Metal foam is cast by nucleating gas bubbles (a foaming agent such as TiH₂, or injected gas) inside a molten metal that is 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:
v = (2/9) · Δρ · g · r² / η
Δρ = density difference (liquid metal − gas)
g = local gravitational acceleration
r = bubble radius
η = 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), the buoyancy term in Stokes' law nearly vanishes. Bubbles barely drift, drainage and convection are suppressed, and far fewer escape before the solidification front locks them in place. The result — confirmed by ESA/NASA foam-casting experiments such as Foamcast — is a foam with a much more uniform, isotropic closed-cell structure at a given porosity, which translates directly into more predictable strength and stiffness for structural lightweight components.
- Gravity slider — scales buoyant rise speed and convective drift from 0 (orbital microgravity) to 1 g (Earth casting).
- Gas injection rate — how many new bubbles nucleate per second inside the still-liquid melt.
- Melt viscosity — higher η slows Stokes rise and coalescence (thicker alloys 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.
- Uniformity CV — coefficient of variation of bubble volume fraction across four horizontal layers of the frozen foam; 0% would be a perfectly uniform foam.