Triple-Phase Contact Line in Partial Wetting: Theories, Experimental Measurements, and Nanoscopic Approach

One of the overlooked topics in classical wetting, which is the main subject of numerous recent researches, is the concept of the triple-phase contact line (TPCL). In this article, after defining the TPCL and emphasizing its significance in wetting studies, this concept and its characterization methods using experimental techniques and molecular dynamics simulations are reviewed and investigated. First, the ideal Young’s model was revised based on the generalized theory of capillarity, and line tension, Γ, was assigned to TPCL as a defining physical parameter. Because ideal models cannot correctly determine the sign and value of Γ, real and non-ideal surfaces were used. Furthermore, the width of the TPCL was investigated by the obtained data from structural analysis of the droplet’s edge using optical and environmental-scanning electron microscopy. The corresponding scaling law from the former lead to a power greater than 0.66, whereas the latter high-resolution method resulted in a value of ~ 0.62. Conversely, MD simulations have illustrated that it is possible to locally use the minimum particle distance for the Lennard-Jones potential as the effective width of the TPCL. In the final section, the most important interaction of the TPCL with its vicinity, known as pinning, was discussed. From the experimental perspective, the focus is on the adhesion force measurements which results in the force-based characterization of the pinning phenomenon. However, on the nanoscale, due to the available kinetic energy, this phenomenon is defined as the drastic slow-down of the motion of the TPCL, directly contradicting the macroscopic view.