Analysis of stress relaxation profiles recorded for exchanged protein layers revealed further insight into their mechanical behaviour (Figure and Supplementary Figure ). Indeed, although addition of free b-casein at the surface of adsorbed BSA layers resulted in a significant reduction in interfacial shear storage moduli and BSA density (Figures and), the stress retention of corresponding exchanged layers remained comparable to that of the pristine BSA layers (Figure ). In contrast, whereas pristine b-casein interfaces displayed negligible elastic components, exposure to free BSA solutions resulted in a significant enhancement of their elastic contribution.