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“Drop Your Thesis!” 2012

Experiment report

Team Name: GAGa DropT (Granular Anisotropic Gases in Drop Tower Experiments) Version: 4.1 Date: 23. 12. 2013

1. Executive summary

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. During the last years, research interest in granulates composed of anisometric grains rose immensely. In a gas of elongated grains (granular anisotropic gas), the energy distribution on translations and rotations is measureable. In contrast to atomic gases, here different degrees of freedom contain a different amount of kinetic energy. To date, only few simulations of granular gases of anisometric grains exist in the literature. In previous experiments of our department, the first experimental data were gathered in a sounding rocket experiment. The results motivated us to explore the properties of granular anisotropic gases in a more systematic quantitative study. Our Drop Your Thesis!-team GAGa DropT (Granular Anisotropic Gases in Drop Tower Experiments) consist of 4 physicists at the Otto-von-Guericke University in Magdeburg, Germany. We have developed a technical scheme to prepare a homogeneous distribution of rods in the container under drop tower conditions, tested several granular materials and obtained some full 3D position, orientation and velocity data from recombination of video data recorded from 2 perspective views. An extension of the latter is work in progress. The full results will provide important insights into the fundamentals of granular dynamics.

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2. Student team description The GAGa DropT team consists of 3 young physicists and one additional physics PhD student, affiliated at the Otto-von-Guericke University in Magdeburg, Germany. We are currently working on our PhDs in the Department of Nonlinear Phenomena, chaired by Prof. Ralf Stannarius. Sandra Wegner obtained her Diploma in Physics in 2011, she is working on sheared granular materials. The main focus of her work is on effects of grain anisometry on packing, flow and orientation of the grains. Studies are mainly performed using excavation, optical imaging and, in particular, 3D Magnetic Resonance Imaging and Computer Tomography. Kathrin May has been working on ferrofluids (paramagnetic colloidal particles in suspensions which consequently show large magnetic interactions) and on the dynamics of thin bubbles prior to her graduation in March 2012. For her PhD, she switched topics to investigate collective dynamic and static effects in suspensions of nanometer-sized elongated colour pigments. Kirsten Harth is working mainly on liquid crystalline films, which can be considered as 2D anisotropic fluids. She is investigating static and dynamic patters as well as hydrodynamics of these confined systems. Besides that, she is involved in some experiments on granular materials. From 2009, she has been leading the REXUS team GAGa, who first investigated a granular gas of anisometrically shaped grains in micro-gravity on a sounding rocket flight in February 2011. Torsten Trittel is a young physicist, working on this project together with us. He investigated the aging of soap and liquid crystal foams for his graduation at the University of Magdeburg. Currently, he is working on his PhD on the dynamics of bursting bubbles and films. He is involved in several micro-gravity experiments of the Department of Nonlinear Phenomena. He is a member of the former REXUS team GAGa. Professor Ralf Stannarius obtained his PhD in Physics from the University of Leipzig, Germany, in 1985. He worked in Leipzig mainly oh thin film dynamics and liquid crystals until he became full professor at the Faculty of Natural Sciences in Magdeburg in 2003. In his Department of Nonlinear Phenomena, various systems displaying nonlinear effects are investigated, e.g., liquid crystals, granular materials, ferrofluids or general hydrodynamic problems as the bubble oscillations or the Faraday Instability. Research on granular matter focussed on pattern formation in rotated tumblers and shearing of granular materials, before. Since 2008 he is a collaborator in the OASIS (Observation and Analysis of Smectic Islands in Space) project. Together with colleagues at the University of Boulder, an experiment for a NASA ISS mission on dynamics and Oswald ripening of domains and particles on thin spherical surfaces is designed. This project activity also allowed two students in the “GAGa DropT” team to gain micro-gravity experience, and some general granular gas experiment testing during three DLR parabolic flight campaigns was performed. In addition, he obtained a DLR grants for additional sounding rocket missions (continuing the research started with the REXUS-10 project GAGa), where all of the GAGa DropT team members are involved. Ralf Stannarius has backed the REXUS team GAGa since 2009 and supports us in this project.

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The 2012 “Drop Your Thesis” team GAGa DropT in the lab in Magdeburg (left) - Ralf Stannarius, Kirsten Harth (back), Sandra Wegner and Kathrin May (front), and with Torsten Trittel in front of the experiment capsule in Bremen (right).

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3. Background and scientific objectives Granular materials show a lot of remarkable features which are not found in conventional solids or fluids. During the last decades, granulate research has moved from engineering and earth sciences into the focus of contemporary physics. Similar to conventional materials, different aggregate states are observed in granular matter depending on the excitation. Granular gases represent dilute ensembles of macroscopic particles interacting through inelastic collisions. In nature, they are found, e.g., in avalanches, in sandstorms or dust devils, in the formation and motion of dunes, in the planetary rings or the asteroid belt. A better understanding of the fundamentals of granular materials could essentially improve engineering of transport, soil and pharmaceutical processes. The dissipative nature of grain collisions leads to the formation of a large variety of patterns. Despite some specific problems have been solved and explained, a comprehensive dynamic theory is still lacking.

Figure 3.1: several examples of isotropic (I) and anisotropic (A) granular materials, from left to right: rice (A), chewing gum (I), poppy seeds (more or less I), glass beads (I) and cylindrical pearls (A), spaghetti (A).

