Contribution à la compréhension de l'évolution des propriétés multiéchelles des émulsions

The optimization of the stability of emulsions, which are thermodynamically unstable, is one of the major challenge of the cosmetic industry. For more than 20 years, many non-invasive methods and techniques have been developed to measure as objectively as possible the physicochemical properties of these emulsions. These methods aim at evaluating their stability (temporal evolution), their efficiency and their safety (non-toxicity), and become more and more sophisticated as the elaboration processes of these products become complex and innovative. The thesis seeks to define an innovative strategy to optimize the stability of emulsions by a multi-scale, multi-physics approach. It studies more particularly simple cosmetic emulsions, stabilized by an emulsifier. Classically, characterization techniques at macroscopic and microscopic scales are used. Nevertheless, these techniques alone do not allow to anticipate the demixing process. It is indeed necessary to use in complement new non-destructive techniques allowing to follow the structural evolution at the mesoscopic scale. The study carried out highlights new viscoelastic and dielectric quantities characteristic of their structure at this scale. They are based on a better theoretical and experimental consideration of multiple relaxation times, related to destabilization phenomena. From a 6 months study of more than a dozen emulsions of different composition and formulation, controlled and repeatable, the follow-up of their evolution tends to show the relevance of these techniques. They allow not only to trace the variations of interfacial properties, but also to have a simultaneous follow-up of structural modifications. Ultrasonic microrheology and radiofrequency impedancemetry by induction, for example, allow to trace the critical micellar concentration of surfactants and the phase inversion temperature of emulsions. With the aim of using these techniques in industry, the analysis of formulations in tanks was compared to stability tests in tubes. Thanks to the multi-scale and multimodal approach developed, trends in emulsion evolution can be deduced in a predictive manner.