Bubble nucleation and growth on microstructured surfaces under microgravity

The dynamics of surface bubble nucleation and growth are crucial for understanding heat transfer in various technologies, including electronics cooling, refrigeration, and nuclear reactors. While extensively studied under terrestrial gravity, the impact of microgravity on these processes remains less understood. Gravity significantly influences heat transfer through thermal convective flow, where hotter, less dense fluid rises, dissipating heat from the substrate. In microgravity, this effect is greatly diminished, leading to potentially different bubble dynamics. Previous microgravity studies have observed non-linear bubble volume growth and highlighted the increased significance of the Marangoni effect. However, a detailed comparison of bubble dynamics in microgravity and terrestrial environments, especially considering the influence of surface microstructures, is lacking. This research aims to address this gap by conducting experiments on the International Space Station (ISS) to study bubble nucleation and growth on microstructured surfaces under microgravity and comparing the results with terrestrial experiments. The study also investigates how surface microstructure geometries influence bubble nucleation, considering their role in heat transfer and cooling.

Extensive research on surface bubble dynamics under terrestrial gravity exists, focusing on models describing bubble growth and collapse. Studies have explored various aspects, including the influence of surface roughness, wettability, and heating conditions. However, research on bubble dynamics in microgravity is comparatively limited. Some preliminary experimental observations in microgravity have reported non-linear bubble volume growth, differing from the linear growth observed on Earth. Investigations into the Marangoni flow's influence on boiling heat transfer in microgravity have also been conducted. Prior work also highlighted the impact of nano/micro-structures on bubble nucleation, showing that microstructure geometry and surface wettability affect nucleation time. However, a comprehensive understanding of the interplay between microgravity, surface microstructure, and bubble dynamics remains lacking. The complexity of accurately predicting bubble dynamics using numerical methods due to limitations in model geometries and computational constraints is also acknowledged.

The study used microstructured copper (Cu) substrates fabricated using a hydrogen bubble template electrodeposition method. By varying the CuSO₄ molarity, substrates with different porosities and characteristic microstructure lengths (100–500 µm) were created. Experiments were conducted both on Earth and on the ISS in a CubeLab instrument. The experimental setup involved immersing the Cu substrate in deionized water within a quartz cuvette and using a Peltier heater to generate surface bubbles. High-speed videography recorded bubble nucleation and growth. Thermofluidic simulations using the finite element method were performed to model bubble nucleation and growth processes under both terrestrial and microgravity conditions. The simulations considered fluid flow, heat conduction, and thermal convection, with gravity effects toggled on or off to represent the two environments. The 2D model simplified the heating substrate for computational feasibility. The characteristic length of the microstructures on the Cu substrates were measured using optical profilometry to determine the average diameter of the micropores. Bubble volume was determined from video recordings by comparing the pixel size to a calibrated scale bar. The heating power, cuvette setup, and water conditions were kept identical for both terrestrial and space experiments to isolate the effect of gravity.

Experiments on the ISS demonstrated significantly faster bubble nucleation and growth rates in microgravity compared to Earth. Bubble nucleation occurred approximately twice as fast in space. Bubble growth rates were significantly enhanced in microgravity, reaching up to 30 times faster than on Earth before the bubble collapsed. The thermofluidic simulations confirmed the role of gravity-induced thermal convective flow in terrestrial conditions, which dissipates heat from the substrate and slows down temperature rise. In microgravity, this convective flow was significantly reduced, leading to localized heat and a much faster temperature increase on the substrate surface. This explains the faster nucleation and growth rates observed in microgravity. The simulations also reproduced the experimental trend of faster nucleation in microgravity. The study also explored the influence of surface microstructure geometry. Experiments using substrates with different characteristic lengths demonstrated that finer microstructures, with larger specific surface areas, lead to slower bubble nucleation due to enhanced heat transfer. The simulations showed that finer microstructures acted as heat-transfer fins, enhancing the dissipation of heat and thus reducing the rate of temperature increase, which explained the slower nucleation rates in those cases. Analysis of bubble growth showed that in microgravity, the local temperature around the bubble interface kept increasing, leading to a nonlinear growth pattern and much larger bubble volumes. The simulations further supported the observation of higher bubble interface temperatures in microgravity due to the lack of convective cooling.

The findings highlight the significant impact of microgravity on surface bubble dynamics. The absence of thermal convective flow in microgravity leads to localized heating, drastically accelerating both nucleation and growth rates. This has significant implications for thermal management in space applications, where efficient heat dissipation is crucial. The influence of surface microstructures on nucleation time provides further insights into designing surfaces for optimized bubble dynamics. The faster bubble growth in microgravity, potentially reaching near-boiling temperatures at the interface before bubble collapse, suggests a different growth mechanism than on Earth. The study contributes to a better understanding of bubble dynamics in microgravity, which may help improve the design of phase change cooling systems and develop more effective bubble-based sensing technologies. The observed bubble collapse in microgravity, along with the generation of smaller bubbles at the nucleation site, indicates a transition to nucleate boiling, suggesting an influence of the significantly elevated temperature.

This study provides a systematic investigation of surface bubble nucleation and growth on Earth and in space. Microgravity significantly accelerates bubble nucleation and growth due to reduced convective heat dissipation, resulting in localized heating and faster temperature increases at the substrate. Surface microstructure geometry also plays a crucial role, with finer structures leading to slower nucleation. These findings have significant implications for thermal management and bubble-based sensing technologies in microgravity environments. Future research could explore the impact of other parameters, such as dissolved gas concentration and surface wettability, and further investigate the mechanisms underlying bubble collapse in microgravity.

The study uses a simplified 2D model for the thermofluidic simulations, which may not fully capture the complexities of the 3D bubble dynamics. The exact nucleation temperature was not experimentally determined, affecting the accuracy of the simulated nucleation times. The limited heating power available on the ISS could have influenced the observed bubble behavior. The surface morphology of the microstructured substrates may have introduced some variability in the results, although the experiments were repeated to ensure reproducibility.