Dynamics of Newtonian and non-Newtonian liquid droplet impact on super-hydrophobic solid surfaces

Abstract

The dynamics of liquid droplet impact on a super-hydrophobic surface was investigated experimentally and theoretically. The reactions of Newtonian droplet impact on a super-hydrophobic surface at low impact velocities (0-1.7m/s) and low Weber numbers (0 - 200) were revisited. The work further extended the investigation to non-Newtonian drops such as shear-thickening cornstarch and shear-thinning blood. The spreading dynamics of pure water, milk, 15wt.% cornstarch colloidal solution and healthy rabbit blood drops were studied by experiments and compared to the previous reported theoretical models. While the Roisman's model agreed well with the Newtonian drop spreading, a large deviation was observed in the experiments with non-Newtonian drops, especially of blood. Further theoretical analysis revealed the effects of the blood shear thinning properties on the droplet spreading. With the knowledge of the experimental spreading dynamics, we developed a rim instability theory to explain the fingering behavior by modifying the classical Rayleigh-Plateau instability. Our model accurately predicted the fingering behavior of water, milk and cornstarch drops, but over-estimated the blood droplet fingering at high impact Weber numbers (greater than 105). Based on the preliminary theoretical analysis, the shear thinning of blood, caused by the red blood cells deformability, was believed as the reason for the deviations in fingering. Following the discussions of spreading and fingering, the stability of the liquid jet was investigated experimentally where the jet breakup modes as well as mode transition were introduced. The jet evolution maps were created for water, milk and cornstarch droplets based on the experiments. Similar to the spreading and fingering, the extraordinary stability of the blood droplet was observed in jetting and was coined as 'shear-thinning inertia-driven stability'. The investigations on jetting filled the vacuum in research on droplet impact. Furthermore, our work initiated the research on the dynamics of blood droplet impact. The 'shear-thinning inertia-driven stability' creates a connection between the blood dynamics and the red blood cells deformability which suggests an important potential application: early diagnosis of blood diseases.