The surface tension of water has profound effects on life (1-3), It makes possible the flow of water to the tops of trees, allows some insects to breathe underwater and others to walk on it, and resists the inflation of lungs in premature infants. Collaboration among biologists, engineers, mathematicians, and physicists has produced exciting advances in our understanding of surface tension's effects in both nature and technology, In a new twist on this theme, on page 931 in this issue, Prakash et al. (4) describe a "capillary ratchet" that explains how some shorebirds feed, highlighting a burgeoning research field that makes practical use of surface tension. Because water molecules are attracted more to each other than they are to air, water acts to minimize its surface energy by minimizing its area of contact with the atmosphere (2, 3). When a liquid drop contacts a solid surface, additional surface energies come into play (2,5), defining an equilibrium contact angle 0 between liquid and solid (see the figure, panel A).
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