3.1 Granular Gases These fundamental laws can be ideally investigated in a granular gas, where all particle collisions can be observed. Similar to classical thermodynamics, a granular temperature can be defined from the mean squared velocity of particles. The distribution of energy on the translational and rotational degrees of freedom can be calculated from the particle motion. To avoid sedimentation, either micro-gravity [1-3,29] or strong external forces are necessary in three-dimensional (3D) experiments (e.g. vibration [4-9], electro-magnetic fields [10,11,30]). Due to the dissipation during grain collisions, a constant energy supply is needed to maintain a steady state. Granular gases show numerous peculiarities even on a very basic level of understanding, e.g., the velocity distributions in a granular gas deviate from the Gaussian shape expected from thermodynamics. The exact shape of the distribution is still a matter of vivid discussions. As granular gases represent a system far from thermodynamic equilibrium, e.g., granular flux from compartments of low granular temperature to such of high granular temperature, as well as clustering and sorting phenomena may occur, e.g. described in the book ‘Granular Patterns’ by I. S. Aranson and L. S. Tsimring [12]. In previous publications, nearly exclusively granular gases of spherically or irregularly shaped grains were considered. Today, the number of papers dealing with theoretical predictions clearly outnumber those of experimental results. Analytical theory is tested by extensive numerical simulations, based on the same initial modelling assumptions. From this, no conclusion on the appropriateness of the model itself can be drawn. Experiments are hard to perform without introducing large velocity or density gradients in the system. Excitation of a granular gas of spherical grains through vibrating walls frequently leads to the formation of one cluster in the center of the container. In many experiments, the granulate is confined to two dimensions. Particle tracking measurements are scarce, and 3D data are not available. Experimental confirmation is needed to validate our present picture of granular dynamics. During the last years, research interest in granular materials of anisometric, especially in elongated grains grew immensely, see Ref. [31] for a recent review. On a large scale, ordering and some other remarkable effects have been observed in fluidized anisotropic granular beds [13-16]. Upright ordering and vortex motion is observed for dense vertically vibrated rods in a container [17]. For more dilute systems, different ordered patterns may form [18]. Quasi nematic, smectic and cubic states have been observed in vertically shaken monolayers [19]. In addition to the excitation strength, the aspect ratio (that is, the ratio of length to diameter

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of a particle) and the shape of the particle ends strongly influence the order formation. Our general goal is an investigation of the properties of dilute granular gases of rodlike particles in 3D.

Figure 3.2: (From left to right) dust devil; unstable clusters formed in 2D numerical simulation of a granular gas of anisometric grains (Kanzaki et al. [21]); cluster in a vibrationally agitated granular gases of spherical grains in microgravity (top: Falcon et al. [2]; bottom: M. Sperl [28]); granular anisotropic gas in microgravity (GAGa team, Uni Magdeburg); orientation of pegs in shear flow in split-bottom couette geometry (S. Wegner, Diploma thesis).

3.2 Previous Research on Granular Anisotropic Gases A one-dimensional granular gas of rodlike particles was numerically simulated by Aspelmeier et al. [20]. The particles interact on a circle, their aspect ratio being irrelevant for the results. The authors study velocity distributions, energy decays and the formation, interaction and breakup of clusters. Alignment in and cooling of a comparatively dense granular gas of infinitely thin needles was studied by Huthmann et al. [21]. Very recently, a numerical simulation of the free cooling (only initial energy input, no constant energy supply) of a gas of anisometric viscoelastic grains in two dimensions was performed by Kanzaki et al. [22]. These authors consider a comparatively dense system with periodic boundary conditions. The decay rates of the kinetic energy in rotational and translational degrees of freedom are compared. Different exponents (for the translational energy deviating from Haff’s law) calculated. From this, it is concluded that there is no equipartition of energy between translational and rotational degrees of freedom. This conclusion has also been drawn by other authors [20-24], but never been measured experimentally. Clusters form, but are unstable and particles tend to show orientational order within these clusters for sufficient aspect ratios and low enough coefficient of restitution. For granular gases of circular or spherical grains, the situation is not that clear – an experiment with a dense gas of disks showed equipartition [9], while simulations predict its breakdown as soon as friction between the particles is introduced [32,33], which is present in all “real” systems.

Up to now, there have been only few attemtps to observe the dynamics of granular gases of anisometric grains. These were obtained from ensembles with a low number of particles and/or in two dimensions. Experiments showed nearly Gaussian velocity distributions for strongly vertically vibrated dimers of spherical grains confined between two vertical plates[23,25]. Cylindrical grains levitated by airflow through a grid at comparatively high area coverage (~35 %) exhibit equipartition between translational and rotational energy, but also correlations of translational and rotational motion [26]. Only the GAGa (Granular Anisotropic Gases, see Ref. [27]) experiment carried out within the REXUS/BEXUS programme in 2010 delivered first experimental insights into the dynamics of a dilute ensemble of rodlike grains in a 3D container. A dilute granular gas of rods of length to diameter ratio 10:1 was observed at two different excitation strengths during the rocket flight. The scientific results (Refs. [34, 35], partially unpublished) can be summarised as: 1. The velocity distributions in a weakly excited granular gas are usually not Gaussian. What excactly determines whether Gaussian or non-Gaussian velocity distributions are observed in experiments, has not been clarified and represents a current matter of scientific discussion. Under vibrational excitation, the excitation direction is observed to be clearly distinguishable from the other directions. In the GAGa experiment, the velocity distribution in the direction where energy input is only provided by grain-grain

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collisions as well as the distribution of the rotational velocities scale approximately with a streched exponential ~exp(-|v|1.5). 2. Equipartition of energy between rotational and translational degrees of freedom does not hold in our 3D system of elongated cylinders. This is unexpected in comparison with the few previous experimental results for disks in 2D. The average kinetic energy in the degree of freedom corresponding to motion in the direction of excitation is much larger than the average kinetic energy in the degree of freedom corresponding to motion in the direction perpendicular to it. This is expected due to the massive energy input during particle vibrating-wall collisions. For the rotational energy, only the sum of the two degrees of freedom (rotations about the 2 perpendicular axis to the long axis) can be measured. This was done by selecting particles with idential length in consecutive frames, only – due to a lack of 3D information. The resulting rotational energy per degree of freedom is significantly smaller than the translational kinetic energies for both degrees of freedom. 3. The homogeneity of the grain distribution in the container depends on the excitation strength, transient density fluctuations have been observed. 4. There are several peculiarities in the statistics of the granular gas – most strikingly at the lower excitation acceleration the average energy of grains in the gas is larger than at the high excitation acceleration. From the experiments, one can speculate that the average rod energy scales with the square of the maximum plate velocity. Velocity distributions have a larger standard deviation and grains are, on average, faster at lower excitation acceleration. This effect is counterintuitive considering most known results about granular gases of spherical grains and might be enhanced by the elongated shape of the grains. 5. From the period following the frequency change, the time scale for the adaption of the system to the dynamic equilibrium corresponding to new excitation parameters has been estimated to be less than 3 seconds. Thus, the length of the microgravity period during the drop tower experiment is appropriate for a study of the steady state dynamics of the system.

