Table of contents

In this paper, I review some of the recent results obtained, using molecular dynamics simulations, on the out-of-equilibrium behaviour of glass-forming systems. Both the ageing (evolution after a fast quench in the glassy phase) and the driven (evolution under uniform shear flow) situations are considered. The theoretical concept of effective temperature that was shown to characterize such nonequilibrium states well is also discussed.

We have used a variety of different applied fields to control the density, growth, and structure of colloidal crystals. Gravity exerts a body force proportional to the buoyant mass and in equilibrium produces a height-dependent concentration profile. A similar body force can be obtained with electric fields on charged particles (electrophoresis), a temperature gradient on all particles, or an electric field gradient on uncharged particles (dielectrophoresis). The last is particularly interesting since its magnitude and sign can be changed by tuning the applied frequency. We study these effects in bulk (making 'dielectrophoretic bottles' or traps), to control concentration profiles during nucleation and growth and near surfaces. We also study control of non-spherical and optically anisotropic particles with the light field from laser tweezers.

We present series of experiments based on near field laser velocimetry, developed to characterize the friction mechanisms at fluid–solid interfaces. For polymers, entangled polymer melts are sheared against smooth solid surfaces, covered by surface attached polymer chains of the same chemical species, having a controlled surface density. Direct measurements of the interfacial velocity and of the shear force allow identification of the molecular mechanisms of friction. Depending on the value of the inverse of the shear rate experienced by the polymer compared to the reptation time, the transition between a regime of high and a regime of low friction observed when increasing the shear rate can be related to disentanglement or to the extraction of the surface chains from the bulk polymer. Surfaces with adjusted friction properties can thus be designed by choosing chain anchored length and surface density. For simple fluids, the direct measurements of the interfacial velocity show that, contrary to the usual hypothesis of hydrodynamics, a simple fluid can exhibit slip at the wall. Both the surface roughness and the strength of the fluid–solid interactions fix the amplitude of wall slip, acting in an antagonistic way.

Biomimetic membranes such as lipid bilayers with several molecular components exhibit intramembrane domains. These domains are formed after the membranes are quenched into a two-phase coexistence region. Solid domains tend to form facets or cylindrical segments whereas liquid domains tend to form spherical buds. In the latter case, one can distinguish three tension regimes; budding always occurs in the low tension regime for which we present preliminary experimental results for vesicle membranes composed of phospholipid, sphingomyelin and cholesterol. A multicomponent vesicle which adheres to a substrate surface consists of a bound and an unbound membrane segment which differ in their compositions. This shift in composition can induce domains within the bound segment. Such domains are also formed when membranes adhere via sticker molecules and can be driven (i) by the interplay of sticker adhesion and shape fluctuations and (ii) by the competition between sticker and repeller molecules.

Two systems that are not fluids but that have properties normally associated with fluids will be discussed: nematic elastomers, which are liquid crystalline rubbers whose shear modulus vanishes because of a spontaneous broken symmetry, and granular materials under flow. Kinetic–hydrodynamic equations for the latter materials will be used to discuss flows in Couette cells and down rough inclined planes.

Self-assembly in soft-matter systems often results in the formation of locally cylindrical or chain-like structures. We review the theory of these systems whose large-scale structure and properties depend on whether the chains are finite, with end-caps or join to form junctions that result in networks. Physical examples discussed here include physical gels, wormlike micelles, dipolar fluids and microemulsions. In all these cases, the competition between end-caps and junctions results in an entropic phase separation into junction-rich and junction-poor phases, as recently observed by electron microscopy and seen in computer simulations. A simple model that accounts for these phenomena is reviewed. Extensions of these ideas can be applied to treat network formation and phase separation in a system of telechelic (hydrophobically tipped, hydrophilic) polymers and oil-in-water microemulsions, as observed in recent experiments.

Liquid foam is an example of soft matter (or a complex fluid) with a very well-defined structure, first clearly described by Joseph Plateau in the 19th century. Current research addresses many aspects of the fluid dynamics of this system. How is liquid transported through it in response to a pressure gradient or gravity? How does it respond to stress, particularly above the yield stress? What is the nature of the local fluid flow in the Plateau borders and their junctions? Simple first-order answers to many such questions exist but ongoing experiments continue to challenge our understanding.

How far can one supercool a liquid before it crystallizes? How much can one stretch it before cavitation occurs ? In order to answer such questions, we have studied liquid helium, a model system. In this review, we show the limitations of the elementary 'standard nucleation theory'. We then show that the existence of 'spinodal' limits needs to be considered in the framework of 'density functional' methods. We also briefly consider the possibility of nucleation by quantum tunnelling. The main emphasis is on cavitation and crystallization in liquid helium, but we also mention several connections with more classical systems, in particular water.

