A theoretical and experimental investigation of reservoir-fed, rigid-roll coating

An industrially-important variant of reverse roll coating is studied in whi ch the metering gap sits beneath a large, liquid reservoir, the influence o f which is investigated via complimentary experimental, analytical (lubrica tion) and computational (finite element) methods, and for which gravitation al effects are shown to be influential. Experimental measurements for both the flow rate and wetting line position are given over a wide range of roll speed ratio and capillary number and it is shown that, provided the wettin g line is sufficiently far from the nip, the flow rate depends linearly on the reservoir level. A key feature of the mathematical models is that, unli ke previous reverse roll coating studies. the variation of dynamic contact angle with metering roll speed has been accounted for. The lubrication mode l also uses boundary conditions which incorporate free surface, Surface ten sion and wetting line effects and the predictions from both models are foun d to be in generally good agreement with experiment. Finally, streamlines o btained from Finite Element solutions of the flow in the reservoir and wett ing line regions are found to compare wall with corresponding experimental flow visualisations. The flow in the reservoir is recirculating in nature, the size and number of recirculations depending on the reservoir geometry. (C) 2001 Elsevier Science Ltd. All rights reserved.