Rheology and structure of drying emulsions
Résumé
Direct (water-in-oil) emulsions are used in various fields, in particular for their ability to dry and finally form uniform liquid or solid surfaces: cosmetics, paints, paving industry (bitumen emulsions), foodstuffs (spray-drying). In these applications the emulsion films used are thin, which makes direct observations difficult. In order to get a deeper insight in the evolution of the microstructure and rheological properties of drying emulsions we here focus on thick (centimetric) emulsion layers. We look at the local properties of these materials with the help of MRI (Magnetic Resonance Imaging): more specifically, we image water and oil separately to obtain their spatial distribution inside the drying emulsion. We also follow the evolution of the rheological properties of the emulsion layer with the help of a specific set-up allowing drying in a rheometer. It appears that a slight water concentration gradient forms within the emulsion, while an oil layer develops at the surface, which explains the strong decrease of the drying rate in time. Our measurements show that the rheological behavior of the emulsion layer below this liquid film is similar to that of an emulsion directly prepared at the same average concentration. Neither the initial emulsion composition nor the drying airflow speed affect the concentration distribution and the material strength evolution. Indeed, the elastic modulus of the emulsions studied follow a master curve, indicating that a drying emulsion fundamentally behaves like an emulsion that is progressively concentrated in oil. Confocal microscopy observations of the emulsion structure in time show that during this concentration coalescence is minor, until the concentration reaches a critical value, in agreement with the above results. Ultimately, the insight provided by this work allows the establishment of a comprehensive drying model for emulsions.