Why is air-water contact angle usually assumed to be 0 ?

[Sorade]

I am a Ph.D student currently interested in fluid flow in porous media (rocks in my case).

I am currently trying to find the range of values for the contact angle in air-water-sandstone systems at shallow reservoir conditions (say 7.5MPa and 35 degrees C). The only value I have managed to come across is 0 degree. Most papers out there seem to assume zero degree, although no reason for this assumption is given.

I was wondering if any of you had any idea as to why such assumption is made and when is it valid ?

My understanding is that the assumption is made to represent complete wetting of the rock by the water. If that is the case wouldn't that change as more and more air is injected in the rock ?

 

[256bits]

I am a Ph.D student currently interested in fluid flow in porous media (rocks in my case).

I am currently trying to find the range of values for the contact angle in air-water-sandstone systems at shallow reservoir conditions (say 7.5MPa and 35 degrees C). The only value I have managed to come across is 0 degree. Most papers out there seem to assume zero degree, although no reason for this assumption is given.

I was wondering if any of you had any idea as to why such assumption is made and when is it valid ?

My understanding is that the assumption is made to represent complete wetting of the rock by the water. If that is the case wouldn't that change as more and more air is injected in the rock ?

That the rock should be completely saturated makes sense.
With bubbles of air in the fluid there should be a contact angle.

Do you have an example a paper that assumes zero degree contact angle?

 

[Sorade]

This paper for example, assumes air-water-rock contact angle to be zero (pdf p7 of 17): http://ac.els-cdn.com/S030193220200...t=1447770342_7de5f60190f4aa04a2089dfb9cf7ca37

My issue is that I am looking at cyclic injections, so technically I should observe a hysteresis effect. Which would translate to, as you mention, having a contact angle when both air and water are present. The value (or relationship) for this angle is to be found nowhere however.

 

[Astronuc]

This paper for example, assumes air-water-rock contact angle to be zero (pdf p7 of 17): http://ac.els-cdn.com/S030193220200...t=1447770342_7de5f60190f4aa04a2089dfb9cf7ca37

The article on ScienceDirect requires a subscription or purchase to read.

I presume the contact angle is with respect to the surface, as opposed to the surface of the grains in the rock, which form porous channels. Correct?

 

[256bits]

This paper for example, assumes air-water-rock contact angle to be zero (pdf p7 of 17): http://ac.els-cdn.com/S030193220200...t=1447770342_7de5f60190f4aa04a2089dfb9cf7ca37

My issue is that I am looking at cyclic injections, so technically I should observe a hysteresis effect. Which would translate to, as you mention, having a contact angle when both air and water are present. The value (or relationship) for this angle is to be found nowhere however.

Is the determination of actual contact angles the impetus important in your research, or just a side issue that you wish to resolve?
A zero is probably uncommon, as that would mean the molecules liquid-solid have as much attraction for each other as they do for liquid-liquid and solid-solid.

For an advancing flow, new liquid-solid bonds have to be formed, so it stands to reason that the advancing contact angle is greater than that for receding flow where bonds have to be broken for the liquid to remain cohesive. For a porous medium, capillary action follows, in which case the voids are linked together with smaller sized pores between the particulate matter. For complete wetting, a thin film could most likely be accounted for to engulf all particles regardless of contact angle. Hydrophobic fluid over solid may leave some voids or pores vacant of fluid, but that is just a surmise, without evidence on my part. Hydrophillic fluid may be difficult to remove completely from the matrix, as it can settle in corners quite happily.

For sandstone, quartz should be a major constituent, and someone surely has investigated water-quartz.

My bet though, is that the zero is used due to all particles eventually being submerged in the fluid and then assumed to be surrounded by a thin film, which when a secondary fluid is injected does encounter. Such as CO2 sequestration, or oil/gas water reservoirs. 100% bet not sure.

Wetting though should not need only a zero contact angle to occur.

Very involved subject by the way.

 

[Sorade]

Thank you both for your replies.

Astronuc, the contact angle is assumed to be with the surface of the channels of the porous media, yes.

256bits, thank you for your explanation at a molecular level. Well, the exact angle is not needed as I am looking at idealised scenarios. I would however like to narrow down the range if possible to reduce uncertainty where I can. In terms of impact on the system I am testing, a sensitivity analysis performed yesterday, showed that the impact of the contact angle on the output pressure was much less significant than that of the interfacial tension between air-water.

Thank you all.