Drop size dependence of contact angles and line tensions of solid-liquid systems

Through a literature review, this paper outlines the current understanding of the drop size dependence of contact angles and line tension of solid-liquid systems. Generally, the drop size dependence of contact angles on solid surfaces have been interpreted in terms of line tension. Owing to the complexity of the contact angle measurements, great discrepancies with respect to the apparent sign and value of line tensions of solid-liquid systems have been reported. The ambiguity regarding the sign and value of line tension may be caused by the roughness and the heterogeneity of real solid surfaces on which the contact angles are measured. However, for a sessile drop on an ideal solid surface, the line tension is shown to be a positive quantity as required by the condition of stable thermodynamic equilibrium. in addition, two techniques to evaluate line tensions of solid-liquid systems by measuring the dependence of contact angles on the radius of the three-phase contact line are also reviewed.