Retention forces and contact angles for critical liquid drops on non-horizontal surfaces

Retention forces and drop parameters are investigated for drops on the verge of sliding on vertical and inclined surfaces. Using earlier observations of drop geometry, the retentive-force factor relating surface-tension forces to contact-angle hysteresis is reliably determined. The retention force for a drop is found to be insignificantly affected by the aspect ratio of its contour. The maximum size of a drop is predicted with good accuracy, based on the two-circle method for approximating shapes of drops. The Bond number of a critical drop is found to be constant for a given surface and liquid. A general relation is proposed between the characteristic advancing and receding contact angles. The relation is supported by a large set of contact-angle data. In the absence of θRθR data, the relation allows estimating the receding contact angle and the critical drop size, using only the advancing angle.

Graphical abstractPredictions of retentive forces, based on a force-balance model are compared to measurements from the literature [C.W. Extrand, Y. Kumagai, J. Colloid Interface Sci. 170 (1995) 515] for critical drops on several surfaces. The proposed k=48/π3k=48/π3 (Eq. (13)) fits the data with (r=0.98r=0.98).Figure optionsDownload full-size imageDownload as PowerPoint slide