title: Equilibrium contact angle or the most-stable contact angle?
abstract: It is well-established that the equilibrium contact angle in a thermodynamic framework is an "unattainable" contact angle. Instead, the most-stable contact angle obtained from mechanical stimuli of the system is indeed experimentally accessible. Monitoring the susceptibility of a sessile drop to a mechanical stimulus enables to identify the most stable drop configuration within the practical range of contact angle hysteresis. Two different stimuli may be used with sessile drops: mechanical vibration and tilting. The most stable drop against vibration should reveal the changeless contact angle but against the gravity force, it should reveal the highest resistance to slide down. After the corresponding mechanical stimulus, once the excited drop configuration is examined, the focus will be on the contact angle of the initial drop configuration. This methodology needs to map significantly the static drop configurations with different stable contact angles. The most-stable contact angle, together with the advancing and receding contact angles, completes the description of physically realizable configurations of a solid-liquid system. Since the most-stable contact angle is energetically significant, it may be used in the Wenzel, Cassie or Cassie-Baxter equations accordingly or for the surface energy evaluation.
authors.affiliation: Laboratory of Colloid and Surface Chemistry (LCSC), Department of Inorganic and Analytical Chemistry, University of Geneva, Switzerland.
authors.forename: F J
authors.initials: FJ
authors.lastname: Montes Ruiz-Cabello
authors.affiliation: Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Campus de Fuentenueva, E-18071 Granada, Spain. Electronic address: marodri@ugr.es.
authors.forename: M A
authors.initials: MA
authors.lastname: Rodríguez-Valverde
authors.affiliation: Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Campus de Fuentenueva, E-18071 Granada, Spain.
authors.forename: M A
authors.initials: MA
authors.lastname: Cabrerizo-Vílchez
keywords: Equilibrium contact angle; Mechanical vibration; Sessile drops; The most-stable configuration; Tilting