DELAYED-ONSET OF RIBBING INSTABILITY DUE TO FINITE-SIZE EFFECTS

A fingering pattern forms at the interface between air and a thin laye r of oil, confined between two diverging cylindrical surfaces, when on e of the cylinders rotates about its axis. We present results of measu rements on this pattern near its onset, and show that the onset values : of both the control parameter (the cylinder rotation speed) and the pattern wavelength are strongly influenced by effects due to the finit e length of the experimental system. In a short system, the onset of t he fingering instability occurs at substantially higher values of the rotation speed than predicted by calculations performed for the infini tely long system. (C) 1995 American Institute of Physics.