2012 Dominique Langevin

Dominique Langevin (Paris)

 The 2012 Overbeek Gold Medal Winner is professor Dominique Langevin from Paris, France. Dominique Langevin is an outstanding scientist who has made leading contributions across a wide range of colloid and interface science. The contributions in the very soft systems of foams, emulsions and microemulsions have rested on great expertise in physical techniques and the behaviour of molecules at fluid interfaces. On reading the summary of her outstanding scientific achievements (below) it is clear obvious why Dominique is such a worthy winner of the 2012 Overbeek Gold Medal. 
   Dominique is not only a scientist, but her work led to the formation of several groups, since the 70’s the surfactant group in the Laboratoire de Physique de l’Ecole Normale Supérieure (ENS), Paris and later in the 90’s, the Groupe de Recherches “Films de tensioactifs flexibles” (1991-1995) [an equivalent to what would be a French Colloid Society]. From 1994 to 1998, Dominique was director of Centre de Recherche Paul Pascal in Bordeaux, one of the largest laboratories in colloid science, where she created a surface of fluids group. She is now leading the liquid interface group in the Laboratoire de Physique des Solides (LPS) in University Paris Sud. 
    Dominique was recently awarded the CNRS Silver Medal, the l’Oreal Prize for Women in Science, the Gentler-Kastler award of the French and German Physical Societies and the Kash Mittal Award for Surfactant in Solution Science. 
    Dominique contributed greatly to our community and was ECIS President from 1992 to 1993. Apart from many French students, Dominique has mentored a number international scientists who have since become very successful in their own right, including Bernard Binks, Michael Gradzielski, Vance Bergeron, Thomas Hellweg, Francisco Monroy, Regine v.Klitzing, Cosima Stubenrauch and Antonio Stocco. Many of Dominique’s closest colleagues are mentioned in the appropriate areas in the section below: 
Dominique Langevin’s Scientific Career
    During her PhD project at ENS, Dominique, together with Jacques Meunier and her supervisor, Marie Anne Bouchiat, worked to establish the experimental and theoretical bases of the technique of light scattering by liquid surfaces. Dominique edited a book on the subject in 1991. Dominique’s PhD work focused on the surface of liquid crystals, which led in particular to a method for determining the angle of the molecules on the surface, and on monomolecular films on the surface of the water, shedding light on the previously ignored frequency variation of viscoelastic coefficients. 
    During her post-doctoral fellowship in the Pierre Gilles de Gennes laboratory in College de France, she investigated with Francis Rondelez the mesh structure of semi-dilute solutions of polymers just postulated by de Gennes, through the measurement of the sedimentation coefficient of nanoparticles in these solutions. Pierre Gilles de Gennes anticipated that this sedimentation coefficient should vary between the values corresponding to the macroscopic viscosity of the solution and that of pure water as exp (R/ξ), R being the radius of the particle and the ξ polymer mesh size. Dominique verified this prediction which has now been demonstrated theoretically and is widely used to predict the mobility of nanoparticles in the cytoplasm of cells and other gels. 
    Back to ENS, she started work on microemulsions with Anne-Marie Cazabat, Jacques Meunier, Alain Pouchelon and Didier Chatenay. They were the first to show that low interfacial tensions γ between these media and water or oil were due to a monolayer of surfactant. Previously, these low tensions were attributed to the proximity of a critical point. Their results made optimizing oil recovery much easier. They showed that the low tensions were associated with a lack of wetting of bicontinuous microemulsions (in water and in oil, which coexist with both water and oil) at the water-oil interface. This was later explained by the theoretical Ising models of microemulsions of Benjamin Widom and many others which followed. The models stimulated a series of works (including work by Dominique) to locate the wetting transition, following the evolution of microemulsions to molecular mixtures, which wet the water-oil interface. In addition, Dominique, together with Anne Marie Cazabat clarified the phenomenon of percolation in water in oil microemulsions (sudden increase in the electrical conductivity of water in oil microemulsions at a certain volume fraction of water droplets typically ranging between 10 and 30%). They attributed the differences in behaviour to interaction forces between droplets. Finally, with Jacques Meunier, she clarified the dependence of the characteristic size in bicontinuous microemulsions on the elastic modulus of curvature of the surfactant layer. This modulus was measured by ellipsometry: the surfactant layers are very rough and they showed that the ellipsometric signal is dominated by the roughness. This result (which they struggled to publish) is now used by researchers to interpret the X-ray reflectivity of the liquid interfaces. They could also connect the low interfacial tensions to the elastic modulus of curvature, which subsequently led to numerous theoretical and experimental works. 
    Dominique also studied wormlike micelles with Albrecht Ott, Didier Chatenay and Wladimir Urbach. They demonstrated that these micelles continuously cut and recombine, as postulated by theorists. They made the first experimental demonstration of the existence of Levy-type anomalous (accelerated) diffusion, a finding that stimulated activity in the area. Then Dominique started a research programme on emulsions and foams. She supervised the construction of several devices for measuring interfacial rheology, to extend the frequency range accessible by light scattering (1kHz-100kHz) with electrically excited capillary waves (10Hz-1000Hz) and compression waves excited mechanically (0.01Hz-10Hz). She was then able to study in an unprecedentedly large frequency range the variation of surface compression viscoelastic coefficients. While studying monolayers of soluble ionic surfactants, for which the frequency dependence is linked to exchanges between surface and bulk, she demonstrated the existence of barriers to adsorption, due to the repulsive electrostatic surface potential that builds up during the adsorption process. This result, the importance of which was not appreciated at the time, is now recognized as important in the generation of foams and emulsions. In collaboration with Emmanuelle Rio, Dominique demonstrated that the adsorption barrier also controls the thickness of films drawn on by plates (Landau-Levich problem) or frames (Frankel problem). 