The main aims of this paper are: (i) to present molecular dynamics (MD) results for time correlation functions, calculated with the help of a new symplectic algorithm that was proposed by us recently; (ii) to discuss the results obtained with the main focus on the interplay between the spin and liquid subsystems as well as on the influence of an external magnetic field on the properties of the system considered; (iii) to compare the time correlation functions, calculated in MD simulations, with theoretical predictions. Our results provide evidence of the interesting interplay of the two subsystems, causing new phenomena not found in nonmagnetic fluids and magnetic lattice models.

We have carried out an ab initio molecular dynamics simulation of liquid oxygen, a molecular fluid in which the individual O₂ units carry a molecular magnetic moment. In addition to the atomic and electronic structures, our simulation describes the evolution of the noncollinear magnetic structure. The atomic structure shows a strong preference for parallel alignment of first-neighbour molecules. The magnetic structure shows strong short-range antiferromagnetic correlations, in agreement with spin-polarized neutron diffraction data. The short-range correlations, observed in both the structural and magnetic properties, primarily result from appropriate trajectories of colliding O₂ molecules. Our simulation also reveals the occurrence of several long-living O₄ units which survive for time periods longer than four times the average residence time observed during collisions.

We discuss, on a microscopic level, the effects of confinement on structural as well as dynamic properties of quantum liquids. The most evident structural consequences of confinement are layer structures found in liquid films, and free surfaces appearing in liquid drops and slabs. These structural properties have immediate consequences: new types of excitation such as surface phonons, layer phonons, layer rotons, and standing waves can appear and are potentially observable in neutron scattering spectra as well as in thermodynamic properties.

Atom scattering experiments provide further insights into structural properties. Methods have been developed to describe elastic and inelastic atom scattering as well as transport currents. The theory has been applied to examine scattering processes of ⁴He and ³He atoms impinging on ⁴He clusters, as well as ⁴He scattering off ⁴He films and slabs.

An overview of recent developments and applications of a specific density functional approach that originates from Rosenfeld's fundamental measure theory for hard spheres is given. Model systems that were treated include penetrable spheres that interact with a step function pair potential, the Widom–Rowlinson model, the Asakura–Oosawa colloid–polymer mixture, ternary mixtures of spheres, needles, and globular polymers, hard-body amphiphilic mixtures, fluids in porous media, and random sequential adsorption that describes non-equilibrium processes such as colloidal deposition and random car parking. In these systems various physical phenomena were studied, such as correlations in liquids, freezing and demixing phase behaviour, the properties of fluid interfaces with and without orientational order, and wetting and layering phenomena at walls.

We have measured the microscopic structure of liquid para-hydrogen by means of a neutron diffraction experiment on the D4C liquids diffractometer at Institute Laue-Langevin (Grenoble, France). This is the first direct neutron diffraction measurement of the static structure factor of hydrogen. The present determination of the microscopic structure of hydrogen is consistent with previous experimental determinations carried out on liquid deuterium and with path integral Monte Carlo simulations. The comparison with recent x-ray determinations is also satisfactory.

We have adapted the transition path sampling algorithm for the study of activated diffusive processes in complex systems, in which the system has to overcome a high and rough free energy barrier. The new scheme is based on the stochastic shooting procedure and employs molecular dynamics trajectories with an Andersen thermostat as a noise generator. The algorithm is illustrated on a diffusive process in a dense Lennard-Jones fluid.

Hydrogen/deuterium isotopic substitution neutron scattering techniques and empirical potential structure refinement have been used to investigate the structure in a 0.86 mole fraction solution of tertiary butanol in water. The resulting model structure suggests that in this mixture, the relatively small quantity of water molecules tend to associate in small hydrogen bonded clusters of two or three molecules, solvated by the alcohol hydroxyl groups. An interesting observation on investigating this novel local water environment is that the tetrahedral hydrogen bonding motif of water still strongly influences the location of both the water and alcohol first neighbours. The combination of the small number of water molecules in the mixture and the bulky molecular structure of the alcohol appears to result in the incomplete satisfaction of the water molecule's full hydrogen bonding capabilities.

Ultrafast vibrational motions and the underlying microscopic couplings in hydrogen-bonded cyclic dimers of acetic acid are studied by pump–probe and photon echo experiments in the mid-infrared. Upon femtosecond excitation of the O–H stretching mode, we demonstrate coherent nuclear motions along the in-plane dimer stretching mode which persist for picoseconds. The anharmonic coupling of the O–H stretching and the low-frequency mode is isolated in the nonlinear vibrational response, whereas other couplings are suppressed. Three-pulse photon echo experiments demonstrate a dephasing of O–H stretching excitations on a femtosecond timescale and quantum beats due to the anharmonically coupled dimer stretching mode.

