We know that a metal paper clip floats on water when it is carefully placed over the surface. This is attributed to the phenomenon of surface tension. I understand that, due to its weight it creates a depression in the fluid as seen in the image below:
Image source: Wikipedia
Due to this depr...
@GuruVishnu DavePHD's answer to the insect question is misleading, because the hydrophobicity or otherwise of the surface is directly related to the contact angle.
Ok. But I also saw other sources claiming insect's legs are hydrophobic with this query. I understand this also depends on the contact angle, however I don't see any contradiction.
If its legs were hydrophilic, then I'd expect the second case of the following diagram to happen:
Insects legs are not a smooth surface. Typically they are covered with hairs so they behave like superhydrophobes. Paper clips have yet to learn this trick :-)
I agree. With this and the acute contact angle in mind, why do the metal paper clips float on water? I expect them to sink, but they don't. So there must be some mistake in my reasoning which I don't see what it is.
@GuruVishnu most surfaces are not perfectly smooth so as the water/air/surface interface moves the angle it makes with the surface continually changes.
@GuruVishnu irregularities on the surface can mean that the interface gets stuck because it can't move away from a local irregularity without changing the contact angle away from the optimum.
> Under most industrial conditions advancing contact angles between 40° and 80°, and receding contact angles smaller than 20° can be expected, and the contact angle hysteresis is large.
What this means is that suppose the drop size is increasing, e.g. we are adding liquid to it, then as the edge of the drop moves outwards the apparent contact angle is larger than the real contact angle due to the pinning.
Conversely if we such liquid out then as the edge moves inwards the contact angle is less than the real contact angle again due to the pinning.
If we had a perfectly smooth surface the apparent contact angle would always by equal to the actual contact angle, but on real surfaces the irregularities change the apparent contact angle.
I can attempt to draw a diagram to show this if you want ...
Even though I didn't get what 'pinning' is completely, I can understand this intuitively. So do you mean the contact angle in case of metal paper clip on water is close to 80°?
Yes sir. It's easy to understand this from your excellent diagram! Is the surface roughness we're concerned about an artefact of the metal or the surface contaminants like oil we saw earlier?
Ok sir. It sounds reasonable. Earlier, I thought these surface irregularities are much smaller than the dimensions of the water molecules/drops. Now it seems they have significant impact on our analysis.
I guess, it would be hard to get acute contact angles in reality.
Ok sir. This cleared a lot of doubts. So, I think an ultrahydrophobic paper clip (of same mass and dimensions of the metal counterpart) would more happily float on water.
Ok sir. Just to confirm whether my understanding is right or not: If we were to place a glass paper clip with contact angle close to zero degrees, will it float or sink? I'd expect it to sink. Is this conclusion correct?
We know that a metal paper clip floats on water when it is carefully placed over the surface. This is attributed to the phenomenon of surface tension. I understand that, due to its weight it creates a depression in the fluid as seen in the image below:
Image source: Wikipedia
Due to this depr...
