Role of voids in taking humans to the bottom of the sea – 1960 vs. 2012 a vehicle perspective  

It seems hard to believe that in this age of ever-expanding technology that 52 years elapsed between humans’ journey to the deepest depths of the ocean.  A total of two vehicles have carried three people to the very bottom of the ocean; a place called Challenger Deep of the Marianas Trench off the coast of Guam. In comparison, during these intervening years, 12 men propelled by 6 spacecraft have visited the Moon, 133 successful Space Shuttle missions took place, Skylab, Mir and now a permanently occupied International Space Station is orbiting the Earth.

Buoyancy has played a huge part in man’s ability to visit and explore the deepest parts of the ocean. This story is about the evolution of this buoyancy and the step change that occurred in mid 1960s that allowed for improved design of subsea vehicles. This is a story about the transition from gasoline to syntactic foams as buoyancy for deep sea applications.

The end of gasoline

We will start the story in the early 1960s where the use gasoline buoyancy was coming to an end, but it did so in a historic fashion. On January 26, 1960 a human occupied vehicle (HOV) called Trieste descended to the farthest depths of the ocean a place called Challenger Deep, the deepest part of the Marianas Trench, see Fig 1. Two men, Jacques Piccard and Don Walsh, took the bathyscaphe down 11 km to the very bottom of the ocean. It stayed there for only 20 minutes. It wasn’t until 52 years later that another human would visit.

Trieste was basically a blimp or dirigible designed for the water. Buoyancy for the Trieste was provided by 128,700 liters (34,000 gallons) of gasoline along with iron shot or pellets.   Because gasoline is compressible, Treiste would lose buoyancy as it descended. The iron shot would slowly empty from ballast hoppers to compensate for the loss of buoyancy. Once at the bottom the iron pellets would empty from the ballast hoppers at a faster rate for assent back to the surface. Although Trieste II was built 1964, it was clear that the end was coming for this type of buoyancy

Hollow microspheres and syntactic foams

Displacing the Trieste bathyscaphe (a complicated buoyancy system) was the Alvin class submersibles which used a new type of buoyancy agent, a syntactic foam. A syntactic foam is a composite foam consisting of a hollow filler material dispersed in a binder phase. Meant for deep sea applications, this hollow phase in the syntactic foam was in the form of hollow glass microspheres (HGMS) or glass microballoons (GMB), see Fig 2 and the binder phase was an epoxy matrix. This GMB average diameter is 50-75 μm, the thickness of a human hair.

The emergence of syntactic foam as a buoyancy material is inextricably linked to the large scale manufacturing of GMBs. The two men responsible for the industrial scale production of GMBs were Cherry L. Emerson and William R..Cuming. They founded Emerson & Cuming in 1948 near Boston MA. Starting in the 1950s they produced GMBs for specialty electronic, thermal and aerospace applications. With the launch of Alvin in 1964 the deep sea applications of GMBs and syntactic foams quickly expanded. Today these materials are the mainstay for deep sea buoyancy for offshore oil and gas, scientific exploration, and naval applications.

In the 1960s the buoyancy on the Alvin-class submersibles did not have the depth capability of Trieste, however, they were much more maneuverable and amenable for search-and-recovery and scientific exploration. In the following decades, the depth capability of syntactic foams have improved to the point that syntactic foams can now reach ocean bottom while providing more buoyancy than gasoline. The second HOV to reach Challenger Deep made significant use of syntactic foams, not only for buoyancy but as a structural material. James Cameron in his vehicle, Deepsea Challenger made the 11km descent on March 12, 2012. He spent total of 3 hours at 11km. About 70 percent by volume of Deepsea Challenger was GMB/epoxy syntactic foam, see Fig 3:

Design of syntactic foams and reinforced voids

The greatest challenge, of course, operating in the deepest regions of earth’s oceans known as the hadal zone, is the large hydrostatic pressure from the seawater. At 11 km, the hydrostatic pressure is around 116 MPa (1140 atm). Compare this to the pressure at sea level of only 0.101 MPa (1 atm), which is the pressure most systems on earth are required to function in. Any component must either be strong enough to withstand the immense hydrostatic pressure, or be inside a protective enclosure (pressure vessel/housing) at 0.101 MPa – which means this enclosure must withstand the pressure.