3.3 Objectives of our Drop Tower Experiment In our “Drop your Thesis” project, we develop an experimental setup and excitation to explore fundamental aspects of the physics of granular gases under drop tower conditions. We will compare the final results to literature data and the previous REXUS experiment. Classical statistical theories are expected to fail as they do not account for the dissipative character of grain collisions. With our experiments, we provide an analysis of a 3D granular gas under different well-defined external conditions, which will be extended in further drop tower campaigns and during future sounding rocket missions. In all experiment runs, we use the same setup, consisting of two identical shaker boxes and a third box with different dimensions and camera positioning. We vary several experimental parameters to find the optimal parameter set and to gain insights into statistical properties of the granular gas. These are the filling fraction (number of rods in the container), the rod length, and excitation parameters (frequency, amplitude). We have performed one experiment with a second material. Using rods in granular gas experiments is a new feature (except the REXUS experiment GAGa). It has several advantages over spheres: rotations and translations can be tracked relatively easy in 3D, rods interact more frequently than spheres at identical filling fraction – the mean free path of a rod is much smaller than that of a grain. This simplifies identification and tracking. The main objective of our project is to observe, quantify, analyze and understand fundamental properties of such a granular gas, exploring the effect of shape anisometry on the collective dynamics of the ensemble. There are several aspects that may be considered: 1. In previous experiments, spherical grains were usually observed to form a single cluster near the center of the container at filling fractions and excitation strengths comparable to our experiment [2, 28]. From the GAGa experiment, it is evident that the rods are much more susceptible to vibration from the boundary than spheres. They distribute almost evenly in the box. By varying the rod aspect ratio (while all other parameters are held constant), we find and indication of the influence of the grain shape on the granular gas dynamics.

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2. The above observation hints at a different characteristic of the energy transfer between rods and the walls or other rods compared to spherical grains. By varying the excitation amplitude and frequency, some quantitative characterization of this effect discovered in the GAGa experiment can be achieved. We recorded videos at a higher frame rate than in the REXUS GAGa experiment. This allows to resolve the particle trajectories and especially the collisions in more detail. An analysis of individual collision scenarios can lead to a deeper understanding of the collective energy transfer properties in these systems. Additionally, the data analysis can be aided by numerical reconstruction of the collisions between a small number of rods. We expect a much richer source of data for various parameter sets than in the previous rocket experiment. However, this evaluation has not finished to date. 3. In nature and applications, the constituents of a granular gas usually differ in size and material. Due to this, we test different aspect ratios and two materials in the experiment. In future experiments, it will be interesting to investigate the behaviour of mixtures of different particle shapes / materials. We focussed on a selection of parameters: the number of rods in the container and excitation parameters. First, the excitation scheme and the setup were optimized for the drop tower conditions. To achieve this, the experimental conditions must be prepared first, so that 4. Due to the temporal restrictions under drop tower conditions (9 s), a special driving scheme and a setup preparation method must be developed. In the previous GAGa experiment, the granular material had more than 60 s time before the homogeneous state was reached and the evaluated data were taken. Here, a spatially homogeneous distribution of the rods and an equilibration of the “granular temperature” must be achieved on a much shorter time scale. This is realised using a specific driving scheme of the walls and a the appropriate initial positioning of the rods.

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4. Experimental setup In principle, our drop tower experiment will consist of two identical test chamber / camera setups and a third setup with different chamber dimensions and camera positioning / model. That setup is part of a different experiment. The two sub-setups mainly consist of a container with three independently controllable vibrating walls (shaker box) containing an amount of rodlike granular material. The vibration scheme will be identical for the two opposing walls, the third wall is inserted safety mechanism (to insert more energy and redistribute the material), that proved to be disturbing rather than useful in the drop tower. The rodlike grains are monitored by two colour high-speed cameras per setup (from top and front in two setups, from the front at an angle respective to each other in the 3rd setup) and illuminated by 12 High-Power LEDs at oblique incidence. The electronics are controlled using micro-controller kits. The mechanical and optical parts of the setup are partially adopted from the 2011 REXUS experiment [27] shown in Figure 4.1. In particular, we maintained the basic shaker construction (with minor technical changes) and actuation mechanism using voice coil actuators. The box is again monitored from front and top in two setups. These cameras are positioned facing the box from top and front for both cameras instead of using a mirror in top view. We learned from REXUS and from trials with a the new prototype, that the optical tunnel is not necessary. In the REXUS experiment, we used small commercial cameras with no fixed focus due to spatial and funding restrictions. This led to problems in the automatic evaluation, due to a low frame rate (30 fps), a lack of synchronization and a too long exposure time for the top view camera. These are replaced by the High-Speed cameras available at ZARM Bremen (512 x 512 pixel2 resolution, 500 frames per second), and the shaker box dimensions are adjusted to fit the camera images. In the third setup, we use commercial digital reflex cameras (Canon EOS 600D or 550D), where all imaging parameters are adjustable and the recording speed at HD resolution is 60 frames per second. The illumination is realized with High-Power LEDs placed nearby the shaking box at an angle of 45 degrees to the front and top plates to avoid reflections. Electronics are entirely newly designed.

Figure 4.1: left: Experimental setup of the REXUS experiment GAGa, right: shaker box in detail; base plate diameter 35 cm, total experiment height ≈20 cm, box dimensions 8 x 6 x 10 (width x depth x height)

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4.1. System description Our setup can be divided into a mechanical, an optical and an electrical part. Additionally, we will have granular rods as particles in the container.

