A Molecular Expression for "Line Tension".

Area of Science:

• Physical Chemistry
• Surface Science
• Nanotechnology

Background:

• Young's equation describes contact angles, but nanodroplets require modifications like "line tension" to explain size-dependent contact angles.
• The physical interpretation of "line tension" remains debated, with challenges in its mechanical analogy and reported small, uncertain values.

Purpose of the Study:

• To investigate the physical basis of "line tension" using molecular simulations.
• To develop a new model for nanodroplet contact angle size dependence without invoking line tension's curvature dependence.

Main Methods:

• Molecular simulations were employed to systematically analyze the relationship between "line tension" and free energy at the three-phase contact line.
• Young's equation was rederived, explicitly accounting for interfacial molecules.

Main Results:

• No direct relationship was found between "line tension" and the free energy per unit length of the three-phase line.
• A new model was developed that explains nanodroplet contact angle size dependence based on interfacial molecules, not line curvature.
• An approximate form of the new model yields a quantity analogous to line tension but with clear molecular interpretations.

Conclusions:

• The conventional concept of "line tension" as a restoring force due to contact line curvature is physically unfounded at the molecular level.
• A new model provides a physically interpretable explanation for the size dependence of nanodroplet contact angles.