AN EXPERIMENTAL INVESTIGATION OF MENISCUS ROLL COATING

A two-roll apparatus is used to explore experimentally the detailed fl uid mechanics of meniscus roll coating in which inlets are starved and flow rates are small. Both forward and reverse modes of operation (wi th contra-and co-rotating rolls) are investigated using optical sectio ning combined with dye injection and particle imaging techniques. That part of parameter space where meniscus coating occurs is identified b y varying the roll separation and roll speeds and hence flow rate and capillary number. Key features of the flow structures identified in th e forward mode include two large eddies (each with saddle point, separ atrix and sub-eddies), a primary fluid transfer jet and the existence of two critical flow rates associated with the switching-on of a secon d fluid transfer jet and the switching-off of the primary transfer jet followed by a change in the flow structure. In the reverse mode, the key features are a single large eddy consisting of two sub-eddies, a s addle point and separatrix, a primary fluid transfer jet and once agai n two critical flow rates. These correspond to (i) the switching-on of a secondary transfer jet and (ii) the disappearance of a saddle point at the nip resulting in the merger of the primary and secondary trans fer jets. Measurements of film thickness and meniscus location made ov er a range of speed ratios and capillary numbers are compared with the oretical predictions. A plate-roll apparatus is used to confirm the pr esence, for very small flow rates, of a sub-ambient, almost linear, pr essure profile across the bead. Investigated also is the transition fr om inlet-starved to fully flooded roll coating as flow rate is increas ed and the changes in how structure and pressure profile are observed.