Studying Ciliary Metachronal Synchronization in Thin Films Using a Hydrodynamic Framework 

The lung surface is carpeted with microscopic, actively beating hairs, cilia, that generate collective fluid flows to drive mucus transport and maintain airway function. Their coordination, known as metachronal synchronization, is crucial for efficient mucociliary clearance. While existing models have explored hydrodynamic coupling in simple or unbounded fluids, the effects of a layered fluid medium with realistic boundary conditions reflecting the thin mucus and periciliary layers of the airways remain poorly understood. To address this, we developed a hybrid hydrodynamic framework that models cilia tips as Stokeslets with a distance-dependent interaction kernel. For cilia separated by distances smaller than the fluid layer thickness, we use the classical unbounded Stokeslet kernel to capture local interactions. For more distant cilia, we employ a Green’s function that incorporates the layered fluid structure, accounting for a no-slip boundary at the cell surface and a no-shear boundary at the mucus–air interface. This multi-kernel approach captures both near- and far-field interactions, bridging the gap between oversimplified unbounded models and computationally intensive full-fluid simulations to quantify hydrodynamic coupling in realistic airway conditions.