This is the 2D companion to the 3D droplet-breakup view: instead of a steady-state morphology, it runs the melt forward through processing time and shows the two real mechanisms by which immiscible-plastic domains coarsen once shear stops actively breaking them up.
Contact coalescence. Couette shear flow (vx = γ̇·y) advects domains at different rates depending on their vertical position, so they drift into contact. Whether a collision actually merges two domains depends on whether the thin melt film trapped between them drains before the next collision separates them — the same film-drainage physics as in real extrusion. A compatibilizer layer at the interface sterically resists that drainage, so the coalescence probability on contact is tied directly to the interfacial tension:
σ(φc) = σ₀ · exp(−k·φc), P(merge | contact) = σ(φc) / σ₀
On merge, area is conserved exactly: r_new = √(r₁² + r₂²).
Ostwald ripening. Even without contact, smaller domains have higher Laplace pressure (Gibbs–Thomson: Δμ ∝ σ/r) and slowly feed material to larger ones through the matrix by diffusion — classic Lifshitz–Slyozov–Wagner (LSW) coarsening, with rate constant proportional to σ. Total dispersed-phase area is conserved at every step; the simulation just redistributes it toward fewer, larger domains.
- Compatibilizer loading — lowers σ exponentially, which suppresses both mechanisms simultaneously and keeps the microstructure fine.
- Shear rate — sets how fast domains drift into contact; higher shear means more coalescence attempts per second.
- Dispersed-phase fraction — sets how much of the 100 μm × 100 μm melt slice starts out as minority-polymer domains (denser field, more contacts).
- Processing time speed — fast-forwards the simulated extruder residence time so coarsening is visible in real time.
Real-world relevance: an uncompatibilized recycled-plastic blend coarsens quickly under both mechanisms, leaving large, weakly-bonded domains that embrittle the part. A compatibilizer keeps the microstructure fine enough — often sub-micron to a few μm — to hold useful mechanical properties, which is exactly why it is the standard lever the plastics-recycling industry pulls.