Our scientific advisory committee, the Suborbital Applications Researchers Group (SARG), brings together scientists and engineers from a variety of scientific disciplines. Today, we kick off the first of a series of occasional columns in which we’ve invited SARG scientists to discuss the research potential of next-generation commercial suborbital vehicles for their particular scientific disciplines.

Our first columnist is Dr. John Pojman, a Professor in the Department of Chemistry at Louisiana State University, who has led microgravity experiments using parabolic aircraft, a sounding rocket, and the International Space Station. Below, Dr. Pojman offers some thoughts on how microgravity experiments aboard commercial suborbital spacecraft will contribute to his research discipline:

Dr. Pojman: “There are many problems involving chemistry that can only be studied in the type of weightlessness afforded by suborbital flights, or as we like to say, “There is no way on Earth to do the experiment!”. Why should gravity be an issue for any chemical process? Gravity does not affect the fundamental atomic and molecular interactions, but it can drastically alter the macroscopic transport of heat and matter through convection, or fluid motion.

“A clear example of the role of gravity is in combustion. Hot air rises but not in weightlessness. Combustion processes occur very differently in weightlessness. Suborbital flights can allow testing how something will burn on a spacecraft in weightlessness. Suborbital flights can also be used to study the fundamental nature of some combustion processes without the interferences of gravity-induced convection. Figure 1 shows a candle burning under 1 g; Figure 2 shows the same candle burning in weightlessness.

Figure 1 (left). A candle burning in 1 g.
Figure 2 (right). The same candle burning in weightlessness.

“A process that is dramatically affected by gravity is frontal polymerization in which a localized reaction zone propagates from the coupling of thermal diffusion and the Arrhenius-dependence of the reaction rate of an exothermic polymerization. The reaction spreads like a flame but through a liquid monomer converting that liquid to a solid polymer or liquid polymer, depending on the type of monomer.

“Extremely complicated behavior can be observed if the reaction also produces gas (from the decomposition of the initiator used for the reacation). Buoyancy cause bubbles to rise but polymer to sink. Parabolic flights with frontal polymerization on NASA’s KC-135 revealed interesting effects of gravity. Figure 3 below shows a front propagating down (the image rotated to allow reading the information) under 1.5 g. Figure 4, further down, shows the same front in weightlessness. Interesting bubble patterns emerged but given the short duration of weightlessness (only 20 seconds) it was difficult to glean much quantitative results. Suborbital flights would allow studying such a system in greater detail.

Figure 3 (top right). Frontal polymerization of hexyl acrylate under 1.4 g on the KC-135 aircraft. The front is descending in the direction of the acceleration vector, which is to the right in the image.

Figure 4 (bottom right). Frontal polymerization of hexyl acrylate under approximately 0.01 g on the KC-135. Notice the unusual bubble patterns.

“Gravity is also very important for systems with different phases that can sediment, as in colloids, emulsions and blends. Performing experiments with such systems on suborbital flights can be a method to prepare “benchmark materials” that can serve as a goal for ground-based processing to achieve.

“Overall, suborbital flights can allow chemists the chance to determine how important gravity is to a process and thus to learn about aspects of a chemical system that would be complicated by buoyancy-driven convection on earth.”

For more information about SARG member Dr. John Pojman and his microgravity chemistry research, please see http://www.pojman.com, and stay tuned for future columns from other SARG scientists. For information about the upcoming Next-Gen Suborbital Researchers Conference, please click here.