Structure, dynamics and transport properties of microemulsions

Keywords: Droplets Bubbles Microfluidics Encapsulation Emulsion a b s t r a c t In this paper, we emphasize our long series of experiments proving that the physical processes along fluid interfaces can be exploited for creating unusual fluidic objects. We report for the first time a couple of new fluidic objects so-called “liquid onions” and “mayonnaise” droplets. The study starts from the observation of antibubbles, exhibiting unstable liquid‐air‐liquid interfaces. We show that the lifetime of such a system has the same origin as floating/coalescing droplets on liquid surfaces. By analyzing such behaviours, we created droplets bouncing on a liquid bath. The methods and physical phenomena collected in this paper provide a basis for the development of a discrete microfluidics. Open questions are underlined, experimental challenges and future applications are proposed.

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Composition, structure and mechanical properties define performance of pulmonary surfactant membranes and films.

The response of interfacial layers to deformations in size and shape depends on their composition. The corresponding main mechanical quantities are elasticity and viscosity of dilation and shear, respectively. Hence, the interfacial rheology represents a kind of two-dimensional equivalent to the traditional bulk rheology. Due to growing interest in the quantitative understanding of foams and emulsions, more works are dedicated to studies on interfacial rheology. This overview presents the theoretical basis for traditional and recently developed experimental tools and discusses their application to different interfacial systems. While dilational rheology provides information on the composition of mixed interfacial layers, the shear rheology gives answers essentially on structures formed at an interface. The most frequently used methods at present are the oscillating drop and bubble tensiometry methods for dilational deformations and oscillating ring/bicone rheometers for shear deformations.

Rheology of interfacial layers

Thermodynamics, adsorption kinetics and rheology of mixed protein-surfactant interfacial layers.

β-Lactoglobulin (BLG) adsorption layers at air–water interfaces were studied in situ with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry as a function of bulk Ca2+ concentration. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy – from the ubiquitous air–water interface through thin foam films to macroscopic foam. For concentrations 30 mM Ca2+, micrographs of foam films show clear signatures of aggregates which tend to increase the stability of foam films. Here, the interfacial layers have a higher surface dilatational elasticity. In fact, macroscopic foams formed from BLG dilutions with high Ca2+ concentrations where aggregates and interfacial layers with higher elasticity are found, showed the highest stability with much smaller bubble sizes.

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Specific effects of Ca2+ ions and molecular structure of β-lactoglobulin interfacial layers that drive macroscopic foam stability

Foam films as instrumentation in the study of amphiphile self-assembly

Aqueous solutions of associative poly(acrylates): Bulk and interfacial properties

The revised articles outline the potential of microscopic foam film instrumentation as an investigation tool in studying the amphiphilic nanostructures in aqueous surfactant solutions. The impact of amphiphilic nanostructures on the drainage behaviour and stability of foam films is traced for surfactant solutions of concentrations orders of magnitude above CMC (micellar solutions) to about two orders of magnitude lower than CMC (premicellar solutions). It is found that in the high-concentration domain the micellar entities affect mainly the stability of the films. In the low-concentration domain, the presence of smaller crumbly aggregates (premicelles), plays a significant role for the kinetic stability of the films. Through the mechanism of Marangoni effect, an enhanced coupling of the specific film hydrodynamics and the mass transfer of the surfactant is obtained. The result is a sharp rise in the kinetic stability of the foam films. The importance of this trend of research is related to providing better insight into the self-assembling phenomena and into the factors that determine the drainage and the stability of thin liquid films. The results have potential and actual applications in food, cosmetic and pharmaceutical industries, as well as in biology and medicine.

Amphiphilic nanostructures in foam films

The present review is focused on most recent studies, in which the interconnections of interfacial layer and foam film properties are innate for the performance of the respective systems and have been clarified to highest possible degree These investigations cover fundamental issues as the initial onset of premicelles in single-surfactant solutions, explore intricate topics as the interplay of competitive and cooperative adsorption in surfactant and protein–surfactant mixtures and give hints for fine-tuning the design of various formulations to be applied in industrial applications

Elena Mileva

Papers

Specific effects of Ca2+ ions and molecular structure of β-lactoglobulin interfacial layers that drive macroscopic foam stability

Foam films as instrumentation in the study of amphiphile self-assembly

Aqueous solutions of associative poly(acrylates): Bulk and interfacial properties

Amphiphilic nanostructures in foam films

Impact of adsorption layers on thin liquid films