Many useful alloys (Al–In, Al–Bi, Cu–Co "monotectics") are made of two metals that don't dissolve into each other in the liquid state — like oil and water. While molten, they exist as microscopic droplets of one phase suspended in the other. What happens to those droplets before the alloy freezes decides the final microstructure:
Stokes settling velocity:
v = (2/9) · Δρ · g · r² / μ
Δρ = density difference between phases
g = gravitational acceleration
r = droplet radius
μ = melt dynamic viscosity
On Earth, g ≈ 9.8 m/s² drives dense droplets down and light droplets up. They collide and coalesce as they migrate, growing larger (which makes them settle even faster, since v ∝ r²) and coarsening into two separated bands before the melt solidifies — exactly what you don't want in a bearing alloy or dispersion-hardened composite.
In orbital microgravity, the effective g the melt "feels" from residual station drag and vibration is roughly 10⁻⁴–10⁻⁶ of Earth's, so the settling term above collapses to nearly zero. Droplets still drift and collide from thermal (Brownian-like) jitter, but with no directional pull they stay finely and uniformly dispersed all the way to solidification — a microstructure that is essentially impossible to cast on the ground.
- Δρ slider — how different the two metals' densities are; a bigger gap accelerates Earth-side settling.
- Viscosity slider — a thicker melt resists settling more (v ∝ 1/μ).
- Droplet radius slider — sets the starting droplet size; settling scales with r², so this has the strongest effect of all.
- Coalescence — same-phase droplets that touch merge into one larger droplet, conserving total volume (r_new³ = r₁³ + r₂³), mirrored in both crucibles equally.
- Stratification index — normalized vertical separation between the phases' centers of mass (0% = fully mixed, 100% = fully layered).
This is the physical reason orbital manufacturing proposals (ISS materials-science racks, and commercial successors) target immiscible alloys and metal-matrix composites specifically — it is one of the few product classes gravity itself actively prevents from being made well.