The effect of surface tension on the shape of fingers in a Hele Shaw cell

The experimental results of Saffman & Taylor (1958) and Pith (1980) on fingering in a Hele Shaw cell are modelled by two-dimensional potential flow with surface-tension effects included at the interface. Using free streamline techniques, the shape of the free surface is expressed rn the solution of a nonlinear integro-differential equation. The equation is solved numerically and the solutions are compared with experimental results. The shapes of the profiles are very well predicted, but the dependence of finger width on surface tension is not quantitatively accurate, although the qualitative behaviour is correct. A conflict between the numerics and a formal singular perturbation analysis is noted but not resolved. The stability of the steady finger to small disturbances is also examined. Linearized stability analysis indicates that the twodimensional fmgers are not stabilized by the surface-tension effect, which disagrees with the experimental observations. A possible rewon for the discrepancy between theory and experiment is suggested.