Metastability at the displacement of a fluid in a Hele-Shaw cell

The displacement of silicon oil by air (with a constant flow rate) in a radial Hele–Shaw cell has been studied experimentally. The initially circular air-oil interface is perturbed controllably by a harmonic law. The sub-sequent evolution of such perturbations has been examined. It has been shown that there are three characteristic regions in the displacement process: stable, metastable (where perturbation can either disappear or grow), and unstable. It has been found that the width of the metastable region increases with the amplitude of perturbation. The results quantitatively confirm the theoretical values of the binodal and spinodal (boundaries of the metastable region) previously predicted for this morphological transition.