Radial displacement of a fluid annulus in a rotating Hele-Shaw cell

The radial displacement of a fluid annulus in a rotating circular Hele-Shaw cell has been investigated experimentally. It has been found that the flow depends sensitively on the wetting conditions at the outer interface. Disp lacements in a prewet cell are well described by Darcy's law in a wide rang e of experimental parameters, with little influence of capillary effects. I n a dry cell, however, a more careful analysis of the interface motion is r equired; the interplay between a gradual loss of fluid at the inner interfa ce, and the dependence of capillary forces at the outer interface on interf acial velocity and dynamic contact angle, result in a constant velocity for the interfaces. The experimental results in this case correlate in the for m of an empirical scaling relation between the capillary number Ca and a di mensionless group, related to the ratio of centrifugal to capillary forces, which spans about three orders of magnitude in both quantities. Finally, t he relative thickness of the coating film left by the inner interface, ai, is obtained as a function of Ca. (C) 1999 American Institute of Physics. [S 1070-6631(99)02503-9].