Lagrangian chaos enhances stirring in the alveoli

Stirring in a two-dimensional viscous environment presents challenges due to the simple, predictable, and deterministic nature of streamline topology. Aref (1984) reported that Lagrangian chaos, or chaotic advection, could be a method to induce chaotic pathlines to enhance stirring in these flows. To better understand the fluid kinematics and stirring mechanisms within lung alveoli—where the flow is both viscous and unsteady—we conduct experiments and simulations to analyze whether asymmetric oscillation effectively stirs the fluid, leveraging the principles of Lagrangian chaos. Our experimental findings indicate a strong dependence on the asymmetry parameter in disrupting reversible flow characteristics, which leads to fluid stirring. However, this stirring does not become chaotic unless the asymmetry parameter and oscillation frequency are optimally tuned. Through numerical simulations, we systematically demonstrate the coexistence of "Moffatt eddies" and an open streamline topology, along with their periodic switching, which facilitates the stretch-fold mechanism leading to fluid stirring. We also quantitatively assess this stirring using Finite Time Lyapunov exponent calculations, identifying a region of chaotic stirring within the parameter space defined by the asymmetry parameter and oscillation frequency.