4.1.1 Mechanics The main piece of our setup is a box with 3 periodically vibrating walls, which is filled with our granular material. The two side walls and the back wall of the container are held by small linear roll bearings and springs, they walls are individually agitated using voice-coil actuators. This is implemented using a ArduinoCC program copied to a microcontroller (Arduino). The plates oscillate in phase and with identical frequencies / amplitudes. The shaker box schematic is shown in Figure 4.2, photographs in Figure 4.3. The inner dimensions of the container are 90.0 x 85.0 x 80.0 mm³ (WxDxH), which are chosen to match the camera resolution. The front and top walls of the container consist of anti-reflection coated acrylic, all other walls are flat painted aluminum. White color enhances the color reproduction of the cameras and produces some diffuse scattered light in the container. A box with black background is not useable with the Photron High-Speed cameras, as it does not produce sufficient color contrast. In the REXUS experiment, there used to be no problems with electrostatic charging – whereas we encountered problems in this experiment (see below). The mechanics are identical to those of the third setup (borrowed from a different project), which we brought to our meeting with the engineers at ZARM Bremen in September 2012, see Fig. 4.4. In that case, also the top wall of the shaker box is made of aluminum, and the container size is 12 x 6 x 10 cm3. A scheme of this setup is shown in Figure 4.5.

Figure 4.2: scheme of the shaker box for the GAGa DropT experiment, inner dimensions ~9 x 8.5 x 8 cm3 (w x d x h)

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Figure 4.3: one shaker box for the GAGa DropT experiment, inner dimensions ~9 x 8.5 x 8 cm3 (w x d x h), LED panels not yet installed. top left: corner view as in Figure 4.2; top right: front view; bottom left: top view; bottom right: back corner

Figure 4.4: left: Example setup similar to that for GAGa DropT during our September visit in Bremen, referred to as third setup in the text; right: high-speed camera that was used during the campaign, test granular material

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Figure 4.5: Sketch of the third, borrowed setup with two front cameras at angles of ±20°

4.1.2 Optics and Cameras The shaker boxes in the front/top view setups are illuminated from front and top, the third setupis illuminated from the front only. For this, we place two panels with 3 high power LEDs each at angles of ±45° in front of each face which must be illuminated. The mounts for the top LED panels are attached to the shaker body, see Fig. 4.2. We chose this setup to avoid reflections on the box, which could be seen in the recorded images of the REXUS experiment. 6 LEDs per camera perspective are sufficient to guarantee a good illumination. It turned out that there are moving shadows, which are very unfortunate for an automatic evaluation of the images. We use the 4 high-speed colour cameras (Photron Fastcam MC2) available at ZARM for the two front / top view setups. Two identical setups are integrated into the capsule. To obtain good results, we need two pairs of time-synchronized cameras to obtain our data. The setup size is adjusted to the 512 pixel x 512 pixel resolution. In principle, top and front view images should look as shown in Fig. 4.6 (here taken with different camera), with flying rods instead. The possible recording speed of 500 frames / seconds was used to have images with small time difference to track the particles form image to image automatically. Camera objectives are adjusted to a fixed focus prior to the experiment, scalings were measured. In the third setup we used commercial Canon EOS cameras, where also the objectives were adjusted and glued before integration of the whole setup and the camera parameters (white balance, exposure time) were set and saved.

Figure 4.6: Schematic images of the front (left) and top perspectives similar to what they look like in the experimental data, the granulate was moving throughout the container, the filling fraction was higher in the experiment

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Image reconstruction parameters (camera focus, corrections for optical aberrations, …) are determined by moving a square patterned plate in the later position of the shaker and using the images of both cameras as input for the parameter calculation. We have tested the camera with an example setup and example granulate (see Fig. 4.4) during our visit in Bremen in September, the resolution and color quality are fine for our experiment. Exact positioning, etc., was performed during the preparation week in Bremen for the high-speed cameras, the third setup was completely preassembled and fixed by us. It resembles the scheme in Figure 4.7. Slightly different objectives were used for tow and front views (due to availability). The distance between the platforms is about the distance between the front view camera and the shaker front plate plus the shaker height. Positioning most of the equipment on the bottom platform was agreed on during our visit in Bremen. The third experiment was integrated on an additional platform between the base platform of the top/front view shakers and the platform, where the top cameras were mounted. Large holes (approx. 10 cm diameter) were drilled, so that the optical path for these setups was undisturbed. A photo of the assembled capsule is shown in Figure 6.1.

Figure 4.7: Schematic of the component integration into the catapult capsule, exact positioning was performed during the campaign preparation week in November, top row: components on the bottom and top plates, yellow rectangles represent the position of the electronics; bottom: left: approximate positioning of the experiment components on the bottom platform (without LED panels, dots represent camera positions), right:schematic of the experiment in the capsule, the distance between top camera objectives and top shaker plate was approximately equal to the distance between camera objective and shaker front wall in front view.

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4.1.3 Electronics The high-speed cameras are controlled with software available at ZARM Bremen. The remaining electronics of the experiment are controlled by a microcontroller board (Arduino Nano V3.0). By programming the microcontroller the frequencies and amplitudes of the shaking walls can be adjusted and the LEDs can be switched on/off. In the third setup, the EOS cameras are switched to video mode beforehand, and the recording is triggered by an ADRUINO signal as well. The Voice Coils moving the shaking walls are driven by three motor drivers (Pololu 18V15). The shaking frequency is generated by a square wave signal switching a digital output alternating high (5V) and low (0V). Since the tone function of the Arduino can only give out frequencies above 32Hz, the frequency is quartered by two cascaded toggle-Flip Flops. The square signal with quartered frequency is then given to all three motor drivers as input. The amplitudes of each shaking wall can be controlled by PWM given out by three outputs of the microcontroller. The amplitudes of the individual walls can be varied independently by changing the PWM duty cycle given to the associated motor driver. The LEDs are powered with four npn-bipolar transistors BD441. Switching on/off of the LEDs is realized by the Arduino. The brightness of the LEDs can be varied independently by potentiometers. The supply voltage for the microcontroller, the motor drivers and the LEDs is provided by a voltage converter (TracoPower TEN60-2412), which converts the provided voltage of 24V to 12V.

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Figure 4.8: top: Schematic sketch of the electronics controlling the experiment Bottom: Photo of the implemented electronics on the platine, with regions marked. The connectors on the right are from top to bottom: camera switching out (not used in front/top view setups), shaker signal out, LED current out, power input.

