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The collision of a liquid drop against a small target results in theformation of a thin liquid sheet that extends radially until it reaches amaximum diameter. The subsequent retraction is due to the air-liquid surfacetension. We have used a time- and space-resolved technique to measure thethickness field of this class of liquid sheet, based on the grey levelmeasurement of the image of a dyed liquid sheet recorded using a fast camera.This method enables a precise measurement of the thickness in the range$(10-450) \, \mathrm{\mu m}$, with a temporal resolution equal to that of thecamera. We have measured the evolution with time since impact, $t$, and radialposition, $r$, of the thickness, $h(r,t)$, for various drop volumes and impactvelocities. Two asymptotic regimes for the expansion of the sheet areevidenced. The scalings of the thickness with $t$ and $r$ measured in the tworegimes are those that were predicted in \citet{Rozhkov2004} fort theshort-time regime and \citet{Villermaux2011} for the long time regime, butnever experimentally measured before. Interestingly, our experimental data alsoevidence the existence of a maximum of the film thickness $h_{\rm{max}}(r)$ ata radial position $r_{\rm{h_{max}}}(t)$ corresponding to the crossover of thesetwo asymptotic regimes. The maximum moves with a constant velocity of the orderof the drop impact velocity, as expected theoretically. Thanks to ourvisualization technique, we also evidence an azimuthal thickness modulation ofthe liquid sheets.

Free radially expanding liquid sheet in air: time- and space-resolved measurement of the thickness field