Surface Tension, Wax and Water Drops

September 14, 2016


Photographer: Dale Hugo
Summary Author: Dale Hugo

After a recent rain in Arlington Heights, Illinois, I looked out to see these tiny domes of water regularly arrayed on my new patio table. Yes, just days ago, I did seal the table with butcher block wax to protect the wood. What caused this? In chemistry, we know that water has polarity, that is, it's a polar covalent compound. Therefore, water molecules pull together unlike say, gasoline or paint thinner. This pulling together, called hydrogen bonding is a result of differing electronegativities of hydrogen and oxygen, plus the small size of the water molecule. I tell my chemistry students that you can expect hydrogen bonding in the nitrogen, oxygen and fluorine compounds. In fact, a Periodic Table will show them all together in the upper right-hand corner.

The differences in size of the domes relate to the imperfections in the surface of the wood. We don't notice them, but the variations in angle, shape and orientation of low spots in the slats add up to the size differences that we see. When the domes become too large then they spill between the gaps in the wood. When a rain event ends, the surface tension draws the domes together to the center of the boards. The better my waxing job, the steeper the wetting angle at the edge and the higher the dome. If there were no water repellant surface, the rainwater would be evenly wet over the entire surface and alas, soaking quite well into the wood, hastening its decay and deterioration.

This mutual attraction of water molecules gives water surface tension, which amounts to a skin on water that allows water bugs (water striders) to walk on ponds and also makes domed puddles like these. Surface tension also helps explains the relatively high boiling point of water (compared to say, methane which boils hundreds of degrees lower than water's boiling point of 100 C).

Photo Details: Camera Maker: Apple; Camera Model: iPad 2; Lens: Apple; Focal Length: 2.0mm (35mm equivalent: 44mm); Aperture: ƒ/2.4; Exposure Time: 0.010 s (1/100); ISO equiv: 64.