Foaming of Binary Mixtures: Link with the Nonlinear Behavior of Surface Tension in Asymmetric Mixtures.

The lifetimes of single bubbles or foams that are formed in mixtures of liquids can be several orders of magnitude larger than the ones formed in pure liquids. We recently demonstrated that this enhanced stability results from differences between bulk and interfacial concentrations in the mixture, which induce a thickness dependence of the surface tension in liquid films, and thus a stabilizing Marangoni effect. Concentration differences may be associated with nonlinear variations of surface tension with composition and we further investigate their link with foamability of binary mixtures. We show that, for asymmetric binary mixtures, that is, made of molecules of very different sizes, strong nonlinearities in surface tension can be measured, that are associated with large foam lifetimes. When the molecules that occupy the largest surface areas have the smallest surface tension, the surface tension of the mixture varies sublinearly with composition, reflecting an enrichment in this species at the interface with air, as classically reported in the literature. In contrast, when they exhibit the largest surface tension, superlinear variations of surface tension are observed, despite a similar enrichment. We discuss these variations in light of a simple thermodynamic model for ideal mixtures and we demonstrate why foam stability is enhanced for both sublinear and superlinear surface tension variations, thus, shedding new light on foamability without added surfactants.

Modification of the Fluctuation Dynamics of Ultrathin Wetting Films.

We report on the effect of intermolecular forces on the fluctuations of supported liquid films. Using an optically induced thermal gradient, we form nanometer-thin films of wetting liquids on glass substrates, where van der Waals forces are balanced by thermocapillary forces. We show that the fluctuation dynamics of the film interface is strongly modified by intermolecular forces at lower frequencies. Data spanning three frequency decades are in excellent agreement with theoretical predictions accounting for van der Waals forces. Our results emphasize the relevance of intermolecular forces on thermal fluctuations when fluids are confined at the nanoscale.

Understanding Frothing of Liquid Mixtures: A Surfactantlike Effect at the Origin of Enhanced Liquid Film Lifetimes.

The formation of froth in mixtures of liquids is well documented, particularly in oil mixtures. However, in nonvolatile liquids and in the absence of surface-active molecules, the origin of increased liquid film lifetimes had not been identified. We suggest a stabilizing mechanism resulting from the nonlinear variations of the surface tension of a liquid mixture with its composition. We report on experimental lifetimes of froths in binary mixtures and show that their variations are well predicted by the suggested mechanism. We demonstrate that it prescribes the thickness reached by films before their slow drainage, a thickness which correlates well with froth lifetimes for both polar and nonpolar liquids.

Dual Marangoni effects and detection of traces of surfactants.

We report controlled and tunable Marangoni flows resulting from concentration gradients that are induced by placing a droplet of volatile solute above the surface of a liquid. Condensation of the solute at the liquid surface results in a surface tension gradient that drives a permanent flow of surface velocity up to a few centimeters per second. Depending on the sign of the variation of surface tension with solute concentration, inward or outward surface flows can be obtained. We show that, in the region close to the vertical axis of the droplet, the flow rate varies with the droplet height following a power law, whose exponent depends on the nature of the transfer of the solute in air. In the case of a purely diffusive transfer we establish an analytical law for the velocity rate, which is in very good agreement with the experimental data. In addition, we discuss the effect of convection on the found scaling laws. Finally, we demonstrate that the Marangoni flow is modified by the addition of a very small quantity of surfactants, which themselves induce a Marangoni flow opposing the primary one. We suggest it could provide a simple method to detect traces of surfactants, of increasing sensitivity with decreasing surfactant solubility.

Control of particle morphology in the spray drying of colloidal suspensions.

Powders of nanoparticles are volatile, i.e. easily disperse in air, which makes their handling difficult. Granulation of nanoparticle powders provides a solution to that issue, and it is generally performed by spray drying the nanoparticles that have been suspended in a liquid. Spray drying of a colloidal suspension consists of atomising the suspension into droplets by a fast flowing and hot gas. Once the droplets dried, the resulting dry grains/microparticles can be used in a wide range of applications - food, pharmaceutics, fillers, ceramics, etc. It is well known that the grains resulting from spray-drying may be spherical but may also exhibit other diverse morphologies. Although different influencing parameters have been identified, no clear overview can be found in the literature for the driving mechanisms of grain shaping. In the present work, we review the assumptions made in the literature to explain the different morphologies. We analyse the orders of magnitude of the different effects at stake and show that the grain shape does not result from a hydrodynamic instability but is determined by the drying stage. However, we emphasize that neither the drying time nor the associated Péclet number are critical parameters for the determination of shape morphology. In light of those results, we also review and discuss the single droplet experiments developed to mimic spray drying. Generalising our previous works, we further analyse how the control of morphology can be achieved by tuning the colloidal interactions in the suspension. We detail the model we have developed that relates the colloidal interaction potential to a critical pressure exerted by the solvent as it flows, and we provide a quantitative prediction of the grain shape. Finally, we offer perspectives with regard to spray drying of systems such as molecular solutions, widely performed in e.g. the pharmaceutical industry.

