J. Meunier

Microscope at the Brewster angle: Direct observation of first‐order phase transitions in monolayers

A microscope employing the characteristics of the reflection at the Brewster angle has been built for the study of first‐order phase transitions in monolayers and the growth of two‐dimensional domains without adding fluorescent impurities. It takes about 2.4 s to constitute an image.

Textures and phase transitions in Langmuir monolayers of fatty acids. A comparative Brewster angle microscope and polarized fluorescence microscope study

Brewster angle microscopy (BAM) and polarized fluorescence microscopy (PFM) are used to observe the distinctive textures of and the transitions between condensed phases in Langmuir monolayers of n‐alkanoic acids. BAM is sensitive to film anisotropy even when the molecules are not tilted as long as the unit cell is anisotropic. Every transition is visible with one or both of the techniques, either as a dramatic change in the degree of contrast or as a sudden alteration of the mosaic domain texture. The two techniques are generally consistent, although the presence of the fluorescent probe impurity (for PFM) causes a subtle difference in the appearance of one transition and small shifts in transition surface pressures.

Critical behavior in microemulsions

Abstract Interactions in dispersions have been studied using light scattering techniques applied to microemulsions. In these systems, hard sphere interactions are dominant. The remaining interactions (van der Waals, etc.) are usually attractive and short-ranged and can be treated as perturbations. However, close to phase transitions where the microemulsion separates into two other microemulsions, the attractive part of the potential becomes large and behaves as if long range interactions were present; the characteristics of the scattered light can also be interpreted by assuming that the system is close to a critical consolute point. The low interfacial tensions (measured between the two microemulsions in equilibrium using surface light scattering techniques) and the large interfacial thicknesses (deduced from optical reflectivity) are consistent with the picture in terms of critical phenomena.

Viscous fingering in non-Newtonian fluids

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.

Phase transitions in Gibbs films: Star textural defects in tilted mesophases

The formation of adsorbed films at the free surface of aqueous solutions of sodium hexadecanoate was observed by microscopy at the Brewster angle. A phase transition between a gas and a tilted phase occurs during these slow formations. The tilt of the molecules creates an observable optical anisotropy. We observed circular domains separated into regions of uniform tilt direction by straight defect lines, i.e., ‘‘star’’ textural defects very similar to those observed in freely suspended liquid crystal thin films and in Langmuir films of esters. The coalescence of domains leads to large regions of uniform tilt direction separated by straight or kink lines. A second transition towards a nontilted phase is observed. Quantitative information about the coefficients of the continuum elastic theory can be derived from our experiments.

Viscous fingering in a shear-thinning fluid

We study the Saffman–Taylor instability in a rectangular Hele-Shaw cell. The driven fluid is a dilute (or semidilute) polymer solution, with a viscosity that exhibits shear thinning. Other non-Newtonian properties such as elastic effects are negligible under the present experimental conditions; the system thus allows for separate investigation of the influence of shear thinning on the instability. The experiments show that, for weak shear-thinning, the results for the width of the fingers as a function of the capillary number collapse onto the universal curve for Newtonian fluids, provided the shear-thinning viscosity is used to calculate the capillary number. For stronger shear thinning, narrower fingers are found. The experiment allows also for a study of the applicability of Darcy’s law to shear thinning fluids. For Newtonian fluids, this law gives the finger velocity as a function of the pressure gradient. For weakly shear-thinning fluids, we find that an effective Darcy’s law, in which the constant v...

Capillary Waves and Bending Elasticity of Monolayers on Water Studied by X-Ray Reflectivity as a Function of Surface Pressure

Thermally induced capillary waves on a monolayer of behenic acid on water have been investigated by x-ray reflectivity. The corresponding roughness is measured as a function of the surface pressure. The experimental data are discussed using a theory of thermal fluctuations. A strong decrease in the roughness is observed at the transition to the solid state. This striking feature is attributed to the rigidity of the monolayer in the solid phase whose value has been determined.

Properties of surfactant monolayers in relation to microemulsion phase behaviour

Abstract The relationship between the properties of surfactant monolayers at oil-water interfaces and the phase behaviour in bulk of mixtures of oil + water + surfactant is discussed. Such monolayer properties include the spontaneous curvature, c o the interfacial tension, I γ, the elasticity K (or rigidity) associated with the mean curvature, and the elasticity K associated with the Gaussian curvature. The model system chosen for investigation is the anionic surfactant AOT + aqueous NaCl + n-alkane at 20°C. In such systems, inversion of microemulsion type from oil-in-water (o/w) to water-in-oil (w/o) is possible with increasing electrolyte concentration. The tension, γ, passes through an ultralow minimum value at conditions corresponding to the formation of three phases. Using small angle neutron scattering, we have determined the structure of surfactant-rich third phases (c o ~ 0) formed with the different alkanes. Lamellar phases consisting of surfactant monolayers separated alternately by oil and water appear with short alkanes, whereas L 3 and bicontinuous phases form in systems containing longer alkanes. The bending elasticity K has been measured for planar monolayers at the oil-water interface by ellipsometry. K is independent of salt concentration but depends markedly on alkane chain length N, falling from ~ 1 k B T for N B T for N = 14. This is discussed in terms of the differing extents of oil penetration into the surfactant chains. Higher rigidities favouring lamellar phases and lower rigidities favouring bicontinuous microemulsions are in line with the theoretical predictions of de Gennes and Taupin. Estimates of the constant K have been obtained in droplet microemulsions (w/o) from a knowledge of their size, K and γ. The sign of the constant is in agreement with the geometry of the phases formed in three phase systems. Finally, the ideas and concepts developed in the oil-water systems described above are used to explain the wetting behaviour by alkanes of AOT monolayers at the air-water surface.

Effects of Alkane Chain Length on the Bending Elasticity Constant K of AOT Monolayers at the Planar Oil-Water Interface

In systems containing AOT + aqueous NaCl + normal alkane, the change of the microemulsion type from oil-in-water to water-in-oil through a surfactant-rich phase can be effected by increasing the aqueous phase salt concentration. We have investigated the multiphase behaviour (in Winsor systems) for a range of alkanes. The low oil-water interfacial tensions enable us to measure the bending elasticity constant K of the monolayer by ellipsometry. Values of K are independent of salt concentration but decrease with increasing alkane chain length. This is discussed in terms of the differing extents of oil penetration into the surfactant chain region and its consequences on the structure of the third phase.

Low interfacial tensions in three-phase systems obtained with oil-water surfactant mixtures

Abstract We have studied the quaternary mixture brine-toluene-SDS-butanol, in the domain where it gives three phases, a microemulsion in contact with an organic and an aqueous phase, and measured three low interfacial tensions γmo, γXXX and γow. We find γow stays equal to the largest of the two others and conclude that the origin of low interracial tensions between the microemulsion and organic or aqueous phases are due to different phenomena.