Experimental study of splashing patterns and the splashing/deposition threshold in drop impacts onto dry smooth solid surfaces

Drop impact onto dry solid surfaces is experimentally investigated over wide ranges of Reynolds and Weber numbers (100 <= Re <= 20,000 and 16 <= We <= 2000). The observed outcome regimes are identified and characterized on a We-Re diagram. At low Reynolds numbers, there is a wide region of We-Re combinations, between the splashing and deposition regimes, for which the impact outcome is characterized by a lifted thin film that moves horizontally on a thin air cushion with no ejection of secondary droplets from its rim. Correlations between the dimensionless numbers that define the deposition/lifted thin film/splashing (Re less than or similar to 1000) and deposition/splashing (Re greater than or similar to 1000) thresholds are deduced from the experimental results and compared with those proposed by other authors. Attention is focused on clarifying the influence of the drop liquid viscosity on the onset of splash, an issue about which contradictory findings have been reported in the literature. In the present work it is found that viscosity promotes splashing and thin film lifting (for high and low Re numbers, respectively), whereas it inhibits the formation of secondary droplets from the rim of the lifted thin film for Re less than or similar to 1000. In the splashing regime, the variations in the outcome pattern are described in detail for the wide range of Re and We numbers considered. In particular, we discuss the occurrence of an abrupt change in the splash pattern when a certain Re-dependent critical We number is exceeded. This finding has been observed for a very wide range of Ohnesorge numbers and, to our knowledge, has not been reported previously. (C) 2012 Elsevier Inc. All rights reserved.