Exploring the strength of wet granular materials via dynamic ball indentation

The aim of this work is to validate the applicability of the Dynamic Ball Indentation (DBI) technique to the study of wet powder dynamics. Up to now, this method has been applied only to dry powders in initial studies. This work is divided in two main parts. The first, and core one, is focused on the study of wet pharmaceutical powders, with the goal of identifying the key variables and dimensionless groups governing the mechanical response of partially saturated powders under the indenter stress. The second part, extend the state of the art in the application of DBI to dry powders, retrieving the flow functions of 9 different powders relying solely on data from DBI. For wet powders, DBI successfully reproduced classical trends from literature, such as the identification of flow regimes based on the inertial number (I) and the analytical relationship between the dimensionless strength (Str) and the capillary number (Ca), validating its applicability to partially saturated systems. Furthermore the classical relationship between Str and Ca has been expanded by accounting for inertial effects through the inertial number, leading to an excellent data collapse onto a master curve described by Str as a function of I^{0.8} x Ca^{0.13}. For dry powders, a new definition of the constraint factor (C) has been proposed, based on the theoretical definition of flowability. This formulation allows the flow function of a powder to be derived directly from indentation data. The proposed C definition was first validated against the classical one, and then used to derive flow functions, which were compared with reference shear cell results. The DBI-based classification matched the benchmark in 7 out of 9 cases, with minor discrepancies in the remaining two. In conclusion DBI is a novel method with highly promising features, its ability to study both wet and dry powder makes it unique. Moreover, the newly proposed definition of Str, integrating capillary and inertial effects, opens promising directions for quantitative prediction of wet powder behavior. Future work should aim to refine the apparatus and compaction protocol, establish quantitative correlations between hardness and torque rheometry, and further validate both the analytical relationship between Str and I^{0.8} x Ca^{0.13} and the DBI-derived method for flow functions characterization.