Search

The beauty of snowflakes originates from the very different ways the ice crystals grow in different environments (1, 2). The amazing variety of snowflake crystal shapes occurs despite the fact that the crystalline structure of snow is always the same: the thermodynamic equilibrium shape is a simple hexagonal crystal. Therefore, the path taken in the nucleation and growth of the ice appears to be more important in determining the final result than the thermodynamic equilibrium state. In PNAS, Lee et al. (3) show that the nucleation pathway for salts precipitating from aqueous solutions is also rather more complicated than the simple formation of a crystalline nucleus that spontaneously forms and subsequently grows. There is still a lot of debate on the conditions under which perfect single crystals are formed, as opposed to the formation of “imperfect” or multiple crystals of different shapes, sizes, and crystalline structures (4⇓⇓⇓⇓⇓⇓⇓⇓⇓–14). Crystallization is traditionally very important for many processes, from the production of steel to the purification of chemicals. In the pharmaceutical and chemical industry high purity is a requirement and is achieved through multiple recrystallizations. For crystallography, the structure of many important biological molecules, such as DNA, and many proteins was unraveled by X-ray diffraction that necessitates good-quality single crystals; all this necessitates a good comprehension of crystal nucleation and growth. Lee et al. (3) follow the crystallization of potassium dihydrogen phosphate (KDP) in evaporating, levitated droplets of aqueous solutions of this salt using a very clever experimental set-up that integrates electrostatic levitation with in situ micro Raman spectroscopy and wide-angle X-ray scattering. In this way, the authors are able to investigate the evolution of salt solutions before and during crystal precipitation and reveal the sequence of events that lead to … [↵][1]1To whom correspondence should be addressed. Email: N.Shahidzadeh{at}uva.nl. [1]: #xref-corresp-1-1

Salt crystallization is a major cause of weathering of rocks, artworks and monuments. Damage can only occur if crystals continue to grow in confinement, i.e. within the pore space of these materials, thus generating mechanical stress. We report the direct measurement, at the microscale, of the force exerted by growing alkali halide salt crystals while visualizing their spontaneous nucleation and growth. The experiments reveal the crucial role of the wetting films between the growing crystal and the confining walls for the development of the pressure. Our results suggest that the measured force originates from repulsion between the similarly charged confining wall and the salt crystal separated by a ~1.5 nm liquid film. Indeed, if the walls are made hydrophobic, no film is observed and no repulsive forces are detected. We also show that the magnitude of the induced pressure is system specific explaining why different salts lead to different amounts of damage to porous materials.

We study the process of phase separation of colloid polymer mixtures in the (spinodal) two-phase region of the phase diagram in shear flow. We use a counter-rotating shear cell and image the system by means of confocal laser scanning microscopy. The system is quenched from an initially almost homogeneous state at very high ( 200 s(-1)) shear rate to a low shear rate (gamma)over dot. A spinodal decomposition pattern is observed. Initially, the characteristic length scale increases linearly with time. As the structure coarsens, the shear imposes a certain length scale on the structure and a clear asymmetry develops. The domains become highly stretched along the flow direction, and the domain width along the vorticity axis reaches a stationary size, which scales as approximate to(gamma)over dot(-0.35). Furthermore, on quenching from an intermediate (6.7 s(-1)) to a low shear rate the elongated structures become Rayleigh unstable and break up into smaller droplets. Still, the system eventually reaches the same steady state as was found from a direct high to low shear rate quench through coarsening.

The seemingly simple problem of the coalescence of droplets was already studied at the end of the 19th century [1]. Coalescence intervenes in many physical situations. For example, the droplet size distribution in rain is (among other processes) determined by it [2]. A detailed understanding of coalescence is important in a large number of industrial applications, such as emulsion stability, ink-jet printing, coating applications, and the dynamics of multiphase flows [3]. The last few years have witnessed a renewed interest in the breakup of free-surface flows under the influence of surface tension [4]. Notably, significant progress has been made in the study of the hydrodynamic singularities that occur in these problems. For the coalescence of a droplet, at the moment of contact, a singularity occurs due to the inversion of one of its two radii of curvature. Detailed theory [5] and numerics [6] have been able to capture the singularity, and an understanding of the hydrodynamics of the coalescence has been developed. However, experimentally, because of the rapidity of the phenomenon, very few quantitative studies of the initial stages of coalescence exist. In this Letter we provide a detailed experimental study of the coalescence of two droplets and of a droplet with its bulk fluid phase, and compare with the theoretical predictions. We succeed in observing purely viscous coalescence, where the radius R of the liquid bridge grows linearly with time t, as opposed to inertial coalescence for which R / �� p . The latter has recently been observed in other experi

