Forced Wetting and Hydrodynamic Assist

Introduction It is well known that air entrainment sets the ultimate limit to coating speeds. It is also known that air entrainment can be postponed to higher line speeds by manipulating coating flows to generate what has been termed ‘hydrodynamic assist’ [1]. There is increasing evidence that the benefit is associated with geometric or hydrodynamic confinement of the moving contact line [1-3]. In addition, experiments have shown that the conditions that allow higher line speeds in curtain coating also reduce the apparent dynamic contact angle, suggesting a direct link [4,5]. However, the mechanism by which the flow might affect wetting speeds and the dynamic angle has not been established – although several theories have been advanced. Here we develop an earlier suggestion [6] that an intense shear stress in the vicinity of the moving contact line can assist surface tension forces in compensating for contact-line friction. This reduces the velocity-dependence of the contact angle and so postpones air entrainment. Hydrodynamic assist is then a natural consequence of forced wetting that emerges when the contact line is driven by a very strong and highly confined flow.