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Surface tension refers to the tension along the surface of the liquid surface layer acting on any boundary due to the imbalance of molecular attraction. Liquids have cohesiveness and adsorbability, both of which are manifestations of molecular gravity. Cohesion allows liquids to resist tensile forces, while adsorption allows liquids to stick to other objects.
Since surface tension exists only at the free surface of a liquid or at the interface between two immisgible liquids, it is generally measured by the surface tension coefficient σ. Sigma is the value of the tension on a surface per unit length, expressed in N/m. The surface tension of various liquids covers a wide range and its value decreases slightly with the increase of temperature.