Hydrophobic/Hydrophylic Balance

If you were to wet a brand new clean window glass surface with pure water you will notice that the surface is not equally funtionalized.  The water will sheet in places of the same plate and bead up in others.  This is not necessarily because there are contaminants unevenly covering different areas of the window.  It is likely because the physical integrity of the near surface is different.  The more 'rough' a surface is the more it will soak up and hold onto water.  Water will form larger beads when it condenses as fog on the surface too.  So the more rough a surface is the more hydrophylic it is.  The opposite is also true.  The more smooth the surface the more hydrophobic it is.  Condensed water or fog will also form smaller beads when looked at through a thirty power handheld lighted microscope.  You can prove this by polishing one side of a mirror plate with a cerium powder of five microns particle size, and then the other side with an aluminum oxide of 100 to 400 nanometers.  You will be able to feel the difference with your fingertip.  In both cases the chemistry of the surface is exactly the same.  The only difference is how smooth each side is.  The side polished with the submicron/nanometer scale aluminum oxide will be much more smooth than the opposite side polished with the cerium oxide of five microns.

Now.  What does this mean to a window cleaner using a water fed pole?  Simply that the results will be different from one window to the next.  Also.  When we choose an inline chemical to functionalize the surface as we clean, it would be best to choose one that will effect the correct balance between hydrophylic and hydrophobic.  This is because of two reasons.  First we do want to functionalize the surface equally across every square inch.  Second water will sheet best from a balanced surface.  If the surface were super hydrophylic it likely wouldn't sheet at all, but would rather cling to the window leaving a light film.  Also the very best hydrophobic surface is not superhydrophobic so little beads of water will form.  It is therefore more time consuming when cleaning an ordinary hydrophobic surface with a water fed pole.  The very best functionalized surface for the WFP user will be both hydrophobic and hydrophylic.  This balance can actually be measured by measuring the contact angle of a drop of pure water.  Also.  The eveness of the 'new' surface can be analyzed by fogging the entire window.  You will be able to easily see at a glance the pattern. And to reiterate the more hydrophylic surface will have the smaller water drop when the window is fogged.  By looking at the size of the water drop on the new surface under a microscope it is possible to quantify different chemical treatments to find the one that works the best.  Or at least to be able to label them from most hydrophylic to the least.  Then to choose the one that we prefer.  Almost like someone chooses a soap/cleaner depending on the level of suds action, and glide.

Written by Henry Grover Jr

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