Viscoelastic properties of silica nanoparticle monolayers at the air-water interface

We have investigated the rheological behaviour of silica nanoparticle layers at the air-water interface. Both compressed and deposited layers have been studied in Langmuir troughs and with a bicone rheometer. The compressed layers are more homogeneous and rigid, and the elastic response to continuous, step and oscillatory compression are similar, provided the compression is fast enough and relaxation is prevented. The deposited layers are less rigid and more viscoelastic. Their shear moduli deduced from the oscillatory uniaxial compression are much smaller than those deduced from pure shear deformation suggesting that the effective shear rate is smaller than expected in the compression measurements.

Amorphous freezing in two dimensions: from soft coils to rigid particles

The topic of the gel transition in two dimensions is revisited by considering data on the shear elasticity of Langmuir monolayers of different spherical objects. Amorphous freezing can be associated to structural percolation in a lattice able to resist shear stresses. The shear modulus and its dependence on the packing fraction are found to strongly depend on the details of the interaction potential and largely differ from expectations for entropic networks. This behaviour can be interpreted in terms of more elaborated percolation theories including central forces and bond-bending forces.

Small-angle neutron scattering study of crude oil emulsions: structure of the oil-water interfaces

We have used SANS techniques to study in situ interfaces between crude oil and water in emulsions. These emulsions were stabilized by asphaltenes, which are natural surface-active molecules in viscous crude oils. By combining SANS and UV-vis spectrometry, we measured both the interfacial thickness and the adsorbed amount as well as the size of the asphaltene aggregates in the oil phase. We found that this size is comparable to the interfacial thickness, suggesting that the interface is covered by adsorbed aggregates. The thickness is a minimum at the pH at which the charge of asphaltenes in contact with water is zero (IEP). This suggests that asphaltene layers in contact with water are swollen and stretched at both low and high pH. The effect of salt addition on the interfacial characteristics is minor at the IEP, but a clear swelling is seen at high pH for 0.1 M salt, an effect that remains to be understood. Emulsion stability was found to correlate well with large interfacial thicknesses.

Neutron reflectivity study of the complexation of DNA with lipids and surfactants at the surface of water

Complexation of lipids and surfactants with short DNA fragments at the air-water interface has been studied by neutron reflectivity. Complexation with zwitterionic lipids occurs in the presence of divalent cations, and ion specificity has been demonstrated (binding is less effective with Ba2+ than with Mg2+ or Ca2+). One and two DNA layers have been observed for dilute and more compact lipid monolayers, respectively. Two DNA layers have also been found with the soluble cationic surfactant dodecyltrimethylammonium bromide (DTAB), except close to the precipitation boundary. This result is opposite to that found in ellipsometry where very thick layers are found in this region. It is possible that the ellipsometry signal is due to highly hydrated bulk complexes adsorbing at the surface, not seen by neutrons because of unfavorable contrast conditions. Long DNA was found to be less keen to form surface complexes than short DNA fragments.

On the origin of the remarkable stability of aqueous foams stabilised by nanoparticles: link with microscopic surface properties

We have performed a quantitative study of the coarsening of foams stabilised by partially hydrophobic silica nanoparticles. We have used a variety of techniques: optical and electron microscopy, microfluidics, and multiple light scattering. Using earlier studies of planar particle monolayers, we have been able to correlate the interfacial properties and the macroscopic temporal evolution of the foam. This has shed light on the origin of the absence of coarsening of particle-stabilised foams. Such particle-stabilised foams appear to be the only known foam system where coarsening is inhibited by surface elasticity.

Sorption properties of functionalized liquid crystalline networks

Functionalized polydomain chiral elastomers were obtained by cross-linking side-chain liquid crystalline polysiloxanes bearing acid functions. Sorption experiments were performed by the use of an electronic microbalance, in the presence of one enantiomer of a chiral amine molecule, able to interact with the acid groups. The results showed that Fick's diffusion law is not valid anymore as soon as an interaction between the material and the molecule is present. Moreover, it was demonstrated that the grafting of interacting groups on a chiral elastomer enhanced both the capacity and selectivity toward one enantiomer.

