Bubble nucleation and growth on microstructured surfaces under microgravity

The study investigates how microgravity affects nucleation and growth of surface bubbles on heated substrates, a process central to heat transfer in technologies such as electronics cooling, refrigeration, nuclear reactors, and metallurgical processes. Prior work largely focused on terrestrial gravity, where gravity-driven thermal convection near the nucleation site significantly influences surface temperature and thus bubble dynamics. In microgravity, buoyancy is negligible and thermal convection is largely suppressed, potentially altering both nucleation and growth. Previous microgravity studies reported non-linear bubble volume growth and stronger Marangoni effects, with altered temperature fields and enhanced growth rates. However, experimental studies in true microgravity remain scarce due to technical and cost challenges, and predictive simulations are difficult because of complex, coupled transport phenomena. This work addresses these gaps via ISS experiments comparing terrestrial and microgravity bubble dynamics and via thermofluidic simulations isolating gravity’s role. It also examines how surface microstructure characteristic length (serving as fin-like geometries) affects nucleation timing when wettability is similar and feature sizes are an order of magnitude smaller than bubble radii.

Classical and modern studies have characterized bubble nucleation, growth, collapse, and associated heat transfer on Earth, including models of inertial growth, pool boiling correlations, and the role of surface topography. Space-based and reduced-gravity studies have observed departures from terrestrial behavior: non-linear growth, altered Marangoni flows, and differences in detachment and coalescence. Micro/nanostructured surfaces can strongly influence nucleation, with characteristic length and wettability dictating site activation and heat transfer. When microstructures are 5–100× smaller than bubble radii, their geometric effects become pronounced. Prior work also distinguishes two growth stages: rapid vaporization-driven (stage I) and slower dissolved-gas–driven (stage II). Existing numerical methods face challenges due to geometric and property simplifications, especially when modeling coupled convection, conduction, and interfacial transport under microgravity.

Experimental: Microstructured copper (Cu) substrates (C1–C4) were fabricated using hydrogen bubble template electrodeposition in H2SO4/CuSO4 electrolytes (H2SO4 at 0.8 M; CuSO4 at 0.2, 0.4, 0.8, 1.0 M). Cu substrates (35 mm diameter, 0.5 mm thickness) were cleaned and used as cathodes; a Cu plate 2 cm away served as anode. Deposition at 1 A·cm−2 for 60 s produced microporous structures whose porosity increased with CuSO4 molarity; substrates were rinsed, dried, and sintered at 710 °C for 30 min in reducing atmosphere to strengthen structures. Characteristic lengths were determined by fitting pore sizes and computing average diameters; surface morphologies were measured via optical profilometry to assess effective surface area and roughness. For boiling experiments, each Cu substrate was epoxied to the inner top wall of a quartz cuvette (10 mm H × 20 mm W × 43.75 mm L; wall thickness 1.25 mm). A 10 mm × 10 mm Peltier heater was mounted externally to the cuvette wall; heat conducted through quartz and epoxy to the Cu substrate. The same heater and placement were used for both terrestrial and ISS experiments to ensure consistency. Each boil used ~4.1 V and ~1.3 A. Videos were acquired at 110 FPS with 2 MP resolution and a ~10° viewing angle; a 1 mm grid was imaged for pixel-to-length calibration. Terrestrial tests oriented the heated Cu surface facing downward to reduce natural convection and prevent buoyancy-driven detachment, matching ISS power constraints and video window; experiments in microgravity were run on the ISS at ambient pressure using Space Tango’s CubeLab. Each condition was repeated three times to confirm nucleation time reproducibility. Simulations: Transient finite element thermofluidic models (2D) included heat conduction and, when gravity was enabled, thermal convection. Geometry comprised a water domain (60 mm × 20 mm) bounded by two 1 mm SiO2 walls and an immersed Cu substrate (5 mm × 0.2 mm) with microstructure represented per C4 characteristic length for nucleation studies; gravity toggled on/off for terrestrial/space cases. Velocity fields, temperature profiles, and maximum surface temperatures were computed over time to assess nucleation conditions (boiling to spinodal range used as reference). Additional simulations modeled microstructure effects by representing C1 and C4 as fin arrays with spacing equal to measured characteristic lengths; gravity off to mimic microgravity; identical heat generation for both. For growth-stage analysis, a 3 mm-radius surface bubble with experimentally informed contact angle was added; interface temperature at the bubble base near the nucleation site was set to 373 K for microgravity runs; the terrestrial case used identical heating power/efficiency with gravity enabled. Interface temperatures were extracted along the arc from top to bottom of the bubble.

