Hi experts,
What excatly is "capillary-active"? I have searched on the Internet but it seems there is no a clear definition.
Some materials, such as fiber glass, I know it is a porous material, and it can absorb water. However, it is not a capillary active material.
Why? The water it absorbs must somehow migrate from one location to another (from exterior surface to the inner). However, this process is not because of the capillay forces? Because it is not capillary active? Then what makes the water migrate? Or the water is not migrating?
Thank you in advance to consider my question.
The different between capillary-active, absorbent and hydrophobic
Re: The different between capillary-active, absorbent and hydrophobic
Hi WEI LI,
true, there is no strict definition for capillary-activity. Only water-repellent materials are certainly not capillary-active (a water droplet on such a surface has a contact angle greater than 90°).
In general, all other porous materials, when exposed to pressureless liquid water for a sufficiently long time, should cover their surfaces and fill their pore spaces with water (due to the contact angle being less than 90°). However, pressureless water only exists in theoretical treatises, and if a porous material with weak capillary forces tries to take up water against the force of gravity, the uptake process will soon stop when the capillary forces can just support this weight of water but not more. So it will not be very useful in practice to call this material capillary-active although in theory it is. On the other hand, gravity may cause water to seep into some mineral wool, but this transport process is caused by the hydrostatic pressure in the water, not by the capillary forces of the mineral wool.
Also, the uptake process may be extremely slow, just an almost imperceptible creeping of the water film over the surface. In a practical scenario other processes, such as vapour diffusion, may then be much more important.
In practice, therefore, we call a material capillary-active if its capillary properties are pronounced enough so that it is useful or necessary to take them into account. The detailed criteria may be different for different materials, for different scenarios and for investigations with different focus.
Regards,
Thomas
true, there is no strict definition for capillary-activity. Only water-repellent materials are certainly not capillary-active (a water droplet on such a surface has a contact angle greater than 90°).
In general, all other porous materials, when exposed to pressureless liquid water for a sufficiently long time, should cover their surfaces and fill their pore spaces with water (due to the contact angle being less than 90°). However, pressureless water only exists in theoretical treatises, and if a porous material with weak capillary forces tries to take up water against the force of gravity, the uptake process will soon stop when the capillary forces can just support this weight of water but not more. So it will not be very useful in practice to call this material capillary-active although in theory it is. On the other hand, gravity may cause water to seep into some mineral wool, but this transport process is caused by the hydrostatic pressure in the water, not by the capillary forces of the mineral wool.
Also, the uptake process may be extremely slow, just an almost imperceptible creeping of the water film over the surface. In a practical scenario other processes, such as vapour diffusion, may then be much more important.
In practice, therefore, we call a material capillary-active if its capillary properties are pronounced enough so that it is useful or necessary to take them into account. The detailed criteria may be different for different materials, for different scenarios and for investigations with different focus.
Regards,
Thomas
Re: The different between capillary-active, absorbent and hydrophobic
Thanks, Thomas. It is now a bit more clear to me.