Effect of particle hydrophobicity on the properties of silica particle layers at the air-water interface

This article describes a study of fumed silica particle layers adsorbed at the air-water interface. We have performed surface pressure, ellipsometry, and Brewster angle microscopy measurements. These determinations were complemented by surface viscoelasticity studies, using capillary waves to measure the compression moduli and an oscillating disc to measure the shear moduli. Our results show a strong influence of the particle hydrophobicity and surface density on the properties of the layers. Under compression-expansion, the particle layers rearrange quasi-instantaneously, and at high density, they buckle and/or collapse. Shear measurements show a transition from viscous to elastic behavior for particles with contact angles close to 90 degrees. The surface compression moduli are quite small and most likely not related to the stability of the foams made with these particles, in contrast to the case of more common surfactant foams.

Enantioselective Absorption of Chirally Doped Liquid Crystalline Networks Studied by the Use of an Electronic Microbalance

A polydomain cholesteric elastomer was obtained by cross-linking a nematic side-chain polysiloxane in the presence of a chiral dopant. After extraction of the chiral dopant, sorption experiments were performed, by the use of an electronic microbalance, in the presence of each enantiomer of a chiral amine molecule. The sorption kinetics corresponds to a Fickian diffusion behavior. They allowed us to determine the diffusion coefficients and to show that the doped polymer has a more pronounced affinity toward one of the enantiomers.

Nanostructures of colloidal complexes formed in oppositely charged polyelectrolyte/surfactant dilute aqueous solutions

Small-angle neutron scattering measurements were performed on dilute solutions of carboxymethylcellulose/DTAB complexes in water in order to determine their size, shape and internal structures. At low polymer content, the complexes are spherical, rather monodisperse and probably made of polymer chains intercalated between surfactant micelles. Moreover, we show that these micelles have a similar cubic arrangement than found in polymer/surfactant precipitates formed at higher surfactant concentrations. At larger polymer content, in the semi-dilute polyelectrolyte regime, the complexes are larger, softer and polydisperse. However, they possess a similar internal structure in both regimes. Carboxymethylcellulose/CTAB complexes are also large, soft and polydisperse but do not seem to exhibit well-defined internal structures.

The sponge phase of a mixed surfactant system

We study the sponge phase of the mixed non-ionic/ionic surfactant system C14DMAO-TTAB-hexanol-brine. Our aim is to determine if this phase exists in this mixed system and if it preserves or changes its structure when the relative amount of the charged surfactant is increased in the mixture. SAXS, FFEM, and conductivity results show that for the same bilayer volume fraction the sponge phase preserves its global structure. We propose a method to determine the geometrical obstruction factor from electrical conductivity measurements in ionic sponge phases. Analysis of lamellar phases in the same system shows that the bilayer thickness increases when the ionic surfactant concentration is increased.

Diffusive liquid propagation in porous and elastic materials: the case of foams under microgravity conditions

We report the results of fluid transport experiments in aqueous foams under microgravity. Using optical and electrical methods, the capillary motion of the foam fluid and the local liquid fractions are monitored. We show that foams can be continuously wetted up to high liquid fractions ( approximately 0.3), without any bubble motion instabilities. Data are compared to drainage models: For liquid fractions above 0.2, discrepancies are found and identified. These new results on foam hydrodynamics and structure can be useful for other poroelastic materials, such as plants and biological tissues.

Co-adsorption of carboxymethyl-cellulose and cationic surfactants at the air-water interface

We investigated the interaction between an anionic polyelectrolyte (carboxymethylcellulose) and cationic surfactants (DTAB, TTAB, and CTAB) at the air/water interface, using surface tension, ellipsometry, and Brewster angle microscopy techniques. At low surfactant concentration, a synergistic phenomenon is observed due to the co-adsorption of polyelectrolyte/surfactant complexes at the interface, which decreases the surface tension. When the surfactant critical aggregation concentration (cac) is reached, the adsorption saturates and the thickness of the adsorbed monolayer remains constant until another characteristic surfactant concentration, C0, is reached, at which all the polymer charges are bound to surfactant in bulk. Above C0, the absorbed monolayer becomes much thicker, suggesting adsorption of bulk aggregates, which have become more hydrophobic due to charge neutralization.