Boundary Condition in Liquid Thin Films Revealed through the Thermal Fluctuations of Their Free Surfaces.

We investigate the properties of nanometric liquid films with a new noninvasive technique. We measure the spontaneous thermal fluctuations of the free surfaces of liquids to probe their hydrodynamic boundary condition at a solid wall. The surface fluctuations of a silicon oil film could be described with a no-slip boundary condition for film thicknesses down to 20 nm. Oppositely, a 4 nm negative slip length had to be introduced to describe the behavior of n-hexadecane, consistently with previous surface force apparatus data on the same system. Our results demonstrate that at vanishing flow a nanometric solidlike layer close to the wall may exist according to the nature of the liquid.

Controlling the buckling instability of drying droplets of suspensions through colloidal interactions.

The present study focuses on the drying of droplets of colloidal suspensions using the Leidenfrost effect. At the end of drying, grains show different morphologies: cups or spheres depending on the ionic strength or zeta potential of the initial suspension. High ionic strengths and low absolute zeta potential values lead to spherical morphologies. A model based on the calculations of DLVO potentials has been implemented to extract a critical pressure, which provides a quantitative criterion for buckling whatever the initial formulation is. Particularly, the buckling time is quantitatively predicted from the interparticle interactions and shows an excellent agreement with experimental values.

Evaporation of an emulsion trapped in a yield stress fluid.

The present work deals with emulsions of volatile alkanes in an aqueous clay suspension, Laponite, which forms a yield stress fluid. For a large enough yield stress (i.e. Laponite concentration), the oil droplets are prevented from creaming and the emulsions are thus mechanically stabilized. We have studied the evaporation kinetics of the oil phase of those emulsions in contact with the atmosphere. We show that the evaporation process is characterized by the formation of a sharp front separating the emulsion from a droplet-free Laponite phase, and that the displacement of the front vs. time follows a diffusion law. Experimental data are confronted to a diffusion-controlled model, in the case where the limiting step is the diffusion of the dissolved oil through the aqueous phase. The nature of the alkane, as well as its volume fraction in the emulsion, has been varied. Quantitative agreement with the model is achieved without any adjustable parameter and we describe the mechanism leading to the formation of a front.

Probing thermal waves on the free surface of various media: surface fluctuation specular reflection spectroscopy.

Thermal motion gives rise to fluctuations in free surfaces; measurement of the thermally excited waves on such surfaces provides information on the mechanical properties of the medium. We have developed an optical tool to probe the thermally excited waves on free surfaces: surface fluctuation specular reflection (SFSR) spectroscopy. It consists in measuring the fluctuations in the position of a laser beam that is specularly reflected onto the free surface of a medium. The position of the reflected beam is sensitive to the roughness of the probed surface; the thermal waves are detected by subtracting the light intensities collected on the two quadrants of a photodiode, on which the beam is centered. We show how the measured signal is related to the medium properties. We also present measurements performed on Newtonian liquids as well as on a viscoelastic solid; we show that in all cases, there is a very good agreement between experimental and computed spectra. SFSR thus applies to a broad range of materials. It moreover offers a very good temporal resolution and should provide a useful tool for dynamical measurements on complex fluids.

Structuring sedimentation in a shear-thinning fluid.

We study the sedimentation of suspensions of monodisperse non-Brownian particles in a shear-thinning polymeric fluid. We observe the formation of particle-rich structures and show that they are associated with a well-defined flow pattern. The results suggest that the observed structuring is a consequence of particle aggregation that amplifies concentration heterogeneities, the flow pattern being further driven by a gravitational instability.

Rapid settling of a colloidal gel.

We study the rapid collapse of gels formed from strongly aggregating colloidal suspensions. This gravity-driven collapse is associated with the apparition of fractures in the bulk of the gels that provide an easy route to the gel-supernatant interface for the solvent and are the cause for the strong increase of the settling velocity. We propose a model that connects the apparition of a fracture in the gel to the settling velocity of the interface. This description takes into account the microscopic structure of the gel and is consistent with the experimental results.

