  Experimental and modelling study is presented for the effect of a wide range
of liquid viscosities on the droplet impact on a smooth solid surface at
atmospheric pressure. A non-monotonic variation of threshold between droplet
deposition and splash was observed experimentally. Specifically, the critical
Weber number separating deposition from splashing decreases and then increases
with increasing the Ohnesorge number. The observations of splash in low
viscosity region and receding breakup in high viscosity region were analyzed
qualitatively from the perspectives of Kelvin-Helmholtz instability and
Rayleigh-Taylor instability, respectively. Based on instability analysis for
the viscosityinduced nonmonotonicity, a new semi-empirical correlation of
droplet splashing thresholds is proposed by fitting experimental results from
previous and present data and shows better performance than previous
correlation formulas.
