Saffman-Taylor instability in radial injection
The Saffman-Taylor instability occurs when a less-viscous fluid displaces a more-viscous one in a Hele-Shaw cell (two closely-spaced plates). A flat interface is unstable: perturbations grow because the pressure gradient in the viscous fluid amplifies protrusions. Surface tension provides a stabilizing cutoff at short wavelengths. The competition selects a characteristic finger width ∼ √B. Simulated here via a diffusion-limited model with noise-seeded finger selection.