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Capillary Systems (Part 2 of 4): Surface Energy/Contact Angle



Hi, my name is Ken Milam.  I’m an application engineer here at ThermoPore.  Welcome to back Thermo.TV.  In the first segment of this series, we described a water molecule as a molecule bearing a positive and a negative side.  In this second segment, we’ll build upon that knowledge to explain surface energy and contact angle.

Okay, so here’s the premise that we'll use for this tutorial - everything in nature has a desire to reduce its energy level or its energy state as much as possible.  Let me say this again - everything in nature has a desire to reduce its energy level or its energy state as much as possible. Here are a few examples that illustrate this principle - things that are hot, cool off.  

Things that are in high concentration dissipate.  Things that are at high elevations fall to lower elevations.  Everything in nature has a desire to reduce its energy level to ZERO. So you’re thinking, okay, but what does this have to do with the water molecule?  Everything.  

Let me start by modeling a group of water molecules as group of magnets oriented into a chain where the positive side of magnet “A” is in contact with the negative side of magnet “B”.  For each magnet - their positive side is aligned with another magnet’s negative side…right?  Wrong. What about the magnets on the end of this chain?  Aren’t there two magnets on the end that are lacking mates?  Of course there are.  There is a positive and a negative end that is simply exposed.   As a result, the magnets on the end of the chain are uneasy and restless because their unbalanced charges are exposed! 

Now, think about this same model in 3D space.  All of the magnets in the middle of the mass will have positive and negative ends that are in close contact with one another – so they’ll be balanced.  Okay, but what about the magnets on the surface of the cluster?  Do you think that they might lack a neighboring opposite charge?  As it turns out, they do and this charge imbalance occurs over the entire outer surface of the cluster….a charge imbalance occurs wherever a magnet’s end is exposed without a corresponding mate. 

Now think small for a second.  What is a water droplet?  It’s a collection of water molecules that behave like magnets.  Where do the charge imbalances lie in a droplet of water?  If you said “on the droplets surface” then give yourself two points.  As a mater of fact, you can calculate the TOTAL amount of charge imbalance or unbalanced energy by measuring the surface area of a water droplet.

So let’s try to connect some dots here.  We said earlier, in the tutorial that everything in nature is constantly trying to reduce its energy level to zero.  We also said that the energy level of a drop of water is dictated by its surface area. Can you guess which geometric shape has the lowest surface area per unit volume?  If you guessed the sphere - give yourself another two points.  As a result, and in an effort to reduce its surface energy to zero – raindrops and water droplets are spherical in shape.  So, let’s now build upon this knowledge of surface energy and discuss what happens when a drop of water comes into contact with solid body. 

Pour a cup full of water onto a car’s metal surface – and the water spreads out into a thin film.  Pour the same cup full of water onto a car’s metal surface that has been freshly waxed, however, and the water turns into hundreds of beads.  The water is the same in both cases.  So, what’s different?  Well, it turns out that it’s the energy level of the car's surface.  Solids can have surface energy just like liquids.  These solids, however, are unable to reconfigure themselves into different shapes because they’re rigid bodies.  The important concept to understand at this point is that the solids also embody surface energy. 

You might be wondering "what happens when a drop of water comes into contact with a solid body?" This is an excellent question.  First the force of gravity tries to flatten out the water droplet.  But doing so increases the droplets surface area, so the liquid fights back by trying to pull itself back into a spherical shape.  So there is a bit of tug-of war that exists between the liquid and gravity.  But there’s also another tug of war taking place.

Solids with high surface energies, are not content….at all…..they’re miserable….and they’re interested in lowering their own energy level.  When a lower surface energy fluid comes into contact with a higher surface energy solid a second tug-of-war erupts.  This time it’s between the solid and the liquid.  The solid finds comfort by covering itself with a lower surface energy liquid.  Even though this action increases the surface area and hence the surface energy of the liquid, it’s precisely how a solid with high surface energy reduces its energy level – by pulling this symbolic low energy comforter over its own high energy surface.   This is the primary driving force that causes a liquid to spread over a solid's surface.

Now look at another example, this time whereby we have a solid with low surface energy.   Solids with low surface energy levels have no desire to become involved with a fluid carrying a higher surface energy.  So instead of pulling the fluid over its own surface, low energy solids push back on the droplet’s edge to minimize the fluid’s footprint that is in contact with the surface.  This is the primary driving force that causes a liquid to erupt into hundreds of water droplets. It's all a simultaneous effort of the liquid to minimize its own surface area and the solid to minimze its contact with the liquid.

In all cases, the shape of the liquid at rest on a surface can tell us much about the surface energies of both materials.  As a matter of fact, the angle that exists between the liquid and the solid, the contact angle, foretells what’s going to happen when a liquid comes into contact with a porous body made from the same material. 

So let’s put all of this together to make some predictions about a porous materials interaction with a liquid.  If the contact angle of a liquid is small, then the liquid will spread, or it will “wet” the solid.  If the solid is a porous material, the liquid will get absorbed into the porous material’s structure.  Put another way, the liquid will be wicked into the porous structure.  If on the other hand, the contact angle is large, the liquid will not spread, or it will not “wet” the solid.  In this situation, a liquid will not wick into porous structure. 

So, this concludes part two of our four part video tutorial and I hope you better understand the role that surface energy plays with the interaction of a liquid and a solid.  Tune in to Thermo.TV for the third tutorial to learn about surface energy’s role in a capillary system.

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