Steady recirculating flow near static contact lines

Recirculating flow near a static contact line, encountered in the productio n and deposition of thin liquid coatings, is undesirable since it constitut es a means by which process defects can arise. Here an idealised model for the steady flow near a static contact line in the slide coating process is considered in which the local free surface shape is assumed to be planar, p rompted by experimental observations, and the flow is driven by a moving li d/boundary. The resulting nonlinear boundary value problem is solved numeri cally for Reynolds numbers, Re is an element of [0, 100] using a finite ele ment formulation of the governing Navier-Stokes equations, enabling the inf luence of both the value of the static contact angle, theta(s), and the ine rtia of the flow close to the static contact line to be explored. Computational results show that the how field is characterised by a sequenc e of distinct eddies, the relative sizes and strengths of which depend stro ngly upon theta(s), while inertia effects have only a minor influence. More over, the predictions are in close accord with Moffatt's classical theory f or the Stokes how regime and, in particular, show that for theta greater th an or equal to 35 degrees the sequence of secondary eddies adjacent to the contact line diminish in size rapidly and eventually disappear for theta(s) greater than or equal to 80 degrees. On the basis of these results it is p ostulated that increasing theta(s) in slide coating systems, by some means, could reduce the frequency of defects usually associated with recirculatin g how near the static contact line. In addition, although the model is moti vated by the slide coating process, it is expected that the results will al so be relevant for other coating flows, such as in slot and curtain coating systems. (C) 1999 Elsevier Science Ltd. All rights reserved.