SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY
Capillary attraction and depression: capillary tubes dipped into water (dyed red) and mercury. Water wets the glass and rises in the tube; mercury, however, gives a capillary depression. The difference may be explained by considering adhesive and cohesive molecular forces. Cohesive forces exist between molecules of the same substance, whereas adhesive forces bind molecules of different substances. Adhesion of water molecules to glass is stronger than their own cohesion - consequently, the meniscus (surface) curves upwards and the water rises. Conversely, cohesion of mercury is greater than its adhesion to glass, causing a capillary depression.
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