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4.1.4 Granular Material As particles, coloured rods of different length to diameter ratio were used. We have tested different rod materials (copper, aluminum, brass, glass, plastic, wood, isolated copper wire). The first test was simply dropping the rod onto an aluminum plate and watching it jump up again. “Good” rods jump high, “bad” rods do not jump at all. This limited the selection to brass rods, glass rods and the isolated wire pieces. The most favorable would be glass. However, flat coloured (almost non-reflecting) ones are not available. We tried etching their surfaces with HF, but this did not the desired results. Also, glass rods are not available for many different aspect ratios (length to diameter ratio). Brass rods jump comparably well as the wirepieces, but are hard to manufacture in a large number. We prepared two sets of flat painted jewellery pearls (glass) available in sizes 1.2 cm length, 2 mm diameter and 1.5 cm length, 2.5 mm diameter. The wire pieces and some untreated jewellery pearls are seen in Fig. 4.9. Our favorite choice of material are self-fabricated isolated (coloured) copper wire pieces. This choice was also made in the REXUS experiments, as these are cheap and can be produced easily, display comparably little reflections, guarantee direct comparability to the results of the previous experiment, and the tracking of individual rods is much simplified by the colour coding. An example image is shown in Figure 3.2 for the REXUS experiment. We could reduce reflections from the rod surfaces by putting them into a sieving machine together with bird sand for several hours, to roughen their surfaces slightly. After this, the rods were washed and sorted according to their length and straightness. We tested different rod colors, and found that the optimal set of colors was yellow, red, green, blue, purple for tracking.

Figure 4.9: granular materials: jewellery glass pearls of length 1.2 cm (green) and 2 cm (blue) and insulated copper wire pieces of length 1.5 cm In the earlier REXUS 10 experiment situations occurred where even manual tracking of rods from frame to frame was difficult as too many rods of the same colour were incidentally gathered in one region, overlapping each other. To avoid this, we use a background filling material (a number of white rods), that will not be detected or tracked and add a small number (about 15) of rods of each tracked color. An example snapshot is shown in Figure 4.10: Here, the box was filled with some of the wire rods shown in Figure 4.9 right (in test case not yet anti-reflection treated), and the experiment was shaken by hand while the test cameras were recording. White rods are hardly discernable from the background, and coloured rods can be well identified. The rod diameter is 1.3 mm, the length can be varied. We prepared used sets of 12 mm, 15 mm and 18 mm length. 15 rods of each color were inserted for tracking, the remaining particles were white. In our experiments, we worked at volume fractions (all rod volume / container volume) below 3 %.

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Figure 4.10: Snapshot of coloured rods moving in a manually shaken container, rod length approx. 1.5 cm.

4.1.5 Equipment list Shaking box (2x): 3x VoiceCoil Maccon CVC 30-15 12x linear roll bearings, Minitec SM 05 G 24x spring, D = 10mm, L = 14 mm 1x bottom wall Aluminium 1x top wall plexiglass 1x front wall plexiglass 3x shaking plate (8 cmx8 cm) Aluminium 12x Linearguide 3x fixture Voicecoil 3x fixture Shaking Plate 2x angle 30mm x 30 mm x 80 mm Other parts (2x): 4x fixture with 3LEDs each 2x electronics box 2x cameras with holders Granulate Third setup (borrowed) Shaking box parts identical to 1 shaking box above, except size 3x fixture with LEDs 1x electronics box with camera control unit 2x EOS 600D camera with holder and cables

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4.2. Choice of drop tower mode and capsule type We used the catapult mode, as we needed as long microgravity duration as possible. The equilibration of the granular gas to a certain temperature, corresponding to a given excitation, takes part of this time, approximately 4 s with the optimized protocol described in the results section. The remaining time gives statistically relevant data. The dimensions of the catapult capsule are sufficient for our experiment.

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5. Experimental procedure Between subsequent drops, some preliminary evaluation of the previous experiment run is performed and the decision on the next set of parameters was made. The amplitudes / frequencies of the shakers were calibrated beforehand. The general procedure directly before/during/after a launch was as follows: Before the launch: 1. Open the shakers, fill them with the selected number of rods, position them and close the setup again. 2. Adjust the frequency / amplitude and timing protocol for the voice coils and transfer it to the micro-controller. Run the shaker system and check/readjust shaking amplitudes of individual walls. For this, we would record movies of the shaking walls in 1g and obtain the amplitudes. The selected method was working well on the previous experiment, so that with a normal adjustment there are no large deviations expected for zero g. Left and right shaker plate should move with the same amplitude, the back vibration was switched off in several experiments. During the drop: 3. We monitor the videos to get a first impression of the experimental results. The experiment is running independently during this phase. After the capsule landing: 4. Download the videos from the cameras. Pre-analyze a part of the videos and decide on parameter modifications (amplitude, frequency, number and positioning of rods) for the subsequent drop.

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6. Drop tower campaign Figure 6.1 gives some impressions from the drop tower campaign – tasks that we and the engineers Jan and Simon from ZARM Bremen usually performed for each catapult shot.

Figure 6.1: Typical impressions from the drop tower campaign: a) The GAGa DropT team working on their setups in the opened capsule (e.g. replacing and repositioning the granular material). b) Jan and Simon from ZARM are placing the casing of the experiment before it is brought into the vacuum tube. c) The capsule is recovered from the deceleration cask after the shot. d) Kathrin removes some styrofoam beads. e) GAGa DropT and Jan from ZARM during the capsule recovery. f) The capsule is lowered to the bottom of the drop tower, from where it was brought the workspace of the experimentalists.

6.1. Before the first drop The experimental setups were integrated into the catapult capsule as described in the previous section. After integrating all setups, the capsule was balanced by the ZARM engineers. Figure 6.3 (left) shows the added setup 3, with two front view cameras while it is not yet integrated into the drop tower capsule, Fig. 6.3 (right) gives a view onto the platform of setups 1 and 2. The images in Figure 6.3 were taken in Bremen during the integration of the experiment. A photo of the partially assembled capsule is shown in Figure 6.4. After this, the shaker boxes were removed and the focus of the cameras was adjusted such that rods in the midplane would appear clearest on the images. A sequence of calibration images was recorded for each setup. This is used to calculate the 3D positions of the rods from the data in the two perspective views. Last, the granular material was sorted and inserted, see Figure 6.5.

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Figure 6.2: Left: Positioning and preparation of setup 1 and 2 and the corresponding front view cameras on the capsule platform. Right: The catapult capsule while only the setups 1 and 2 (front and top view setups) are integrated. Setup 3 will be inserted at the level in between, where the empty holders for a platform are already mounted.