We calculate the interfacial tension and the wetting behavior in phase separated colloid–polymer mixtures both for ideal and excluded volume interacting polymers. Within the recently developed extension of the free volume theory to include polymer interactions [Aarts, Tuinier, and Lekkerkerker, J. Phys.: Condens. Matter 14, 7551 (2002)] the interfacial tension of the free interface is calculated by adding a van der Waals squared gradient term. The wetting behavior at a hard wall is calculated following a Cahn–Fisher–Nakanishi approach taking the one- and two-body colloid-wall interactions into account. Comparing results for interacting polymers with those for ideal polymers we find that for interacting polymers the interfacial tension does not increase as steeply as a function of the gas–liquid colloid density difference. Furthermore, the wetting transition shifts to higher polymer concentrations, even to above the triple line. The predictions for both the interfacial tension and the wetting are compared to recent experiments.

The surface-active ions tetraphenylarsonium (Ph4As+) and tetraphenylboron (Ph4B-) have a similar structure but opposite charge. At the solution-air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph4As+ compared with Ph4B-, in addition to opposite alignment of interfacial water molecules. At the water-oil interface, the interfacial tension is 7 mN/m lower for Ph4As+ than for Ph4B- salts, but this can be fully accounted for by the different bulk solubility of these ions in the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measurements.

The shear flow of microfibrillated cellulose dispersions is still not wholly understood as a consequence of their multi-length-scale heterogeneity. We added carboxymethyl cellulose, a charged polymer, that makes cellulose microfibril dispersions more homogeneous at the submicron and macro scales. We then compared the yielding and flow behavior of these dispersions to that of typical thixotropic yield-stress fluids. Despite the apparent homogeneity of the dispersions, their flow velocity profiles in cone–plate geometry, as measured by rheo-MRI velocimetry, differ strongly from those observed for typical thixotropic model systems: the viscosity across the gap is not uniform, despite a flat stress field across the gap. We describe these velocity profiles with a nonlocal model, and attribute the non-locality to persistent micron-scale structural heterogeneity.

A new method for the elaboration of a novel type of catalytic microsystem with a high specific area catalyst is developed. A silver nanocluster catalytic microreactor was elaborated by doping a soda-lime glass with a silver salt. By applying a high power laser beam to the glass, silver nanoclusters are obtained at one of the surfaces which were characterized by BET measurements and AFM. A microfluidic chip was obtained by sealing the silver coated glass with a NOA 81 microchannel. The catalytic activity of the silver nanoclusters was then tested for the efficiency of water purification by using catalytic ozonation to oxidize an organic pollutant. The silver nanoclusters were found to be very stable in the microreactor and efficiently oxidized the pollutant, in spite of the very short residence times in the microchannel. This opens the way to study catalytic reactions in microchannels without the need of introducing the catalyst as a powder or manufacturing complex packed bed microreactors.

We study the effect of addition of polyethylene oxide (PEO) on the rheological behavior of suspensions of Laponite. Experiments were performed on mixtures of PEO and Laponite at different concentrations. These mixtures can exhibit very strong shear thickening behavior: under shear, the suspension can become a yield stress material sufficiently strong to support its own weight. Depending on the polymer concentration, we observe continuous or discontinuous shear thickening mechanisms; it is the discontinuous shear thickening that leads to the formation of yield stress materials.

A thin stream or rope of viscous fluid falling from a sufficient height onto a surface forms a steadily rotating helical coil. Tabletop laboratory experiments in combination with a numerical model for slender liquid ropes reveal that finite-amplitude coiling can occur in four distinct regimes (viscous, gravitational, inertio-gravitational, and inertial) corresponding to different balances among the three principal forces acting on the rope. The model further shows that the onset of coiling has distinct viscous, gravitational, and inertial modes that connect smoothly with the corresponding finite-amplitude regimes. In addition to steady coiling, slender liquid ropes falling onto surfaces can exhibit a remarkable variety of nonstationary behaviors, including propagating spiral waves of air bubbles, supercoiling, the leaping-shampoo (Kaye) effect for non-Newtonian fluids, and the fluid-mechanical sewing machine in which the rope leaves complex stitch patterns on a moving surface.

Yield-stress liquids are materials that are solid below a critical applied stress and flow like mobile liquids at higher stresses. Classical descriptions of yield-stress liquids, which have been the basis for asymptotic and computational studies for five decades, are inadequate to describe many recent experimental observations, and it is clear that the time dependence of microstructure must be taken into account in the description of many real yield-stress liquids.

