Spreading-splashing transition of nanofluid droplets on a smooth flat surface

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.