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

Highlights

• The role of emulsification on the evaporation from an aqueous water-in-oil emulsion.

• Dynamics for different water-in-oil based systems.

• Introduced correction factor for evaporation.

• Model methodology to derive retained liquid amount after evaporation.

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.

Introduction

Aqueous liquid evaporation from a water-in-oil emulsion is a feature of many industrial and laboratory processes. One such application of emulsion chemistry is the commonly practised offset printing technology, in which the inked image area is differentiated from the non-image (non-printed) area in respect to surface energy by the use of a water-based fountain solution, which prevents the transfer of oil-based ink onto the wetted non-image area of the printing plate in the press. This technology provides an excellent study platform for investigating phase demixing and evaporation from a sheared thin film, and practitioners have long sought an understanding of how the aqueous fountain solution phase behaves during film print transfer. Parallels with other thin film forming technologies range from, for example, lubrication pressurised films in the presence of invaded water, water-bitumen-water coatings to mesophase wetting of porous media, thin film polymer in water extrusion and pharmaceutical drug development.

Tack as measured on a twin-roll device such as a TackOscope (IGT testing systems, The Netherlands) is a rheological parameter indicative of internal cohesion of the fluid as it is split at the exit of the twin-roll nip given intact adhesive boundary conditions with the roller surfaces (ASTM standard for tack measurement: D 4361-97). While ink tack is defined as the resistance to separate an ink sample under film splitting between two ink adhering surfaces, it has a unit which is application specific, e.g. energy expended in the separation, force required to complete the split, torque needed to maintain the rotation of the rollers hindered by the film splitting, pressure in respect to force per unit area of film split, length over which the split occurs, dimensionless ratio of these etc. [1].

The evaporation mechanism of water from emulsions has been extensively studied. Under conditions when the evaporation rate is controlled by mass transfer across a vapour phase, the evaporation can be slowed by repulsive inter-droplet interactions [2]. As well, water evaporation may be limited by diffusion in the network of water films within the emulsion. The same authors also stated that a compression of the drops, such as under dynamic conditions, may lead to coalescence of the emulsion drops and the formation of a macroscopic oil film at the emulsion surface, which serves to prevent further water evaporation and resulting in a phase separation. The stability of thin films have been studied by Bibette, showing that the stability of such films in concentrated emulsions is governed by the microscopic pressure acting on the oil-water interface. As an example, large droplets are more stable than small droplets [3]. Bouchama et al. claim that the initial phase of evaporation of an oil-in-water emulsion is fast evaporation of free water [4]. The same authors have defined five different regimes of evaporation based on changes in conductivity of the emulsion. They report that the drying of the film evolves in steps where water first evaporates until an oily skin is formed on top of the film under which the emulsion experiences a phase inversion to water-in-oil constituting the final film. Clint et al. used a gravimetric technique to measure the rate of evaporation of water from micro-emulsions. The evaporation rates show that water drop diffusion within the bulk micro-emulsion is rate limiting while the subsequent processes of water transfer across the liquid/vapour surface are not [5]. We may expect, therefore, that a water-in-ink emulsion will also show an initial diffusion determined demixing phase prior to more rapid evaporation, enhanced by the action of repeated film splitting, until a final slow evaporation phase as the remaining micro-emulsion becomes more and more stable, changing only by a combination of diffusion and the continued creation of fresh surface by the action of film splitting.

The liquid-containing ink as distributed on rollers and paper surfaces in air means that fountain solution is evaporated progressively due to thin layer evaporation. Thus, since the ink tack is not directly correlated with print properties, due to the evaporation limitations in transfer to the print substrate surface, we choose rather to analyse the influence of the aqueous liquid titration and evaporation procedure to record ink tack as a function of added fountain solution, and so, in turn, accomplish the aim of modelling the evaporation dynamic as a function of fount solution content in the ink.