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

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1. Nov 17, 2015

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 ?

2. Nov 17, 2015

### 256bits

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?

3. Nov 17, 2015

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.

4. Nov 18, 2015

### Astronuc

Staff Emeritus
5. Nov 18, 2015

### 256bits

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.

6. Nov 18, 2015