Conversation started May 28, 2020 at 5:45.
Guru Vishnu
Guru Vishnu
May 28, 2020 05:45
0
Q: Why does a metal paper clip float on water despite having an acute contact angle?

Guru VishnuWe 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...

fluid-statics surface-tension
John Rennie
John Rennie
I'm working for a few minutes but I shouldn't be too long
John Rennie
John Rennie
May 28, 2020 06:12
@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.
Guru Vishnu
Guru Vishnu
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:
user image
John Rennie
John Rennie
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 :-)
Guru Vishnu
Guru Vishnu
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.
John Rennie
John Rennie
My recollection of seeing water drops on metal is that the angle isn't acute.
As I recall it's around 90°, and pinning will stop the metal/air/water interface moving.
Guru Vishnu
Guru Vishnu
May 28, 2020 06:30
What does 'pinning' mean? Is that the standard method to measure surface tension as mentioned by Farcher below your post?:
This is one of the standard methods of measuring surface tension Method 3 Wilhelmy plate or ring method in this link zzm.umcs.lublin.pl/Wyklad/FGF-Ang/… — Farcher Mar 20 '16 at 13:44
John Rennie
John Rennie
@GuruVishnu most surfaces are not perfectly smooth so as the water/air/surface interface moves the angle it makes with the surface continually changes.
Guru Vishnu
Guru Vishnu
May 28, 2020 06:49
Ok sir.
John Rennie
John Rennie
@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.
Guru Vishnu
Guru Vishnu
Ok sir. Since the pin is not moving, do we need to take these 'pinning' effects into our consideration?
John Rennie
John Rennie
In your question you have the quote:
> 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.
Guru Vishnu
Guru Vishnu
May 28, 2020 07:04
Yes and we're concerned about the advancing contact angles, right?
John Rennie
John Rennie
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 ...
Guru Vishnu
Guru Vishnu
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°?
John Rennie
John Rennie
I've never measured it, and I don't have a paper clip on hand to test.
But in real life most surfaces are covered in a layer of crud e.g. oil from human skin.
And that increases the contact angle.
Guru Vishnu
Guru Vishnu
I think it'll be very hard to measure it without sophisticated instruments.
I have some paper clips here, but not the accuracy to measure the contact angle.
Ok sir. If the outer layer has few molecular thickness of oil, then it makes sense on why the contact angle must be obtuse.
John Rennie
John Rennie
user image
That's my attempt to show what happens on a rough surface.
In both cases the actual contact angle (shown by the red squares) is 90°, but in the lower case the angle appears to be greater that 90°.
Guru Vishnu
Guru Vishnu
May 28, 2020 07:20
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?
John Rennie
John Rennie
It's just because all metal objects have been moulded or machined and that leaves a surface with microscopic irregularities.
Guru Vishnu
Guru Vishnu
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.
John Rennie
John Rennie
No, they are typically on the micron length scale. About 1000 times bigger than molecules.
Guru Vishnu
Guru Vishnu
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.
John Rennie
John Rennie
Yes. Ultrahydrophobes usually use an extreme form of pinning i.e. their surfaces have highly irregular surfaces ona very small scale.
Guru Vishnu
Guru Vishnu
May 28, 2020 08:20
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?
John Rennie
John Rennie
Yes, it will sink.
Guru Vishnu
Guru Vishnu
Ok sir. Thank you.
$\Huge\ddot\smile$
John Rennie
John Rennie
@GuruVishnu or you can just add surfactant to the water to reduce the contact angle.
If you get the paper clip floating then add some washing up liquid you'll see the paper clip sink.
Guru Vishnu
Guru Vishnu
Ok sir. Now, I understood the point.
Thank you.
 
Conversation ended May 28, 2020 at 8:29.