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.