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. This 2D view is a vertical cross-section through the same twin crucibles as the 3D model: one on Earth, one in orbit.
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) exactly as real spherical droplets would, even though this view renders them as flat circles.
- 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.