Figure 6.3: Left: Setup 3 with two front-view cameras at angles of ±20° respective to the normal of the front window of the shaker box prior to its integration into the capsule. Right: view onto the platform with mounted setups 1 and 2 (front and top view videos), an electronics box can be seen on the far left, next to it one of the cameras and facing it the shaker box with some granular material inserted.

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Figure 6.4: The fully assembled interior of the catapult capsule for the GAGa DropT experiment

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Figure 6.5: Left: Rodlike granular (wire pieces of 1.5 cm length) material in the shaker box before the first launch. Right: Sorting the self-fabricated granular material (in this case glass rods) for homogeneity of paint and equal length was necessary.

6.2. Between the launches Before each launch, the granulate containers were opened and the granular material replaced or redistributed. Also, the ARDUINO was programmed according to our selection of vibration amplitudes and frequencies. A list of the parameters of the experiments is given in the next section.

6.3 Technical Problems and Modifications During the 5 drops, several modifications of details of the experiment routine were necessary to improve the output data.

6.3.1 Change of Initial Rod Distribution and Excitation Scheme In the first catapult shot, the granular material was placed at random in the container, as was also done in the previous REXUS experiment. Due to the short available microgravity time, this proved to be no good initial condition: Right after the launch, the material slowly rises from the bottom of the container, see Figure 6.6. As the side walls were already vibrating initially, they had pushed the material away from their reach and their motion has practically no effect on the rod distribution. Consequently, the rods collectively reach the top of the shaker box after about 0.5 s and are only weakly reflected. No acceptable distribution of the material is achieved. In the second catapult shot, the rods were initially placed near the vibrating side walls. The vibrations were switched on only 0.5-1 seconds after lift-off at a large amplitude. At this moment, the “carpet” of granular material has risen approximately through half the box height. Due to the strong excitations, rods become more homogeneously distributed in the container after another 2 seconds. Identically to the previous REXUS experiment, the back wall of the shaker box was also vibrating. This led to an

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accumulation of more particles in the front region of the container, which should be avoided (see Fig. 6.7). An excitation of the back wall proved not to be necessary in drop tower conditions. In the third launch, the initial distribution of the rods was similar to the 2nd launch, except rods were slightly more distributed towards the middle of the box. This proved to be the best rod positioning and was kept for all following measurements. The temporal dependence of the spatial distribution of rods in the container is shown in Figure 6.8. All three distributions and the resulting rod distributions in the container at later stages of the experiment are summarized in Figure 6.9. The excitation scheme was similar and the back wall vibrations were switched off in two of the three setups. Some rods remained at the bottom of the container. After this launch, it was necessary to clean the acrylic front and top walls.

Figure 6.6: The assembly of rods rises slowly from the bottom of the container while the side-wall vibrations have practically no effect on the redistribution of the granulate. Data taken from Day 1, Setup 1.

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Figure 6.7: Accumulation of rods near the front wall of the container due to the vibrations of the back wall, from Day 2, Setup 1 (15 mm rod length) and from Day 3, Setup 2 (12 mm rod length)

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Figure 6.8: Distribution of rods over container with the developed mechanism, lift-off occurs at frame 1278. Data from Day 3, Setup 1, recorded at 500 fps.

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Figure 6.9: Distribution of 250 rods of 15 mm length in the container at different stages of the experiment: top row: before the launch in top view, middle row: in frame 2000 in front view, bottom row: top and corresponding front view images of the container shortly before the end of the microgravity period (frame 5700)

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6.3.2 Electrostatic Charging In contrast to the previous REXUS experiment with a similar container, we encountered problems with electrostatic charging of the rods respective to the container walls in the drop tower experiments, especially in launch 4 after the acrylic front and top walls of the granulate container were cleaned. This resulted in a significant number of rods sticking to the walls. The reduction of the number of rods in the container is unfavourable, as is reduces the number of rod-rod collisions in the experiment as well as it results in less data points for the velocity statistics. The major problem, however, was that these rods partly block the field of view onto the other particles in the container. This problem could not be solved completely without reconstructing parts of the setup for follow-up experiments. However, in launch 5, it could be reduced by applying antistatic spray right before the capsule was prepared for launch in the catapult. Image sequences from a video are shown in Figure 6.10.

Figure 6.10: Still images from Setup 1 (left) and Setup 2 (right) during the 4th launch. Particles got stuck

to the front wall due to electrostatic charging.

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7. Scientific results 7.1. Collected data

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7.2. Data processing and data analysis First, we give an overview of the quality of the collected video data during the five catapult shots. The setup shown in Figure 4.6 is referred to as front-top view setup, the one in Figure 6.1 as front-front view setup. Excitation parameters were given in Table 7.1. The results in this section are preliminary in the sense that not all data were completely evaluated and that work on a more accurate and efficient detection algorithm is a long-term issue and work in progress.

7.2.1 Overview of recorded video data The image quality of both sets of cameras is very different: The high-speed cameras in Setup 1 and Setup 2 record synchronous video material at a frame rate of 500 fps. Their colour reproduction quality and image resolution is not as well suited for the experiment as that of the commercial Canon EOS 600D cameras used in Setup 3. The main drawback of the latter is that they cannot be synchronized in video mode, which leads to a maximum time lag of 1/120 s between corresponding images in both views (recording at 60 frames per second). This becomes relevant only in the 3D reconstruction of fast moving rods, especially during the initial phase of the experiment. Both setups represent an immense improvement over the original REXUS 10 GAGa experimental setup [27]. In Figure 7.1, representative images of the recordings, for which no data was shown in Figures 6.8 and 6.9 are given.

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Figure 7.1: Representative front and top view images (frame 4000) of experiments taken on the five catapult shots in the front-top view setup. Images from the recordings Day 1, Setup 1 and Day 3, Setup 1 are shown in Figure 6.9.

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Figure 7.2: Representative front view images of the left camera extracted from the movies recorded by the front-front view setup (frame number 400) from Days 1, 2, 4 and 5. An image from Day 3 is shown in Figure 7.3 below. The images from the right view camera are comparable.