We investigate the effect of wall roughness upon the entrapment of oil (alkanes) by water flooding in a microchannel. We use fluorescence microscopy to track the in situ oil displacement process in these channels of controlled wall roughness. We find that the viscosity contrast between water and oil determines whether the alkane phase is partially retained in the microchannel. Oil recovery rates are found to be controlled by the wall roughness and the flow rate in the experiment. We also perform the displacement experiments in novel microfluidic 2D porous networks and show that a small variation in the pore-size distribution is also a representation of the solid medium's roughness. We observe that the trapped oil in the porous network follows the same trend as in the rough microchannels, i.e., viscosity contrast, flow speed, and roughness govern the quantity of the trapped oil. We propose a scaling law to quantify the trapped fluid volume based on our experimental observations, which accounts for both the flow rate and the characteristic roughness of the system.

We study the wetting of graphene nanopowders by measuring the water adsorption in nanopowder flakes of different flake thicknesses. Chemical analysis shows that the graphene flakes, especially the thin ones, might exist in the partially oxidized state. We observe that the thinnest graphene nanopowder flakes do not adsorb water at all, independent of the relative humidity. Thicker flakes, on the other hand, do adsorb an increasing amount of water with increasing humidity. This allows us to assess their wetting behavior which is actually the result of the competition between the adhesive interactions of water and graphene and the cohesive interactions of water. Explicit calculation of these contributions from the van der Waals interactions confirms that the adhesive interactions between very thin flakes of graphene oxide and water are extremely weak, which makes the flakes superhydrophobic. "Liquid marble" tests with graphene nanopowder flakes confirm the superhydrophobicity. This shows that the origin of the much debated "wetting transparency" of graphene is due to the fact that a single graphene or graphene oxide layer does not contribute significantly to the adhesion between a wetting phase and the substrate.

The glass transition is perhaps the greatest unsolved problem in condensed matter physics: the main question is how to reconcile the liquid-like structure with solid-like mechanical properties. In solids, structure and mechanics are related directly through the vibrational density of states of the material. Here, we obtain for the first time the density of states of colloidal glasses and supercooled liquids from a normal-mode analysis of particle displacements measured using confocal microscopy. We find that the spectrum of the (non-linear) vibrations has many ‘soft’, low-frequency modes, more abundant and very different in nature from the usual acoustic vibrations of ordinary solids. This results in an anomalous low-frequency peak in the density of states which approaches zero frequency as one goes deeper into the glass. The observed soft modes are due to collective ‘swirling’ particle motions, that extend over surprisingly long length scales.

This study investigated in male European hamsters (Cricetus cricetus ) whether entrainment of circannual rhythms follows the principles of the nonparametric entrainment model. In 2 experiments the times of the year when long (LP) or short photoperiod (SP) are able to synchronize the reproductive cycle were determined, by recording phase response curves (PRCs). A total of 28 groups of 10 hamsters were synchronized by SP, before being subjected to 2 converse experiments: a) 14 groups were transferred to constant LP, only interrupted by SP for 1 month (SP-pulse), the pulse being increasingly delayed between groups by 2 weeks or 1 month steps; and b) the remaining 14 groups stayed in constant SP interrupted by LP for 1 month (LP-pulse) at different phases of the cycle. In a 3rd experiment 5 groups of 10 European hamsters were subjected to constant LP interrupted by 1-month SP-pulses in regular non-365-day zeitgeber intervals (circannual T-cycles) differing between groups (c). The reproductive state was checked every 2 or 4 weeks. The PRCs revealed that an SP-pulse had a very strong phase-resetting capability of -180 degrees to at least +81 degrees in subjective summer (a). During subjective winter when the animals hibernate, a SP-pulse had only weak effectiveness (a) whereas an LP-pulse advanced the circannual clock by up to +41 degrees (b). In the latter conditions a further advance of up to +156 degrees was achieved by the decrease in photoperiod at the return to SP conditions, which terminated the reproductive phase already after 4 to 5 weeks. In different circannual T-cycles the animals entrained for at least 2 cycles (c). In conclusion, 1) the circannual rhythm of European hamsters can be entrained by one photoperiodic signal per cycle, 2) the decrease in photoperiod is most important for its synchronization, and 3) as in circadian clocks the resetting of circannual clocks follows the principles of the nonparametric entrainment model.

We present a study of hydraulic jumps with flow predominantly in one direction, created either by confining the flow to a narrow channel with parallel walls or by providing an inflow in the form of a narrow sheet. In the channel flow, we find a linear height profile upstream of the jump as expected for a supercritical one-dimensional boundary layer flow, but we find that the surface slope is up to an order of magnitude larger than expected and independent of flow rate. We explain this as an effect of turbulent fluctuations creating an enhanced eddy viscosity, and we model the results in terms of Prandtl's mixing-length theory with a mixing length that is proportional to the height of the fluid layer. Using averaged boundary-layer equations, taking into account the friction with the channel walls and the eddy viscosity, the flow both upstream and downstream of the jump can be understood. For the downstream subcritical flow, we assume that the critical height is attained close to the channel outlet. We use mass and momentum conservation to determine the position of the jump and obtain an estimate which is in rough agreement with our experiment. We show that the averaging method with a varying velocity profile allows for computation of the flow-structure through the jump and predicts a separation vortex behind the jump, something which is not clearly seen experimentally, probably owing to turbulence. In the sheet flow, we find that the jump has the shape of a rhombus with sharply defined oblique shocks. The experiment shows that the variation of the opening angle of the rhombus with flow rate is determined by the condition that the normal velocity at the jump is constant.

