Particle Aggregation in Microgravity: Informal Experiments on the International Space Station Stanley Love NASA Lyndon B

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln NASA Publications National Aeronautics and Space Administration 2014 Particle aggregation in microgravity: Informal experiments on the International Space Station Stanley Love NASA Lyndon B. Johnson Space Center,, [email protected] Donald Prettit NASA Lyndon B. Johnson Space Center, Scott esM senger NASA Lyndon B. Johnson Space Center Follow this and additional works at: http://digitalcommons.unl.edu/nasapub Love, Stanley; Prettit, Donald; and Messenger, Scott, "Particle aggregation in microgravity: Informal experiments on the International Space Station" (2014). NASA Publications. 142. http://digitalcommons.unl.edu/nasapub/142 This Article is brought to you for free and open access by the National Aeronautics and Space Administration at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in NASA Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Meteoritics & Planetary Science 49, Nr 5, 732–739 (2014) doi: 10.1111/maps.12286 Particle aggregation in microgravity: Informal experiments on the International Space Station Stanley G. LOVE1*, Donald R. PETTIT1, and Scott R. MESSENGER2 1NASA Lyndon B. Johnson Space Center, Mail Code CB, 2101 NASA Parkway, Houston, Texas 77058, USA 2Robert M. Walker Laboratory for Space Science, Astromaterials Research and Exploration Science Directorate, NASA Lyndon B. Johnson Space Center, Mail Code KR, 2101 NASA Parkway, Houston, Texas 77058, USA *Corresponding author. E-mail: [email protected] (Received 07 November 2013; revision accepted 22 February 2014) Abstract–We conducted experiments in space to investigate the aggregation of millimeter- and submillimeter-sized particles in microgravity, an important early step in planet formation. Particulate materials included salt (NaCl), sugar (sucrose), coffee, mica, ice, Bjurbole€ chondrules, ordinary and carbonaceous chondrite meteorite fragments, and acrylic and glass beads, all triply confined in clear plastic containers. Angular submillimeter particles rapidly and spontaneously formed clusters strong enough to survive turbulence in a protoplanetary nebula. Smaller particles generally aggregated more strongly and quickly than larger ones. We observed only a weak dependence of aggregation time on particle number density. We observed no strong dependence on composition. Round, smooth particles aggregated weakly or not at all. In a mixture of particle types, some phases aggregated more readily than others, creating selection effects that controlled the composition of the growing clumps. The physical process of aggregation appears to be electrostatic in nature. INTRODUCTION Marshall and Cuzzi 2001; Wurm et al. 2001; Marshall et al. 2005; Guttler€ et al. 2010; Weidling et al. 2012; A crucial step in planet formation is the growth of and references therein), and numerically modeled solid bodies in the millimeter to meter size range: too magnetic particles (e.g., Nuth and Wilkinson 1995; large to condense directly from the gas phase and too Dominik and Nubold€ 2002; Wang et al. 2010). small to interact strongly through mutual gravitation. Here, we present the results of particle The widespread existence of planets demonstrates that aggregation experiments carried out in microgravity on some growth mechanism operated in that size regime, board the International Space Station (ISS). Despite but the process is not well understood. Whatever it was, their simplicity, these experiments illustrated the it worked despite nebular turbulence (e.g., behavior of millimeter- and submillimeter-sized particles Weidenschilling and Cuzzi 1993; Sekiya 1998; Cuzzi in low-density, low-viscosity gas (air) without the et al. 2001; Weidenschilling 2010) that was probably influence of gravity, conditions that are difficult to strong enough to disrupt small-scale gravitational replicate on Earth for more than a few seconds. We collapse via the Goldreich and Ward (1973) mechanism found complex and often counterintuitive effects and to break dust structures held together by weak related to particle size, number density, composition, surface forces. Work on this topic (reviewed by shape, and surface texture, as well as evidence that Beckwith et al. 2000; Cuzzi and Weidenschilling 2006; may help identify the physical mechanism that Dominik et al. 2007; Blum and Wurm 2008) has underlies the observed behavior. Because of the included the sticking properties of ice and frost (e.g., unsophisticated nature of the experiments it describes, Supulver et al. 1995, 1997; Bridges et al. 1996), silicate this report is intended primarily to inform more dust in microgravity environments (e.g., Wurm and formal and realistic future investigations of weightless Blum 1998; Blum and Wurm 2000; Blum et al. 2000; particle aggregation. © The Meteoritical Society, 2014. 