Solution scattering from iodine in different solvents is used to derive the cross-correlation between iodine and the solvent. In parallel, molecular dynamics simulations are performed and the findings compared to the experimental results. The close correspondence shows that it is possible to describe the structural rearrangements of the solvent around a solute molecule. Time-resolved studies of photoexcited molecules will therefore allow one to study the relaxations of the solute as well as the solvent molecules.

Molecular dynamics results on water confined in a silica pore in the low-hydration regime are presented. Strong layering effects are found due to the hydrophilic character of the substrate. The local properties of water are studied as a function of both temperature and hydration level. The interaction of the thin films of water with the silica atoms induces a strong distortion of the hydrogen bond network. The residence time of the water molecules is dependent on the distance from the surface. Its behaviour shows a transition from a Brownian to a non-Brownian regime on approaching the substrate in agreement with results found in studies of water in contact with globular proteins. The confined liquid at low hydration shows upon supercooling an anomalous behaviour which is rather different to that in the high-hydration regimes already studied.

The surface energy and entropy of liquid Ga–Bi and Ga–Pb alloys have been studied by means of surface light scattering measurements at various compositions and temperatures between the respective eutectic and monotectic points. Analysis of these results using the Gibbs adsorption equation gives evidence for wetting and prewetting transitions in these alloys completely consistent with a tetra-point wetting scenario (Dietrich S and Schick M 1997 Surf. Sci.382 178). Surface freezing transitions are observed for conditions near the liquidus curves. In view of their viscoelastic properties and their relation with the wetting film characteristics, we suggest a simple explanation for the observed surface freezing phenomena in terms of nucleation of strongly undercooled wetting films.

The structure of liquid 1, 3-dimethylimidazolium hexafluorophosphate is described in detail and compared with the structure of 1, 3-dimethylimidazolium chloride. In each case, the data were obtained from neutron diffraction experiments and analysed using an empirical potential structure refinement process. Overall, the structures are similar; however, significant differences arise from the variation in anion size.

In this study, we perform first-principles molecular dynamics simulations of the eutectic alloy Ge₁₅Te₈₅ at five different densities and temperatures. We obtain structures in agreement with the available diffraction data and obtain a new view of the molten Ge chalcogenides. We show that the anomalous volume contraction observed in the liquid 30 K above the eutectic temperature corresponds to a significant change of the Ge–Te partial structure factor. The detailed structural analysis shows that volume variations observed upon melting in Ge₁₅Te₈₅, as in liquid GeSe and GeTe, can be explained in terms of the competition between two types of local environment of the germanium atoms. A symmetrical coordination octahedron is entropically favoured at high temperature, while an asymmetrical octahedron resulting from the local manifestation of the Peierls distortion is electronically favoured at lower temperatures.

The one-dimensional smectic ordering of the liquid crystal 10CB incorporated in the pores of a silica aerogel has been investigated via x-ray scattering. Although the smectic order is made short-ranged by the aerogel host and the amplitude of the associated Bragg-like peak grows continuously with decreasing temperature, part of the first-order character of the 10CB's direct isotropic–smectic phase transition is retained in the discontinuous temperature dependence of the smectic correlation length. This behaviour contrasts with that of materials where the smectic phase develops from a locally orientationally ordered nematic and can be interpreted as a nucleation-type process.

We have investigated the phase behaviour of pUC18 plasmid solutions with phase separation experiments and polarized light microscopy. Furthermore, the configuration of the superhelix is monitored with small-angle neutron scattering. The phase diagram is interpreted with liquid crystal theory including the effects of charge, orientation entropy, excluded volume, as well as the elastic, entropic and electrostatic contributions to the molecular free energy.

We review the novel features of the Stokes drag of a spherical particle dispersed in a nematic solvent. It is anisotropic, couples to rotations of the particle, and exhibits strong non-linearities.

Aqueous solutions of apoferritin, which consists of 24 proteins assembled into a spherical shell (outer diameter 12 nm), represent highly monodisperse systems. In low-salt solutions, a pronounced peak in the static structure factor S(q) indicates intermolecular interactions due to the net negative charge (pH ≈ 5) of apoferritin. We have investigated both the structure and dynamics of ordered solutions for the first time in the vicinity of the S(q) peak at q*. Coherent small-angle neutron scattering experiments were performed at the Institute Laue-Langevin (Grenoble) and the dynamics was studied using the high-resolution neutron spin echo spectrometer IN15. The dynamics at q > q* coincides with extrapolated results of previous experiments. For low-salt solutions, the normalized intermediate-scattering function has a shape that qualitatively resembles that of S(q) in the vicinity of q*. However, the inverse effective diffusion constant differs quantitatively from S(q) below q*.

