MATLAB Microfluidic Path Correction

I already have a working MATLAB model that generates particles, defines the micro-channel geometry, and steps the simulation forward, but the trajectories are drifting away from the intended channel centrelines. Your task is to bring those paths back into proper alignment with the device.

What I need
• Correct the alignment of particle trajectories by adjusting the entry points and modifying particle flow direction within the code.
• Tackle the root cause: the current channel geometry parameters are influencing the solver in a way that deflects particles, so the fix must happen where those geometric terms are fed into the velocity field or boundary conditions.
• Return a clean, well-commented .m file (or set of functions) that reproduces the corrected behaviour, plus a brief note that highlights which sections were changed and why.

Acceptance check
When I run the updated script on my existing test cases, every particle should remain centred in its designated micro-channel lane throughout the full simulation length with no unexplained displacements.

Tools in play
Please work entirely in MATLAB; no external CFD suites are involved. If you want to prototype in Simulink or use additional MATLAB toolboxes, that is fine as long as the final deliverable remains executable as plain MATLAB code on R2021a.