Active Gel — Actomyosin

Active gels model the actin–myosin cortex: a polar, out-of-equilibrium fluid where ATP-driven myosin motors generate internal stresses. The order parameter is the local actin density ρ(x,y,t). The simplified model: ∂ρ/∂t = D∇²ρ − ∇·(ζρ²∇ρ) + η, where ζ is the active contractility (motors pulling actin toward high-density regions). Above a critical ζ, the homogeneous state is unstable — actin condenses into clusters, rings, and cortical waves seen in dividing cells. This active-matter instability is distinct from passive phase separation; energy is continuously injected by ATP hydrolysis.