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Capillary Systems (Part 1 of 4)


Do you ever wonder why a paper towel or a sponge soaks up water?  How does Rain-X or Scotch guard work and why does water bead up on a car after it has been waxed.  Well, these are just a few of the basic questions that you will be answer at the completion of this video tutorial. 

Hi, my name is Ken Milam.  I’m an application engineer here at ThermoPore.  Welcome to Thermo.TV.  In this four part series, we’ll be discussing the material science behind porous materials and their interaction with various liquids.  You will learn the key parameters that dictate if a fluid will be pulled or wicked into a porous material or if the material will resist the fluid’s entry.  You’ll also understand the parameters that quantify one response or the other. More importantly, at the end of this series, you‘ll understand the material science behind these interactions so that you will be empowered to answer a host of related technical questions based on the same material science fundamentals.

Our tutorial will cover topics in the following order:  In part one of this series, we’ll cover the water molecule.  In part two, we’ll explain surface energy and contact angle.  Then, in part three, we'll introduce capillary systems.  Lastly, in part four, we’ll discuss the capillary force formula in a quantitative fashion. 

So let’s start with the first part of the series to learn about polar molecules.  But first, we’ll need a quick review of some basic chemistry in order to build a solid foundation for future discussions.  There are 138 elements that make up everything in our world and the simplest form of each element is the atom.  Every atom is composed of three basic three basic components:  protons, neutrons, and electrons.    

For the purpose of this tutorial, we’ll be discussing the water molecule which is composed of Hydrogen and Oxygen.  Hydrogen is the simplest element on the periodic table.  It is made up of one proton and one electron.  Oxygen, on the other hand, is composed is 8 protons and 8 electrons.  In all cases, electrons orbit around the atom’s nucleus and they maintain their position in what we’ll refer to as the atom’s orbital shell which can be represented as a simple thinned walled sphere that the electron’s are free to travel within. 

Now, each element has a unique number of electrons and there are only a certain number of electrons that will fit into each atom’s shell.  So some elements have one shell while other elements have multiple shells.  For the most part, atoms populate their orbital shells from the inside out.  So once an element fills it’s first inner most shell, it starts to fill a second outer shell.  This process continues until all of the electrons are loaded into shells. 

In the case with oxygen, it dumps two electrons into its innermost shell and six into its second outer shell.  As for Hydrogen, well, it simply has one electron orbiting in its outer shell.  The funny thing about atoms is this – there all about maintaining a good image when viewed by their neighboring atoms.  Yes, there’s a bit of a vanity affair taking place at the atomic level and it’s all about what’s on the outside.  In an atoms case, it’s all about their outer shell.

So in the atomic world of fashion, a good looking outer shell is one that is either completely full or complete empty of electrons.  Oxygen’s outer shell contains 6 electrons – but its outer shell holds 8 electrons.  So you’ve got Oxygen really wanting two more electrons so as to maintain good standing within the atomic community.  As a matter of fact – it’s desperate for two additional electrons - constantly in search of two additional electrons in order to uphold its image.

And then there’s Hydrogen – how does Hydrogen’s appearance measure up?  As we said before, Hydrogen has one electron in its outer shell but its outer shell holds two electrons. So you’ve got unhappy Hydrogen that either 1) wants to add an electron to its outer shell or 2) give up its one electron to someone else. 

So you’ve got unhappy campers in Hydrogen and Oxygen in there native state.  Hydrogen atoms are all over the place with their outer electron for sale – and you’ve got Oxygen out there looking to purchase two additional electrons.  So, as you might suspect, Oxygen strikes a deal with two Hydrogen atoms. 

These two Hydrogen atoms and Oxygen get together and decide that they’re happiest when the three of them work together to mutually satisfy each of their requirements.  In short, two Hydrogen atoms decide to give up their electron to Oxygen.  In this arrangement, each Hydrogen atom now has an empty outer shell and Oxygen finally has a full outer shell.  So it takes two Hydrogen atoms to satisfy the need of one Oxygen atom.  Two Hydrogen, one Oxygen, Two Hydrogen, one Oxygen - also known as H2O. 

But there’s a catch – there are some strings attached to this deal.  So let’s explore the consequences.  We’ve just discussed the way that hydrogen and oxygen atoms work together to create a molecule.  What we have not yet discussed is the fact that electrons and protons carry charges.  Electrons are negatively charged and protons are positively charged.   So what impact does this fact have on the molecular creation of H2O?  A lot.

You see, in their natural state, atoms carry a neutral charge.  They have an equal number of positively charged protons as they do negatively charged electrons.  Parallel to this, however, is their desire to have stable and respectable outer shells.  Their desire to look good on the outside, however, creates some charge imbalances once they engage in this electron swapping lifestyle.

When an atom gives away an electron, it then has more protons then it does electrons.  In other words, it has more positively charged components than negatively charged components.  So the atom then carries a positive charge.  Conversely, the atom that acquires one or more electrons from another atom now has more electrons as compared to protons -more negatively charged components as compared to positively charged components.  So the atom carries a negative charge.

As most of you probably have experienced with magnets – opposite charges attract.  In our earlier example with Hydrogen and Oxygen, two Hydrogen atoms gave up their electrons and became positively charged.  One Oxygen atom obtained two electrons and became negatively charged.  As a result of their differences in charge, the positively charged Hydrogen atoms are attracted to the negatively charged Oxygen atom and visa versa.

So it turns out that these water molecules all start to behave like magnets with each water molecule having a positive side and a negative side, or a positive and negative pole.  Just like magnets and as a result of the opposite and attractive magnetic charges, water molecules have a tendency to orient themselves such that their positive and negative sides of neighboring molecules are close to one another.  Then, these attractive forces help water molecules resist forces that otherwise would separate them from one another.  This is important, so let me say it again, the attractive forces in a water molecule help water molecules resist forces that otherwise would separate one H2O molecule from a neighboring H2O water molecule.

These molecular attractive forces can best be represented by thinking about water's surface features.  Water bugs can walk on water because of the water molecule's attractive forces to one another.  Belly flops at the local swimming pool hurt and water can be siphoned all because of a water molecule's attractive forces to it neighbor.

So, this concludes part one of our four part video tutorial.  Tune in to Thermo.TV for the second part of the tutorial to learn about surface energy and the resulting contact angle that exists when water comes into contact with another surface.