Since droplet retraction is involved in many applications but receives less attention than droplet spreading, this article reports a detailed study of the retraction dynamics for water droplets with varying velocities impacting surfaces ranging from hydrophilic to superhydrophobic.
The morphologies of retracting droplets are compared in detail. Two distinct retraction modes are classified: the inertial mode and the capillary mode. The retracting droplet with an inertial mode features a rim-lamella structure with a slowly decreasing dynamic receding contact angle while the capillary mode shows a collapsed rim and lamella and capillary wave propagation. During the retraction process, the inertial mode first takes place, and then the capillary mode occurs at a later stage. The inertial mode becomes more dominant with increasing impacting velocities. The dependencies of the retraction rate on impacting velocities and surface wettabilities are justified from the experimental measurements. The inertial-mode retraction rate increases with increasing velocity, while the capillary-mode retraction rate decreases. The retraction rate is generally higher for more hydrophobic surfaces, and its dependence on impacting velocities becomes stronger. An improved shape assumption for the retracting droplet is proposed by introducing an averaged dynamic receding contact angle and a shape factor. This semiempirical model can well explain the dependency of the inertial-mode retraction rate on impacting velocities for water droplets. Last but not the least, the normalized retraction curves generally collapse into a universal curve, revealing certain similarity behaviors for the droplet retraction on solid surfaces. (publisher abstract modified)