Capillary Action

Surface Tension If you have ever tried a fancy dive into the pool and did not complete the rotation, or ever did a "belly-flop", you remember the water did not at that moment feel like a liquid. Suface tension is that tendency of water to act as if it's surface is covered by plastic wrap. The discussion below tells why it happens.

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Surface tension is a phenomenon observed at a liquid-gas interface. It is caused by the difference in forces of adhesion and cohesion. The amount of surface tension is caused by the magnitude of the difference in these forces. The mercury shown in the picture at right has a high surface tension, resulting in small drops that are almost spherical. The larger drops are flattened because mercury has a high specific gravity (density), and gravity is acting on a larger volume of mercury.

The difference in the shape of the drops is more easily seen in the larger image.

The image at left has two drops of distilled water, four drops of distilled water with green food coloring added in a line through the center, and an outer ring of distilled water with green food coloring and dishwashing detergent added.

The shape of the drops is an indication of the amount of surface tension. The drops with the highest domes have the greatest surface tension. So it appears the food coloring enhanced the surface tension of the distilled water by increasing cohesion within the drop. Adding detergent relaxes the surface tension by decreasing the cohesion of water molecules for themselves. Surfactants are materials that decrease the surface tension of water.

Surfactants are regularly used when mixing dissimilar liquids or liquids with dry powders. The decrease in cohesion allows easier and more thorough mixing.

Copyright 2005. Clay Robinson, Ph.D., as to all resources: Materials may not be reproduced without Dr. Robinson's written consent. Students are prohibited from selling (or being paid for taking) notes or webpages during this course to or by any person or commercial firm without the express written permission of the developer of these pages.

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