Juan H. Agui

Absorption of short duration pulses by small, scalable, tapered granular chains

Making shock proof layers is an outstanding challenge. Elastic spheres are known to repel softer than springs when gently squeezed but develop strong repulsion upon compression and the forces between adjacent spheres lead to ballistic-like energy transfer between them. Here we demonstrate that a small alignment of progressively shrinking spheres of a strong, light-mass material, placed horizontally in an appropriate casing, can absorb ∼80% (∼90%) of the incident force (energy) pulse. The system can be scaled down in size. Effects of varying the size, radius shrinkage and restitutive losses are shown via computed “dynamical phase diagrams.”

Multi-scale kinetics of a field-directed colloidal phase transition

Polarizable colloids are expected to form crystalline equilibrium phases when exposed to a steady, uniform field. However, when colloids become localized this field-induced phase transition arrests and the suspension persists indefinitely as a kinetically trapped, percolated structure. We anneal such gels formed from magneto-rheological fluids by toggling the field strength at varied frequencies. This processing allows the arrested structure to relax periodically to equilibrium—colloid-rich, cylindrical columns. Two distinct growth regimes are observed: one in which particle domains ripen through diffusive relaxation of the gel, and the other where the system-spanning structure collapses and columnar domains coalesce apparently through field-driven interactions. There is a stark boundary as a function of magnetic field strength and toggle frequency distinguishing the two regimes. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces. Such directed assembly may be harnessed to create unique materials from dispersions of colloids.

Energy Partitioning and Impulse Dispersion in the Decorated, Tapered, Strongly Nonlinear Granular Alignment: A System with Many Potential Applications

Rapid absorption of impulses using light-weight, small, reusable systems is a challenging problem. An axially aligned set of progressively shrinking elastic spheres, a “tapered chain,” has been shown to be a versatile and scalable shock absorber in earlier simulational, theoretical, and experimental works by several authors. We have recently shown (see R. L. Doney and S. Sen, Phys. Rev. Lett. 97, 155502 (2006)) that the shock absorption ability of a tapered chain can be dramatically enhanced by placing small interstitial grains between the regular grains in the tapered chain systems. Here we focus on a detailed study of the problem introduced in the above mentioned letter, present extensive dynamical simulations using parameters for a titanium-aluminum-vanadium alloy Ti6Al4V, derive attendant hard-sphere analyses based formulae to describe energy dispersion, and finally discuss some preliminary experimental results using systems with chrome spheres and small Nitinol interstitial grains to present the unde...

Impulse Propagation in Granular Systems

We review recent developments on the topic of impulse propagation in assemblies of elastic beads, where the beads interact upon contact via the (nonlinear) Hertz potential. The role of solitary waves in granular systems is emphasized. The effects of impurities, disorder and restitution are discussed. The possible applications of this research in the development of new technologies are mentioned.

Total Temperature Measurements Using a Rearward Facing Probe in Supercool Liquid Droplet and Ice Crystal Clouds

This paper presents an analysis of local total temperature and humidity experimental measurement taken in atmospheric ice cloud flows. The measurements were obtained in a series of tests in NASA’s Propulsion Systems Laboratory. The probe used in the tests is referred to as the Rearward Facing Probe which was designed to mitigate the contamination effects of ice accretion and ingestion into the probe. The data provided important insights in the interaction of the ice cloud and the atmospheric flow. For the majority of the test runs, small temperature drops in the range of 0.6 to 2.8 0C and up to 1.5 g/kg of water vapor rise were found as a result of the interaction. Under certain very low temperature or high TWC conditions, the interaction with the cloud produced a warming of the airflow. A thermal model based on evaporative and convective heat transfer mechanisms between the spray droplets and the airflow showed good agreement with the experimental data. Detailed analyses of the response of the probe under various flow, thermodynamic, and cloud conditions, are provided in the paper.

Testing of Regeneration Filtration Concepts for Future Spacecraft Applications

NASAs future planetary surface and long duration transit missions will require the development of robust and effective filtration systems. Contamination from planetary dust and build up of internally generated particulate matter over long periods will significantly encumber the nominal operation of cabin filtration systems and would involve increased maintenance resources. One option for consideration is a combination of surface prefiltration and inertial/cyclonic particulate separation which can provide critical secondary filtration and regeneration capabilities. To test this concept, an experimental apparatus was devised to demonstrate, using JSC-1af lunar simulant and mono-sized silica particles as challenge aerosols, (a) the performance and loading characteristics of various surface prefilters and (b) regeneration concepts for in-place cleaning of the surface filter. In addition, high-speed reverse flow jets were used behind the pre-filter to break up the accumulated dust cake. The tests consisted of measuring flow velocities, pressure drops across filter elements, and particle counts (filter efficiency). Standard as well as high-speed video imaging was also performed in order to observe particle transport and dust cake breakup. The results from a series of filter loading and regeneration tests, including data from some low gravity tests, are presented in this paper.

Reduced Pressure Cyclone Separation Studies using Synthetic Lunar Regolith

In order to provide a safe and sustainable astronaut crew environment at a lunar or extraterrestrial outpost, dust mitigation techniques appropriate for the crew vehicle or space environment must be developed. Cyclonic separation is an attractive method because of equipment durability and maintainability. Further dust mitigation studies are required to optimize cyclone separator performance under the reduced air pressures anticipated in crew habitat and air-lock environments. In this paper we examine the collection efficiency of several cyclonic separators under ambient and reduced pressures. Performance testing of a commercial cyclone separator is used as a baseline for comparison with a custom-designed cyclone. Details of the experimental test system along with the design, performance, fabrication, and implementation of a custom cyclone separator are examined. We also describe the customization and implementation of specialized optical diagnostic instrumentation providing particle size, count, and flow data. Test results are compared with cyclone computational flow dynamic modeling to evaluate model parameters and optimize collection efficiency under anticipated flow conditions.

Field-Responsive Colloidal Suspensions in Microgravity

When exposed to a magnetic field, paramagnetic colloidal particles acquire dipole moments. Interaction between induced moments leads to the formation of different microstructures that depend on the strength and characteristics of the external magnetic field, particle size, and the suspension concentration. Studies of suspensions of non-spherical paramagnetic particles can expand the range of applications for magnetorheological fluids and provide further insight into their physics of aggregation. Here we present the fabrication of anisotropic paramagnetic polystyrene particles following the particle stretching method of Keville and coworkers.