## Contact Angle and Wettability

EXAIR has been selling high-efficiency, hard-hitting compressed air products since 1983 for the purpose of drying and cleaning materials with compressed air.  The laminar airflow allows for a consistent force in the same direction, which is beneficial for effective cleaning and drying.  If we look closer at the surface of the material and how liquids interact, we can get a better understanding.

First, let’s look at the atoms of a solid.  Since the atoms are tightly packed and arranged in a stable format, the forces are even around each atom, except for the atoms on the outside surface.  These forces are unequal, so they will generate an imbalance, which is called surface tension.  This surface tension will have an atomic interface with other objects.  Let’s look at a liquid.  Liquid is a substance that flows readily.  Since it can flow, the surface tension on the outside will create a sphere in space.  So, what happens when these two surfaces interact?

If we look at a liquid, we can place a drop on a solid surface.  If the surface tension of the solid is weaker than the surface tension of the liquid, then the liquid will bead up.  If the solid has a strong surface tension compared to the liquid’s surface tension, then it will lay flat or wet out.  This phenomenon is important to know if you want things like liquids to wet out with inks and paints versus having the liquid bead up with waterproof materials.

We can measure how much force the solid surface is exerting when interacting with the liquid by the contact angle (refer to the photo above).   This angle will indicate the wettability of the fluid.  For a contact angle greater than 90o, the surface would be considered non-wettable with that liquid.  If the contact angle is less than 90o, then the liquid will wet out onto the surface.  For water, we have a variety of degrees of interaction as related to contact angles:

• >150o – Super-hydrophobic
• 90o – 150o – hydrophobic
• 30o – 90o – normal wetting
• 10o – 30o – hydrophilic
• <10o – Super-hydrophilic

Why is this important to know?  For drying and cleaning applications, it is beneficial to know how easy or difficult the liquid is to remove from the material.  If the contact angle is greater than 90o, then we can help the customer adjust the blowing angle and reduce the air pressure to save money because the liquid is easy to remove.  If the contact angle is less than 90o, then we can adjust the angle to be more direct as well as increase the force level to increase effectiveness.

How does this apply to EXAIR products?  We are the market leaders for non-contact drying and cleaning.  For applications where you need to remove as much water as possible for quicker heating cycles, to reduce labor costs when cleaning parts, or to quickly dry surfaces for labels and print quality, EXAIR has a variety of blow-off products; like the Super Air Knife.  Within these product lines, we offer a kit that includes a regulator and a shim set to vary the force levels to tackle the different degrees of wettability.  EXAIR is a proponent of using the least amount of air to “do the job”.  The regulator can make fine adjustments to the force levels while the shims will make a coarse adjustment.  So, depending on the contact angles, you can “dial in” the correct amount of force to clean and dry your products.  What could be better than this?  For U.S. and Canadian customers, EXAIR offers a 30-day unconditional guarantee on our cataloged items to try them out.  You can see for yourself how effective and efficient our products are.  If you would like to speak to an Application Engineer about your drying or cleaning application, we would be happy to make a recommendation to help solve your issues.

John Ball
Application Engineer
Email: johnball@exair.com