  Even a small fraction of nanoparticles in fluids affects the splashing
behavior of a droplet upon impact on a smooth surface. Nanofluid drop impact
onto a smooth sapphire substrate is experimentally investigated over wide
ranges of Reynolds ($10^2<\mathrm{Re}<10^4$) and Weber ($50<\mathrm{We}<500$)
numbers for three nanofluid mass concentrations (0.01%, 0.1%, 1%) using
high-speed photography. Nanofluids are prepared by diluting a commercial
Al$_2$O$_3$-water nanofluid in aqueous glycerol solutions without dispersants.
In total, 30 samples are prepared and 1799 data points are acquired. Every
sample is experimentally characterized prior to droplet impact measurements in
terms of stability, density, viscosity, and surface tension to demonstrate the
observed outcomes on the We-Re maps. Each droplet impact condition is repeated
at least 3 times to ensure good repeatability. The non-monotonic behavior of
the spreading-to-splashing transition remains the same for nanofluids. However,
nanofluids influence this boundary by promoting splashing at low Reynolds
numbers. We explain this behavior by increased lamella spreading speed and lift
during the lamella spreading stage. Finally, we develop an empirical
correlation which describes the splashing threshold dependency on nanoparticle
concentration for the first time.