7.2.2 Rod detection and tracking in 2D image sequences We are working on an automatic detection routine for tracking the coloured particles in 3D. In the current version, considerable manual work is still necessary. The main problem in the particle detection routine is the overlapping of projected images of the particles in both views. Also, crossing particles of the same colour often have to be identified manually. Different methods have been tried, the so far best scheme is given below. The data analysis is performed using matlab. The videos are first split into single frames and then evaluated. The following steps have been implemented and are applied to each image: A. Detection of the particles in the respective 2D views (applied to each video separately):

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Figure 7.3: Frame from a video sequence taken in the front-front view setup (top) and the result of the colour separation step A1, video data recorded on Day 3.

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Figure 7.4 : Exemplary analysed image with automatically merged rod segments and detected rods.

Figure 7.5: Image sequence from the previous REXUS experiment with tracked rods marked in different colours.

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Figure 7.6: Manually detected green rods in frames 3000-3010 of Setup 1, day 3 in front view. The numbered tags are the assigned rod numbers

7.2.2 Calculation of 3D positions from the 2D data The rough data obtained from the two different setup types (front and top view, front views at different viewing angles) can be compared to the expectations resulting from the previous 2D analysis from the REXUS experiment GAGa [34]. There, determining 3D particle tracks was very difficult as the particles were slightly gathering near the front, the top view images suffer from motion blur, the box was very deep and the frame rate was insufficient to obtain high-accuracy data. Some exemplary trajectories were obtained [35]. The currently applied routines follow the scheme below: B. 3D matching and tracking of rods B1. Enter camera calibration settings B2. Identify all rods of this colour in the first frame of each video, giving the projected images of rods that belong together the identical indices. B3. Calculate the 3D position of the rod ends from the 2D data of both views (see below) B4. Rods are tracked by minimum distance to the next position for the automatically detected particles. Tracks are already well defined in the manually detected particles, because there individual particles were already tracked from frame to frame.

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7.3. Results An excitation protocol to distribute an ensemble of granular rods approximately evenly in a cuboid container during drop tower shots has been developed. To achieve this, the majority of the rods are placed close to the moving walls initially and the walls start moving at maximum amplitude (6 mm peak-peak in our setup) only after the granulate has risen to about half the container height. Good starting conditions for the experiment are realised after about one second of strong excitation. After this, the vibration amplitude is set to its nominal value for the measurement. The back wall is not agitated, it can be replaced by a solid wall in future experiments. Furthermore, we encountered problems with rods sticking to the acrylic front and top walls due to electrostatic charging that were not encountered in our previous experiments. This issue was solved in a recent drop tower experiment by using conductively coated (indium tin oxide - ITO) acrylic instead. Instead of tracking and colouring all rods, only some of the rods are coloured (and tracked) in this experiment, the other ones representing a background gas. As all particles are equal (within production inaccuracies) except their colour. The selected method only leads to less data points in the statistics but no qualitatively or quantitatively different results. In this experiment, we used 15 rods coloured in green, blue, yellow, red and purple, the rest of the granular gas consisted of white rods that are not tracked. This method immensely simplifies the identification and tracking of the particles during the data analysis. It proved to be favourable to use even a smaller number of coloured particles for automatic particle tracking in future experiments (10 each). Even if the background rods (white) are not tracked, the information whether a coloured rod is (partially) blocked from view or a background pixel is seen would be useful for the evaluation. Thus, the container walls and the background rods should have opposite colours in future setups. Black walls and white rods would be ideal for tracking, but white walls and black rods had to be chosen due to camera issues in a follow-up experiment. A change of the camera system (if possible) would improve the data quality: Synchronous high-speed recordings (at frame rate 120 fps or higher) combined with the colour image quality of the EOS cameras used in the additional setup would strongly simplify the colour separation and spatial resolution of the data. Also, the illumination scheme should be altered to front illumination instead of oblique lighting, as moving shadows from the moving walls can be avoided for the cost of few small overexposed spots in the images. This also simplifies the automatic detection. An automatic particle tracking routine was developed, but has to be improved. The current algorithm is based on detection, identification and correct merging of coloured segments in the images of each camera view. These are then matched with their counterpart in the other perspective and the 3D-positions of the rod ends is calculated. Alternatively, the rod ends can be identified by hand. The 3D result is accepted if the rod length is within certain length bounds. While the latter method provides up to 50 % more output data points, it is not efficient. Both methods do not use the full image information, i.e. the full cylinder projection. When both cameras are set up in front view, the 3D positions of the rods carry an unacceptable inaccuracy in the depth (z) –position. The front- and top-view setup is favourable for the evaluation. The evaluation of the video material is still in progress, 3D data of a granular gas are obtained. From this, we will finally compute the statistical data that will give the characteristics of the gas under different driving conditions, or the characteristic properties for the different materials. Not all data have been evaluated yet.

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8. Conclusions Within our Drop Your Thesis! - project GAGa DropT, we performed experiments with granular gases of rodlike grains during five drop tower catapult shots in December 2012. Particles can be (partly automatically) tracked in 3D, the accuracy is strongly dependent on the camera positions. We encountered several problems that were not present in the previous REXUS 10 experiment GAGa. In order to obtain an approximately even distribution of the granular material in the shaker box, the granulate must be initially positioned close to the moveable walls, then left to rise through half the container and afterwards strongly excited. After this routine was applied, the frequency and amplitude settings for the actual experiment can be set. We have conducted experiments using two types (glass cylinders and thin wire pieces) of granular material, concentrating on the wire pieces in most experiments. Three different lengths (1.2 cm, 1.5 cm, 1.8 cm) were used. The number of rods was adjusted such that the mean free path would be approximately identical. Different excitations were applied to otherwise identical systems (number, material and length of rods in the container). A comparison is complicated by the fact that not all rods were moving or that the rods were not equally distributed in all cases. These experiments may sketch some statistical results but must be repeated in future drop tower campaigns with an improved setup. The current routine for the data analysis is an improvement of the previous work, but it still must be improved to give more and more reliable data. This could be realised, e. g. by the use of a method that optimises the position and orientation of a cylindrical rod that is projected from 3D into the planes of view. The use of a front- and a top view camera in the experiments is necessary. The data analysis as well as the development of the computer routine are still in progress. We obtain three-dimensional position, orientation and velocity data of a granular gas of rodlike particles in microgravity. First sets of 3D data have been evaluated, and a preliminary comparison to the previous 2D data [34] might be drawn when the analysis is finished. Additional data recorded under identical driving and filling conditions might be necessary to improve the statistics. Thus, not all objectives have been evaluated yet: The appropriate experimental procedure was developed (objective 4). The distribution of rods in the container would be good, if there were no effects of electrostatic charging (objective 1). This was avoided in a follow-up experiment. Quantitative conclusions to objectives 2 and 3 (influence of the excitation parameters and rod material on the properties of the granular gas) cannot be drawn at the current stage of data evaluation. We conclude that the experimental setup should be improved in the following ways:

1. The colour of the background particles should be clearly discernable from the wall colour, e.g., black walls and white background gas rods. Even fewer coloured particles would improve the automatic detection.

