Kruse-Joanny active gel theory · spontaneous flows · contractile instability
Active gel theory (Kruse, Joanny, Jülicher) describes the cytoskeleton as a polar active fluid. Myosin motors generate contractile active stress σ_active = −ζΔμ·δ (negative = contractile). The 1D momentum equation γv = η∂²v/∂x² + ζΔμ·∂c/∂x drives spontaneous flows when active stress exceeds the instability threshold. Actin density c(x) advects and polymerizes/depolymerizes with turnover rate k. Above ζΔμ_c the uniform state is unstable and density waves form.