The role of adhesion on mesoscale indentation for determining moduli of hydrated materials

Indentation tests utilizing the load (P)-displacement (δ) data have been common for obtaining bulk moduli (E) of hydrated materials, including biological specimens and hydrogels. While experimentally simple to perform, the data analysis can sometimes be complicated, especially when adhesion between the indenter and sample occurs. The adhesion issue for nano/microindentation on hydrated materials has been addressed in several studies, but hardly any studies have reported the involvement of adhesion in analyzing mesoscale (0.1–1 mm) P-δ data. In this study, we evaluated three methods for analyzing experimental P-δ data acquired from mesoscale indentations on hydrated materials to obtain their E values. They were the classical Hertz model, a modified Hertz approach with P and δ values corrected using Hertz relations, and a modified Hertz approach with the correction of contact radius (a) by including the work of adhesion, W, between the indenter and the sample. The experimental P, a, and δ data were simultaneously collected using transparent gelatin gels, and these P-δ and P-a data were used to verify the adequacy of the three analysis methods. In particular, the E values from these methods were checked against the values obtained using the Johnson-Kendall-Roberts model and the P-a data. Accurate moduli resulted only when W was included in the analysis. The analysis with the inclusion of W was applied to obtain the E values of silicone and other model hydrogels, of which only the experimental P-δ data could be obtained, and their moduli were found to be close to the values reported.