    Annie Colin and Dominique’s work on soap film drainage showed that the drainage velocity is much larger than the velocity that had been predicted for immobile surfaces. The film surfaces are in fact mobile and the velocity depends on the surface elastic modulus of compression E. They showed that the modulus which was to be used is the high frequency modulus, because the film is thin and does not contain enough surfactant to allow surface replenishment during the drainage. This is in line with the correct interpretation of the Bancroft rule that says that the continuous phase of the emulsion formed contains the surfactant. Most researchers believe (still) that the emulsion type is controlled by the spontaneous curvature of the surfactant layer while it is the drainage rate of the thin films between drops that matters: when drops containing the surfactant approach, the films between them drain fast because surfactant is directly accessible for replenishment (low value of the compression elastic modulus, E) , while the films between surfactant-free drops drain much more slowly (high E). While studying films containing surfactant and polyelectrolytes, Annie Colin and Dominique highlighted and explained the presence of a stratification, associated to oscillatory forces between film surfaces. The study of the stratification was traced back later to a local viscosity in the film, very different from the viscosity of the polymer solution, in collaboration with Regine v.Klitzing and Emmanuelle Rio. Studies of mixed layers with oppositely charged surfactant and polymer adsorbed at the film surfaces revealed unexpected features : the layers are relatively fluid with flexible polymers, but solid and fragile with rigid polymers. In this last case, the films are very unstable, probably because the surface layers may break during film drainage, and leave the film unprotected. Similar features were proposed to explain the limited stability of protein stabilised emulsions under shear. 
    Dominique also studied bulk complexation in solutions of surfactants with oppositely charged polymers that are often used in industrial formulations. She could distinguish the role of the collapse of the polymer chains when the ions are neutralized by the ions of the surfactant from the aggregation of polymer chains together, phenomena giving rise to opposite changes in size. She was also able to highlight in some cases the existence of monodisperse aggregates with liquid crystalline nanostructure. 
    In the 90’s Dominique started to study foams and foam drainage. At the time, there were two theories, assuming that surface layers were rigid or mobile, confirmed each with different commercial foaming solutions. Using controlled mixtures of surfactants, she was able to show that in practice, the surface layers had an intermediate behaviour, depending on the rheological properties of these surfaces. She then tackled with Arnaud Saint Jalmes and later Emmanuelle Rio the problem of Ostwald ripening: bubble coarsening by gas transfer between smaller bubbles, where the pressure is greater, and larger bubbles where the pressure is lower. The study of Ostwald ripening is difficult because it is coupled with drainage, and experiments are planned in drainage-free conditions in the International Space Station. In parallel, in collaboration with Véronique Schmitt and Fernando Leal at CRPP, Bordeaux, Dominique conducted a study with emulsions where drainage is negligible. They showed that the rate of ripening follows the compression modulus of the interfacial layer. This result is not completely accepted, many other authors thinking that ripening is controlled by the permeability of the interfacial film to the dispersed phase. Nevertheless, Dominique was able to show that the foams stabilized by nanoparticles do not ripen, while the coverage rate of bubbles is only 20%, thus excluding a low permeability to gases. In contrast, the modulus is high enough that the ripening is blocked (Gibbs criterion E> γ/2). Dominique is currently studying a third process of destabilization of foams and emulsions, coalescence, which is still very poorly understood and on which she has written a recent review with Emmanuelle Rio. 
    Dominque recently started the study of surface shear rheology. This allowed her to show that many surface layers (mixed layers of cationic surfactants and DNA, catanionics layers, nanoparticle layers) behave as three-dimensional soft solids (equivalency frequency-shear rate). In collaboration with Francisco Monroy, from Universidad Complutense Madrid, she studied monolayers of various lipids and showed that contrary to current thoughts of biophysicists about free motion of proteins in biological membranes, some lipid layers are not fluid, but viscoelastic, or solid (especially for lipid “rafts” in biological membranes). They also studied monolayers of neutral polymers and demonstrated for the first time a phenomenon of reptation and a glass transition at a temperature well below the Tg in bulk. 
    Dominique also collaborates with Jean François Argillier of Institut du Pétrole Energies Nouvelles, in the field of heavy crudes. They connected the stability of emulsions and the compression elastic modulus E. They have also shown by neutron scattering that the water-oil interface is composed of aggregates of asphaltenes adsorbed from the oil, an issue much debated previously. They are now interested in the low oil-water interfacial tension in the presence of surfactants at high pH. 
    Finally, Dominique started work on the toxicity of nanoparticles within several European networks. Her first results, in collaboration with Michèle Veber, LPS and Adrianna Filoramo and Stéphane Campidelli CEA, concern nanotubes functionalized with proteins and cholesterol derivatives.