Droplet impact dynamics over a range of capillary numbers and surface wettability: Assessment of moving contact line models and energy budget analysis

Abstract

Bouncing and non-bouncing impact dynamics of a droplet on a solid surface are studied experimentally and numerically. High-speed visualization and an in-house dual-grid level-set method based solver are employed. Two established contact angle models, namely, Kistler and Fukai models, are implemented in the solver. While the Kistler model employs a time-varying dynamic contact angle, the Fukai model accounts for a quasi-dynamic contact angle based on contact line velocity. Better agreement between the present numerical result and present as well as published experimental results of a dynamic contact angle is found for the Kistler model, specifically for more transient contact angle variations cases that correspond to the less viscous droplets on the hydrophilic surfaces (Ca = 0.005–0.037 and θeq = 22°–90°). This is because the Kistler model can replicate more dynamic variations of the contact angles during spreading and receding as compared to the Fukai model, while both the Fukai and Kistler models numerical results are found in good agreement with the measurements for less transient contact angle variations cases that correspond to the high viscous droplets on the hydrophilic/hydrophobic surfaces (Ca = 7.596 and θeq = 86°–125°). Finally, the coupled effects of liquid surface tension, liquid viscosity, substrate wettability, and impact velocity during droplet bouncing and non-bouncing are presented through an energy budget analysis. At a given impact velocity, for less-viscous and less-surface tension liquids, the viscous dissipation is substantial irrespective of the surface wettability, whereas for less-viscous and high-surface tension liquids, the viscous dissipation is smaller on hydrophobic surfaces as compared to that on hydrophilic surfaces.