An experimental procedure to obtain the equilibrium contact angle from the Wilhelmy method

The theory of contact angles has been developed using the concept of the Young contact angle, and equilibrium quantity related to perfectly smooth and homogenous surfaces; on real surfaces it is commonly accepted that one can obtain "metastable" equilibrium states, in which the shape of the meniscus along the triple line is not fully equivalent to the "equilibrium" meniscus. In the present paper, a new method is presented to obtain an equilibrium contact angle with a simple modification of the Wilhelmy microbalance. The supply of acoustic energy to the liquid at a frequency which induces the resonant formation of capillary waves, followed by the reduction of the amplitude of the vibration, allows the meniscus to move from the metastable advancing or receding shape to the stable equilibrium one. This result appears to be significantly independent of the initial conditions; it has been possible to confirm the theoretical provision that the energy barriers between metastable states far from equilibrium are lower than the barriers at equilibrium. The equation which relates the equilibrium cosine of the contact angle and the mean of advancing and receding cosines has been confirmed on homogenous surfaces, but has been disproved on macroscopically heterogenous surfaces. The method appears promising also for its simplicity and low cost.