Crack patterns induced by auto-stratification in drying sessile droplets of dairy proteins

Exploring the interfacial mechanisms involved in the evaporation of colloidal solutions is currently an open question with potential biomedical and industrial applications. In biological systems, unraveling evidence of self-arrangement is even more challenging due to the different size, structure and charge of their multi-components. In this work, we study the evaporation dynamics in mixes of dairy proteins, i.e., whey proteins and casein micelles. Combining the observation of crack formation in drying droplets and the evaluation of the elastic response in films during the evaporation, we highlight interfacial stratification resulting in the external accumulation of whey proteins. We also relate such preferential segregation to the mechanical properties of the dry matrices, showing that whey protein overrepresentation confers a brittle behavior to the mixes. These experimental results enable the development of a first predictive model of the elastic modulus for biocolloid binary mixes, taking into account their deformability and different ratio in the samples. Unraveling the link between colloid self-arrangement and the evolution of skin properties during the evaporation is a first step towards controlling structure and use properties of the dry particles composing dairy powders.