Physics Photo of the Week

August 28, 2009

Capillary action
"Dichromate", the mascot cat that lives in the Warren Wilson Chemistry Department is admiring the tiles on the patio in front of the Hamil Science Center at Warren Wilson College. 

Notice that the edges of the tiles near the gaps are wet, whereas the main parts of the tiles are dry.  Having rained the night before the tiles hadn't yet completely dried. Why are the tiles consistently wetter along the edges while drier in the centers?

The tiles are laid on top of a solid  concrete surface.  Water is somewhat slow to drain out from underneath the tiles, thus the bases of the tiles are immersed in about a centimeter of water.  Capillary action on the surface of the concrete tiles acts as a wick to conduct the water at the base up the edges of the concrete.   The water then dissipates along the tops constantly replenishing the water on the surface that evaporates.  See the drawing below.

The roughness of the concrete forms the capillary channels for the "wicking" of the water.  The capillary effect is not caused by the gap between the slabs.  Eventually, the whole tile dries out in the absence of additional rain.  The water on the edges of the tiles has less sunlight to cause evaporation and the water at the bases of the tiles acts as a reservoir.

Capillary action - the adhesion and flowing of water through small orifices - is very important in nature.  Water adhering on the rough surfaces of rocks contributes to the erosion, support and nourishment for plant life, and the eventual production of soil.  Many times we have seen trees growing out of cracks in the rocks, which not only provide a pathway for roots, but also a reservoir for water and nutrients that trees require.  Capillary action is partly enables trees to draw water from the ground to the treetops, but that full explanation is another story... 

Many chemistry students have watched capillary action in thin layer chromatography where a solvent migrates along a paper or silica gel surface and separates components of a mixture deposited on the surface of the gel.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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