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Issue 36, 2018
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Microscopic details of a fluid/thin film triple line

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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 contact 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–Dupré 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 an 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.

Graphical abstract: Microscopic details of a fluid/thin film triple line

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Article information

30 May 2018
26 Aug 2018
First published
27 Aug 2018

Soft Matter, 2018,14, 7492-7499
Article type
Author version available

Microscopic details of a fluid/thin film triple line

T. Twohig, S. May and A. B. Croll, Soft Matter, 2018, 14, 7492
DOI: 10.1039/C8SM01117F

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