The effect of rotational shear on viscous fingering in miscible fluids.

When a lower viscosity fluid invades a more viscous one in a small channel, the fluid flow can become unstable and form fingers: the classic viscous fingering instability. In our experiments, we use miscible fluids in a radial Hele-Shaw geometry and study the effect of applying oscillatory rotational shear to the fluid by rotating one of the Hele-Shaw plates with respect to the other. The application of the shear changes the structure of the interface between the two fluids along the direction spanned by the small gap. We find that the rotational shear suppresses the onset of the instability as well as changes the width of the fingers once they grow. We study the effects of shear frequency, shear amplitude, and the fluid viscosity ratio on the instability onset and growth dynamics.