On the link between foam coarsening and surface rheology: why hydrophobins are so different

Why are hydrophobins so different? To understand this, here we compare the stability of foams made with hydrophobin HFBII, Quillaja saponin, beta-lactoglobulin or beta-casein. Our experimental design eliminates the effects of coalescence and creaming, allowing us to study Ostwald ripening primarily. We observe that in HFBII foams, disproportionation is effectively stopped and the air bubbles size remains practically constant over the time of the experiment. We explain this with the large differences in surface rheological properties of the adsorbed layers, which were studied using small and large deformation surface dilational and shear rheology experiments. At low surface coverage we have mapped the system behaviour to the equivalent 2D polymer network having similar dilational modulus vs. surface pressure dependence, allowing us to introduce an equivalent molecular "hardness''. This comparison indicates that even at low surface coverage, HFBII molecules behave as harder entity at the interface, when compared to other systems. We find similar behaviour at high surface coverage, where HFBII layers can form microscopic wrinkles at the interface. At large surface deformation the HFBII dilational modulus increases nearly monotonically, over a much larger range of surface pressures, and much higher moduli are reached compared to the other three systems. These observations correlate well with the observed difference in the foam behaviour.