Thin-films stability

Formation of superficial waves is observed during the wire coating process, which aims at covering a small-radius cylinder with a thin layer of another material, initially liquid, so as to protect or paint textile fibers, optical fibers, electric wires, etc. The presence of these waves can have consequences on aesthetic finish or on the characteristics of the product (i.e. thermal exchange coefficient because the surface increases).

In order to understand the way in which these instabilities occur, experimental tests are carried out measuring the thickness of the liquid that covers the wire before its drying, and deducing amplitude and characteristic wavelength of the superficial wave. This measurements are obtained through a new technique based on laser plane probes. The use of two equal probes allows the experimental measurement of the wave velocity and the amplification (or damping) factor from the same wave.

In the case of simple withdrawal of the wire from the liquid bath, or when an orifice is used for controlling the final thickness (die coating ), the obtained results agree with the existing theories found in literature. If an annular jet is used to control the final thickness of the coating (annular jet wiping), no existing theories can be applied for comparison. A new model is therefore developed, which reveals very good agreement with experimental data. For certain ranges of the dragging wire velocity and for certain values of jet pressure, the formation of waves is observed, while in other cases this can be controlled or even eliminated.

For more details, see the following related publications.

Zuccher, S. 2009 An experimental study of the liquid film on a vertical wire under the action of an impinging annular jet. Experiments in Fluids, 46, 2, 309-322.

Zuccher, S. 2005 A novel measurement technique for the study of wire coating instabilities. Experiments in Fluids 39, 4, 694-702.

Zuccher, S. & Buchlin, J.-M. 2001 Liquid film instabilities in die coating process. In Advances in Coating Processes. Proceedings of 4th European Coating Symposium 2001, Free University of Brussels, Belgium.

Buchlin, J.-M. & Zuccher, S. 1999 Liquid film instability of wire coating. In Advances in Coating and Drying of Thin Films. Proceedings of 3rd European Coating Symposium 1999. University of Erlangen-Nürnberg, Germany.

Zuccher, S. 1999 Liquid film instabilities of wire coatings, von Karman Institute for Fluid Dynamics, Rhode-Saint-Genèse, Bruxelles.