THEORETICAL AND EXPERIMENTAL STUDIES OF THE FLUID-DYNAMICS OF A 2-ROLL COATER

Two cylindrical rolls are partially immersed in a Newtonian fluid so that the roll axes are parallel to each other and lie in the plane of the free surface of the fluid. The rolls nearly touch each other, and when they counterrotate, fluid is drawn into the region between them. Classical lubrication theory is used to predict the thickness of the fluid film withdrawn from the bath by the rolls. The effect of gravity is accounted for. For large capillary numbers the theory agrees well with data obtained with various Newtonian fluids. Gravity effects become measurable at small capillary numbers and the data show the trends predicted by the model. Some discussion is offered regarding the nature of boundary conditions imposed at the film-splitting stagnation point. It is shown that the use of the more accurate Coyne-Elrod condition does not significantly alter the model based on the approximate but simple Prandtl-Hopkins condition, at least for the range of conditions normally met in the laboratory or in coating practice. © 1979, American Chemical Society. All rights reserved.