Blake Terence D, Dobson Rosemary A, Ruschak Kenneth J
Research & Development, Kodak Limited, Harrow, HA1 4TY, UK.
J Colloid Interface Sci. 2004 Nov 1;279(1):198-205. doi: 10.1016/j.jcis.2004.06.057.
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.
将液体涂覆到固体上是一个重要的工业过程。成功涂覆的一个前提条件是液体能够动态地润湿固体表面。高速涂覆的一个限制因素是动态润湿失效的发生，这会导致空气夹带。在简单的实验中，如将胶带或纤维垂直插入液体池中，空气夹带通常发生在毛细管数Ca < 1时。然而，这个限制并非一成不变。实际上，“流体动力辅助”这一术语已被创造出来，以强调这样一个事实，即涂覆流动可以被操纵以促进润湿，从而推迟空气夹带。商业帘式涂覆通常在0.5 < Ca < 10的范围内运行。该过程的流动可视化表明，对于给定的润湿速度，流体动力辅助会导致动态接触角减小，正是这一点使得更高的涂覆速度成为可能。光纤涂覆方法已经发展到这样一个阶段，即相对粘稠的液体可以在毛细管数约为1000的情况下以非常高的速度成功涂覆。在最近的一篇论文中，雅克明（D. Jacqmin，《流体力学杂志》455 (2002) 347）提出，在这种情况下，夹带进入涂层的空气会在涂层模头中发现的高流体压力（约为1 MPa）下溶解。在此，我们报告了在0.5 < Ca < 50的区间内成功进行的帘式涂覆。这项新研究支持了另一种假设，即空气夹带推迟到非常高的毛细管数是强烈流体动力辅助的结果。
