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

Abstract

Hypothesis: Even a small fraction of nanoparticles in fluids affects the splashing behavior of a droplet upon impact on a smooth surface.Experiments: Nanofluid drop impact onto a smooth sapphire substrate is experimentally investigated over wide ranges of Reynolds (10 2 < Re < 10 4 ) and Weber (50 < We < 500) numbers for several nanofluid concentrations (0.01% < φ < 1%) using high-speed photography. Nanofluid suspensions are prepared at different nanoparticle concentrations by diluting a commercial Al2O3-water nanofluid without dispersants in aqueous glycerol. Every sample is experimentally characterized in terms of density, viscosity, and surface tension to demonstrate the observed outcomes on the We-Re maps.Findings: The non-monotonous behavior of the spreading-to-splashing boundary remains the same for nanofluids. However, nanofluids influence this boundary either positively or negatively depending on the Reynolds number; the nanofluids suppress splashing at high Reynolds numbers whereas they promote splashing at low Reynolds numbers. We have explained this behavior caused by the addition of nanoparticles as resulting from an increase in the lamella expansion speed during the initial spreading stage. Finally, we developed an empirical relationship to describe the dependence of the splashing threshold to the nanoparticle concentration for the first time. Highlights• We-Re map is plotted for various nanofluid concentrations.