Interfacial shear rheological measurements using a bicone-type geometry were conducted to investigate the role of interfacial shear properties in foam stability. Two materials were employed as foam stabilizers: an anionic amino acid-based surfactant and a zwitterionic polyelectrolyte (gelatin). The rheological analysis indicated that their mixture forms a viscoelastic film adsorbed at the air/solution interface. At a surfactant concentration of 5 mmol/dm3 in the presence of gelatin (0.5 mass %) at pH 7, both the interfacial shear storage modulus (G') and loss modulus (G″) reached maximum values, while the loss tangent (tan δ = G″/G') reached a minimum, indicating the formation of an elasticity-dominant, gel-like interfacial film. These results suggest that interfacial gelation occurs between the oppositely charged components. Notably, the highest foam stability was observed at the composition, implying that interfacial gelation enhances foam stability by suppressing liquid drainage and bubble coalescence. Furthermore, interfacial rheological measurements showed that lower pH promotes more pronounced elastic responses, consistent with lower surface tension and improved foam stability. These findings highlight the potential of interfacial shear rheology using the bicone-type geometry as a practical tool for predicting the long-term stability of foams in various industrial applications.
Asai et al. (Fri,) studied this question.