A two-roll apparatus is used to explore experimentally the detailed fluid mechanics of
meniscus roll coating in which inlets are starved and flow rates are small. Both forward
and reverse modes of operation (with contra- and co-rotating rolls) are investigated
using optical sectioning combined with dye injection and particle imaging techniques.
That part of parameter space where meniscus coating occurs is identified by varying
the roll separation and roll speeds and hence flow rate and capillary number.
Key features of the flow structures identified in the forward mode include two large
eddies (each with saddle point, separatrix and sub-eddies), a primary fluid transfer
jet and the existence of two critical flow rates associated with the switching-on of
a second 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 saddle point and separatrix, a primary
fluid transfer jet and once again 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 transfer jets.
Measurements of film thickness and meniscus location made over a range of speed
ratios and capillary numbers are compared with theoretical predictions. A plate-roll
apparatus is used to confirm the presence, for very small flow rates, of a sub-ambient,
almost linear, pressure profile across the bead. Investigated also is the transition from
inlet-starved to fully flooded roll coating as flow rate is increased and the changes in
flow structure and pressure profile are observed
