I have accomplished to render the surface of a rock sample superhydrophobic. The spherical water drops pin to the surface rather than rolling-off on the surface. However, I read that the latter phenomenon is widely attributed to superhydrophobic surfaces. What may the reason for the pinning effect ? Could it be related to high surface roughness, which somehow causes the water drop to pin to the surface ?
@Wen Cao there are perfectly spherical hemispherical water droplets on the surface. Although I have no suitable tool to measure contact angle, the perfect spherical shape probably infers that the surface is superhydrophobic.
The first image shows the water droplets pinned to the surface. The second image shows the bright image of a particle in water, which appears due to the air layer covering its surface.
I still feel that the surface is in the state between hydrophobic and superhydrophobic. Moreover, the measurement contact angle relates the droplet volume. A droplet with the volume of 4 ~ 5 μL is preferred. The shape of droplet could influence the the measurement contact angle. And the shape of droplet depends on the droplet volume. Because under the influence of gravity, the shape of a large droplet could change from spherical to oval. From my experiences, the contact angle of your sample is 130° ~145°. I have observed the shape of ~1 mL water droplet on the surface of sodalime glass sheet with the real contact angle of ~136° (5 μL), and the shape of ~1 mL water droplet is as similar as your observations. From my best knowledge, a superhydrophobic surface usually has a low surface energy and a hierachical structure. For a rock, it seems unable to form a hierachical structure by naturally. Mahmut Camalan
@Wen Cao Thank you very much. Your comments are really enlightening. I initially inferred that a natural rough texture would be sufficient to impart superhydrophobicity. Perhaps, it may be helpful to impart extra roughness by etching the surface with a severely acid or alkaline solution.
I cannot give the exact details yet. However, I can say that we are investigating their sedimentation behaviour.
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