Droplet breakup is a key step in emulsion processing and manufacture. An ability to predict droplet dismption in a known flow field is essential to allow emulsion product formulation with the optimal droplet size, minimise energy input during manufacture and to reduce shear damage of the surface-active species. Conventional theories for predicting droplet dismption are based upon interfacial energy alone and fail when a protein emulsifier is present at moderate concentration. This is because the protein forms an interfacial network having mechanical strength. New equipment, the Cambridge Interfacial Tensiometer (CIT), was designed and constmcted to directly determine the mechanical properties of protein films adsorbed at the air-water and oil-water interfaces. The technique is an interfacial twodimensional analogy of conventional materials testing methodology, but is conducted with high spatial resolution and requires the measurement of micro-Newton forces. Interfacial elasticity modulus values were of order 200 mN/m for P-lactoglobulin networks at the air-water interface, consistent with a calculated ensemble average estimated using Atomic Force Microscopy derived data on the unfolding of individual protein molecules~ Interfacial elasticity modulus increased with protein concentration, although a 31% enhancement in the maximum stress transmitted through the protein film resulted when sub-interfacial P-lactoglobulin concentration was reduced from 1.0 mg/mL to 0.01 mg/mL. Reduced competition for interfacial space at lower protein concentrations is believed to result in greater conformational change, and hence entanglement, on adsorption to the interface from dilute solution. Importantly, this study suggests that network mechanical response is determined by residual protein tertiary stmcture, and that adsorbed networks should therefore be analysed as nanostmctured biomaterials. The mechanical properties of adsorbed layers of de novo peptides were determined with the CIT. The peptides were shown to exhibit the extremes of protein behaviour previously observed, demonstrating the possibility of controlling product form and functionality through combined hydrodynamic and molecular specification. This is the first study to establish the mechanical properties of an adsorbed protein film using conventional stress-strain approaches to high material defmmations. The work demonstrates that droplets surrounded by interfacially adsorbed protein should be viewed as deformable capsules or cells enclosed within a stress-transmitting network. Deformation and dismption could then be predicted by existing theories for such systems, using the constitutive data provided by the CIT stress-strain tests. Such an approach is expected to be superior to existing methods based solely on interfacial energy. t Data from Carrion-Vazquez et al. (1999) and Best et al. (200 1 ).