Rheology of attractive microcapsules

Microcapsules are droplets protected by an elastic membrane and used to control the delivery of active ingredients in an increasing number of applications. Whereas microcapsules were originally designed with petroleum resources (polymers and solvents), the challenge for the microencapsulation industry is to use biobased materials. One generic solution that emerged in the scientific community is to assemble at water-oil interfaces oppositely charged biomaterials (e.g. chitosan, cellulose, alginate, fatty acid). The assembly of these materials is relatively well-mastered but the resulting physical properties are poorly controlled. Very recently, it has been shown that suspensions of different bio-based microcapsules can form gels for volume fraction as low as 10%, which is reminiscent of suspensions of attractive colloidal particles. This behavior is puzzling as it requires the emergence of attraction between athermal particles on length scales of several tens or hundreds of micrometers. Microscopically, microcapsules spontaneously formed aggregates under quiescent conditions, whereas in rheometric experiments, suspensions of these capsules behave as a yield stress fluid. Below the yield stress, the suspension does not flow. At higher stress, the capsules are dispersed in the flow, and the suspension has a quasi-newtonian behavior, such as classic suspensions of non-interacting particles. From a fundamental point-of-view, this behavior is interesting to deepen our understanding of rheology of suspensions and also to gain insights on their Brownian counter-part. From an industrial point-of view, this phenomenon of ‘aggregation’ is deleterious for the stability of the suspension in microencapsulation processes or during their storage. We could also take advantage of this phenomenon to design bio-based suspensions with responsive rheological properties based on athermal particles at low volume fraction. As compared to gels made of Brownian particles, one expects in these athermal systems a flow-induced structuration which is strongly coupled to the flow history. This rather unique feature would make them a new class of yield stress fluids. However, we are limited by the fact that the physical origin of capsule aggregation remains unclear. The objective of this project is to understand the relationships between the rheology of suspensions of attractive microcapsules, the physico-chemistry of their bio-based membrane and the nature and amplitude of their long-range attractions.