Mohsen Karbaschi

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.

Thermodynamic Models for the Adsorption of Alkyl Trimethyl Ammonium Bromides at the Water/Hexane Interface

Based on surface/interfacial tension isotherms measured for the homologous series of alkyl trimethyl ammonium bromides (CnTAB) using the drop profile analysis tensiometry the adsorption behavior at three different liquid-fluid interfaces is discussed: solution/air, solution/hexane vapor and solution/hexane bulk liquid. The adsorption behavior can be described by different models. In the presence of hexane molecules (as a bulk liquid or as vapor in the air phase) the adsorption of the CnTAB molecules can be best described by a competitive adsorption with hexane molecules. This competitive thermodynamic model can be applied successfully to all three interfaces.