Fundamental Physics: Physics of plasmas and solid/liquid dust particles

Aggregation Phenomena
Granular systems

Drop Tower Bremen - DYT2012

2012

K. Harth ⁽¹⁾, K. May ⁽¹⁾, T. Trittel ⁽¹⁾, S. Wegner ⁽¹⁾, R. Stannarius ⁽¹⁾

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AIM
This experiment aims on the investigation of the behaviour of granular anisotropic gases in microgravity. Anisotropic granular gases are encountered in a number of situations - in avalanches, sandstorms, dust devils, the formation and motion of dunes, planetary rings, and the asteroid belt, to name a few. Study of anisotropic granular gases has, until now, been primarily restricted to numerical computer simulations.

BACKGROUND
We encounter granular materials every day, but they still offer a large variety of riddles and problems for scientists and engineers. During the past decades, the field moved from engineering and geosciences into the focus of contemporary physics. A general theory of granulates has not been developed yet. Similar to conventional materials, different aggregate states can be observed in granulates. Granular gases are loose, agitated ensembles of grains - like particles in a sandstorm or in the Saturn rings. In contrast to atomic gases, grains lose kinetic energy in collisions. Thus, energy must be constantly supplied to avoid clustering or sedimentation. A granular gas under stable conditions at low excitation is best maintained in microgravity. The dissipation is expected to affect the shape of the velocity and energy distributions in the gas and can lead to cluster formation. Literature comprises numerous analytical and numerical studies. Experiments are hard to realize and rare. Nearly exclusively granular gases of spherical or completely irregular grains are considered. In 3D investigations of granular gases of spherical grains, the Knudsen regime (mean free path larger than container size) or clustering were analysed.
 
During the last years, research interest in granulates composed of anisometric grains rose immensely. Various soft matter systems display an effective anisotropy due to anisometric shapes of the individual constituents. In granulates, flow alignment, particle ordering and giant density fluctuations were reported. Grains of non-spherical shapes as constituents of granular gases have only been little considered so far.

To date, only few simulations of granular gases of anisometric grains exist in literature. The specific characteristics of the dynamics will depend on, e.g., the aspect ratio, shape and mass distribution of the grains. In previous experiments on a sounding rocket flight (REXUS 10), the first experimental data on a 3D granular gas of rodlike grains were gathered. Rotational as well as translational motion of the rods is measureable. Morover, the mean free path is considerably reduces as compared to a granular gas of spherical grains at comparable filling fraction. Dynamics in the particle-particle collision dominated regimes thus becom accessible.

The results of the REXUS experiment motivated us to explore the granular anisotropic gas in a systematic quantitative study. We will obtain full 3D position, orientation and velocity data from recombination of video data recorded from 2 perspective views. We investigate granular gases of different particles and at different excitations. Our results will provide important insights into the fundamentals of granular dynamics.

GaGa project website in German 

ESA webstory on the team and the progress during the drop campaign.

previous experiment:
REXUS-10
Launch Date: 23 February 2011
GaGa (Granular Anisotropic GAses)

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