Microscopic Details of a Fluid/Thin Film Triple Line

TL;DR

This study investigates the microscopic contact mechanics of a fluid on a thin elastic film over a fluid substrate, revealing a transition from Young-Dupré to Neumann-like behavior as film thickness varies, supported by experimental and modeling insights.

Contribution

It provides the first detailed experimental and theoretical analysis of the microscopic contact angles in a fluid/film/fluid system, clarifying the apparent paradox between macroscopic and microscopic contact angles.

Findings

Young-Dupré law applies to thick films.

Transition to Neumann-like balance occurs as film thins.

Models identify key lengthscales and tension-bending interplay.

Abstract

In recent years, there has been a considerable interest in the mechanics of soft objects meeting fluid interfaces (elasto-capillary interactions). In this work we experimentally examine the case of a fluid resting on a thin film of rigid material which, in turn, is resting on a fluid substrate. To simplify complexity, we adapt the experiment to a one-dimensional geometry and examine the behaviour of polystyrene and polycarbonate films directly with confocal microscopy. We find that the fluid meets the film in a manner consistent with the Young-Dupr\'e equation when the film is thick, but transitions to what appears similar to a Neumann like balance when the thickness is decreased. However, on closer investigation we find that the true contact angle is always given by the Young construction. The apparent paradox is a result of macroscopically measured angles not being directly related to true microscopic contact angles when curvature is present. We model the effect with the Euler-Bernoulli beam on a Winkler foundation as well as with an equivalent energy based capillary model. Notably, the models highlight several important lengthscales and the complex interplay of tension, gravity and bending in the problem.