To infinity, and beyond the normal capillary-viscous behaviors in liquid hydrogen

Liquid hydrogen (LH₂) is a common type of fuel used for space travel. For short trips into space, storing LH₂ is manageable, but for longer trips, it gets complicated: Without Earth’s gravity, the different phases of hydrogen don’t separate neatly, and ingesting hydrogen gas is detrimental to engines.

One solution to this problem is a liquid acquisition device (LAD), which is a completely passive system that relies on capillary action. It separates the liquid and gas phases, drives fuel towards the engine, and keeps out vapors.

Zheng et al. explored the wicking capabilities of LH₂ to ultimately improve the efficacy of LADs in processing fuel.

“The reliability of the entire device is ultimately built from what happens inside thousands or millions of individual microscopic passages, and if these processes are simply averaged into an effective porous-medium property, an important part of the design physics is lost,” said author Yonghua Huang.

The researchers set out to understand LH₂ wicking at the pore scale, allowing them to analyze how individual microstructures influence liquid transport and determine why certain geometries perform better than others.

Their models showed LH₂ wicking could not be fully understood through the classical balance between capillary action and viscous resistance alone. The overall process remains capillary-dominated, but because of LH₂’s exceptionally low viscosity, transient inertia becomes important during movement and can locally become comparable to capillary action.

“This finding provides a more complete physical picture of LH₂ wicking and identifies transient inertia as an essential mechanism that is missing from the classical capillary–viscous description,” Huang said.

Their work provides LAD designers an opportunity to consider the pore geometry of the device’s microstructure, rather than evaluating current devices for performance.

“Based on these optimization results, we hope to design and fabricate representative LAD samples and evaluate them experimentally under cryogenic conditions, ultimately including LH₂ tank experiments,” Huang said.

Source: “Theoretical and numerical investigation on capillary wicking dynamics of liquid hydrogen on micro-structured surface,” by Zhaoqi Zheng, Ming Zhu, and Yonghua Huang, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0349159 .