees C.

We study the effects of addition of the macrocyclic cage CESTO (5,12,17-trimethyl-1,5,9,12,17-pentaazabicyclo (7.5.5)nonadecane) on the aggregation behaviour of lithium dodecylsulphate (LDS) in aqueous solution at a pH approximately=12.7. At this pH CESTO is neutral and binds selectively Li counter-ion. Electron spin-echo modulation results show that CESTO interacts with LDS micelles by decreasing the amount of water at the LDS polar headgroups region. Small-angle neutron scattering shows that LDS forms micelles of quasi-spherical shape. Upon addition of CESTO to 1% LDS micellar solution (1/1 mole ratio) we observed a dramatic lowering of the micellar charge, a small growth of the aggregation number, and increased elongation of micelles.

We investigate the drainage of aqueous foams (water+SDS) by ESR using two different nitroxide labels: a water soluble probe and a labelled surfactant. We show that the rotational motion of the probes slows down as the films of the foam become thinner and that confinement takes place. We also experiment with and discuss the influence of the incorporation of a hydrosoluble polymer (PEO).

Molecular dynamics calculations have been used to investigate the comportment of clusters of n-hexadecane molecules deposited as patches on a non-polar surface formed from a 'self-assembled' monolayer of long-chain alkylthiols with C₁₆ hydrocarbon tails. At room temperature, in contrast to intuition-but in general agreement with experiment-hexadecane does not seem to wet such a surface but rather appears to prefer to form a droplet. The contact angle estimated from the average droplet shape is about 60 degrees , which is about 20% larger than the experimental value. Possible reasons for the discrepancy are discussed.

We present an ellipsometric study of wetting and adsorption at the liquid-vapour interface of the cyclohexane-methanol system both at two-phase coexistence and in the one-phase region of the phase diagram. At bulk coexistence, it is observed that either a very thin or a thick film of the heavy phase intrudes between the lighter phase and the vapour in a certain temperature range. The observed hysteresis is consistent with the assumption that the wetting transition if a first-order phase transition. Also, experimental evidence is presented for the existence of a prewetting transition in the mixed region of the phase diagram. For certain compositions, a first-order thin-to-thick-film transition can be observed as the temperature is decreased towards the separation temperature. The prewetting critical point, where the distinction between the thin and the thick film vanishes, is found to display two-dimensional Ising criticality.

When two similar small particles are attached to a liquid interface they attract each other due to a lateral capillary force. This force appears because the gravitational potential energy of the floating particles decreases when they are approaching each other. This force is proportional to R⁶ (R is the particle radius), so it decreases very fast with particle size and becomes negligible for R<10 mu m. We found that the situation is quite different when the particles (instead of being freely floating) are partially immersed in a liquid layer on a substrate. In this case the energy of capillary attraction is proportional to R² and turns out to be much larger than kT even with particles of diameter about 10 nm. The effect is related to the particle three-phase contact angle, i.e. to the intermolecular forces, rather than to gravity. The experiments show that the lateral capillary forces can bring about the formation of a two-dimensional array (2D-crystal) from both micrometre-size and submicrometre particles: latex spheres, protein globules, etc.

Structural information about monolayers of soluble surfactants adsorbed at the air-liquid interface may be determined by neutron reflection in conjunction with isotopic substitution. An outline of the procedure is given and it is shown that there are two types of structural parameter, one determining the thickness sigma of the distribution of a given fragment and the other the separation delta between the centres of the distributions of different fragments. The former depends on both the roughness of the layer and the intrinsic thickness of the distribution of the fragment. The delta values are independent of the roughness. The results are summarized for a wide range of surfactants and surface concentrations. It is shown that a simple model of the roughness due to thermal excitations accounts for much of the roughness of the layer, that the hydrocarbon chains are strongly tilted away from the surface normal, and that there is a significant incidence of gauche conformations as the chain length increases.

Fluid membranes such as lipid bilayers often form oriented stacks or bunches. Within these bunches, the direct molecular forces are renormalized by thermally excited shape fluctuations. For purely repulsive forces between the membranes, this renormalization depends only weakly on the number N of the membranes within the bunch. In the presence of attractive forces, these bunches undergo continuous unbinding transitions. For freely suspended bunches, all membranes unbind simultaneously at a unique N-independent temperature T_*^s, but the effective critical behaviour depends on N over many length scales. For bunches that adhere to another surface, the membranes can peel off one after another in a sequence of transitions that occur in a finite temperature range above T_*^s.

It is shown experimentally that the first-order phase transition between the SmA- and SmC*-like structures in freely suspended films of the compound ALLO can be induced by transient changes Delta tau in the tension of these films. For a given positive variation Delta tau in the film tension, the width of the temperature interval Delta T=T-T_A-C* in which the transition can be induced is measured as a function of the film thickness. A thermodynamic explanation of the observed phenomena is proposed.

Solitons were discovered and their properties described in the study of ideal, dissipation-free non-linear integrable systems such as the Korteweg-de Vries equation and Toda's lattice. Originally appearing as solitary waves, their peculiar properties of invariance upon translation and their typical elastic collisions with at most a phase shift in trajectories led quite correctly to them being considered particle like. Indeed they behave as 'perfect' particles/molecules/hard spheres. What about real, dissipative media such as a sheared, stably stratified atmosphere or open shallow liquid layers heated from the air side? In the latter case I illustrate how following an instability threshold solitons can be created and maintained thanks on the one hand to a (local) balance between (inertial) non-linearity and dispersion and on the other hand to an input-output (pumping-dissipation) energy balance. Theory and numerical and experimental evidence illustrate the features of such 'imperfect'/van der Waals-like molecules: head-on and oblique elastic or inelastic collisions, wall reflections with and without formation of Mach stems, etc.

We present the results of a Benard-Marangoni convection experiment in a square container with small aspect ratio. When the control parameters are slightly changed, a sequence of bifurcations is observed. Moreover, in some measurable region of parameter space, a chaotic behaviour is evidenced. We show that these dynamics can be explained by assuming that the system is close to a codimension two (Takens-Bogdanov) bifurcation after taking into account the symmetries of the system. The dynamical model deduced from these simple observations reflects many of the features present in the experiment. The characteristics of the chaotic regime are also compared with those of the model.

In non-cohesive granular materials the elementary excitations are the vibrations. Due to the nature of the contacts between individual grains, the sound propagation will be along meandering tortuous paths. We have studied the high-frequency vibrations in such a material. The vibrations experienced by a single grain were exceedingly noisy even when the amplitude was low. At low vibration amplitudes, thermal fluctuations dominate the response, so a local heater can probe the extent of the vibration.