Flow Structure and Transfer Jets in a Contra-Rotating Rigid-Roll Coating System

A comprehensive analysis is presented of the internal flow structures that can arise in the coating bead of a twin roll coater, operating in forward mode, as inlet flux is varied seamlessly from a starved to a flooded condition. This is accomplished via the application of lubrication theory combined with the acquisition of accurate finite element solutions of the full two free surface flow problem. The former is used to map out associated control-space diagrams in terms of the non-dimensional quantities, roll speed ratio, inlet flux and Capillary number, and, further, to predict the distribution of eddies and liquid transfer paths as a function of the main operating parameters. The dynamic contact angle is predicted as a function of the operating conditions for both the computations and analysis. Detailed results are presented which: (i) provide a complete picture of attendant eddy structures; (ii) quantify the strengths of primary, secondary and asymmetric liquid transfer jets; (iii) reveal the presence of a weak secondary asymmetric transfer jet in the intermediate to flooded inlet flow regime; (iv) describe stagnation and free surface locations as a function of inlet flux and Capillary number; (v) demonstrate the unexpected coexistence of the primary and asymmetric jets in the low speed ratio meniscus regime.