Mini-Encyclopedia of Papermaking Wet-End
Part Two: Definitions and Concepts
Critical Surface Tension
The critical surface tension of a solid surface is an indication of its relative water-hating or water-loving character. A low critical surface tension means that the surface has a low energy per unit area. The quantity is based on experiments with a series of pure liquids. These experiments have to be conducted on a flat, non-porous sample of that solid. A small droplet of each liquid is placed onto the surface. One measures the angle of contact at the solid-liquid-air contact line. The angle is drawn through the liquid phase. One plots the cosine of the angles of contact versus the surface tension of each liquid. For example, the surface tension of water, in equilibrium with its vapor at room temperature, is 72 mNm-1. Surface tensions of other liquids are widely available. The critical surface tension equals the surface tension at which the plotted line intersects 1.0.
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