Novel droplet ripening behaviour in ternary oil-alcohol-water systems

Abstract

An ever increasing interest in reducing costs and the energy impact of different manufacturing processes is one of the main drives in industrial fields such as the food and pharmaceutical sectors. In these fields, often at least one of the manufacturing stages will be some form of emulsification, thus the interest in researching into spontaneous emulsification, where emulsions can be formed without adding external energy. In this thesis three different systems in which spontaneous emulsification occurs have been studied, focusing on some of the special features each one presents. The first system we have studied is composed of a bath, made of toluene and ethanol with added silica particles, into which two aqueous droplets are injected. If there is an alcohol concentration difference between the drops, spontaneous formation of small droplets, from the the less alcohol-rich drop all along the path towards the ethanol-enriched drop, will occur until a complete “bridge” is formed. This behaviour has been related graphically to the diffusion paths on the underlying three-fluid phase diagram, arguing that compositional ripening is the reason for the mass transfer. Different experimental techniques were employed to characterize the spontaneously formed droplets. By switching the alcohol present in the previous system to methanol we have obtained a more dynamic system with more extensive emulsification. In this system, experiments used only one injected drop and its diffusion behaviour was studied. Several quantitative parameters, such as the relative growth of the drop, how long it takes before it starts to redissolve and when do the spontaneously formed droplets redissolve, have been related to the initial alcohol content inside the drop, using its own ternary phase diagram. Solvatochromic dyes were used on systems with no particles to follow the different diffusion flows, confirming our hypothesis visually. The last system we have studied is composed of 2,3-dimethylpenthane, 1- propanol, water and Poly(methyl methacrylate) (PMMA) particles. Although its phase diagram is very similar to those of our previous systems, we found that its behaviour is not. Spontaneous emulsification is still present, however, at the surface of the injected Pickering water drop. Additional droplets were found to emerge from these spontaneously formed droplets sitting at the drop’s interface. These droplets, although partially covered by particles, and therefore stabilized, evolve over time increasing their size and exhibit coalescence. We have explored its ternary diagram and found that alcohol, even though increased amounts of alcohol destabilize the drops by increasing its miscibility, a small amount is needed for this e↵ect for the same reason. However, if no alcohol is present in the system, fractal dewetting was observed both at the bottom and top of the cuvette where the drop is located. Varying the size of the particles in this system modified the observed patterns, which were characterized by its fractal dimension, obtained through image analysis.