Q Why does plastic dry slower than glass?

A Ever wonder why Tupperware containers retain huge droplets of water after a dishwasher cycle, or why plastic cups take longer to air-dry than their glass counterparts? UW-Madison physics Professor Clint Sprott explains that two factors are at play: cohesion and adhesion.

"Cohesion is the tendency of the water molecules to attract one another and collect into a spherical droplet," Sprott said. "Adhesion is the attraction of the water molecules to the surface."

The spherical shape of a water droplet results from molecules trying to get as close together as possible. This allows for the most comfortable arrangement and a minimum energy state. When water encounters a surface, a tug of war between cohesion and adhesion causes droplets to flatten to different degrees. "The ratio of cohesion to adhesion dictates whether the water forms droplets or a thin film on the surface," Sprott said.

"The adhesion of water to clean plastic is relatively small compared to the adhesion to glass, and thus the water does not spread across the surface of the plastic as it would for glass," Sprott added.

So what does this mean for the drying time of different materials? "It takes longer for the plastic to dry since the droplets have a smaller surface area than does a thin sheet of water on glass," Sprott said.

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