Olive Oil's Unexpected Behavior in Water

[Yuqing]

I was using extra virgin olive oil recently and noticed something peculiar. I had always thought that oils will spread when in contact with water, but the olive oil didn't seem to do so.

When the oil entered the water, they were pinched into spheres which quickly rose to the surface. The thing is when the spherical drops reached the top, they did not break the water surface and enter the water-air interface. Instead, they remained more or less spherical right under the water surface, with perhaps a tiny area exposed to the air at the top. After a bit of a delay, the spherical droplets will then spread into a larger circular lens. Notably, the larger drops tend to do this more readily than the smaller drops. What causes the drops to remain under the water surface without breaking it, surely buoyancy will cause it to rise further. Secondly, what then causes the oil to spread into a lens? I have looked at spreading and entering coefficients for olive oil and water, and both of these are positive indicating that entering and spreading should be favourable opposed to this behavior.

 

[torquil]

What causes the drops to remain under the water surface without breaking it, surely buoyancy will cause it to rise further.

The strong surface tension of water, I would guess. Same thing that causes some insects not to fall through the surface from above.

Secondly, what then causes the oil to spread into a lens?

I would guess the not so strong surface tension of the oil? Not strong enough to maintain a spherical shape, since the oil is subjected to upward boyancy in the water. But the exact sequency of events and times are interesting. Does the oil drop stay spherical for some time, after which is then gradually changes into a lens? or does a gradual process start immediately?

Torquil

 

[Andy Resnick]

I was using extra virgin olive oil recently and noticed something peculiar. I had always thought that oils will spread when in contact with water, but the olive oil didn't seem to do so.
<snip>
What causes the drops to remain under the water surface without breaking it, surely buoyancy will cause it to rise further. Secondly, what then causes the oil to spread into a lens? I have looked at spreading and entering coefficients for olive oil and water, and both of these are positive indicating that entering and spreading should be favourable opposed to this behavior.

There's two phenomena involved- the first has to do with 'film pressure' or 'disjoining pressure' and viscosity, while the second has to do with balancing wetting and buoyancy.

The second observation first- that the drop forms a lenticular shape is indeed a sign that the oil-water interfacial energy is lower than air-water interfacial energy- that is why the oil displaces the air. That it does not form a uniform, 1-atom thick layer means the oil does not perfectly wet the water- the contact angle is not zero.

Now, for the oil 'breaking through' the water and forming an air-oil interface- this is more complicated. Crudely, the thin film of water in between the oil and air needs time to flow out from in between. There's a variety of ways to describe the dynamics, but that film can be stable for very long periods of time; drops of water can bounce off a air-water interface:



Drop coalescence is an active area of research, you may be interested in reading this, to 'get your feet wet' (I couldn't resist...)

http://pubs.acs.org/doi/abs/10.1021/la034991g

 

[Yuqing]

Thank you for the replys.

@torquil - all the drops do eventually spread to become a lens, but at different times. I've noticed that generally, larger drops progress from sphere to lens faster than smaller drops. Although the delay varies, when the spreading finally happen it does so very fast (like the snapping of an elastic band).

@Andy - thank you for the links they are very interesting.

If anyone can tell me a bit more about the thin film of water between air and oil that would be great, if not then this is enough to satisfy me for the time being.

I'm guessing the thin film effect described by Andy to be similar to the phenomenon described in this video, with the difference in speed being attributed to viscosity.
http://www.youtube.com/watch?v=P9wnmRa-Nrg&feature=related"
Is that right?

Thanks to all.

 

[AngieH33]

Was totally going to say the same... and yeah.those links are super interesting. Thank you for sharing them!

 

[torquil]

I tried it myself. The larger drops seemed to flatten at later times. The flatteing was quite sudden, so it seemed to be a fast process that occurs after some time, e.g. after the water on top of the oil drop has moved away and the oil drop boyancy is able to break the surface.

For some reason that youtube video doesn't play...

Torquil

 

[Yuqing]

Hmm, odd that you found smaller drops to flatten quicker. For me, it definitely seemed like the smaller drops flatten quicker. One more thing I've noticed. I've mentioned in the first post that some (most I think) droplets have a small spherical cap (tiny area) at the top already exposed to the air when they spread. How can there possibly be a stable water layer seperating it and the water-air interface if a part of it is already exposed? Perhaps I've noticed wrong.

edit: On second thought, it could just as easily be a trick of the light. Using the naked eye to determine such fine details is not reliable. Perhaps torquil could give some thought into this.

As for the video, try this link instead:

http://www.youtube.com/watch?v=zRSK4k3D-50"