You can use the rate of uptake in a strip of your paper, using a balance to measure the weight increase, due to water invasion. Then the Washburn equation relates the rate of uptake to the pore size and the contact angle. Google it!

You can use the rate of uptake in a strip of your paper, using a balance to measure the weight increase, due to water invasion. Then the Washburn equation relates the rate of uptake to the pore size and the contact angle. Google it!

But be careful: you may not be measuring the equilibrium contact angle - as the droplet shrinks, you are having a receding edge, and with quite a rough surface the hysteresis of contact angle has to be very high. I feel you need to first ask yourself - why are you trying to measure the contact angle on a surface where the droplet is not in equilibrium?

Instead of measuring the rate of uptake of a droplet, you can also measure the height of penetration of the liquid/solution/mixture in a paper strip that is hanging in a (closed) glass cylinder with the bottom of the strip just entering the liquid. In a more sophisticated version you measure the weight of the strip as a function of time; but for a first impression visually measuring the height will show you how well this method works.

The method has been described in some older papers:

H.G. Bruil en J.J. van Aartsen, Colloid Polym. Sci. 252 (1974) 32 (capillary penetration)

T. Gillespie en T. Johnson, J. Colloid Interface Sci.36(1971) 282 (capillary penetration in paper fabrics).

Before the experiment the paper strip should be equilibrated with the vapor of the liquid. Also the surface tension and viscosity of the liquid should be known. In the case of solutions (or mixtures) the surface tension can be analyzed with the Gibbs eq.

With the Washburn eq. and the known surface tension (+Gibbs' law) and viscosity of the liquid an impression can be gained on how the wetting characteristics change with surface tension. This is valuable information when you want to change the penetration in practice. As mentioned by Terentjev a fundamental interpretation is more difficult because you then also have to consider the kinetics in detail.

See for more detail on the Washburn eq. and the fundamentals of wetting the attached literature.

Thanks a lot ! all of you, for sharing your valuable experience.