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Capillary Systems (Part 3 of 4): Capillary Tube



Hi, my name is Ken Milam.  I’m an application engineer here at ThermoPore.  Welcome to Thermo.TV.  In the first two segments of this series, we described the water molecule as a polar molecule.  We also discussed the contact angle that a water droplet forms when placed into contact with another material.  Now, let’s turn our attention to water’s interaction with a capillary and apply the basic material science fundamentals that we’ve learned thus far to that scenario. 

A capillary can be considered a very small diameter tube.  Capillaries are unique in that they possess a high amount of surface area relative to the capillary’s inner volume.   Actually, as you make the capillary of the tube smaller and smaller, the ratio of surface area to capillary volume increase exponentially.  This relationship is important to remember so say it to yourself – small diameter capillary, high surface area ratio, small diameter capillary, and high surface area ratio.
So why is this relationship important?  I’m glad you asked.

Consider for a second a capillary made from a material with a very high surface energy.  We used metal in a former example – now consider glass as another material with high surface energy.  Glass capillaries were used by many nurses when I was a child for blood sample collections during a routine check-up.  Many of you might recall the same.  Now, you’ll learn about the material science that enabled that glass capillary to draw the blood sample right up the capillary column.  Okay, so let’s look at this scenario in a little more detail.  Recall that a material’s energy level is a function of its surface area. 

A material that carries a high surface energy and a large amount of surface area will have the strength to pull fluids of many types over its surface in order to reduce its energy level.  In other words, this high energy material will win many surface energy tug-of-wars – much to the chagrin of the fluids that it meets along the way. 

So imagine yourself as a water molecule that suddenly gets surrounded by a glass capillary tube.  Everywhere you look, you see the inner diameter of a capillary tube – and because of the tube’s high energy level….it’s very unhappy.  Let’s give one of the molecules in the glass capillary a name – let’s give him a name that starts with an E – as in Energy….Edward.  Let’s assume that Edward is located on the capillary’s inner diameter.  In other words, Edward can see you - remember, you’re a lower energy molecule floating inside Edward’s capillary. 

In recognition of Edward’s discomfort, your low energy state, and your unfortunate close proximity to Edward, Edward reaches out and grabs you and uses you as a cover to lower his own high surface energy level.  But you’ve got neighboring water molecules of your own type nearby - and there exists an attractive force between you and your neighbor.  Let’s give one of these neighboring molecules a name that start’s with N for neighbor….Nancy.  As Edward grabs you and moves you over his surface, you in turn bring Nancy and Nancy’s neighboring molecules along for the ride (remember your pulling your neighbors along like a three dimensional magnetic chain).

The perhaps unintended consequence of your pull on Nancy is this – you just put Nancy in close proximity to Edward’s capillary neighbor – who we’ll call Eli –Eli happens to be the next molecule along the capillary tube.  Because Edward and Eli are similar molecules, Eli is also unhappy with his current energy level.  Eli looks out and sees Nancy – with her low energy state and her close proximity and you can probably guess what happens next.  Eli moves Nancy to reduce his energy level, which causes Nancy to bring along her neighboring molecules.  I think you can see where we are going with this.  The same process repeats itself over and over again.  Each action that is made by the capillary’s molecules moves the fluid further along the capillary.  And this, it turns out is precisely how fluids can be magically drawn up or along a capillary.

Okay, so that covers capillaries made from materials with high energy levels.  Now, let’s turn our attention to a capillary scenario involving a different type of capillary.  So what happens when a fluid comes into contact with a capillary made from a low surface energy material?  Well, I’m glad you asked.  The response of a capillary made from a low energy material, like PTFE (commonly referred to by the trade name Teflon), is opposite of a high energy material like glass.  As you recall, Edward was interested in pulling the fluid over its surface to reduce its energy level.   But low energy materials like Teflon already have very low energy levels, so their response is to resist fluid from contacting its surface.

Let’s recreate our previous example whereby you’re surrounded by a capillary, except this time let’s make the capillary from a low energy material.  The capillary molecule that we’ll personify this time will be Louie.  Louie has a low energy level – much lower than your own energy level. 

From Louie’s perspective, you represent trouble – any contact with you will actually raise Louie’s energy level.  Therefore, Louie attempts to push you and your neighbors away from his proximity.  As a matter of fact, he and his neighbors are actually conspiring to resist you and your neighbor’s entry into the capillary.  So, in the absence of any external force, you simply will not be able to gain entry into the capillary.  Ahh, but what happens if there is an external force…trying to force the liquid into the capillary?  Well, another battle gets set up between you and Louie. 

If the external force pushing you into the capillary is stronger than Louie’s resistance, then you will be forced into the capillary.  However, if Louie’s resistance is greater than the force that is pushing you – Louie and his neighbors will keep you from gaining entry into the capillary.

So let’s think about two real world applications that illustrate these two scenarios.  A paper towel is comprised of numerous paper fibers that are bonded together and they create millions of tiny capillaries.  When water comes into contact with a paper towel, the paper towel draws the fluid into its series of capillaries as a result of the capillaries higher relative surface energy.  When this happens, the paper towel is said to be wetted by the water. 

Now, what about the second scenario that we discussed whereby the capillary resists fluid entry?  If you’ve been skiing in the last twenty years then you know and appreciate the benefit of ski jackets that don’t get wet.  It turns out that these garments are composed of fabric based capillary systems with very low energy levels.  When in contact with water, the capillary system simply resists the water entry.  Put another way, they are not wetted by water.

So, this concludes part three of our four part video tutorial.  Tune in to Thermo.TV for the fourth tutorial where we’ll use our knowledge of capillary systems to explain the Capillary force formula.

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