Demixing and evaporation from a mechanically distributed water-in-oil thin film emulsion

Abstract

Evaporation of water from an alcohol/surfactant stabilised water-in-oil thin film emulsion, including transitioning from a state of excess water providing an oil-in-water precursor phase, has been studied using a printing ink application device. An ink rheology testing technique (TackOscope), incorporating the possibility to apply an aqueous liquid based on isopropyl alcohol and surfactant, termed fountain solution, to mix in an oil-based ink to create an emulsion in a twin roll nip, was used to provide information relating to oil-water balance during emulsification. Internal cohesion of the ink-liquid emulsion is recorded as film split force between the two rollers during titration and evaporation, and defines the intrinsic tack as a function of water content and shear aging of the ink. A mathematical model to derive the retained aqueous liquid solution amount during evaporation is developed and demonstrated. The evolving evaporation is seen to follow two simultaneous exponential defined functions, that of demixing, a delay function, and that of evaporation, a driving function. The ink used shows a continuous tack increase over time, superposed on this trend, and addition of intermediate amounts of fountain solution was shown to decrease the tack of the emulsion monotonically. After evaporation the tack finally returns back to its expected undisturbed level, following an aqueous liquid-free ink tack development, the result being a sigmoidal evolution to this point. A proposed model methodology to derive the retained fountain solution amount after evaporation, for a series of additions over time, has also been developed, demonstrating the effect of discontinuous or continuous liquid addition and intermediate and subsequent progressive evaporation.