ABSTRACT
The theoretical and experimental background of Young's equation for the solid-liquid-vapor contact angle are reviewed and discussed. A recent microgravity liquid phase sintering experiment using a W-Ni alloy provides an opportunity to test the contact angle definition in terms of the surface tensions without the influence of gravity. The resulting configuration can only be described by a force balanced three-vector diagram, not by Young's equation. Microscopically, a three phase contact forms a "valley" rather than a "ridge" on an initially plane solid surface. A main problem of Young's equation is the assumption of thermodynamic equilibrium for what is a metastable system.
