Spreading and retraction of the concentric impact of a drop with a sessile drop of the same liquid: Effect of surface wettability

The concentric impact on a sessile drop is relevant in many applications, including spray coating and icing phenomena. Herein, the spreading and retraction phases yielded during the impact of a coaxial drop with a sessile drop on a solid substrate were empirically and analytically examined. We analyzed the effects of surface wettability on the impact outcomes utilizing five distinctive surfaces (i.e., smooth glass, aluminum, copper, Teflon, and coated glass). The results showed that the merged drop takes longer to attain its maximum spreading diameter at a relatively higher contact angle of the sessile drop with the solid surface. Furthermore, based on energy balance, a model for predicting the maximum spreading diameter of the drop with varying surface wettability was presented. This model considers the assumption of viscous energy loss during the merging of falling and sessile drops and at the maximum spreading diameter. Additionally, the maximum retraction height during the impact on the coated glass surface was investigated. Our model results matched well with the experimental data.