laboratoire de physique statistique
laboratoire de physique statistique




P A R M I :

Precursor films in wetting phenomena - Popescu, M. N. and Oshanin, G. and Dietrich, S. and Cazabat, A-M

Abstract : The spontaneous spreading of non-volatile liquid droplets on solid substrates poses a classic problem in the context of wetting phenomena. It is well known that the spreading of a macroscopic droplet is in many cases accompanied by a thin film of macroscopic lateral extent, the so-called precursor film, which emanates from the three-phase contact line region and spreads ahead of the latter with a much higher speed. Such films have been usually associated with liquid-on-solid systems, but in the last decade similar films have been reported to occur in solid-on-solid systems. While the situations in which the thickness of such films is of mesoscopic size are fairly well understood, an intriguing and yet to be fully understood aspect is the spreading of microscopic, i.e. molecularly thin, films. Here we review the available experimental observations of such films in various liquid-on-solid and solid-on-solid systems, as well as the corresponding theoretical models and studies aimed at understanding their formation and spreading dynamics. Recent developments and perspectives for future research are discussed.
Exploring water and other liquids at negative pressure - Caupin, Frederic and Arvengas, Arnaud and Davitt, Kristina and Azouzi, Mouna El Mekki and Shmulovich, Kirill I. and Ramboz, Claire and Sessoms, David A. and Stroock, Abraham D.

Abstract : Water is famous for its anomalies, most of which become dramatic in the supercooled region, where the liquid is metastable with respect to the solid. Another metastable region has been hitherto less studied: the region where the pressure is negative. Here we review the work on the liquid in the stretched state. Characterization of the properties of the metastable liquid before it breaks by nucleation of a vapour bubble (cavitation) is a challenging task. The recent measurement of the equation of state of the liquid at room temperature down to -26 MPa opens the way to more detailed information on water at low density. The threshold for cavitation in stretched water has also been studied by several methods. A puzzling discrepancy between experiments and theory remains unexplained. To evaluate how specific this behaviour is to water, we discuss the cavitation data on other liquids. We conclude with a description of the ongoing work in our groups.
Shear banding in thixotropic and normal emulsions - Paredes, Jose and Shahidzadeh-Bonn, Noushine and Bonn, Daniel

Abstract : When made to flow, yield stress materials rarely flow homogeneously. This is mostly attributed to the fact that such materials show a transition from a solid- to a liquid-like state when the stress exceeds some critical value: the yield stress. Thus, if the stress is heterogeneous, so is the flow. Here we consider emulsion flows in a cone-plate geometry that, for Newtonian fluids, correspond to a homogeneous stress situation and show that shear banding can also be observed either due to wall slip or to the existence of a critical shear rate. By means of velocity profiles obtained using a confocal laser scanning microscope combined with a rheometer we conclude that the last type of shear banding occurs only in thixotropic yield stress materials.
Post-Tanner stages of droplet spreading: the energy balance approach revisited - Mechkov, S. and Cazabat, A. M. and Oshanin, G.

Abstract : The spreading of a circular liquid drop on a solid substrate can be described in terms of the time evolution of its base radius R(t). In complete wetting, the quasistationary regime (far away from initial and final transients) typically obeys the so-called Tanner law, with R similar to t(alpha T), alpha(T) = 1/10. Late-time spreading may differ significantly from the Tanner law: in some cases the drop does not thin down to a molecular film and instead reaches an equilibrium pancake-like shape; in other situations, as revealed by recent experiments with spontaneously spreading nematic crystals, the growth of the base radius accelerates after the Tanner stage. Here we demonstrate that these two seemingly conflicting trends can be reconciled within a suitably revisited energy balance approach, by taking into account the line tension contribution to the driving force of spreading: a positive line tension is responsible for the formation of pancake-like structures, whereas a negative line tension tends to lengthen the contact line and induces an accelerated spreading (a transition to a faster power law for R(t) than in the Tanner stage).
Post-Tanner spreading of nematic droplets - Mechkov, S. and Cazabat, A. M. and Oshanin, G.

