Fig. 2. Deposition dynamics.

a The temporal evolution of the normalized contact lengths of impacting droplets (D₀ = 2.0 mm, U_i = 0.7 m s⁻¹, c = 60 μm). Silicone oils are used as overlayers. b The normalized impact time appears to be weakly dependent upon the Weber number and decreases as the overlayer viscosity increases. t^* is the characteristic contact time, $t^{*}=\sqrt{\rho R_0^3/{\gamma }_{\mathrm{k}}}$, where R₀ is the droplet radius and subscript k is d for pure water and d/o for overlaid water. Solid lines are drawn to aid the visualization. Insets are schematics showing the spreading and receding of an impacting droplet. Error bars denote the standard deviation over five experiments. c The normalized maximum spreading diameters share a similar slope but decrease as the viscosity increases for We > 10. The purple line is the linear fitting line for pure water with a slope of 0.28. Error bars denote the standard deviation over five experiments. d The normalized maximum out-of-plane height as a function of the Weber number. Solid lines are drawn to aid the visualization. Error bars denote the standard deviation over five experiments. e The maximum aspect ratio of droplets after receding as a function of the Weber number. The green and red regions denote deposited and pinch-off rebounding droplets, respectively. The red dashed line denotes the threshold value above which pinch-off occurs. c = 59  μm for (b–e). f Phase diagram of a water droplet overlaid with 100-mPa s silicone oil. Markers show the experimental outcome after impact (purple squares: deposition, magenta circles: pinch-off rebound, orange diamonds: fingering, transition and green triangles: splashing) as a function of the impact velocity and overlayer thickness. Purple, red, orange and pink solid lines denote the boundary of deposition for the overlayer with viscosities of 500, 100, 20 and 4.6 mPa s, respectively. Phase diagrams of water droplets overlaid with 500, 20 and 4.6-mPa s silicone oils are shown in Supplementary Fig. 6.