Impingement dynamics of water drops onto four graphite morphologies: From triple line recoil to pinning

• Study on triple line dynamics of impact drops on graphite (4 hydrophobicities).
• Relationships between triple line dynamics and surface topographies detailed.
• Pinned state due to random cavities in a smooth surface found for impact drops.
• Typical superhydrophobic surfaces recoil noted for drops on graphite 160°.
• Transition from Cassie-Baxter to Wenzel state identified for graphite 140°.

Droplet impingement experiments at low Weber number (We = 7.61) were conducted by digitizing silhouettes of impacting water drops onto four graphite surfaces with different hydrophobicities. The relaxation of the wetting diameter, the droplet height and the dynamic contact angle were determined for characterizing the peculiar impact and spreading dynamics for the unalike surfaces, typified by four distinctive topographies carefully analyzed by scanning electron microscopy. After the initial inertially dominated phase, during which the drops spreading onto all substrates showed a similar behavior, the expected recoil phase was not observed for droplets impacting onto graphite surfaces characterized by 90° and 120° advancing contact angles. A few milliseconds after the impact, the drops exhibited a pinned configuration due to the peculiar morphology of these graphite substrates: randomly distributed cavities on a generally smooth surface, in fact, stopped the movement of the triple line. This behavior, however, was not detected for water droplets impinging onto graphite surfaces characterized by 160° advancing contact angles, which instead presented the usual retraction phase. Finally, the graphite surface characterized by 140° advancing contact angles showed a mixed behavior due to a transition of the drop configuration from Cassie-Baxter to Wenzel state.

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