Abstract : The quasistationary spreading of a circular liquid drop on a solid substrate typically obeys the so-called Tanner law, with the instantaneous base radius R(t) growing with time as R similar to t(1/10)-an effect of the dominant role of capillary forces for a small-sized droplet. However, for droplets of nematic liquid crystals, a faster spreading law sets in at long times, so that R similar to t(alpha) with a significantly larger than the Tanner exponent 1/10. In the framework of the thin film model (or lubrication approximation), we describe this `acceleration' as a transition to a qualitatively different spreading regime driven by a strong substrate-liquid interaction specific to nematics (antagonistic anchoring at the interfaces). The numerical solution of the thin film equation agrees well with the available experimental data for nematics, even though the non-Newtonian rheology has yet to be taken into account. Thus we complement the theory of spreading with a post-Tanner stage, noting that the spreading process can be expected to cross over from the usual capillarity-dominated stage to a regime where the whole reservoir becomes a diffusive film in the sense of Derjaguin.
Some specificities of wetting by cyanobiphenyl liquid crystals - Delabre, U. and Richard, C. and Cazabat, A. M.

Abstract : The present paper provides an up to date restatement of the wetting behaviour of the series of cyanobiphenyl liquid crystals (LCs) on usual substrates, i.e. oxidized silicon wafers, water and glycerol, at both the macroscopic and microscopic scale, in the nematic range of temperature. We show that on water the systems are close to a wetting transition, especially 5CB and 7CB. In that case, the wetting behaviour is controlled by the presence of impurities. On a mesoscopic scale, we observe for all our (thin LC film-substrate) systems an identical, complex, but well defined general scenario, not accounted for by the available models. In the last part, we present a study on line tension which results from the specific organization of LCs at the edge of the nematic film. We report preliminary results on two-dimensional film coalescence where this line tension plays a major role.
Supersolidity and disorder - Balibar, Sebastien and Caupin, Frederic

Abstract : A solid is called `supersolid' if it exhibits superfluid properties. Supersolidity is a paradoxical phenomenon whose understanding has become a major challenge since 2004, when Kim and Chan first observed what could be mass superflow through solid helium 4. In this review, we describe how successive experiments indicated that what was observed in helium 4 was not intrinsic properties of the crystalline state as originally proposed 35 years before. Disorder coming from how the solid is grown (dislocations, grain boundaries and other interfaces, liquid or glassy regions, impurities...) was shown to play an essential role. However, one does not know yet which type of disorder is involved or by which mechanism it leads to the observed properties. Furthermore, all the experimental features probably cannot be explained by a common mechanism. Recent measurements of the shear modulus of helium 4 crystals could even be explained without the need of any superfluidity. In fact, many theoretical predictions need to be checked experimentally, so the whole issue is far from understood. Even some crucial experiments would need to be repeated more systematically. The present review of the experimental observations and theoretical scenarios raises a series of questions which call for answers.
Phase separating colloid polymer mixtures in shear flow - Derks, Didi and Aarts, Dirk G. A. L. and Bonn, Daniel and Imhof, Arnout

Abstract : 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.
Chaotic dynamics of the magnetic field generated by dynamo action in a turbulent flow - Petrelis, F. and Fauve, S.

Abstract : We present models related to the results of a recent experiment (the `VKS experiment') showing the generation of a magnetic field by a fully turbulent flow of liquid sodium. We first discuss the geometry of the mean magnetic field when the two coaxial impellers driving the flow counter-rotate at the same frequency. We then show how we expect this geometry to be modified when the impellers rotate at different frequencies. We also show that, in the latter case, dynamical regimes of the magnetic field can be easily understood from the interaction of modes with dipolar (respectively quadrupolar) symmetry. In particular, this interaction generates magnetic field reversals that have been observed in the experiment and display a hierarchy of timescales similar to the Earth's magnetic field: the duration of the steady phases is widely distributed, but is always much longer than the time needed to switch polarity. In addition to reversals, several other large scale features of the generated magnetic field are obtained when varying the governing parameters of the flow. These results are also understood in the framework of the same model.
Absence of grain boundary melting in solid helium - Caupin, Frederic and Sasaki, Satoshi and Balibar, Sebastien

Abstract : Crystals are often expected to start melting at their free surface or at the interface between grains. Grain boundary melting corresponds to the situation where the interface between grains is invaded by a thick liquid film at the bulk melting temperature T(m). In some cases, premelting is predicted, with liquid-like layers appearing between grains at temperatures below T(m). We review this topic, and describe our experiments on solid helium 4. We find that grain boundaries are not wetted by the liquid at T(m): they emerge at the liquid-solid interface with a non-zero contact angle. This is consistent with a general argument which predicts that, although systems with short-range forces might show grain boundary melting and premelting, in systems with long-range forces (like helium), grain boundaries can only be wetted incompletely by the liquid at T(m).