Rectified motion of colloids in asymmetrically structured channels.

We set micron size particles into macroscopic motion by submitting them to a low frequency electric field (of zero mean value) in a microfabricated channel exhibiting a topological ratchet-like local polarity. Rectification is induced by the coupling between electrophoresis, electroosmosis, and dielectrophoresis. The macroscopic velocities of the particles are functions of the electric field and of the geometry of the channel; they strongly depend on their size which opens the way to potential separations.

Aggregation of particles settling in shear-thinning fluids. Part 2. Three-particle aggregation.

We have experimentally studied the coaxial settling of three identical non-Brownian spheres in a shear-thinning fluid at small Reynolds numbers. While settling, the particles create corridors of reduced viscosity in their wake and, if they are initially close enough to one another, they can form stable clusters. By analogy with previous results obtained on two-particle interaction in the first part of this work, we show that the particle velocities can be satisfactorily described using a first-order expression and assuming that the reduced viscosity remains constant. We report systematic experiments performed at different initial separation distances between particles and the use of our simple model allows the prediction of the settling behaviour and in particular the conditions for clusters formation. We thus show that particle aggregation can occur even for large initial distances between particles and within times that are small compared to the time scales in Newtonian fluids.

Moving droplets on asymmetrically structured surfaces.

It is shown theoretically and experimentally that a liquid droplet can move on a surface structured with a locally asymmetric pattern when a breathing of the drop is induced by external means. Two different situations can be envisioned: a drop whose volume is modulated and a drop whose equilibrium contact angle is switched between two extreme values. This last case was experimentally investigated using electric fields acting on water droplets in castor oil. The main trends of the theory are verified although a quantitative analysis would necessitate either a simpler experimental geometry or a more elaborate model. The results are discussed with a miniaturization of the setup in mind which would have important potential applications in the field of integrated analysis systems.

Acting on actin: the electric motility assay.

We have developed a novel technique which allows one to direct the two dimensional motion of actin filaments on a myosin coated sheet using a weak electric field parallel to the plane of motion. The filament velocity can be increased or decreased, and even reversed, as a function of orientation and strength of the field. PMMA (poly(methylmethacrylate)) gratings, which act as rails for actin, allow one for the first time to explore three quadrants of the force velocity diagram. We discuss effective friction, duty ratio and stall force at different myosin densities. A discontinuity in the velocity force relationship suggests the existence of dynamical phase transition.

A Pulsed Field Gradient NMR Technique for the Determination of the Structure of Suspensions of Non-Brownian Particles with Application to Packings of Spheres.

The internal structure of systems of particles in a liquid is studied with a novel NMR technique based on the measurement of the squared modulus of the magnetization in presence of a pulsed field gradient. The formalism is analogous to the one used in classical scattering techniques (light, X-rays, neutrons); it allows similar information to be obtained about the structure (in particular, the structure factor S(q)). The main improvement is that the range of particles sizes is 10 µm to 1 mm, as compared with the range of the scattering techniques (<5 µm). The NMR technique was validated by studying packings of spherical particles of mean diameter 240 µm created by sedimentation. The profile of the experimental squared modulus of the magnetization versus the wave vector provides results for the mean size of particles and the compacity. The main feature is that it depends on the pair distribution function, and the present results are in good agreement with a model based on the Percus-Yevick approximation. This technique is then particularly adapted to systems such as non-Brownian suspensions, fluidized beds, porous media, and sediments. Copyright 1998 Academic Press.

Dielectrophoretic ratchets.

We have experimentally applied some concepts of "force-free" motion to micron size particles (latex beads). The coupling of dissipation and local spatial asymmetry of the potential experienced by the beads can put them into motion. The potentials used in these experiments are of dielectrophoretic nature. To that end, electrodes of particular shapes were used in order to submit the considered suspensions to inhomogeneous ac electric fields. Two regimes were explored: i-the Brownian ratchet case in which a Brownian particle is successively trapped in a factory roof-like potential and left free to diffuse. ii-the shifted ratchets case in which two potentials exhibiting similar characteristics are applied successively, one of them being shifted by a fraction of their common period relatively to the other. In both cases, a good agreement with the theoretical predictions was observed. In particular, particles of different sizes were characterized by different macroscopic velocities leading to the prospect of promising separation techniques. (c) 1998 American Institute of Physics.