Microstructure does not matter (sort of): dimensionless groups for viscoplastic drop impact on thin films*

We quantitatively test dimensionless groups for predicting the onset of splash behavior in drop impact of yield-stress fluids on thin films. Results are relevant to fire suppression, painting, and spray coating with viscoplastic fluids. A single dimensionless quantity, IF(D/t), a non-trivial extension of the generalized Reynolds number, collapses high-dimensional data and separates the maps into different impact regimes. Even for fluids with different microstructure, we observe a constant critical value of IF(D/t) for a stick-splash transition, which is only a factor of two larger for ``glassy'' Carbopol microgel suspensions compared to ``gel'' Laponite suspensions. We discuss possible reasons for the mismatch in the critical value and raise a more general issue that dimensionless quantities can be sensitive to the range of available rheology data. We also test thixotropically ``aged'' Laponite, to further test the results. The results demonstrate that microstructure and chemistry do not matter (to leading order) if the right macroscopic rheological properties are selected for the dimensionless quantity. Reference: Sen et al., ``Viscoplastic drop impact on thin films,'' \textit{J. Fluid. Mech.} (2020). https://doi.org/10.1017/jfm.2020.147