Un mince filet de fluide visqueux qui tombe sur une surface s’enroule en helice tournante, alors que le meme fluide en forme de nappe se replie de maniere periodique. Ces instabilites, dites « de flambage », cachent une physique d’une grande quatre regimes qui correspondent aux differents equilibres des forces visqueuse, gravitationnelle et inertielle. La gamme de leurs applications va des industries de l’alimentation et de fabrication des polymeres jusqu’a la dynamique du manteau de notre planete.

International audience; The review deals with applications of magnetic resonance imaging (MRI) techniques to study flow. We first briefly discuss the principles of flow measurement by MRI and give examples of some applications, such as multiphase flows, the MRI rheology of complex fluid flows, and blood flows in the human body.

In the free-energy landscape picture of glassy systems, their slow dynamics is due to a complicated free-energy landscape with many local minima. We show that for a colloidal glassy material multiple paths can be taken through the free-energy landscape. The evolution of the nonergodicity parameter shows two distinct master curves that we identify as gels and glasses. We show that for a range of colloid concentrations, the transition to nonergodicity can occur in either direction (gel or glass), accompanied by "hesitations" between the two. Thus, colloidal gels and glasses are merely global free-energy minima in the same free-energy landscape, and the paths leading to these minima can be complicated. © 2007 The American Physical Society.

We study the rheology of a granular paste, i.e. a dense suspension of non-Brownian particles, quantitatively at steady state, in a cylindrical Couette cell. Previous studies have shown a discrepancy between local and global measurements of the viscosity for these materials, making it impossible to predict their resistance to flow. Using both MRI investigation techniques and classical rheology studies, we show that agreement between local and global measurements can be obtained, provided the migration of particles inside the gap is taken into account. As found by Leighton & Acrivos (J. Fluid Mech. vol. 181, 1987, p. 415), the migration leads to a particle density gradient in the flow, the highly sheared regions being less dense in particles. Here, by comparing the local viscosity and particle density measurements from MRI with the macroscopic relation between viscosity and the volume fraction, it is shown that global and local measurements agree with each other. This consequently allows us to define a viscosity for dense suspensions.

International audience; We study droplet formation in granular suspensions by systematically varying the volume fractions (ϕ) and particle diameters (d). For suspensions with water as the suspending liquid, we find three different regimes. For dilute suspensions (ϕ ≤ 45%), drop formation follows the predictions for inertial breakup and exhibits identical dynamics to that of pure water. The breakup is strongly asymmetrical in this case. Only for more concentrated suspensions (ϕ > 45%) does the presence of particles change the dynamics and two other regimes, a symmetrical inertial regime and a Bagnoldian regime, are uncovered. We construct and discuss a phase diagram that allows us to understand and predict the breakup behavior in granular suspensions.

The microscopic origin of apparent wall slip is studied systematically using a confocal laser scanning microscope coupled to a rheometer. We obtain flow curves on a model emulsion from classical macroscopic measurements that are compared with flow curves obtained from microscopic measurements. By controlling the wetting properties of the shearing walls, we show that the characteristic length used in the so-called fluidity model, proposed by Goyon et al. [Nature (London) 454, 84 (2008)], can be understood in terms of roughness induced by adsorbed droplets on the surface. Additionally, we disentangle two different effects that contribute to the difference between micro- and macrorheology. Both effects manifest themselves as gap-dependent viscosities due to either the formation of a lubricating layer close to the shearing walls or cooperative effects when the flow is strongly confined. Finally, we show that the cooperative effects can also be translated into an effective slip velocity.

Polymer composite materials are widely used for their exceptional mechanical properties, notably their ability to resist large deformations. Here, we examine the failure stress and strain of rubbers reinforced by varying amounts of nano-sized silica particles. We find that small amounts of silica increase the fracture stress and strain, but too much filler makes the material become brittle and consequently fracture happens at small deformations. We thus find that as a function of the amount of filler there is an optimum in the breaking resistance at intermediate filler concentrations. We use a modified Griffith theory to establish a direct relation between the material properties and the fracture behavior that agrees with the experiment.

