In COMSOL, when using Particle Tracing coupled with Laminar Flow, ensuring size-based particle behavior requires careful consideration of both the physics governing particle interactions and how particle sizes are represented within the simulation. Here are some steps to ensure size-based segregation:

1. Enable Particle-Wall Interactions:

• Ensure that particle-wall interactions account for particle size. By default, particles in COMSOL may behave as massless points unless explicitly configured.
• Go to the Particle Tracing Module > Wall Boundary Condition, and set the interaction type: Use Bounce or Stick to simulate realistic particle-wall interactions. For size-based effects, choose Freeze or Stop if the particle diameter exceeds the gap size.

2. Define Particle Size Explicitly:

• In the Particle Properties, assign a finite size (diameter or radius) to your particles.
• Use the Drag Force and enable size-dependent effects. For example: Define the drag force as proportional to the particle size using Stokes' Drag or Custom Drag Laws.
• For dense flows or size-based collisions, enable Particle Collision Modeling (if applicable).

3. Account for Particle-Fluid Interaction:

• Ensure Particle-Fluid Coupling is active (if applicable), especially for finite-sized particles.
• Include effects like pressure gradients and viscous drag on particles that depend on size.

4. Introduce Size-Based Boundary Conditions:

• Use a Conditional Expression to impose restrictions based on particle size. For example: This ensures particles larger than the gap are stopped. Use a boundary condition such as: arduinoCopy codeif(diameter > gap_width, stop, continue)

5. Check Streamline-Based Motion:

• In Streamline-based particle tracing, particle motion depends solely on the flow field and may ignore size effects. Switch to Newtonian or Lagrangian formulations to enable size and mass-dependent motion.

6. Refine Geometry and Meshing:

• Ensure the gap geometry is sufficiently resolved to reflect physical constraints accurately.
• If particle size approaches the gap size, refine the mesh near the gap to capture size-dependent dynamics effectively.

7. Test and Validate:

• Start with simple test cases to validate size-based behavior (e.g., particles of various sizes interacting with a small gap).
• Visualize particle trajectories to ensure size effects are being modeled correctly.

Additional Tips:

• If computational cost allows, consider enabling Two-Way Coupling to allow particles to influence the flow field dynamically.
• For more complex segregation dynamics, consider using Discrete Element Modeling (DEM) in conjunction with fluid flow if available in your COMSOL license.

By explicitly defining particle size and enabling appropriate wall and fluid-particle interactions, you can ensure that the simulation reflects realistic size-based segregation. Let me know if you'd like help configuring specific settings!