What forces cause the wetting of a brick when water is poured over it? When water is poured over a porous material, like brick, which forces are causing the wetting of brick (i.e., absorption of water by brick)? Is it gravity, capillary force, or both? Normally capillary action causes rising of water rather than sucking it downward, so I believe it is only gravity acting in this situation. Is this right?
 A: The Young equation for complete wetting is 
$\psi = 2 \sigma / r p g$
where $\psi$ is the capillary potential, $r$ is the pore radius which is also the curvature of the liquid surface, and $\sigma$ is the surface tension of the liquid. This makes me think that the effects of gravity are really negligible in this situation, compared with forces like surface tension.
I believe that therefore, the capillary force is the main thing to take into account in the wetting of something porous, such as a brick, though gravity may have a small role.
Information about the equation can be found here. Hope this helps!
A: Capillarity is responsible for wetting the brick. It is better to think in terms of surface energy. Water has two surfaces, one in contact with surrounding air, and one in contact with brick. If surface energy of water-brick interface is lower than water-air interface, then water will flow in such a way as to increase its contact with brick and reduce its contact with air, thus reducing its total surface energy. That is how you get wetting. 
Capillary action does not necessarily cause upward motion of water. In a capillary tube, it so happens that to increase its contact with glass surface it must travel upwards. Of course as it rises potential energy of water column increases even as its surface energy is decreasing, so it stops at a height where its total energy is a minimum.
Flow in fine pores is not governed by gravity (in the sense that it is not significant), unless large changes in height is involved. Look up definition of Weber number.
