Experimental and numerical investigation of continuous dip-coating for wire with immiscible two-fluid system

Abstract

Dip-coating is one of the extensively used industrial coating processes. The popularity of this technique coating process is due to its ease of application, versatility in using a wide variety of substrates and coating liquids, low wastage, etc. Understanding this coating process holds economic and industrial importance in having better control of the final film thicknesses. The pioneering work in dip-coating is because of Landau-Levich [19] and subsequent improvements by Derjaguin [8]. The entrainment law given by Landau-Levich stressed the importance of the interaction of capillary and viscous forces. Several modifications and improvements were made to this theoretical work and David Quéré and his group did a voluminous amount of work in understanding the fibre/wire coating process [30]. They incorporated the effect of inertia into the film formation process and showed the possibility of various regimes within the flow. In recent years, the multiphase dip-coating processes become increasingly popular and this present work aims to extend our understanding of fibre/wire coating using a single liquid, involving a liquid-air interface to a two-fluid flow configuration, which contains an evolving fluid-fluid interface. Experimental and numerical work has been performed to study this problem. Experiments are performed by direct visualization of the coating film with varying control parameters. A numerical model is then established by solving fluid flow equations and capturing the fluid fluid interface with the help of a level-set method. The data generated upon this exercise in JNCASR and performed experiments at Universite de Lille, France, we set to discover and understand various flow regimes present in this flow problem. The results show the presence of a visco-capillary regime where the role of inertia and gravity can be assumed to have minimal importance. In this regime, the entrainment is shown to have a close resemblance with the classical LLD theory, which is being followed at higher Ca numbers. The role of inertia is manifested in providing a sharp deviation to the film thickness from the LLD thickness values and the sharpness becomes gentler as viscous, capillary, gravity and inertia all start to play important roles simultaneously. At higher inertia, the film formation is limited by the growth of the boundary layer and this gives rise to a regime solely dominated by boundary layer effects. An alternate visco-gravitational regime is also present suggesting the formation of the film primarily because of the interaction of viscous and gravity forces with negligible inertia and capillary suction. Moreover, film formation over a thin fibre/wire is often susceptible to instabilities and our numerical exercise shows the presence of them for an extensive range of control parameters and non-dimensional numbers.