How emulsified droplets induce the bursting of suspended films of liquid mixtures.

Emulsion droplets of silicone oil (PDMS) are widely used as antifoaming agents but, in the case of non-aqueous foams, the mechanisms responsible for the bursting of the films separating the bubbles remain unclear. We consider a ternary non-aqueous liquid mixture in which PDMS-rich microdroplets are formed by spontaneous emulsification. In order to quantitatively assess the effect of the emulsified microdroplets, we measure the lifetime of sub-micrometer-thick suspended films of these emulsions as well as the time variations of their thickness profiles. We observe that a droplet entering the film reduces its lifetime by inducing a local and fast thinning. In most cases, we ascribe it to the spreading of the drop at one of the film interfaces with air, which drags the underlying liquid and eventually causes the film to burst rapidly. We explain why, despite slower spreading, more viscous droplets cause films to burst more efficiently. More rarely, microdroplets may bridge the two interfaces of the film, resulting in an even more efficient bursting of the film, which we explain.

Straightforward Assessment of Wettability Changes by Washburn Capillary Rise: Toward a Screening Tool for Selecting Water Compositions for Improved Oil Recovery.

Evaluating the wettability state of reservoir rocks is key for understanding and optimizing waterflooding and improved oil recovery techniques that imply the use of low-salinity water. Aside from established petrophysical techniques, such as Amott imbibition tests, we evaluated the Washburn capillary rise method as a low-cost, easy-to-implement, and rapid screening tool for probing the wettability state of rock samples. The well-known limitations of this method are discussed and circumvented. We show that measuring the capillary rise of two liquids -brine and n-octane-is required to assess the evolution of the wettability state of a material induced by various treatments. The wettability state is quantified by the adhesion tension of brine to the solid. The higher the adhesion tension of brine, the more water-wet the sample. An increase in oil-wetness is observed when the sample is contacted with a crude oil or its released waters; an increase in water-wetness is obtained by postcontacting the oil-wet sample with low-salinity brine or surfactant solutions. The Washburn capillary rise is revealed to be a robust method for screening wettability alteration. With a typical duration of 1-10 min, it allows reproducibility check and screening of a wide range of brine compositions in a reasonable time frame. Therefore, it is a relevant tool to identify the most favorable brine compositions to be tested afterward with more time-consuming techniques, such as Amott tests and corefloods.

Microalgae as Soft Permeable Particles.

The colloidal stability of non-motile algal cells in water drives their distribution in space. An accurate description of the interfacial properties of microalgae is therefore critical to understand how microalgae concentrations can change in their biotope or during harvesting processes. Here, we probe the surface charges of three unicellular algae─Chlorella vulgaris, Nannochloropsis oculata, and Tetraselmis suecica─through their electrophoretic mobility. Ohshima's soft particle theory describes the electrokinetic properties of particles covered by a permeable polyelectrolyte layer, a usual case for biological particles. The results appear to fit the predictions of Ohshima's theory, proving that all three microalgae behave electrokinetically as soft particles. This allowed us to estimate two characteristic parameters of the polyelectrolyte external layer of microalgae: the volume charge density and the hydrodynamic penetration length. Results were compared with transmission electron microscopy observations of the algal cells' surfaces, and in particular of their extracellular polymeric layer, which was identified with the permeable shell evidenced by electrophoretic measurements. Noticeably, the algal surface potentials estimated from electrophoretic mobility using the soft particle theory are less negative than the apparent zeta potentials. This finding indicates that electrostatics are expected to play a minor role in phenomena of environmental and industrial importance, such as microalgae aggregation or adhesion.

Uptake kinetics of spontaneously emulsified microdroplets at an air-liquid interface.

Understanding the transfers occurring at the interfaces between emulsions and air is required to predict the properties of foamed emulsions, used for example as antifoaming lubricants or for oil extraction. Whereas bubbling oil-in-water emulsions have been studied in details, oil-in-oil emulsions have received less attention. We consider a phase-separating mixture of three oils being Polydimethylsiloxane (PDMS), decane and cyclopentanol. PDMS is dispersed as submicrometer-sized droplets by spontaneous emulsification. In bulk, we show that the time evolution of the emulsion is driven by undelayed coalescence of the Brownian microdroplets. At the freshly created interface of an air bubble created in the emulsion, we use tensiometry measurements to investigate the uptake kinetics of PDMS-rich microdroplets at the air-liquid interface. Specifically, we evidence two mechanisms of uptake: the advection of droplets at the interface during bubble swelling, followed by their diffusion on a longer time scale. We model the growth of the PDMS-rich layer at the interface and, finally, we establish the surface energy of a thin film of PDMS-rich phase squeezed between air and liquid as a function of its thickness.

Spinodal stratification in micellar films between oil and silica.

We report on the thinning mechanisms of supported films of surfactant (nTAB) solutions above the critical micellar concentration. The films are formed by pressing an oil drop immersed in an aqueous surfactant solution on a silica surface. Depending on the length of the carbon chain of the surfactant and its concentration, two modes of destabilization of the stratified films are observed. The first one proceeds by heterogeneous nucleation, characterized by the lateral expansion of the domain of lower thickness as evidenced long ago in suspended micellar films. In addition, the simultaneous stepwise thinning of several domains, called spinodal stratification, is observed here in supported films. We measure the time evolution of the thickness of the films, and we discuss the selection mechanism of each destabilization mode.

Dramatic Slowing Down of Oil/Water/Silica Contact Line Dynamics Driven by Cationic Surfactant Adsorption on the Solid.

We report on the contact line dynamics of a triple-phase system silica/oil/water. When oil advances onto silica within a water film squeezed between oil and silica, a rim forms in water and recedes at constant velocity. We evidence a sharp (three orders of magnitude) decrease of the contact line velocity upon the addition of cationic surfactants above a threshold concentration, which is slightly smaller than the critical micellar concentration. We show that, with or without surfactant, and within the range of small capillary numbers investigated, the contact line dynamics can be described by a friction term that does not reduce to pure hydrodynamical effects. In addition, we derive a model that successfully accounts for the selected contact line velocity of the rim. We further demonstrate the strong increase of the friction coefficient with surfactant bulk concentration results from the strongly nonlinear adsorption isotherm of surfactants on silica. From the variations of the friction coefficient and spreading parameter with surface concentration, we suggest a picture in which the part of the adsorbed surfactants that are strongly bound to the silica interface is trapped under the oil droplet and is responsible for the large increase in line friction.

Water film squeezed between oil and solid: drainage towards stabilization by disjoining pressure.

The spontaneous drainage of aqueous solutions of salt squeezed between an oil drop and a glass surface is studied experimentally. The thickness profile of the film is measured in space and time by reflection interference microscopy. As the film thins down, three regimes are identified: a capillary dominated regime, a mixed capillary and disjoining pressure regime, and a disjoining pressure dominated regime. These regimes are modeled within the lubrication approximation, and the role of the disjoining pressure is precisely investigated in the limit of thicknesses smaller than the range of electrostatic interactions. We derive simple analytical laws describing the drainage dynamics, thus providing tools to uncouple the effect of the film geometry from the effects of the disjoining or capillary pressures.