J.Y. Won

Dynamics of drops – Formation, growth, oscillation, detachment, and coalescence.

Single drops or bubbles are frequently used for the characterization of liquid-fluid interfaces. Their advantage is the small volume and the various protocols of their formation. Thus, several important methods are based on single drops and bubbles, such as capillary pressure and profile analysis tensiometry. However, these methods are often applied under dynamic conditions, although their principles are defined under equilibrium conditions. Thus, specific attention has to be paid when these methods are used beyond certain limits. In many cases, computational fluid dynamics (CFD) simulations have allowed researchers, to extend these limits and to gain important information on the interfacial dynamics. Examples discussed here are the capillary pressure tensiometry used for short time and profile analysis tensiometry for long time dynamic interfacial tension measurements, the oscillating drop methods for measuring dilational visco-elasticity. For measuring the coalescence of two drops the liquid dynamics of the subsequently formed liquid bridges have to be considered. In this paper, a thorough review of important experimental and computational findings, related to the dynamics of drops, including its formation, growth, oscillation, detachment, and coalescence is presented. Emphasis is however on some selected important developments. In addition, the paper tries to predict the main directions of advancement in interfacial research for the near future.

Completely engulfed olive/silicone oil Janus emulsions with gelatin and chitosan

Janus emulsions, formed by mixing two oil components (i.e., olive oil (OO) and silicone oil (SiO)) with water in the presence of two surface active biopolymers, i.e., gelatin and chitosan, are investigated in more detail. The stability of Janus droplets formed strongly depends on the polymer components used. The mixture of both biopolymers represents an extraordinary effect which can be related to the complex formation of gelatin and chitosan. Taken into account that under the given pH conditions, in the acidic pH range between 4 and 6, below the isoelectric point of gelatin, both polymers are polycations, one can conclude that non-Coulombic interactions are of relevance for the enhanced surface activity of the complexes. Dynamic interfacial tension (γ) measurements by using the drop profile analysis tensiometry (PAT) indicate a strong adsorption of the polymer complexes at the olive oil/water interface in contrast to the silicone/water interface. In a first step, the polymer complexes are adsorbed at the interface, and in a second step, a more rigid skin-like polymer layer is formed. This first example of a polymer-stabilized Janus emulsion opens new perspectives for the application, e.g., in food emulsions or for making scaffold materials.

Adsorption of equimolar aqueous sodium dodecyl sulphate/dodecyl trimethylammonium bromide mixtures at solution/air and solution/oil interfaces

Two oppositely charged surfactants in an aqueous solution interact with each other via electrostatic interactions and produce surfactant complexes—catanionics. However, their quantitative influence on the interfacial tension at the solution/air and solution/oil interfaces is still unknown. We have measured the interfacial tension of sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DoTAB) aqueous mixtures using the du Nouy ring tensiometer and drop profile analysis tensiometry (PAT-1). For the oil phase, we used hexane. The obtained kinetic and equilibrium experimental results were fitted with the common theoretical Frumkin adsorption model. We found that SDS + DoTAB complexes show much higher surface activity than the single surfactants. At solution/air and solution/oil interfaces, SDS and DoTAB produce ion pairs with low water affinity and relatively low oil solubility. SDS + DoTAB partition coefficient between hexane and water phases is 0.32. Such surfactant complexes with advanced surface characteristics may be beneficial in different industrial branches nowadays.