Saffman-Taylor instability (1958): When a less viscous fluid (e.g., air) invades a more viscous one (e.g., oil) in a Hele-Shaw cell,
the interface is unstable — small bumps grow into fingers. Growth ∝ ∇²φ = 0 (Laplace equation) → fractal dimension D_f ≈ 1.71 (DLA universality class).
Surface tension σ regularizes the fingertip singularity — larger σ → wider, fewer fingers.