International audience; Rigid blade coating of glass plates by oil in water emulsions stabilized by surfactants is studied. Complete surface coverage is obtained only for speeds exceeding a threshold velocity dependent on the height between the blade end and the surface. Below this threshold, the emulsion can be inverted in the vicinity of the blade. The inversion dynamics of the oil in water emulsion and the deposition patterns induced by this phase inversion are studied using a microscope mounted set up. We show that these dynamics are universal for different volume fractions and deposition velocities. This inversion as well as the destabilization of the emulsion film deposited at high speeds gives rise to different patterns on the glass surface. These patterns are discussed in terms of the emulsion characteristics as well as the deposition velocity.

International audience; The flow of dry and wet granular media is investigated in a Couette geometry using magnetic resonance imaging in order to test the applicability of the “fluidity model” for nonlocality in these materials. Local volume fraction measurements show that the systems become heterogeneous during flow. We find that the nonlocal rheology of suspensions can be correlated using the fluidity model, but the length scale that emerges is not a material property and the model cannot be used for predictive purposes. Rather, the suspension behavior is fully explained as a consequence of stress-driven particle migration and the resulting concentration gradient. The conclusion is less strong for the dry granular system, but it appears likely that the apparent nonlocal behavior is simply due to the formation of a shear band caused by granular dilatancy.

The area of contact between two objects was detected by using the strong enhancement of the fluorescence of rigidochromic probe molecules attached to one of the surfaces. Confinement of the molecules suppresses nonradiative decay and turns on the fluorescence. The approach is demonstrated by imaging of the contact area of a plastic sphere in contact with a flat glass surface. Our results agree excellently with the prediction of Hertz’s classical theory based on elastic deformation.

Bloodstain Pattern Analysis is a forensic discipline in which, among others, the position of victims can be determined at crime scenes on which blood has been shed. To determine where the blood source was investigators use a straight-line approximation for the trajectory, ignoring effects of gravity and drag and thus overestimating the height of the source. We determined how accurately the location of the origin can be estimated when including gravity and drag into the trajectory reconstruction. We created eight bloodstain patterns at one meter distance from the wall. The origin’s location was determined for each pattern with: the straight-line approximation, our method including gravity and our method including both gravity and drag. The latter two methods require the volume and impact velocity of each bloodstain, which we are able to determine with a 3D scanner and advanced fluid dynamics, respectively. We conclude that by including gravity and drag in the trajectory calculation, the origin’s location can be determined roughly four times more accurately than with the straight-line approximation. Our study enables investigators to determine if the victim was sitting or standing, or it might be possible to connect wounds on the body to specific patterns, which is important for crime scene reconstruction.

The study of the behavior of sessile droplets on solid substrates is not only associated with common everyday phenomena, such as the coffee stain effect, limescale deposits on our bathroom walls , but also very important in many applications such as purification of pharmaceuticals, de-icing of airplanes, inkjet printing and coating applications. In many of these processes, a phase change happens within the drop because of solvent evaporation, temperature changes or chemical reactions, which consequently lead to liquid to solid transitions in the droplets. Here we show that crystallization patterns of evaporating of water drops containing dissolved salts are different from the stains reported for evaporating colloidal suspensions. This happens because during the solvent evaporation, the salts crystallize and grow during the drying. Our results show that the patterns of the resulting salt crystal stains are mainly governed by wetting properties of the emerging crystal as well as the pathway of nucleation and growth and are independent of the evaporation rate and thermal conductivity of the substrates.

We study the rheological behavior of concentrated granular suspensions of simple spherical particles. Under controlled stress, the system exhibits an S-shaped flow curve (stress vs. shear rate) with a negative slope in between the low-viscosity Newtonian regime and the shear thickened regime. Under controlled shear rate, a discontinuous transition between the two states is observed. Stress visualization experiments with a novel fluorescent probe suggest that friction is at the origin of shear thickening. Stress visualization shows that the stress in the system remains homogeneous (no shear banding) if a stress is imposed that is intermediate between the high and low-stress branches. The S-shaped shear thickening is then due to the discontinuous formation of a frictional force network between particles upon increasing the stress., 5 pages + 6 figures

This section addresses the physics of liquid spreading from an energetics point of view. The different modes of spreading are present and several fluid natures (pure or complex) are investigated.

This section presents the contact angle, its determination, hysteresis, and the different approaches to correct it, depending on the substrate roughness.

The maximum spreading of drops impacting on smooth and rough surfaces is measured from low to high impact velocity for liquids with different surface tensions and viscosities. We demonstrate that dynamic wetting plays an important role in the spreading at low velocity, characterized by the dynamic contact angle at maximum spreading. In the energy balance, we account for the dynamic wettability by introducing the capillary energy at zero impact velocity, which relates to the spreading ratio at zero impact velocity. Correcting the measured spreading ratio by the spreading ratio at zero velocity, we find a correct scaling behaviour for low and high impact velocity and, by interpolation between the two, we find a universal scaling curve. The influence of the liquid as well as the nature and roughness of the surface are taken into account properly by rescaling with the spreading ratio at zero velocity, which, as demonstrated, is equivalent to accounting for the dynamic contact angle.

