Observation of discrete concentric surface modulations on free-flying liquid tin sheets

We report on the observation of concentric surface modulations on thin tin sheets formed from laser pulse impact on tin microdroplets (of various diameters D0=27,31,45,55, and 70 μm) at intermediate Weber (We) numbers in the range ∼1 000–7 000. By combining optical inspection in both reflection and transmission modes, we establish that the modulations, with a wavelength on the order of 15 μm, have an amplitude of the order of 10 nm on a sheet (ranging 50 to several 100 μm in radius) with an average thickness of the order of 100 nm. Surprisingly, the modulations appear to be predominantly axisymmetric. The highly reproducible modulations are interpreted as an intermediate We manifestation of the sheet modulations that cause sheet breakup for higher We numbers as investigated by Klein et al. [J. Fluid Mech. 893, A7 (2020)]. We compare the obtained typical wavenumber and amplitude to their model, which relates the instantaneous wavenumber to the key system parameters assuming a Rayleigh–Taylor instability, although questions on the origin of the axisymmetric modulations remain.