Using extensive molecular dynamics simulations we study the behaviour of polyelectrolytes (PEs) in poor solvents, explicitly taking the counterions into account. The resulting pearl-necklace structures are subject to strong conformational fluctuations. These lead to small signatures in the form factor and the force–extension relation, which is a severe obstacle to experimental observations. In addition, we study how the necklace collapses as a function of the Bjerrum length. Finally, we demonstrate that the position of the first peak in the inter-chain structure factor varies with the monomer density as ≈ ρ_m^0.35 for all densities, which shows a pertinent different behaviour as compared to that of PE solutions in good solvent.

Polyelectrolyte multilayers which are built up by alternating adsorption of polyanions and polycations from aqueous solutions at a solid interface are investigated by reflectometry and ellipsometry. Below a degree of charge of about 70% the adsorption stops after a certain number of dipping cycles and no multilayer formation occurs. This indicates an electrostatically driven adsorption process. Below a charge density of 70% an adsorption can take place if the charged segments are combined as a block of the polymer.

We have investigated the interactions between anionic polyelectrolytes and a cationic surfactant at the air/water interface and in bulk, for increasing surfactant concentrations. Mixed aggregates are formed at the air/water surface at extremely low surfactant concentrations. Above a critical aggregation concentration, a viscosity drop indicates that polymer chains undergo a rapid collapse. At higher surfactant concentrations, light scattering shows the existence of larger structures, which are surprisingly monodisperse. Their size increases with surfactant concentration. During this bulk evolution, surface tension remains constant, suggesting that the surface aggregates remain unchanged.

Hairy wormlike micelles are constituted of surfactant micelles onto which amphiphilic copolymers are adsorbed. The copolymer layer induces a steric repulsion between the micelles, which is evidenced by the emergence of a broad peak in the structure factor for semi-dilute solutions. In this paper, we propose three experimental determinations of the thickness of the copolymer layer, h. We compare the results of the different approaches and show that both the numerical values of h and its variation with the surface density of the copolymer layer are in very good agreement with simple theoretical expectations.

In this work, we first briefly review recent results regarding effective interactions of charged star polymers (polyelectrolyte stars) and then we present new results for many-body systems obtained by employing this interaction. The dominant role is played by the trapped counterions that bring about an entropic repulsion between the centres of these macromolecular aggregates. Subsequently, we explicitly derive density-dependent effective interaction potentials between the polyelectrolyte stars and calculate liquid structure factors and solid-state zero-temperature phase diagrams, discovering thereby a large variety of different crystal phases, similar to those found for the case of neutral star polymers.

As is well known, the effective charge of weak polyelectrolytes (PEs) decreases with decreasing salt concentration due to the electrostatic repulsion between dissociated charges. Close to dielectric boundaries, image-charge effects influence the dissociation equilibrium. At low-dielectric-constant substrates, one finds a further charge decrease and repulsion from the interface, while at high-dielectric-constant (e.g. metallic) substrates, the effective charge increases and the PE is attracted to the interface.

We report measurements on gas–liquid phase-separating colloid–polymer mixtures. A horizontally placed optical microscope with long-working-distance objectives enables us to see effects of gravity on phase separation kinetics and hence see the complete phase separation from beginning to end. Furthermore, the static profile near a single wall is analysed giving surface tensions in good agreement with scaling predictions as well as results from another experimental technique. The contact angle remains, however, intangible.

In recent years optical tracer techniques have been developed to determine the micro-rheology of soft viscoelastic materials. Recent theoretical arguments (Levine A J and Lubensky T C 2001 Phys. Rev. E 65 011501) suggest that the correlated fluctuations of a pair of widely separated probe particles should reflect the bulk rheology of the medium that they are embedded in more accurately than the motion of a single particle. We present a experimental test of these arguments. Using optical tweezers techniques (Henderson S, Mitchell S and Bartlett P 2002 Phys. Rev. Lett.88 088302), we measure at high spatial and temporal resolution the thermal motion of a pair of colloidal particles suspended in a semi-dilute viscoelastic solution of non-adsorbing polystyrene in decalin. From the measured particle trajectories we determine both the one-and two-particle correlations and extract the local and bulk rheology. A comparison of the two measurements shows significant differences which are interpreted in terms of the depletion of polymer molecules from the particle surface.

