Wetting at high capillary numbers

The coating of liquids onto solids is an important industrial process. A prerequisite for successful coating is that the liquid dynamically wet the surface of the solid. One of the limits to high-speed coating is the onset of dynamic wetting failure, which leads to air entrainment. In simple experiments in which a tape or fibre plunges vertically into a pool of liquid, air entrainment usually occurs at capillary numbers Ca < 1. However, this limit is not immutable. Indeed, the term "hydrodynamic assist" has been coined to emphasise the fact that coating flows may be manipulated to promote wetting and so postpone air entrainment. Commercial curtain coating typically operates in the range 0.5 < Ca < 10. Flow visualisation of this process has shown that hydrodynamic assist leads to a reduction in the dynamic contact angle for a given wetting speed and it is this that permits the higher coating speeds. Methods of coating optical fibres have evolved to the point where comparatively viscous liquids can be coated successfully at very high speeds with Ca of order 1000. In a recent paper Jacqmin, (D. Jacqmin, J. Fluid Mech. 455 (2002) 347) has suggested that in this case air entrained into the coating dissolves under the high fluid pressures found in the coating die, which are of order 1 MPa. Here we report successful curtain coating over the interval 0.5 < Ca < 50. The new study supports an alternative hypothesis that the postponement of air entrainment to very high capillary numbers is the result of intense hydrodynamic assist. (C) 2004 Elsevier Inc. All rights reserved.