*2.4. Numerical Simulation*

The numerical analysis software COMSOL multiphysics5.5 was used for simulation analysis in this study. The laminar flow module and particle trajectory tracking module in the software were used to simulate the separation process. The particle trajectory tracking module couples the effect of multiple force fields on the particle trajectory, calculating the trajectory of particles in the channel.

First, a 2D sketch of the microfluidic chip was drawn by AutoCAD2021 and then imported into COMSOL multiphysics5.5. The model needed to be meshed before the analysis, and the meshing was finer to achieve a better simulation. According to the actual situation of the subject, the Reynolds number did not exceed 1000, so the fluid was set as laminar flow. The Reynolds numbers in collection area 1 and 2 were 173 and 346, respectively. The particle density was 1.05 g cm−<sup>3</sup> and the aerodynamic diameter was 0.5~8 μm. The inlet flow rate was 12.5 mL min<sup>−</sup>1, and the wall was set to be slipproof. During the simulation, the collisions between the particles and the walls of the microfluidic device were inelastic, and the particles stuck to the walls to calculate the particle collection rate. Particle tracking uses Newton's law of motion to solve differential equations. The research object of this topic is the movement of spores in the air. The spores were mainly driven by drag force in the microfluidic chip. The particle tracking module was used to simulate the particle trajectory. The drag force satisfies Stokes' law, and the temperature and absolute pressure are standard states. Then, the simulation boundary was, the corresponding entrance and exit were set, and 100 particles were released at the entrance each time to ensure the reliability of particle collection efficiency. Since the simulation of this study needs to study the laminar flow and particle motion state in the microfluidic chip, transient and steady-state solvers were configured. The steady-state solver was used to study laminar flow, and the transient solver was used to study the particle. In the case of motion, the steady-state calculation amount is small, and one can choose to solve it directly. The solver can choose PARDISO to meet the requirements, and then choose the multi-threaded nested analysis pre-sorting algorithm, and the configuration of the steady-state solver is completed. The transient solver is more complicated to solve, and it needs to be solved iteratively. The GMRES solver was selected to solve the trajectory of the particle.
