Date of Award

5-2003

Level of Access

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Douglas W. Bousfield

Second Committee Member

Adriaan R.P. van Heiningen

Third Committee Member

J. Larcom Ober

Abstract

The physics of a fluid flow between two rotating cylinders is important in processes such as bearing lubrication, roll coating, and printing. A small amount of dissolved polymer in the fluid can have a large impact on the behavior of the process. Viscoelasticity affects the stability of application and metering processes, and reduces the maximum speed at which a uniform film can be coated onto a substrate. The goal of this work is to characterize the effect of viscoelasticity on the forward roll coating operation. A bench-top apparatus simulated the process. Measurements of the gap between the roll surfaces, pressure profile, and film thickness were made for a known roll speed and external load. Newtonian, Boger, and shear-thinning viscoelastic fluids were characterized and tested. Two-dimensional hnite element analysis of forward roll coating between two rigid rolls was completed for Newtonian, Oldroyd-B, and Giesekus fluids. The results for the Newtonian liquid were consistent with published experimental and theoretical data after the elasticity of the deformable cover was included in the analysis. A dimensionless empirical expression described the results. A lubrication analysis with nonHertzian cover deformation and visco-capillary boundary conditions at the film-merge and film-split corresponded with experimental measurements over a limited load range. The viscoelastic fluids showed a trade-off between shear thinning and elasticity. Thtee fluids were Boger-like in that they exhibited nearly constant viscosity in simple shear but had significant elasticity. Two liquids obeyed the Carreau model but showed significant elasticity. AJl five liquids exhibited varying degrees of the Weissenberg effect. The Boger fluids produced larger gaps and showed increased sensitivity to roll speed compared to the Newtonian liquid. The two shear-thinning elastic fluids produced larger gaps and increased sensitivity to the external load. Dimensionless, empirical expressions described the results. The hnite element analysis revealed the presence of a stress boundary layer at the free surface downstream of the hlm-split for the Oldroyd-B fluid; the Giesekus fluid, under the same conditions, did not produce the stress boundary layer. Competing effects of shear thinning and elasticity were revealed as a reduction of roll separating force produced by the Giesekus fluid.

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