Finally, considering the importance of interfacial mechanics and elasticity to enable processes such as cell adhesion, spreading and proliferation at liquid-liquid interfaces , we characterised the impact of the composition and processing of mixed protein layers on interfacial viscoelasticity. To do so, we applied an interfacial stress relaxation protocol previously reported , based on the application of a defined shear strain on protein interfaces using the du Nouy ring of our interfacial rheometer, followed by relaxation at this constant strain. In agreement with previously reported observations , BSA interfaces (alone) displayed moderate levels of stress retention sR (near 33 %), whereas b-casein interfaces (alone) displayed too fast relaxation profiles to be characterised using our assay. Similarly, at even moderate concentrations of b-casein in the feed solutions (e.g. 25%), interfaces were also found to be too fluid to allow interfacial stress relaxation experiments to be carried out. However, at intermediate b-casein concentrations of 2 and 5% (in the feed solutions), sR was found to increase to near 60%, indicating a significant increase in the elastic component of corresponding interfaces (Figures 8 and Supplementary of 100 ng/cm 2 ) could be attributed to the retention of a percolated BSA network at the interface, whilst achieving sufficient packing of interstitial space with b-casein molecules. The combination of this network architecture and molecular crowding could be proposed to result in the enhancement in elasticity observed.