We introduce a new scheme for investigating temporally heterogeneous dynamics, which is termed time-resolved correlation (TRC). TRC is applied to data obtained by diffusing wave spectroscopy probing the slow dynamics of a strongly aggregated colloidal gel. Other examples of TRC data, collected for different jammed materials in single and multiple scattering, are provided to demonstrate the wide range of applicability of this method. In all cases we find evidence that the slow dynamics results from a series of discrete steps rather than from a continuous motion, suggesting temporal heterogeneities to be a general feature of slow dynamics in jammed systems.

At low ionic strength the interaction between any two charged colloids in a concentrated charge-stabilized suspension is strongly influenced by the charge distributions of other colloids in the neighbourhood of the two interacting colloids. When such many-body interactions become important, a colloidal suspension can no longer be treated as a simple Yukawa liquid. We here (i) discuss experimental data showing that many-body interactions are indeed present in low-salt-content colloidal suspensions, (ii) report on three-body calculations in colloidal systems that help in understanding the concept of 'macroion shielding', (iii) demonstrate the configuration dependence of effective forces and (iv) illustrate, using simulations based on (a) a truncated Yukawa potential with a density-dependent cut-off and (b) a full Poisson–Boltzmann mean-field description, the effect of many-body interactions on the solid–liquid phase behaviour of colloidal suspensions.

We study glass formation in hard spheres with short-range attraction. The system consists of nearly-hard-sphere polymethylmethacrylate particles and non-adsorbing random-coil polystyrene which induced a depletion attraction between the particles. The experiments reveal a re-entrant glass transition and two qualitatively distinct glassy states. Dynamic light scattering, covering eleven orders of magnitude in time, gives insight into the kinds of particle motion responsible for these observations. The possible relevance of our results to generic issues, such as the distinction between fragile and strong glass formers, the nature of the underlying 'free energy landscape', and the relative importance of temperature and pressure, is discussed.

We calculate the depletion potential between two hard spheres of radii R_s and that between one hard sphere and a planar hard wall in a sea of hard rigid spherocylindrical rods of length L and radius R_r. Unlike earlier studies our approach takes into account the true geometry of the problem and gives, to first order in the rod density, the exact depletion potential for all values of R_s, L, and R_r.

Recent research related to foam drainage is surveyed with emphasis on the influence of interfacial rheology. Active research directions are highlighted and the possible impact of these studies on macroscopic rheology is indicated.

We propose a simple approximation scheme for computing the effective charges of highly charged colloids (spherical or cylindrical with infinite length). Within non-linear Poisson–Boltzmann theory, we start from an expression for the effective charge in the infinite-dilution limit which is asymptotically valid for large salt concentrations; this result is then extended to finite colloidal concentration, approximating the salt partitioning effect which relates the salt content in the suspension to that of a dialysing reservoir. This leads to an analytical expression for the effective charge as a function of colloid volume fraction and salt concentration. These results compare favourably with the effective charges at saturation (i.e. in the limit of large bare charge) computed numerically following the standard prescription proposed by Alexander et al within the cell model.

Molecular dynamics simulations are used to characterize the permeation by water of cylindrical nanopores, modelling ion channels, as a function of channel radius R and dielectric permittivity . Intermittent permeation is found in a narrow range around the threshold values of R and . While channel permeation is highly sensitive to channel polarization effects, no effect on structural properties of the confined water is found on varying .

The behaviour of an amphiphilic membrane is determined by the physical and chemical properties of the molecules which form the bilayer and their interactions with the surrounding medium. Bulk or interfacial chemical reactions modify interaction parameters and/or affect directly the chemical composition of the membrane. We monitor the morphological response and the thermal fluctuations of giant lipid vesicles to chemical reactions in the external vesicle medium using phase-contrast microscopy. Observation of vesicle conformations as a function of time allows us to characterize the statics and dynamics of membrane response as well as the underlying chemical kinetics. As two examples, we present (a) a photochemical reaction of hexacyanoferrate which induces an increase in pH and (b) the enzymatic cleavage of phosphatidyl choline by the phospholipase C from Bacillus cereus.

We present an x-ray diffraction study on rod-like molecules, i.e. the normal alkanes C₁₉H₄₀ and C₉H₂₀ condensed in nanoporous Vycor glass. Temperature-dependent diffraction patterns elucidate how the structure and phase behaviour of these molecules are affected by the random substrate disorder and the geometric confinement. For the medium-length alkane C₁₉H₄₀ a quenching of the lamellar ordering is observed, whereas this geometric ordering principle survives in the case of the short-length C₉H₂₀, although in a modified fashion.

