Rosace patterns in drop impact

We report an experimental study of the instability of the corolla for drop impacts on liquid surface for moderate Weber numbers (We) and millimetric liquid layers (of thickness h), where no splash is observed. Thanks to a Fourier Transform Profilometry technique (FTP), we exhibit and analyze for the first time the formation of a rosace-like pattern originated from an hydrodynamic instability. Using the shallow water approximation, we explain the main mechanisms leading to these patterns: it consists in the linear instability of the self-similar axisymmetric radial solution of the equations. We found that the number of folds scales like We/h at the power 2/7 as observed in our experiments.