(499j) Computational Investigations Into the Effects of Viscoelasticity On Liquid Transfer During a Gravure Process

Liquid coating and printing processes are attractive for bringing large-scale manufacturing capabilities to potentially important advanced functional materials such as structured films for which fabrication is typically limited to the lab-scale. For example, gravure coating of dilute block copolymer solutions has been shown as facile fabrication method for thin nano-pillared films. Accordingly, there is an emerging interest in modeling the transfer of liquids during gravure-like processes, both experimentally and computationally.

Two salient features of the action of liquid transfer during gravure are the extensional nature of the flow (c.f. axisymmetric filament stretching) and de-wetting of the liquid from the gravure cell wall associated with emptying. Coating liquids often exhibit complex rheologies due to the presence of polymers or particles; in the block copolymer example above, viscoelasticity of the liquid can be expected to play an important role. While filament stretching studies abound in the literature, the consideration of dynamic wetting in such flows is lacking, and the problem of computing viscoelastic flows with dynamic (de)-wetting in general is largely an open one.

We extend the method of computation for viscoelastic filament stretching flows [1] with a model for dynamic de-wetting based on the commonly used Navier-slip law at the solid-liquid boundary and a dynamic contact angle. For ALE methods such as ours, the slip law causes fatal problems at the dynamic contact line when computing the evolution of the conformation tensor due to unresolvable stress there. Allowing the contact angle to be dynamic can mitigate these problems.

Recent experiments [2] demonstrate the effects of viscoelasticity in a model process whereby liquid is pulled out of a cavity (truncated cone) onto a moving flat piston. Fraction of liquid transferred is examined as a function of piston speed for Newtonian and viscoelastic liquids. Viscoelasticity is shown to affect liquid transfer either adversely or favorably depending on the interplay of gravitational and inertial effects. We discuss computational results in relation to the experiments, and show qualitative success in predicting the enhancement or reduction in liquid transferred out of the cavity when the liquid is viscoelastic.

[1]        Bhat, P., Basaran, O., & Pasquali, M. (2008). Dynamics of viscoelastic liquid filaments: Low capillary number flows. J. Non Newtonian Fluid Mech., 150(2-3), 211–225.

[2]        Sankaran, A. K., & Rothstein, J. P. (2012). Effect of viscoelasticity on liquid transfer during gravure printing. J. Non Newtonian Fluid Mech., 175-176(C), 64–75.