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Mid-gap invasion in two-layer slot coating

Published online by Cambridge University Press:  17 July 2009

JAEWOOK NAM  and
MARCIO S. CARVALHO
JAEWOOK NAM
Affiliation:
Coating Process Fundamentals Program, Department of Chemical Engineering and Materials Science, University of Minnesota, MN 55455, USA
MARCIO S. CARVALHO*
Affiliation:
Department of Mechanical Engineering, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marques de Sâo Vicente 225, Gávea, RJ 22453-900, Brazil
*
†Email address for correspondence: [email protected]
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Abstract

Multi-layer, continuous liquid coating is the most efficient way to manufacture films that require more than one layer for optimal performance. Dual-layer slot coating is one of different coating methods largely used to deposit two thin, uniform liquid layers on to a moving substrate. The two liquid phases are separated by an inter-layer that starts at the separation point (or line, in three dimensions) attached to the die surface. The stability of the two-phase flow and the location of the separation point are directly related to the quality of the final product. Ideally, the separation point should be attached to the downstream corner of the mid die piece of a dual slot-coating die. However, its location may change as operating conditions vary, leading to undesired flow states, with microvortices and periodic oscillation. The movement of the separation point from its desired location along the die surface is usually referred to as mid-gap invasion and can be associated with the onset of coating defects. It is crucial to determine the set of flow conditions at which it occurs. We study the evolution of the separation-point location and the inter-layer configuration as a function of operating conditions by flow visualization and by solving the two-dimensional Navier–Stokes equation for free-surface flows. The results reveal two different mechanisms for mid-gap invasion, depending on the viscosity ratio of the two liquid layers. They also show that the most critical parameter responsible for the onset of mid-gap invasion is the bottom-layer wet thickness (flow rate). Although the movement of the separation point involves an evolution of complex flow states, a simple but accurate criterion based on rectilinear flow approximation is proposed.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 631 , 25 July 2009 , pp. 397 - 417
Copyright
Copyright © Cambridge University Press 2009

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References

REFERENCES

Armi, L. 1986 The hydraulics of two flowing layers with different densities. J. Fluid Mech. 163, 27–58.CrossRefGoogle Scholar
Bolstad, J. H. & Keller, H. B. 1986 A multigrid continuation method for elliptic problems with folds. SIAM J. Sci. Stat. Comput. 7, 1081–1104.Google Scholar
Cohen, D. 1993 Two-layer slot coating flow visualization and modelling. Master's thesis, University of Minnesota.Google Scholar
Maenosono, S., Okubo, T. & Yamaguchi, Y. 2003 Overview of nanoparticle array formation by wet coating. J. Nanopart. Res. 5, 5–15.CrossRefGoogle Scholar
Mavridis, H., Hrymak, A. N. & Vlachopoulos, J. 1987 Finite-element simulation of stratified multiphase flows. AIChE J. 33, 410–422.Google Scholar
Musson, L. C. 2001 Two-Layer Slot Coating (PhD thesis, University of Minnesota). University Microfilms International.Google Scholar
Romero, J. R., Scriven, L. E. & Carvalho, M. S. 2006 Effect of curvature of coating die edges on the pinning of contact line. AIChE J. 52, 447–455.Google Scholar
de Santos, J. M. 1991 Two-Phase Cocurrent Downflow Through Constricted Passage (PhD thesis, University of Minnesota). University Microfilms International.Google Scholar
Sartor, L. 1990 Slot Coating: Fluid Mechanics and Die Design (PhD thesis, University of Minnesota). University Microfilms International.Google Scholar
Sartor, L., Huff, S., Kishi, C. N., Meyer, D. & Ercillo, J. C. 1999 Method of multilayer die coating using viscosity adjustment techniques. U.S. Patent 5 962075.Google Scholar
Scanlan, D. J. 1990 Two-slot coater analysis: inner layer separation issues in two-layer coating. Master's thesis, University of Minnesota.Google Scholar
Schlichting, H. & Gersten, K. 2001 Boundary Layer Theory, 8th edn. McGraw-Hill.Google Scholar
Silliman, W. J. & Scriven, L. E. 1980 Separating flow near a static contact line: slip at a wall and shape of a free surface. J. Comput. Phys. 34, 287–313.CrossRefGoogle Scholar
Taylor, S. D. & Hrymak, A. N. 1999 Visualization and flow simulation of a two-slot coater. Chem. Engng Sci. 54, 909–918.CrossRefGoogle Scholar
Thompson, J. F., Warsi, Z. U. A. & Mastin, C. W. 1985 Numerical Grid Generation: Foundations and Applications. North Holland.Google Scholar
Vinokur, M. 1983 On one-dimensional stretching functions for finite-difference calculations. J. Comput. Phys. 50, 215–234.Google Scholar
Wilson, Mark C. T., Summers, Jonathan L., Kapur, N. & Gaskell, P. H. 2006 Stirring and transport enhancement in a continuously modulated free-surface flow. J. Fluid Mech. 565, 319–351.CrossRefGoogle Scholar