Thermodynamics of Capillary Rise: Why Is the Meniscus Curved?

The capillary rise (or depression) phenomenon is analyzed thermodynamically with the aim to propose a more transparent pedagogic approach than the mechanical equilibrium picture that is currently standard in most textbooks. The driving force lifting the liquid column in the gravitational field is identified as the surface free energy gained when the liquid wets the inner wall of the capillary. The curving of the meniscus and the accompanied pressure drop across it according to the Young–Laplace equation is a consequence of the constant chemical potential throughout the system at physicochemical equilibrium in the gravitational field. As an illustration we show how meniscus shapes can be calculated numerically for different situations.