We present a comprehensive review of the physical behavior of yield stress materials in soft condensed matter, which encompass a broad range of materials from colloidal assemblies and gels to emulsions and non-Brownian suspensions. All these disordered materials display a nonlinear flow behavior in response to external mechanical forces, due to the existence of a finite force threshold for flow to occur: the yield stress. We discuss both the physical origin and rheological consequences associated with this nonlinear behavior, and give an overview of experimental techniques available to measure the yield stress. We discuss recent progress concerning a microscopic theoretical description of the flow dynamics of yield stress materials, emphasizing in particular the role played by relaxation time scales, the interplay between shear flow and aging behavior, the existence of inhomogeneous shear flows and shear bands, wall slip, and non-local effects in confined geometries., Comment: Review article: V1: 58 pages, 38 figs, 487 refs. V2: Final version 44 pages, 27 figs, 449 refs. Accepted for publication in Rev. Mod. Phys

International audience; The evaporation of droplets on a substrate that is wetting to the liquid is studied. The radius $R(t)$ of the droplet is followed in time until it reaches zero. If the evaporation is purely diffusive, $R \propto \sqrt{t_0\,{-}\,t}$ is expected, where $t_0$ is the time at which the droplet vanishes; this is found for organic liquids, but water has a different exponent. We show here that the difference is likely to be due to the fact that water vapour is lighter than air, and the vapour of other liquids more dense. If we carefully confine the water so that a diffusive boundary layer may develop, we retrieve $R(t) \propto \sqrt{t_0\,{-}\,t}$. On the other hand, if we force convection for an organic liquid, we retrieve the anomalous exponent for water.

We study the fingering instabilities that occur in the debonding of model adhesives. We perform a linear analysis of the instability in the lifting Hele-Shaw cell, relevant for the failure of soft adhesives. The analysis is done for a Newtonian fluid but can be extended to shear-thinning fluids without any problem. When compared to experiment, significant differences are found between the linear theory and the experimental results. We therefore, discuss three-dimensional effects, that are found to improve the agreement; nevertheless significant differences between theory and experiment persist.

Pushing a fluid with a less viscous one gives rise to the well known Saffman–Taylor instability. This instability is important in a wide variety of applications involving strongly non-Newtonian fluids that often exhibit a yield stress. Here we investigate the Saffmann–Taylor instability in this type of fluid, in longitudinal flows in Hele–Shaw cells. In particular, we study Darcy's law for yield stress fluids. The dispersion equation for the flow is similar to the equations obtained for ordinary viscous fluids but the viscous terms in the dimensionless numbers conditioning the instability now contain the yield stress. This also has repercussions on the wavelength of the instability as it follows from a linear stability analysis. As a consequence of the presence of yield stress, the wavelength of maximum growth is finite even at vanishing velocities. We study Darcy's law and the fingering patterns experimentally for a yield stress fluid in a linear Hele–Shaw cell. The results are in rather good agreement with the theoretical predictions. In addition we observe different regimes that lead to different morphologies of the fingering patterns, in both rectangular and circular Hele–Shaw cells.

We study the nonlinear rheological behaviour and the microscopic particle dynamics for a colloidal glass. The measurements allow us to relate the microscopic diffusion dynamics to the macroscopic viscosity of the system. We demonstrate that the competition between the spontaneous restructuration (ageing) and the destruction of the internal structure by the shear ('shear rejuvenation') leads to a bifurcation in rheological behaviour. For a stress smaller than a (time-dependent) critical value, the viscosity increases in time and the material eventually stops flowing. For slightly larger stresses the viscosity decreases continuously with time and the flow accelerates. Thus the viscosity jumps discontinuously to infinity at the critical stress.

