The impalement of water drops impinging onto hydrophobic/superhydrophobic graphite surfaces: the role of dynamic pressure, hammer pressure and liquid penetration time

Abstract Droplet impingement experiments at low Weber numbers were conducted by digitizing silhouettes of impacting water drops onto unlike graphite substrates, typified by different advancing water contact angles ( θ a ): 140 and 160°. The relaxation of wetting diameter, dynamic contact angle, and drop shapes were measured. The purpose was to carefully investigate the phenomenology and possible causes of the failure of the superhydrophobicity. During impact and spreading phases, all the drops impinging onto both graphite substrates showed a similar behavior. Then, after an initial free recoil, drops impinging at lower impact velocities onto graphite substrates characterized by θ a = 140° clearly exhibited time intervals in which the wetting diameter appeared to be almost constant. The duration of this pinned phase was observed decreasing with increasing the impact height and almost completely disappearing for drops impinging at higher impact velocities. This behavior has never been reported before, and, contrariwise, water droplets impinging at lower impact velocities onto hydrophobic and superhydrophobic surfaces have been generally observed more freely retracting, and ultimately rebounding, compared to drops impacting at higher velocities. In the present study, this latter behavior was recorded just for drops impinging onto graphite surfaces characterized by θ a = 160°. A theoretical description of the experimental results was proposed, specifically investigating the role of dynamic pressure, hammer pressure and liquid penetration time during the impact, spreading and recoil stages.