Water Cohesion: A Sticky Situation

April 03, 2019

PattiW_raindrops_IMG_5454 (2)

Photographer: Patti Weeks 
Summary Author: Patti Weeks 

After a particularly long period of rainy weather where I live in eastern North Carolina, I emerged from a building one afternoon to see my car covered in raindrops. It had stopped raining, so the drops were stuck in small, distinct spheres on the surfaces of the car. After some research, I discovered I was observing four interesting phenomena of water.

The most obvious phenomenon, as seen in the photo above from inside the driver’s seat window, was the refracted and inverted image of the trees in the background in each drop of water. What’s up is down, what’s right is left.

In addition, each drop was demonstrating the principle of cohesion, the attraction of like substances. As with the attraction of opposite magnetic poles, a water molecule forms a polar bond of its two slightly positively charged hydrogen atoms with its one slightly negatively charged oxygen atom. And the bonded hydrogen atoms are strongly drawn to oxygen atoms of adjacent water molecules, holding the water molecules of raindrops together and causing the water to stick to itself.

The principle of adhesion, the attraction of unlike substances, was demonstrated by the pull of the drops of water to the surface of the car. In a similar way that water sticks to itself, water is attracted to objects that have charges.

And the principle of surface tension was at play. The molecules on the surface of water drops are not only pulled in toward the interior molecules but are also drawn sideways toward their adjacent exterior molecules. This gives the exterior molecules a higher energy level than those in the interior, thus strengthening their integrity. (This property gives water striders the ability to walk on the surface of a pond.) Because the interior water molecules are in a more favorable condition of cohesion, they'll avoid being on the harder working exterior. This tends to make a raindrop minimize its surface area by pulling itself into a rounded shape. And because the water’s cohesive force was stronger than the adhesive property of the car, and the raindrops were not yet yielding to gravity or evaporation, they stayed relatively spherical long enough for me to take several photographs. Photo taken February 20, 2019.

Photo Details: Camera: Apple iPhone 7; focal length: 3.99 mm; aperture: f/1.8; exposure: 1/805 second; ISO 20.