• Nucleation acceleration in microgravity: Space bubble nucleated at ~76 s after heating start versus ~161 s on Earth under identical setup (approximately 2× faster nucleation; reduced heating time by about half).
• Growth enhancement: At equal times post-nucleation (150 s), space bubbles were much larger than terrestrial; space bubble volumes reached ~10–20× those on Earth during growth.
• Growth rate amplification: The space bubble volume growth rate increased by ~2 orders of magnitude over the growth period, reaching ~30× the terrestrial growth rate just before collapse; terrestrial growth rate remained relatively steady.
• Collapse behavior: Space bubbles collapsed at ~213 s after nucleation, with subsequent ejection of many smaller bubbles from the nucleation site (evidence of nucleate boiling). Terrestrial bubbles did not collapse within ~600 s of heating.
• Thermal-convective mechanism: Simulations showed maximum substrate surface temperature rises much faster without gravity, exceeding terrestrial by ~50 K after ~15 s of heating. Terrestrial velocity fields exhibited strong convection with |u| ~10^-3 m/s; microgravity flows were ~10^-6 m/s (3 orders lower), indicating conduction-dominated heat transfer in microgravity.
• Elevated local interface temperatures: Growth-stage simulations showed bubble interface temperatures in microgravity up to ~20 K higher than on Earth, with the highest near the bubble base; the local interface temperature in microgravity can approach 373 K by the end of growth, reducing local air solubility drastically and driving the observed high growth rates.
• Stage-II dominance with rising temperature: Space bubble growth does not follow V ∝ t^0.5 (stage I); rather, it is consistent with stage II (dissolved-gas–driven) with increasing local temperature T and decreasing local solubility Cs, leading to non-linear acceleration of growth.
• Microstructure effect on nucleation time: In microgravity, finer microstructures (denser fins, higher specific surface area) enhanced heat transfer to the liquid, cooling the surface and delaying nucleation. Observed nucleation times: C1 ~288 s, C2 ~272 s, C3 ~241 s, C4 ~210 s (with reduced heating power to magnify differences). Simulations corroborated slightly longer time to reach nucleation temperature for fine (C1) versus coarse (C4) fins.
• Reproducibility: Two ISS missions (SpaceX CRS-22 and Northrop Grumman NG-17) produced consistent nucleation times for similar substrates (e.g., ~70–76 s for C4).

The work demonstrates that suppressing gravity eliminates buoyancy-driven thermal convection near the heated substrate, localizing heat and accelerating substrate surface temperature rise. This leads to earlier attainment of nucleation temperature and faster nucleation in microgravity. During growth, the elevated local temperature around the bubble base in microgravity reduces dissolved gas solubility, enhancing stage-II gas exsolution and causing non-linear acceleration of bubble volume growth. The higher interface temperatures and lower solubilities explain the observed ~30× higher growth rates and larger volumes prior to collapse. Conversely, on Earth, convection (and Marangoni effects) efficiently removes heat, maintaining a relatively constant boundary-layer temperature and a nearly constant growth rate. Surface microstructures act as heat-transfer fins; finer, denser fins enhance conduction into the liquid and more effectively cool the substrate, thereby delaying nucleation. Collectively, these findings clarify the thermofluidic mechanisms by which microgravity modifies bubble dynamics and suggest strategies for engineered heat transfer in space systems and for bubble-based sensing where controlled nucleation and growth are desirable.

Experiments aboard the ISS, combined with thermofluidic simulations, show that microgravity markedly accelerates bubble nucleation and growth by suppressing convection, raising local temperatures at the nucleation site and bubble base, and reducing gas solubility. Space bubbles nucleate in about half the time, grow to volumes 10–20× larger, and achieve growth rates up to ~30× greater than terrestrial cases before collapsing. Surface microstructures modulate nucleation timing: finer, higher-area fins improve heat transfer to the liquid and delay nucleation relative to coarser structures. These insights provide guidelines for thermal management in microgravity (e.g., phase-change cooling) and development of bubble-based sensing technologies. Future work could quantify local temperature fields and solubility near interfaces in situ, refine 3D simulations with resolved microstructure geometries and interfacial physics, explore a broader design space of fin geometries and wettabilities, and extend to different fluids and pressures relevant to space systems.

• Experimental constraints on the ISS limited heating power and required standardized hardware; terrestrial tests used a downward-facing heated surface to mitigate convection and buoyancy-driven detachment within recording windows.
• Camera temporal resolution (~9 ms) precluded resolving the very brief stage-I explosive growth.
• Simulations used 2D geometries and simplified representations of microstructures and material properties; exact surface morphologies and 3D effects were not fully captured, affecting absolute nucleation times though trends matched experiments.
• Exact nucleation temperatures and local interfacial temperatures/solubilities could not be directly measured; growth-rate-based back-calculation was not possible due to unknowns such as precise heating efficiency and local solubility.
• Ambient-pressure ISS conditions were used; results may differ under other pressures or fluids relevant to space systems.