Conceptual aspects of line tensions

TL;DR

This paper examines the conceptual foundations of line tensions in three-phase contact systems, analyzing their definitions, dependencies, and invariance under different interface choices, with implications for equilibrium contact angles.

Contribution

It clarifies the conditions under which line tension is uniquely defined and introduces the concept of line stiffness constants to ensure conceptual consistency.

Findings

Line tension for a liquid lens is independent of Gibbs dividing interface choices.

Two definitions of line tension for a drop are identified, with only one being independent of interface choices.

Equations for equilibrium contact angles incorporate line tension, Tolman length, and stiffness constants.

Abstract

We analyze two representative systems containing a three-phase-contact line: a liquid lens at a fluid--fluid interface and a liquid drop in contact with a gas phase residing on a solid substrate. We discuss to which extent the decomposition of the grand canonical free energy of such systems into volume, surface, and line contributions is unique in spite of the freedom one has in positioning the Gibbs dividing interfaces. In the case of a lens it is found that the line tension is independent of arbitrary choices of the Gibbs dividing interfaces. In the case of a drop, however, one arrives at two different possible definitions of the line tension. One of them corresponds seamlessly to that applicable to the lens. The line tension defined this way turns out to be independent of choices of the Gibbs dividing interfaces. In the case of the second definition,however, the line tension does depend on the choice of the Gibbs dividing interfaces. We provide equations for the equilibrium contact angles which are form-invariant with respect to notional shifts of dividing interfaces which only change the description of the system. Conceptual consistency requires to introduce additional stiffness constants attributed to the line. We show how these constants transform as a function of the relative displacements of the dividing interfaces. The dependences of the contact angles on lens or drop volumes do not render the line tension alone but a combination of the line tension, the Tolman length, and the stiffness constants of the line.