Metal foam is cast by nucleating gas bubbles (a foaming agent such as TiH₂, or injected gas) inside a molten metal that is thickened with fine particles and then cooled until it solidifies with the bubbles frozen in place. Each bubble's rise speed comes from Stokes' law for a sphere in a viscous fluid, computed every frame from the real formula:
v = (2/9) · Δρ · g · r² / μ
Δρ = density difference (liquid metal − gas) ≈ 2400 kg/m³
g = local gravitational acceleration
r = bubble radius (this mold: 1.5–4 mm)
μ = melt viscosity
On Earth (g ≈ 9.8 m/s²) this buoyant rise is fast: bubbles drift upward, collide and coalesce into larger, unevenly sized cells, and many burst at the free surface before the melt solidifies — a process called drainage. The result is a foam with a coarse, graded pore structure: denser and finer near the bottom, coarser and weaker near the top. Buoyancy-driven convection also stirs the melt, further disrupting uniformity.
In microgravity (≈10⁻⁶ g aboard the ISS), g in the formula above is essentially zero, so the buoyant rise term drops to zero too — bubbles only wander by slow diffusion-like jitter. Drainage and convection are suppressed, far fewer bubbles escape before the solidification front locks them in place, and the frozen pore distribution is far more uniform at every height. Save an Earth run and a microgravity run below to compare the frozen patterns and their uniformity numbers side by side.
- Gravity slider — sets g directly in the Stokes formula, from 0 (orbital microgravity) to 1 g (Earth casting); at 0 the rise velocity is exactly zero.
- Gas injection rate — how many new bubbles nucleate per second inside the still-liquid melt.
- Melt viscosity — higher μ slows Stokes rise and coalescence (thicker, particle-stabilized melts foam more evenly even on Earth).
- Cooling rate — how fast the solidification front climbs from the mold floor, freezing whatever pore structure exists at that height.
- Pore-height variance — statistical variance of the vertical position of every frozen bubble, normalized by the mold height squared and shown as a percentage; a lower value means frozen pores are spread more evenly from floor to top instead of clustering toward one end.