Occurrence and formation mechanisms of bubbles entrapped into water drops impinging on graphite

• The entrapped bubble occurrence was studied for water drops impacting on graphite.
• Five diverse bubble formation mechanisms were observed and minutely detailed.
• The bubble is entrapped when the top liquid layer pinches off a central air cavity.
• The substrate hydrophobicity influences the entrapped bubble occurrence/absence.
• The substrate hydrophobicity does not strongly affect the entrapment mechanism.

A study on the occurrence and formation mechanisms of air bubbles entrapped into water drops impinging onto four graphite substrates, typified by distinctive hydrophobicities and surface topographies, was conducted by digitizing silhouettes of the impacting droplets. The images were taken sequentially immediately after the drop impingement by using a high-speed video camera system, and then processed to determine the drop edge coordinates. The profiles of the impacting droplet, the air cavity and the entrapped bubble were, thus, identified. The formation of the entrapped bubble was meticulously analyzed, and five diverse mechanisms were observed and minutely detailed in this study. During the retraction phase, the bubbles were trapped since the top liquid layer of the drop closed down an air cavity previously formed at the center of the drop. The bubble was caught into the drop just when the flow of the liquid over the top of the air cavity moved inward faster than the upwards movement of the fluid below the cavity. The investigation was conducted for different impacting droplet sizes (1.91 ≤ D0 ≤ 4.87 mm), and for a wide range of impact velocities (0.11 ≤ Vi ≤ 0.74 m/s), so that the system conditions for occurrence and formation mechanisms of the entrapped bubble were clearly identified.