732 Particle aggregation in microgravity 733 EXPERIMENTS contained particles that differed from a protoplanetary nebula. The containers also kept the particles confined One of us (D.R.P.) conducted simple microgravity at high density: approximately 10À3 g of solids cmÀ3, experiments in off-duty time on the ISS during versus approximately 10À11 g of solids cmÀ3 in a Expedition 6, Expedition 30, and the docked phase of protoplanetary nebula (although local turbulence-driven Space Shuttle mission STS-126. The investigations used particle concentrations may have been up to 1059 materials that were either freely available on board the higher; Cuzzi et al. 2001). Finally, the containers ISS or carried as part of the crew’s limited personal kit. affected the experiments through particle–wall collisions, Studying small particles in a human-operated although these generally resulted in simple reflections of spacecraft poses logistical challenges. Without gravity, particle flight paths, suggesting that the walls were airborne particulates can easily disperse on ventilation dynamically similar to neighboring volumes containing currents. They may find their way into crewmembers’ the same mixture of air and particles. eyes or lungs, creating potentially serious medical The air in the experiment containers was at cabin problems. Mitigating these risks requires at least three temperature (21 °C) and pressure (745–750 mmHg). Its levels of containment. composition was 79% N2, 20% O2, and 0.5% CO2 at Fortunately, the ISS is well stocked with clear 40% relative humidity. For the experiments carried out polyethylene bags. Some are manufactured with double in air, the stopping distance of the particles was 10– walls; others can be nested to create a second protective 20 m, orders of magnitude larger than the container. layer. A third level of confinement is achieved by This indicates that viscous forces played a minor role in keeping the bag near an inlet filter of the ventilation these experiments. For a single trial conducted with lm- system, which provides suction velocities of 0.5– scale particles in water, viscosity was a dominant factor. 1.0 m sÀ1. The first set of experiments was conducted on ISS Both the American and Russian space programs use during Expedition 6 in 2002–2003. It used only double-walled polyethylene bags with heat-sealed edges materials readily available on the Station. Zip-lock to hold beverages. Some of the experiments reported bags, Russian drink bags, and culture flasks served as here used Russian drink bags as containers. When containers, and salt, sugar, instant coffee, and mica empty and flat, a Russian drink bag is an irregular were employed as granular solids. Five conditions were rectangle with dimensions about 8.5 9 27 cm. When tested: salt (NaCl) crystals 0.5–1.0 mm in size, filled with fluid, its volume is 550 Æ 5cm3 and its contained in a polyethylene bag with air at a number effective thickness is 2.4 cm. Most new drink bags density of approximately 10 cmÀ3;1–6 mm salt crystals contain dry beverage powder whose free-floating in air with approximately 1 cmÀ3; 0.5–1.0 mm sugar aggregate behavior can be studied by simply inflating (sucrose) crystals in air with approximately 40 cmÀ3; the bag with air. A drink bag of sweetened tea contains 0.1 mm powdered coffee particles in air with a commercial tea bag and an average of 10.3 g of loose approximately 5000 cmÀ3; and 5 lm mica flakes in a sugar (sucrose) crystals. The tea bag can be trapped in culture flask of water with approximately 108 cmÀ3. The one end of the drink container with a spring clip to particles used in these tests were 101–1049 larger than allow study of the sugar alone. the approximately 1 lm primary dust particles of a Other experiments were contained in food-overwrap protoplanetary nebula. The materials used are obviously packages, stiff polyethylene bags with heat-sealed seams not important in planet formation, but they do that form approximately rectangular boxes represent a crystalline ionic solid, crystalline and 24.0 9 28.0 9 12.0 cm in size, with an internal volume noncrystalline organic solids, and a hydrated silicate. of about 11,000 cm3. A food-overwrap bag offers a The second set of experiments was conducted in chance to study the behavior of particles in a larger 2008, while Space Shuttle mission STS-126 was docked volume, with less interaction with the container walls. to the ISS. It investigated the aggregate behavior of Two other experiments used standard polyethylene zip- chondrules from the L/LL4 chondrite Bjurbole€ (sample lock bags of two sizes, 800 and 4000 cm3. A single USNM 610). Bjurbole€ and its chondrules have been aqueous experiment used a Costar #3075 polystyrene very thoroughly studied (e.g., Caffee et al. 1982; culture flask with a nominal volume of 250 cm3 and a Rietmeijer and Mackinnon 1984; Nava 1994; measured volume of 266 cm3. Wasilewski et al. 1995; Kuebler and McSween 1996; All of the container materials were dielectrics, Kuebler et al. 1997; Kletetschka et al. 2001). In which acted as electron scavengers, gaining negative preparation for flight, the chondrules were charge through contact friction, and as charge barriers disaggregated, washed with absolute ethanol in an able to retain an induced local charge.

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