We review recent experiments on the growth of cryogenic fluids adsorbed on various substrates structured in different ways. On a very well defined array of microscopic linear wedges sculpted on thin Si wafers, the film mass is found to diverge as a power law in the chemical potential difference from saturation with an exponent x = −1.96 ± 0.10, in very good agreement with recent scaling analysis results. For the other, more irregular patterns, the observed exponents range from −0.95 to −2. In any case, they are always much smaller than those found for flat or rough surfaces.

We investigated the dynamics of layering transitions and other structure formation processes in molecularly thin liquid films upon reducing the separation between two atomically smooth mica substrates. Using a newly developed surface forces apparatus with two-dimensional imaging capability, we followed the hydrodynamic processes during drainage with unprecedented precision. Depending on the substrate elasticity and the approach rate, drainage occurs either in a series of consecutive layering transitions or in a single step. In the latter case, nanoscopic amounts of liquid are trapped inside the contact area transiently. The experimental observations are explained qualitatively by combining hydrodynamic effects with elastic deformations of the substrates. Furthermore, we present evidence for anisotropy in the fluid dynamics induced by the lattice symmetry of the substrates.

We present real-time optical microscopy observations of the pattern evolution in self-destruction and subsequent dewetting of thin polymer films based on experiments with polydimethylsiloxane films sandwiched between silicon wafers and aqueous surfactant solutions. A clear scenario consisting of four distinct stages has been identified: amplification of surface fluctuations, break-up of the film and formation of holes, growth and coalescence of holes, and droplet formation and ripening. Besides a linear dependence on film viscosity and surface tension, the time τ for film rupture varied significantly with film thickness h (τ ∼ h⁵), as expected from theory. While the role of long-range forces is dominant only in the first stage, the later stages are controlled by the combination of interfacial tensions resulting in the contact angle characterizing the three-phase contact line. During the first stage, the characteristic distance of the pattern remains constant, represented by a time-independent wavevector. In all subsequent stages, this wavevector decreases with time as a consequence of hole opening, coalescence, and ripening on droplets. The later stages of evolution are a function of the contact angle at the three-phase contact line. Only a clear distinction between stages before and after film break-up allows a correct interpretation of the observed pattern evolution in unstable thin films.

Despite its large economic impact, lubrication has not been well understood up to now. The present paper shows the possible impact of quasielastic and inelastic neutron scattering in this field. Liquids under shear have been investigated using the backscattering instrument IN16 (ILL). Macroscopic and microscopic modes, which can be addressed within the same measurement, have been explored. For a commercial motor oil a macroscopic velocity distribution with surface slip has been found. For a polymer solution (P85 in deuterated water) we report diffusion to slow down under shear. Additionally, diffusion becomes anisotropic under shear for both samples. The experimental data are evaluated quantitatively by using computer simulations which show the limitation of this technique: the quasielastic linewidth should not be larger than the inelastic energy shift from Doppler scattered neutrons.

Combination of synchrotron radiation sources and large-volume presses enables us to carry out in situ observations of structural change in liquids and glasses at high pressures up to several gigaPascals and high temperatures above 1200^oC. In this report, we present two examples: liquid Sn and SiO₂ glass. X-ray diffraction measurements were carried out on liquid Sn at 2.0 and 5.3 GPa. With increasing pressure, a shoulder of the first peak in the structure factor became less prominent and the ratio of the position of the second peak to that of the first peak decreased. These changes are attributed to a diminishing of the covalent structures remaining in the liquid state. Data on the temperature dependence of the x-ray diffraction by SiO₂ glass were measured up to 560^oC at 17 GPa. The sample crystallized to stishovite above 560^oC. While no drastic change was observed in the short-range order, a shift of the position of the first sharp diffraction peak to higher wavenumber and a sharpening of it were observed with increasing temperature. The shift and the sharpening suggest some relaxation in the intermediate-range order.

We review the inherent structure thermodynamical formalism and the formulation of an equation of state (EOS) for liquids in equilibrium based on the (volume) derivatives of the statistical properties of the potential energy surface. We also show that, under the hypothesis that during ageing the system explores states associated with equilibrium configurations, it is possible to generalize the proposed EOS to out-of-equilibrium (OOE) conditions. The proposed formulation is based on the introduction of one additional parameter which, in the chosen thermodynamic formalism, can be chosen as the local minimum where the slowly relaxing OOE liquid is trapped.

We use confocal microscopy to directly visualize the dynamics of ageing colloidal glasses. We prepare a colloidal suspension at high density, a simple model system that shares many properties with other glasses, and initiate experiments by stirring the sample. We follow the motion of several thousand colloidal particles after the stirring and observe that their motion significantly slows as the sample ages. The ageing is both spatially and temporally heterogeneous. Furthermore, while the characteristic relaxation timescale grows with the age of the sample, nontrivial particle motions continue to occur on all timescales.