We carried out systematic rheometrical tests under controlled stress with smooth and rough parallel disks, along with magnetic resonance imaging (MRI) tests in coaxial cylinder geometry, with foam and a model concentrated emulsion. At low shear stress wall slip appears to occur but the bulk fluid remains static, as proved by the fact that in this regime the apparent shear rate obtained for a given shear stress is inversely proportional to the gap between the disks. At high shear stress data with different surface types and gaps coincide, suggesting that wall slip is negligible in this regime. In parallel, MRI results show that, in contrast with the apparent, simple, yielding behavior observed in usual rheometry, there is an abrupt transition from a finite shear rate to a static one at critical stress. This critical shear rate precisely corresponds to the transition between the two regimes of slip. This suggests that different flow regimes occur with these materials: (1) at low stress, with smooth surfaces a layer of a different material is sheared along the solid surfaces whereas the rest of the fluid does not flow; with rough surfaces there is no flow; (2) beyond a critical stress, for both surface types, the bulk fluid starts to flow but the shear is localized in a thin layer; then the thickness of this layer increases when stress is applied; (3) for both surface types homogeneous flow is obtained only beyond slightly larger stress, which is associated to a critical, apparent shear rate., Des expériences de rhéometrie avec des surfaces lisses et rugueuses ont été réalisées en géométrie plan-plan ainsi que des expériences d'IRM en géométrie à cylindres coaxiaux, sur une mousse et une émulsion concentrée modèle. Aux faibles contraintes un glissement apparaît à proximité du mobile qui induit le cisaillement alors que le reste du matériau demeure immobile. Aux fortes contraintes les données coïncident quelle soit la nature de la surface, ceci nous indiquant que le glissement est négligeable dans ce régime. Un gradient de cisaillement critique correspond précisément à la transition entre ces deux régimes d'écoulement pour les différents types de surface.

We investigate the nonlinear rheological behavior of colloidal suspensions of Laponite, a synthetic clay, driven by a steady and homogeneous shear strain. We show that the external drive leads to a drastic slowing down of the aging dynamics or even, in some cases, in the rejuvenation of the system. Under shear, after a surprisingly long time, the spontaneous aging process observed at rest is suppressed. The system then reaches a non-equilibrium stationary state, characterised by a complex viscosity depending on the applied shear rate. In addition, the glass exhibits a non-Newtonian shear-thinning behavior. These rheological behaviors confirm recent numerical and theoretical predictions., Comment: 17 pages, Latex, 6 figures, figures included in the body of the text, to appear in Journal of Rheology

Measurement of the resistance to a stretching motion, the so-called elongational viscosity η e , has proven to be difficult for dilute polymer solutions, although it is important for a large number of applications. We attempt to deduce η e of dilute solutions of flexible polymers in two different ways. First, η e can be inferred from measurements of the shear viscosity η and the first normal stress difference N 1 , invoking an appropriate constitutive equation. Second, it is measured in the opposing jet elongational viscometer. Although the results of the latter method have been much debated they agree reasonably well with those from the first method. The combination of the two methods thus allows to obtain the correct order of magnitude of η e .

The cohesive failure of soft adhesives is studied using a yield stress fluid as a model adhesive, which allow to relate the viscoelastic properties of the fluid directly to its adhesive performance. We derive a theoretical expression for the force—distance curve as a function of the yield stress, which describes our experimental results very well. The theoretical prediction is obtained by assuming a circular air—adhesive interface; surprisingly, good agreement between theory and experiment is also obtained when strong fingering instabilities are observed in the experiment. This result is confirmed by the fact that we do not find a significant reduction in the work of adhesion when fingering instabilities are present. In addition, we discuss the morphology of the fingering instabilities.

The Stokes–Einstein (SE) relation is a form of the fluctuation-dissipation theorem (FDT), and relates the diffusion coefficient of colloidal particles to the viscosity via the thermal energy kBT. We study the diffusion coefficient and viscosity of concentrated hard-sphere colloidal model systems in the frequency domain. We show that for these metastable fluid systems strong deviations from the SE relation occur, contrary to predictions of mode coupling theory. Using the FDT, these deviations can be quantified by an effective temperature, Teff. The deviation Teff/T−1 exhibits power-law behavior over several decades in frequency.

A series of benchmark experiments on the effect of the wetting state on the flow properties in porous media were performed, allowing us to relate the wetting properties at the pore scale to the macroscale hydrodynamics. Drainage of n-alkanes (oils) displaced by air in a model porous medium consisting of water-wet sand was studied using gamma-ray densitometry and weight measurements. The enormous advantage of our system is that we know and control the wetting properties perfectly: we can tune the wetting properties by changing the salinity of the water. This allows us to perform porous medium flow experiments for the different wetting states without changing the transport properties (viscosity and density) of the oil. Drainage is found to be more efficient, and consequently oil recovery more important for partial wetting.

