Capillary Action

Angel Lugo Lugo

I would like to discuss something that was not explained in our course but always had my attention. Since I was a kid I had a lot of curiosity about why when I had a straw in a glass with liquid I always observed that the level of the liquid was a little higher inside the straw. Now that I studied physics and as a university student I am able to do research and learn about why of the phenomena I know this is called capillary action. 

The capillary action can be observed first of all in a capillary tube and laboratory glassware forming a meniscus. This is caused by adhesion, which is the attraction between different molecular species, when it pulls the liquid column up until there is a sufficient mass of liquid for gravitational forces to overcome the intermolecular forces. After some research I learned that the capillary action is not only present in tubes, I did learned that this is the way tears comes out when we cry. This is because inside our eyelids we have the lacrimal ducts which are of very tiny diameter, then acting as capillary tubes. This is also observed in paper towels, sponges, fabrics, well in most of the material which are sufficient porous to absorb liquids. This characteristic of the materials is even used for chromatographical separations of liquid and gases in chemistry and physics laboratories.

But then I asked myself: It has to be physical explanations which allow me by some sort of equation describe the capillary action. Well I founded an explanation for my beloved liquid inside the straw and why depending the wide of the straw varies the level of the liquid inside it. The height h of a liquid column is given by: 

where gamma is the liquid-air surface tension (force/unit length), θ is the contact angle, ρ is the density of liquid (mass/volume), g is local gravitational field strength(force/unit mass), and r is radius of tube (length). Another thing I really found as a curious detail when I got in a chemistry laboratory is the upside down meniscus of the mercury. After researching about it I found it was because of the polar surface in contact with.

As for the another applications of capillary action that I mentioned before I founded that there is also a physical description of the volume of liquid absorbed by the porous material.  A porous material absorbs liquid by a rate that decreases over time, in other words as more time passes less liquid will be absorbed due saturation of the media. The volume absorbed is described by the following equation

where S is the sorptivity of the medium, with dimensions m/s1/2 or mm/min1/2 , A is the area of the porous object and V is the cumulative volume. The sorptivity of the material is mainly given by tables. So if you can do some research of the porous material you are using, calculate the area of the piece of material being used you will be able to know how much liquid will by absorbed by it.

I found this topic quite interesting and useful in the today’s industry, and very practical and favorable in laboratories technology.