We present comparisons of theoretical and simulation results for static and dynamical correlation functions for a very simple model of attractive colloidal systems, the short-ranged square-well potential. In the region of the phase diagram investigated, the system displays slow (glassy) dynamics. In particular, we compare the static structure factor calculated by Percus–Yevick closure versus the simulation results, both in the small-and large-wavevector ranges. For the same model, we also compare the non-ergodicity parameter, i.e. the long-time limit of the dynamical density correlators, as calculated by mode-coupling theory and by simulation, confirming the presence of two distinct glassy phases.

The local order in a supercooled monodisperse colloidal fluid is studied by direct imaging of the particles with a laser scanning confocal microscope. The local structure is analysed with a bond order parameter method, which allows one to discern simple structures that are relevant in this system. As expected for samples that crystallize eventually, a large fraction of the particles are found to sit in surroundings with dominant face-centred cubic or hexagonally close-packed character. Evidence for local structures that contain fragments of icosahedra is found, and, moreover, the icosahedral character increases with volume fraction ϕ, which indicates that it might play an important role at volume fractions near the glass transition.

Compared to pure fluids, binary mixtures display a very diverse phase behaviour, which depends sensitively on the parameters of the microscopic potential. Here we investigate the phase diagrams of simple model mixtures by use of a microscopic implementation of the renormalization group technique. First, we consider a symmetric mixture with attractive interactions, possibly relevant for describing fluids of molecules with internal degrees of freedom. Despite the simplicity of the model, slightly tuning the strength of the interactions between unlike species drastically changes the topology of the phase boundary, forcing or inhibiting demixing, and brings about several interesting features such as double critical points, tricritical points, and coexistence domains enclosing 'islands' of homogeneous, mixed fluid. Homogeneous phase separation in mixtures can be driven also by purely repulsive interactions. As an example, we consider a model of soft particles which has been adopted to describe binary polymer solutions. This is shown to display a demixing (fluid–fluid) transition at sufficiently high density. The nature and the physical properties of the corresponding phase transition are investigated.

Viscoelastic phase separation is a new type of phase separation, which may be universal in any dynamically asymmetric mixture composed of slow and fast components. In such a dynamically asymmetric mixture, phase separation generally leads to the formation of a long-lived 'interaction network' (transient gel) of the slow components if the concentration is high enough and the attractive interactions between the components are strong enough. Then, domains rich in fast components are nucleated in a transient gel and they grow. Transiently, a network pattern of the phase rich in slow components is produced even if it is a minority phase. This is a unique feature of viscoelastic phase separation. Pattern evolution is basically controlled by the nucleation kinetics of domains rich in fast components under elastic interactions, the volume shrinking kinetics and the self-induced elastic stress. We discuss the roles of bulk and shear stresses on pattern formation and the mechanisms of network formation.

Effective interactions in soft-matter physics result from a formal contraction of an initial multicomponent system, composed of mesoscopic and small particles, into an effective one-component description. By tracing out in the partition function the degrees of freedom of the small particles, a one-component system of mesoscopic particles interacting with a state-dependent Hamiltonian is found. Although the effective Hamiltonian is not in general pairwise additive, it is usually approximated by a volume term and a pair-potential contribution. In this paper the relation between the structure, for which the volume term plays no role, and the thermodynamics of a fluid of particles interacting with a density-dependent pair potential is analysed. It is shown that the compressibility equation differs from that of atomic fluids. An important consequence is that the infinite-compressibility line derived from the thermodynamics does not coincide with the spinodal line stemming from the divergence of correlations.

A recently developed first-principles approach to the non-linear rheology of dense colloidal suspensions is evaluated and its results compared to those from simulations of sheared systems close to their glass transitions. The predicted scenario of a universal transition of the structural dynamics between yielding of glasses and non-Newtonian (shear-thinning) fluid flow appears well obeyed, and calculations within simplified models rationalize the data over variations in shear rate and viscosity of up to three decades.

We examine linear viscoelastic, and translational and rotational diffusion properties of colloidal model dispersions. Theoretical results are discussed, in comparison with experiments, for monodisperse suspensions of charged and neutral colloidal spheres, and for binary dispersions of differently sized tracer and host particles. The theoretical methods employed comprise a mode-coupling scheme for Brownian particles, and a rooted cluster expansion scheme of tracer diffusion with two- and three-body hydrodynamic interactions included. We analyse in particular the validity of various empirical generalized Stokes–Einstein–Debye (SED) relations between the (dynamic) shear viscosity and translational/rotational diffusion coefficients. Some of these generalized SED relations are basic to microrheological measurements aimed at characterizing the viscoelasticity of complex fluids on the basis of the diffusional properties of immersed tracer particles.

