How do reactive boundaries affect the transport of gyrotactic microorganisms in oscillatory channel flow? Insights from dual Lagrangian–Eulerian modeling

How reactive boundaries affect the transport of gyrotactic microorganisms in oscillatory flows is a biologically, environmentally, and technologically significant question. Many motile aquatic species, such as Chlamydomonas reinhardtii, swim upward via gyrotaxis and interact with boundaries that partially absorb, trap, or inactivate cells, which are behaviors relevant to photobioreactors, mucosal tissues, and oceanic interfaces. To investigate these effects, we study the dispersion of microorganisms in an oscillatory vertical channel flow, with reactive boundaries modeled using Robin-type conditions. We use a dual modeling approach: a continuum (Eulerian) model describes population-level concentration dynamics, while a stochastic (Lagrangian) model based on Langevin dynamics tracks individual swimmer trajectories under shear, gravity, and boundary interactions. We explore how wall reactivity, oscillation frequency, and swimming parameters influence accumulation, dispersion, and net flux. This study provides mechanistic insight into biological systems such as gut microbiota transport, mucus clearance in airways, and reproductive fluid transport including sperm migration, environmental flows such as algal migration in ocean layers and biogeochemical transport in tidal mixing, and engineered microsystems such as photobioreactors and biofunctionalized microchannels, where boundary reactivity significantly affects microorganism dispersion and accumulation.