A simple mean-field theory is presented which describes the basic observations of recent experiments revealing rich wetting behaviour of n-alkane/methanol mixtures at the liquid-vapour interface. The theory, qualitative and in part heuristic, is based on a microscopic lattice-gas model from which a Cahn–Landau approach is distilled. Besides the physics associated with the short-range components of the intermolecular interactions, effects of the long-range tails of the net van der Waals forces between interfaces are also taken into account. Further, gravitational thinning of the wetting phase is incorporated. The calculation of the spreading coefficient S is extended to the experimentally relevant situation in which the bulk adsorbate is slightly away from two-phase coexistence due to gravity. Analysis of this novel approximation to S for systems with short-range forces leads to the conclusion that the surface specific heat exponents α s =1,1/2, and 0, for first-order wetting, tricritical wetting and critical wetting, respectively, are robust with respect to (weak) gravitational thinning, consistently with experiment. For three different systems the adsorption is calculated as a function of temperature and compared with the experimentally measured ellipticity. Including weak long-range forces which favour wetting in the theory does not visibly alter the critical wetting transition for the nonane/methanol mixture, in contrast with the generic expectation of first-order wetting for such systems, but in good agreement with experiment. For decane/methanol weak long-range forces bring the transition very close to the prewetting critical point, leading to an adsorption behaviour closely reminiscent of short-range tricritical wetting, observed experimentally for alkane chain length between 9.6 and 10. Finally, for undecane/methanol the transition is clearly of first order. First-order wetting is also seen in the experiment.

We study the viscous fingering or Saffman–Taylor instability in two different dilute or semi-dilute polymer solutions. The different solutions exhibit only one non-Newtonian property, in the sense that other non-Newtonian effects can be neglected. The viscosity of solutions of stiff polymers has a strong shear rate dependence. Relative to Newtonian fluids, narrower fingers are found for rigid polymers. For solutions of flexible polymers, elastic effects such as normal stresses are dominant, whereas the shear viscosity is almost constant. Wider fingers are found in this case. We characterize the non-Newtonian flow properties of these polymer solutions completely, allowing for separate and quantitative investigation of the influence of the two most common non-Newtonian properties on the Saffman–Taylor instability. The effects of the non-Newtonian flow properties on the instability can in all cases be understood quantitatively by redefining the control parameter of the instability.

The surfactant sodium bis(2-ethylhexyl) sulphosuccinate (AOT) in water forms lamellar phases at low salinities and sponge phases at higher salinities. For the lowest salinities, the lamellar phase contains many onion defects and is very viscous and elastic. At higher salinities but still in the lamellar phase, the sample has a low viscosity. The sponge phase is a Newtonian fluid with a very low viscosity. When either the low-viscosity lamellar or the sponge phase are sheared, they exhibit a transition, after a long time, to a phase which has the same rheological properties and the same texture as the very viscous lamellar `onion' phase. The random nature of the transition suggests that the formation of the shear-induced phase is due to activated processes, which we discuss here. In addition, the relaxation of the shear-induced phases is very slow; some of its relaxation mechanisms are described.

A mean-field theory is presented which describes the basic observations of recent experiments revealing rich wetting behaviour of n-alkane/methanol mixtures at the liquid-vapour interface. The theory is based on a microscopic lattice-gas model from which a Cahn-Landau phenomenological approach is derived. Besides the physics associated with the short-range components of the intermolecular interactions, the effects of the long-range tails of the net van der Waals forces between interfaces are also taken into account. The tricritical wetting transition, which is featured in the theory when long-range forces are ignored, is replaced by a critical prewetting transition when these forces are included. The experimental observations agree to a large extent with the predictions of the theory for short-range tricritical wetting.

Most concentrated colloidal suspensions such as cement, drilling fluids, paints, muds, etc., have been considered until now thixotropic fluids with a flow curve of an ideal yield stress fluid. We start by showing from inclined plane tests, intended to determine the yield stress, that these systems in fact exhibit peculiar properties. Unlike ideal yield stress fluids, they stop flowing abruptly below a critical stress, and start flowing at a high velocity beyond a critical stress, which in addition increases with the time of preliminary rest. In order to clarify these features we carried out a complete set of rheometrical tests with a model fluid, a bentonite suspension. Our results show that under controlled stress, in some cases after significant flow, there is bifurcation of the behavior towards either stoppage or rapid shear, depending on the relative values of the imposed and critical stresses. As an immediate consequence, we find that no (homogeneous) steady state flows at a shear rate below a critical value can be obtained. These results can be qualitatively predicted by a simple theoretical model that assumes that the viscosity of the material results from the competition between aging and shear rejuvenation, associated to, respectively, the organization or disorganization of the network of particle interactions. This shows that the flow curve in the steady state of concentrated colloidal suspensions and, more generally, of structured fluids, is strongly affected by their thixotropy.

We study the mobility of H+ ions for the quasi-binary liquid mixture of water and isobutyric acid in the vicinity of the critical point Tc of the mixture. In the one-phase region above Tc, a continuous but very drastic decrease in ion mobility is observed for compositions in isobutyric acid exceeding the critical composition. The results are interpreted as the formation of dynamic clusters. The "percolation" line for cluster formation is found to start at the critical point and remains at the critical composition for all temperatures T > Tc studied.