The research objective of this Interdisciplinary Research (IDR) collaborative award is to explore interdisciplinary parallels between volcanology and thermal spray materials processing, notably the behavior of molten materials at extreme temperatures and pressures. Volcanic magma is subjected to high (700-1200°C) temperatures and pressures approaching 1000 atmospheres. Thermal spray involves impinging molten (1000-2000°C) metallic or ceramic micro-particles at 100-200 m/s onto substrates, where instantaneous pressures can reach 10000 atmospheres underneath the impacting "splat". The collaborative link stems from a recent discovery of highly nanoporous structures on the underside of frozen splats, the hypothesized mechanism of which is bubble nucleation due to rapid depressurization; this is known to occur in magma during volcanic eruption. Accordingly, existing geologic models and controlled thermal spray processing will be used to better understand this phenomenon for a wide range of materials and environmental variables. The approach taken will be to (1) utilize volcanologic modeling techniques and experimental parameter space to explore the capability of thermal spray to produce "geo-inspired" nanoporous thin films and coatings on a useful scale and (2) use thermal spray processing, specifically splat formation, to develop microscale experiments in which the high-temperature properties of volcanic materials may be measured systematically.
If successful, the benefits of this research will include robust design tools to fabricate useful nanoporous surfaces and structures of a wide range of materials using thermal spray, a multi-billion dollar industry with significant infrastructure. This would also introduce a new technological "pull" for thermal spray driving further fundamental studies and innovation. In addition, it would create a reliable tool to address highly debated topics in magma behavior, important for understanding and prediction of eruptions. For outreach, the program will take advantage of the highly visible and dynamic nature of both fields, to facilitate dissemination of complex scientific concepts to undergraduates, under-represented pre-collegiate students, and senior citizens.
The project constitutes an interdisciplinary collaborative study investigating the parallels between thermal spray processing and volcanic eruptions, both of which represent harsh environments. During volcanic eruptions magma is subjected to high temperatures and pressures, whereas thermal spray processing involves impinging molten micro?particles, of similar size as volcanic ash, at high velocities onto substrates producing high instantaneous pressures underneath the impacting particles. This scientific collaboration began with a recent discovery of highly nanoporous structures on the underside of frozen ‘splats’ formed during thermal spray processing, and the recognition that magma during explosive volcanic eruptions,is also highly porous, due to micro-size vesicles. In both cases the mechanism leading to this porosity is rapid depressurization of the molten material, causing rapid nucleation of nano?scale gas bubbles that are subsequently quenched into pores. The goal of the project was twofold: (1) explore the capability of thermal spray processing to produce ‘geo-inspired’ nanoporous thin films and coatings; and (2) to develop microscale experiments in which the high?temperature properties of volcanic materials may be measured systematically. The rationale behind the use of geological materials was their abundance and the fact that they are silicate glasses. This outcomes report focuses on the volcanological aspects of the project, where the initial aim was on characterizing and producing volcanic glass powders that are suitable for thermal spray processing. Early on it was recognized that – contrary to expectation – most of the volcanic glasses produced during explosive volcanic eruptions contain up to several weight percent of dissolved water. It appeared that this water nucleates vapor bubbles during thermal spray processing, but they rapidly grow to larger in size than observed during thermal spray processing of metals. Prior to eruption, magma contains several weight percent of water dissolved within the silicate melt. Upon eruption, pressure and water solubility decrease, resulting in the nucleation of vapor bubbles. Because water is less dense in the vapor phase than when dissolved in the melt, this makes the magma more buoyant and amenable to rapid ascent. It also profoundly increases magma viscosity, thus hindering the growth of bubbles. As the magma ascends rapidly toward the Earth’s surface and pressure decreases, bubbles therefore remain at considerably higher pressure than the surrounding melt. This overpressure ultimately causes the magma to rupture in a brittle manner, a process called fragmentation. The resultant abrupt release of the compressed vapor from within bubbles is the key mechanism that gives rise to explosive volcanic eruptions. The discovery of high water contents in volcanic glasses produced by explosive eruptions was perplexing, given that it was expected that most of the water would have formed bubbles, in order to cause the magma to fragment explosively. Consequently, this provided a strong incentive to better understand the underlying cause for the unexpected high water contents, as well as the implications for explosive eruptions and for thermal spray processing. The ensuing investigation became the major effort for the earth-science part of this project. The outcome of this research is the discovery that the vast majority of the water dissolved within volcanic glasses is not of magmatic origin. Instead, it is meteoric water (also called secondary water), which diffused into the glass after the fragmented magma has cooled and been deposited. A new integrated method was developed to discriminate between magmatic and secondary water, using thermo-gravimetric analysis and numerical modeling. The discrimination between magmatic and meteoric water has allowed us to constrain the amount of water lost from magma during explosive eruptions. This important constraint was then used to model magma degassing and fragmentation during the eruption of Medicine Lake Volcano, California. This volcano is the largest active volcano in the Cascades volcanic arc, with its last explosive eruption in 1060 CE. It has served as an ideal case study, not only in terms of providing the necessary materials for analysis and thermal spray processing, but also for studying the degassing processes associated with explosive eruptions. The analysis of water content in samples from Medicine Lake Volcano, as well as the pore structure of the micro-porous pyroclastic samples, was combined with detailed numerical modeling of eruptive magma ascent and ensuing bubble formation, magma degassing and ultimate fragmentation. We find that most of the magmatic water had formed bubbles by the time the magma fragmented. Interestingly, most of the magmatic water is lost from the magma within a few seconds after fragmentation. In fact, this rapid release of this water results in a transformation of the erupting magma from a highly viscous silicate melt with dispersed vapor bubbles to a dilute mixture of magmatic water vapor that carries the fragmented magma from several hundred meters depth within the volcanic conduit high into the Earth’s atmosphere, where it forms a tall eruption column of gases and volcanic ash.