Bubble motion measurements during foam drainage and coarsening

We have studied bubble motion within a column of foam allowed to undergo free drainage. We have measured bubble motion upward with time and as a function of their initial positions. Depending on the gas used, which sets the coarsening and drainage rates, different bubble upward motion types have been identified (constant speed, acceleration or deceleration) and explained in relation with liquid downward flows. The proofs of the consistency between bubble upward motion and liquid downward flow are obtained both by comparing the bubble motion curves to the liquid drainage ones, and by comparing the time variations of the liquid fraction extracted from bubble motion to direct liquid fraction measurements by electrical conductimetry. The agreement between bubble position tracking and electrical conductivity shows in particular that it is possible to determine the drainage regime from such simple bubble motion measurements. This work also allowed us to demonstrate a special case of foam coarsening and expansion, occurring when the foam gas is less soluble than the outside one, caused by diffusion of this external gas into the foam. All these results allow us to build a picture of drainage and coarsening seen from the bubble point of view.

Viscosity effects in foam drainage: Newtonian and non-newtonian foaming fluids

We have studied the drainage of foams made from Newtonian and non-Newtonian solutions of different viscosities. Forced-drainage experiments first show that the behavior of Newtonian solutions and of shear-thinning ones (foaming solutions containing either Carbopol or Xanthan) are identical, provided one considers the actual viscosity corresponding to the shear rate found inside the foam. Second, for these fluids, a drainage regime transition occurs as the bulk viscosity is increased, illustrating a coupling between surface and bulk flow in the channels between bubbles. The properties of this transition appear different from the ones observed in previous works in which the interfacial viscoelasticity was varied. Finally, we show that foams made of solutions containing long flexible PolyEthylene Oxide (PEO) molecules counter-intuitively drain faster than foams made with Newtonian solutions of the same viscosity. Complementary experiments made with fluids having all the same viscosity but different responses to elongational stresses (PEO-based Boger fluids) suggest an important role of the elastic properties of the PEO solutions on the faster drainage.

Domain shape relaxation and local viscosity in stratifying foam films

We studied the dynamics of two different types of domain shape relaxation in a stratifying foam film composed of an anionic polymer and cationic surfactant. Those films thin in stepwise fashion: circular domains of lower film thickness are formed, expand and coalesce until they cover the whole film surface. We found that the shape relaxation of coalescing domains is governed only by 2D dissipation, and the measurement of the time scales allows to determine the ratio between the driving force (line tension) and local film viscosity. Further, we analyzed the withdrawal of stripes and modeled it by a moving disc pulled by an external force. Here, 3D dissipation can not be neglected (Stokes paradox) and the equilibrium velocity depends logarithmically on the viscosity of the surrounding 3D air. The evaluation of both kinds of relaxation events yields the orders of magnitude of film viscosity and line tension. For the investigated system we found that the film viscosity is at least 30 times larger than the bulk viscosity, which can be explained by the local molecular ordering and strong interactions with film surfaces.

Surface complexation of DNA with insoluble monolayers. Influence of divalent counterions

DNA interacts with insoluble monolayers made of cationic amphiphiles as well as with monolayers of zwitterionic lipids in the presence of divalent ions. Binding to dioctadecyldimethylammonium bromide (DODAB) or distearoyl-sn-glycero-3-phosphocholine (DSPC) monolayers in the presence of calcium is accompanied by monolayer expansion. For the positively charged DODAB monolayer, this causes a decrease of surface potential, while an increase is observed for the DSPC monolayers. Binding to dipalmitoyl-sn-glycero-3-phosphocholine preserves most of the liquid expanded-liquid condensed coexistence region. The liquid condensed domains adopt an elongated morphology in the presence of DNA, especially in the presence of calcium. The interaction of DNA with phospholipid monolayers is ion specific: the presence of calcium leads to a stronger interaction than magnesium and barium. These results were confirmed by bulk complexation studies.

Quantitative description of foam drainage: transitions with surface mobility

We have performed forced drainage experiments on aqueous foams of bubble diameters D varying from 0.18 to 8 mm, and made with different surfactant and protein solutions (providing different surface viscoelastic properties). Changing bubble size or surface properties allows to evolve between two drainage regimes, the respective dimensionless permeabilities also varying with these parameters. We show that the bubble size and surface properties can be incorporated into a single surface mobility parameter that controls the transition between the two drainage regimes. The permeability measurements indicate how do the hydrodynamic resistances of the foam channels and nodes depend on surface mobility. Taking advantage of the large range of experimental conditions, leading to a variation of the mobility parameter over more than 3 decades, a simple and consistent description of both the drainage regimes and the transition in between them is obtained. For the smallest bubbles (D < 0.5 mm) anomalous behaviors are observed and discussed.