The cell cortex is an active gel: actin filaments crosslinked by myosin motors that consume ATP to generate contractile stress. This drives cell shape changes, cortical flows, and self-organization of the cytoskeleton.
Actin-Myosin Network Simulation
Stress & Order Parameter
Active stress: σ_a = ζΔμ (motor activity × ATP free energy)
Active gel theory (Kruse–Joanny–Jülicher 2005): the polarity field p⃗ satisfies ∂ₜp + (v·∇)p = Ωp + h/γ where h is the molecular field. Contractile instability (ζ > ζ_c) leads to spontaneous flow and aster formation.