Measurement of microscopic rheological properties in oil-in-water emulsions via spherical nanoindentation

Techniques for evaluating the micromechanical properties of materials are crucial in engineering fields. In previous studies, many researchers have utilized atomic force microscopy (AFM) to address these subjects. However, there are few data on dispersion systems, such as slurries and creams, due to the AFM tip having a nanoscale length. These materials are essential in industrial and engineering settings, requiring an accurate evaluation in a manner similar to AFM. Hence, we focus on ultrahigh accuracy and sensitive spherical nanoindentation (SNI), allowing the measurement of tissue-level features at the surface layer to characterize this soft matter. In this study, we show that SNI potentially measures the local spatial properties of concentrated dispersion fluids, especially oil-in-water (O/W) emulsions with various multilamellar structures. We set the parameter t_e for considering the organization of an equilibrium state consisting of the energy release rate and the work of adhesion on the Johnson–Kendall–Roberts (JKR) predictions. An important consequence of introducing t_e is that the results obtained by SNI match the theoretical JKR values for large t_e, suggesting that we can evaluate the microscopic properties more accurately using the classical JKR model. We find that the local features are affected by the lamellar bilayers and the work of adhesion Δγ grows monotonically with increases in space occupied by lamellar structures. Since viscosity effects, such as mechanical energy dissipation and interpenetration, appear as a part of Δγ, the behavior of Δγ clearly shows the microscopic characteristics of the O/W emulsions.