In this short overview we report on our ongoing work on the dynamics of bubbles in various flows. Three different situations are explored: the competition between acoustic and hydrodynamics forces in a vertical pipe (Rensen J, Bosman D, Magnaudet J, Ohl C D, Prosperetti A, Tögel R, Versluis M and Lohse D 2001 Phys. Rev. Lett.86 4819), a rising bubble on which shape oscillations have been induced (de Vries J, Luther S and Lohse D 2002 Eur. J. Phys. B 29 503), and a bubble in a rotating horizontal cylinder. Whereas for the first two situations the standard bubble force models (Magnaudet J and Eames I 2000 Annu. Rev. Fluid Mech.32 659) are consistent with our measurements, modifications for the lift force model seem to be required in the last case.

We describe preliminary results of experiments and simulations concerned with the dewetting of thin polystyrene films (thickness < 7 nm) on top of silicon oxide wafers. In the experiments we scratched an initially flat film with an atomic force microscopy (AFM) tip, producing dry channels in the film. Dewetting of the films was imaged in situ using AFM and a correlated pattern of holes ('satellite holes') was observed along the rims bordering the channels. The development of this complex film rupture process was simulated and the results of experiments and simulations are in good agreement. On the basis of these results, we attempt to explain the appearance of satellite holes and their positions relative to pre-existing holes.

Metal foams are prepared by mixing a metal powder and a gas-releasing blowing agent, by densifying the mix to a dense precursor and finally foaming by melting the powder compact. The foaming process of aluminium foams is monitored in situ by x-ray radioscopy. One observes that foam evolution is accompanied by film rupture processes which lead to foam coalescence. In order to elucidate the importance of oxides for foam stability, lead foams were manufactured from lead powders having two different oxide contents. The two foam types were generated on Earth and under weightlessness during parabolic flights. The measurements show that the main function of oxide particles is to prevent coalescence, while their influence on bulk viscosity of the melt is of secondary importance.

The boiling crisis (BC) is well known in the world of heat and mass transfer. It is a transition from nucleate boiling (i.e. boiling in its usual sense) to film boiling, where the heater is covered by a continuous vapour film. The BC is observed when the heat flux from the heater exceeds a critical value. Heat exchange then falls down and endangers the exchanger whose temperature rises abruptly. The physical mechanism of the BC is still under debate. We propose the recoil force (the thrust of vapour production) at the solid–liquid–vapour contact line to lie at the origin of the BC. At large heat flux, the recoil force tends to spread the vapour bubble that otherwise would not wet the solid. We give both analytical and numerical analysis in support of this idea. We also report experiments under microgravity conditions performed with near-critical fluids (SF₆ and CO₂). The absence of gravity effects and the vicinity of the critical point where the liquid–vapour surface tension vanishes emphasize the influence of the recoil force: during heating, the vapour drop is indeed seen to spread.

This special issue of Journal of Physics: Condensed Matter contains the Proceedings of the Fifth Liquid Matter conference held in Konstanz, Germany, 14-18 September 2002. These conferences are organized every three years by the Liquids Section of the Condensed Matter Division of the European Physical Society. Previous meetings were held in Lyon, Firenze, Norwich and Granada. The aim of the conferences is to bring together scientists working on the liquid state of matter. This rapidly growing field includes the physics, chemistry, biology and chemical engineering of liquid matter as well as various applied research areas.

The conference at Konstanz had 512 registered participants from four continents. The scientific programme consisted of 12 plenary lectures, 84 symposia talks and 506 poster contributions. This volume of the proceedings contains 60 of the oral communications.

Similar to observations at previous Liquid Matter Conferences there is an increasing trend to use and expand concepts and methods originally developed for simple liquids to study and understand properties of more complex liquid systems. This applies in particular to the area of soft condensed matter such as colloidal suspensions, polymeric systems and biological materials. Research in this area is a good example of truly interdisciplinary activities, where traditional borders between physics and its neighbouring sciences have disappeared. As a consequence of this development a significant number of the participants of the conference come from other disciplines than physics, so that this meeting provided a very useful forum for the exchange of ideas and results among scientist with different backgrounds.

The conference was held at the campus of the University of Konstanz. The organizers of the conference are very grateful to the University and its Rector Prof. G. von Graevenitz for the substantial help received and for sponsoring the conference. Finally, it is a pleasure to acknowledge the work of many students, of secretaries and of collaborators and colleagues, who helped to run the conference smoothly.

The Board of the Liquids Section of the European Physical Society decided that the Sixth Liquid Matter Conference will be held in Utrecht, The Netherlands, 2-6 July, 2005.