Capillary Structures for Exploration Life Support (Capillary Structures) - 05.10.17

Overview | Description | Applications | Operations | Results | Publications | Imagery

ISS Science for Everyone

Science Objectives for Everyone
Current life-support systems on the International Space Station require special equipment to separate liquids and gases, including rotating or moving devices that could cause contamination if they break or fail. The Capillary Structures for Exploration Life Support (Capillary Structures) investigation studies a new method using structures of specific shapes to manage fluid and gas mixtures. The investigation studies water recycling and carbon dioxide removal, benefiting future efforts to design lightweight, more reliable life support systems for future space missions.
Science Results for Everyone
Information Pending

The following content was provided by Mark Milton Weislogel, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details

OpNom: Capillary Structures

Principal Investigator(s)
Mark Milton Weislogel, Ph.D., IRPI LLC, Wilsonville, OR, United States

John Graf, Ph.D., Johnson Space Center, Houston, TX, United States
Michael Callahan, Ph.D., NASA Johnson Space Center, Houston, TX, United States

NASA Johnson Space Center, Houston, TX, United States
IRPI LLC, Wilsonville, OR, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
Technology Demonstration Office (TDO)

Research Benefits
Space Exploration, Earth Benefits, Scientific Discovery

ISS Expedition Duration
April 2017 - September 2017

Expeditions Assigned

Previous Missions
Information Pending

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Experiment Description

Research Overview

  • Deep space human exploration missions require highly reliable Life Support Systems that are lightweight and easy to maintain.
  • Life Support Systems typically include processes that involve interaction between liquids and gases and require special treatment than would occur on Earth due to the microgravity environment.
  • Water recovery systems generally incorporate rotating equipment and membranes to separate liquids from gases. These systems tend to be susceptible to contamination which naturally occurs as water is recovered from the waste leaving a concentrated brine.
  • The carbon dioxide removal system currently utilized on the International Space Station (ISS) is a reusable sorbent. To make it microgravity compatible, a solid sorbent was created that is less efficient and less robust than the liquid variant that would more likely be used on Earth.
  • The Capillary Structures for Exploration Life Support (Capillary Structures) experiment explores the use of capillary structures to passively separate gasses and fluids in microgravity. Capillary structures are likely to be more robust and simpler than current systems since they rely on geometry instead of rotation or materials properties to separate liquids and gases.
  • This experiment demonstrates the Capillary Brine Residual in Containment woven cell evaporator. This is a water recovery system that allows water to be evaporated from a “bucket”; the capillary structure makes this “bucket” work in microgravity.
  • This experiment also demonstrates a regenerable Liquid CO2 Sorbent system. This CO2 removal system uses a network of “water falls” to bring a liquid sorbent in contact with air, allowing the CO2 molecules to adsorb onto the liquid. The capillary structures allow for the “water fall” to work in microgravity.


The Capillary Structures for Exploration Life Support (Capillary Structures) payload supports demonstration of advanced life support systems technologies that utilize capillary structures for fluid containment and management, and that cannot be fully demonstrated in terrestrial or short duration microgravity environments. The research gained by this experiment is associated with two life support system technologies: a water recovery technology, and a carbon dioxide removal technology. There are science and technology demonstration components for each of the technologies being evaluated. The science components are designed to validate SEFIT stability models and evaporation performance prediction models.
The water recovery system being evaluated is the Capillary Brine Residual in Containment (CapiBRIC); specifically, the evaporator element. The CapiBRIC is designed to passively recover water from the brine that is left over from the ISS Urine Processor Assembly (UPA). The capillary structure holds the brine and maintains a free evaporative surface over which a sweep gas is flown to carry away the moisture evaporated from the brine. The condensing heat exchanger in the air revitalization system (ARS) condenses the moisture and sends it to a water processor where it is conditioned for use as technical water for other systems, or as potable water for the crew. This experiment focuses on characterizing both containment and evaporation performance of the capillary structure within the CapiBRIC, called the “Capillary Evaporator”. The science component of the Capillary Evaporator evaluates the effect of pore shape, connectivity, depth, and contact line length on stability and drying performance. The technology demonstration component uses the Capillary Evaporator engineering article to demonstrate infill, drying, and fluid stability of a matrix of cells that comprises a portion of the full scale CapiBRIC evaporator. A non-toxic ersatz is used to mimic the characteristics of the ISS wastewater brine that most impact fluid flow and containment.
The carbon dioxide removal system evaluated in this experiment is the Capillary Liquid CO2 Sorbent System, designed to remove CO2 from air using a liquid sorbent, and to regenerate the sorbent. Capillary structures are used to form a direct liquid-air interface at the contactors, and to move fluid across these contactors so that it might be passed on to the next process. This experiment focuses on evaluating flow across manifolded capillary channels as a function of total system liquid volume, and motive force (i.e. pump speed). The science component of the Capillary Sorbent experiment evaluates distribution of flow across channels and varied levels of bifurcation, and the flow profile across the channel given varying pump speeds. The technology demonstration components include demonstration of flow of a fluid of similar viscosity to liquid sorbent across parallel channels, and demonstration of two simple contactors in series as proof of concept of the microgravity regenerable liquid sorbent system.

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Space Applications
Water recovery and air purification systems are the most important elements of a crewed spacecraft, but they are also among the heaviest and most complex. Future space missions that aim to send humans deeper into the solar system require lightweight, simple life support technology. Capillary Structures studies a passive system that can separate gases and fluids in microgravity, which would be impossible to test on Earth because of the influence of gravity. The investigation demonstrates a system that allows water to be evaporated from a bucket-like container, and a regenerating system that collects carbon dioxide so it can be removed from cabin air. The systems use capillary action, which use surface tension to contain fluids, and fluid dynamics to move liquids and gases around.

Earth Applications
Capillary Structures tests the use of capillary action to separate and move fluids and gases, whose function in microgravity would be impossible to test on Earth. Similar technology could be used in water recovery systems, desalination plants and other facilities on Earth. Capillary systems can be simpler than current water-purification and air-cleaning systems, because they rely on specific geometric shapes and fluid dynamics rather than complex machinery.

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Operational Requirements and Protocols
Four experiments comprise the minimum science objectives for this payload: 1) Evaluation of containment and drying performance in various macro-scale pore geometries. 2) Evaluation of flow bifurcation and consolidation across open capillary channels. 3) Demonstration of a subscale capillary evaporator. 4) Demonstration of a series liquid sorbent contactor system. Each experiment requires a crew tended component performed on the Maintenance Work Area. For each experiment, the crew fill the test articles with a working fluid ersatz. An HD Video camera is used to capture the capillary fluidics interactions. A still camera on interval setting is used to capture untended drying of the Capillary Evaporator test articles.

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Decadal Survey Recommendations

Information Pending

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Results/More Information

Information Pending

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Related Websites

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