Syntactic foam contains reinforced voids and are the key to creating a light but strong foam capable of withstanding the pressures experienced at these ocean depths. These are significantly different than common single-phase foams such as those found in furniture cushions and insulated coolers. The design of the rigid syntactic foams for buoyancy requires balancing of competing design crite­ria, minimizing density of the of the foam and maximizing resistance to hydrostatic pressure at ocean depth. The strength of the syntactic foam is derived from three primary sources; the GMBs, resin and interface between them. In a hydrostatic environment, the strength of the GMB will increase as the ratio of the wall thickness to diameter (t/d) increases. But as t/d increases so does the density, thus reducing buoyancy. The strength of the binder phase as well as the interface needs to be considered as they both reinforce the strength of the GMB. In the case of Deepsea Challenger submersible where the syntactic foam is structural and needs to handle more complex loading than just hydrostatic pressure, a more detailed analysis of the strains was needed to design a appropriate syntactic foam.

Also, the lifetime performance of any buoyancy will depend on the pressure cycle(s) it experiences. Will syntactic foams be continuously cycled as with an Alvin type submersible; descending and ascending hundreds of times a year? Or will it be at a single depth for a decade or more as are many applications in offshore oil and gas production? Physical aging (free volume), water ingress, and poly­mer and glass degradation are other factors that need to be considered.

The design of voids as well knowledge of intrinsic voids is essential to the proper functionality of deep sea buoyancy. In this book, Voids in Materials: From Unavoidable Defects to Designed Cellular Materials, we provide details on the design, processing, characterization, and functionality derived from atomic scale defects up through macroscale porosity. The basic premise of the book is that,at some level, all solids contain voids, whether intrinsic or intentional. Sometimes the voids are ignored, at other times they are taken into account, and other times they are the focal point of the research. In this book, we take due notice of all of these occurrences of voids, whether designed or unavoidable defects. We define these voids (or empty spaces in materials), categorize them, characterize them, and describe the effect they have on material properties.

Gladysz and Chawla’s upcoming book Voids in Materials: From Unavoidable Defects to Designed Cellular Materials is now available for pre-0rder on the Elsevier Store. Use discount code “STC3014” and save up to 30% on your very own copy!

About the Authors

Dr. Gary Gladysz (Twitter: @GMGladysz) is an Associate at Empyreus Solutions, LLC, Seattle WA, USA, where he consults and leads university and government technical interactions. He received his PhD from the New Mexico Institute of Mining & Technology where he participated in the NATO Collaborative Program with the German Aerospace Institute (DLR).   Since receiving his PhD, he has led research efforts in university, government and industrial settings. He has extensive research experience designing and characterizing fibrous composite materials, ceramic composites, polymers, composite foams, and thin films.

As a Technical Staff Member at Los Alamos National Laboratory (LANL), he was technical lead for Rigid Composites and Thermoset Materials. In 2005 he was awarded the LANL Distinguished Performance Group Award for his work leading materials development on the Reliable Replacement Warhead Feasibility Project. He has served on funding review boards for LANL, National Science Foundation, ACS, and the Lindbergh Foundation. He has been guest editor on four issues of leading materials science journals, including Journal of Materials Science and Materials Science & Engineering: A. Dr. Gladysz has organized five international conferences/symposia on syntactic foams and composite materials. He started and currently chairs the ECI international conference series on Syntactic & Composites Foams. Dr. Gladysz currently lives near Boston, Massachusetts, USA.

Krishan Chawla is Professor Emeritus of Materials Science and Engineering at the University of Alabama at Birmingham, USA. He received his Ph.D. from the University of Illinois at Urbana-Champaign. His research interests encompass processing, microstructure, and mechanical behavior of materials. He has taught and/or done research at several universities around the world. Professor Chawla has served as a Program Director for metals and ceramics in the Division of Materials Research, National Science Foundation.

He is a fellow of ASM International. Among his other awards are: Distinguished Researcher Award at New Mexico Tech, Distinguished Alumnus Award from Banaras Hindu University, Eshbach Society Distinguished Visiting Scholar award at Northwestern University, Faculty Fellow award at Oak Ridge National Laboratory. Professor Chawla is the author or coauthor of various textbooks in the area of materials and serves on the editorial boards of a number of journals. He is editor of the journal International Materials Reviews.