Fully resolved simulations of rigid particle focusing in serpentine microfluidic devices

In recent years, lab-on-chip technology has become a particularly interesting topic in the field of medical diagnostics. In this context, the ability of such devices to sort particles at the micrometric scale without any external forces is quite intriguing. Due to their small length scale, they operate with flows at low Reynolds numbers, typically in the range 1–100. In this flow regime, the particle dynamics is affected by inertial effects, including wall-lift force and shear-gradient lift force. Secondary flow can arise in curved channels due to radial pressure distribution. This study focuses on rigid particle transport in microfluidic devices, employing a computational framework to better understand the complex interplay between channel geometry, cross section, particle characteristics, and flow conditions. The flow solver is based on a lattice Boltzmann method and includes a fluid structure interaction technique. The particle rigid motion is evaluated starting from the computation of the hydrodynamic forces acting on the particle and repulsive forces taking into account any particle-particle or wall-particle interactions. Kinematic and dynamic boundary conditions are imposed at the fluid-solid interface by means of an immersed-boundary treatment. The model is validated against benchmark data: a single sphere settling under gravity, and rotations of spheroidal particles in a three-dimensional Couette flow at low-to-moderate Reynolds number. Finally, simulations of particle transport in serpentine channels with 10 repeating units have been carried out for two particle sizes at different flow conditions, demonstrating the effectiveness of the solver for particle-focusing applications within serpentine microdevices. To the authors' knowledge, the results reported in this paper are the first obtained with fully resolved simulations of fluid-structure interaction applications in microchannels with complex realistic geometry.