Nanofiltration Membrane 2D: Pore-Scale Diffusion & Rejection

This is a 2D cross-section companion to the 3D nanofiltration membrane simulator, built on genuinely distinct 2D mechanics rather than a flattened camera view. Solute particles — a neutral glucose-size molecule, Na⁺, Cl⁻ and SO₄²⁻ — undergo real Brownian dynamics (a Stokes–Einstein diffusion coefficient that shrinks with solute radius) plus convective drift from the applied pressure, so each one must actually random-walk through the feed channel until it stumbles into one of a handful of real pore gaps in the membrane line; particles that hit the solid membrane between gaps simply bounce off. Only at a pore mouth does the same Ferry steric-sieving and Donnan charge-exclusion test from the Donnan-Steric Pore Model decide whether the particle is admitted. Adjust pore radius, membrane fixed charge and applied pressure to see size- and charge-selective rejection emerge from first-principles 2D transport.