2. The illumination should be realised at normal incidence from top and front (as in the front view of the GAGa experiment [27]). This avoids moving shadows of the vibrating walls in the container at the cost of few overexposed image areas. Illumination shall be implemented from top and front.

3. The data analysis can be improved by using more than two cameras, e.g. two cameras at different angles in front view and a top camera. A top view camera is necessary to obtain an acceptable measurement error in all three spatial coordinates. It would be favourable to find a camera system that can be synchronized at record at least 120 frames per second and which also has a better colour resolution than the currently used high-speed cameras.

4. Electrostatic charging of rods with respect to the container walls must be avoided, e.g. by using conductively coated acrylic.

Improvements 1, 2, and 4 have been already implemented in consecutive drop tower experiments within the DLR-projects GAGa and (tests for) Equipage, which is led by our advisor Prof. Stannarius. Whereas the tracking problem partly remains unsolved (improved, but unfinished), a much better colour reproduction and very homogenous distributions of particles without sticking to any walls could be achieved. These experiments are to be continued in 2014 in further drop tower catapult shots and two scheduled sounding rocket launches.

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References [1] Evesque et al., Microgravity Sci. Technol., XIV-I:p. 280, 2005 [2] Falcon et al., PRL 83: p. 440, 1999 [3] Falcon et al., EPL 74: p. 830, 2006 [4] Rouyer and Menon, PRL 85: p. 3676, 2000 [5] Olafsen and Urbach, PRE 60:R2468, 1999 [6] Olafsen and Urbach, PRL 81:4369, 1998 [7] Mikkelsen et al., PRE 70:061307, 2004; Phys. Fluids, 159:S8, 2003 [8] Viridi et al., PRE 74:041301, 2006 [9] Nichol and Daniels, PRL 108:018001, 2012 [10] Maaß et al., PRL 100:248001, 2008 [11] Aranson and Olafsen, PRE 66:061302, 2002 [12] I. S. Aranson and L. S. Tsimring , Granular Patterns, Oxford University Press, Oxford, 2009 [13] Chate et al., PRL 96:180602, 2006 [14] Börzsönyi et al., PRL 108:228302, 2012; Wegner et al., Soft Matter 8: p. 10950, 2012 ; Börzsönyi et al., PRE 86:051304, 2012 [15] Kudrolli et al., PRL 100:058001, 2008 [16] Ramaioli et al., PRE 76:021304, 2007 [17] Blair et al., PRE 67:031303, 2003 [18] Galanis et al., PRL 96:028002, 2006 [19] Narajan et al., J. Stat. Mech. 01:P01005, 2006 [20] Aspelmeier et al. PRE 57: p. 857, 1998 [21] Huthmann et al., PRE 60: 654, 1999 [22] Kanzaki et al., J. Stat. Mech. 06:P06020, 2010 [23] Costantini et al., J. Chem . Phys. 122: 164505, 2005 [24] Villemot and Talbot, Granular Matter 14: 91, 2012 [25] Wildman et al., EPJ ST 179: p. 5, 2009 [26] Daniels et al., PRE 79: 041301, 2009 [27] Harth et al., Proc. 20th ESA PAC Symposium,SP-700 p. 493, 2011 [28] M. Sperl, private communication, unpublished [29] Sack et al., Phys. Rev . Lett. 111,:018011, 2013 [30] Falcon, et al. , Europhys. Lett. 103: 64004, 2013 [31] Börzsönyi and Stannarius, Soft Matter 9, p. 7401, 2013 [32] Huthmann and Zippelius, PRE 56: R6275, 1997 [33] Brilliantov, et al. , PRL 98: 128001, 2007 [34] Harth et al., PRL 110: 114102, 2013 [35] Harth et al., AIP Conf. Proc. (Powders and Grains) 1542, p. 807, 2013

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A. Appendices

A.1. Technical description

A.1.1. Components description

Component Weight (kg)

Electronics box (2) (1x Board, 1xTracopower TAE60-24-12, 3x Pololu 18v15,

1x Arduino Nano) 2*0.6

holder with 3 LEDs (4) 4*0.25 Shaker Box (2)

(with 3xVoiceCoils CVC30-15, 2xLED holder) 2*1.65

Photron Fastcam MC2 heads (4) Approx. (4*0.25)

Platform (2) 2*15.5 Total 37.5

A.1.2. Products used for the experiment

There are no chemicals, gases or dusts used in our experiment.

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A.1.3. Electrical and Electronic systems The electronics of the experiment needed only one electricity plug-in per setup (2 all in all) , switched on with the catapult drop-off. The maximum power consumption is about 200W. We use a D-sub (9 Pins) as power connector. The internal electronics scheme is shown in Figure 4.8. High-speed cameras were controlled using electronics provided by ZARM.

A.1.4. Special equipment

We used the 4 colour high-speed camreas at ZARM. No other special equipment was needed.

A.2. Mechanical strength of structures A.2.1. Characteristics of the payload

Platform Overall

dimensions (X×Y×Z in mm)

Mass (Kg)

Position of the Center Of Mass (X,Y,Z in mm)

1 655x655x152 21.5

(12, 0, ?) (z not calculated or measured)

2 655x655x68 16

(2.9, 0, ?) (z not calculated or measured)

A.2.2. Maximum acceleration and deceleration

We presented a mechanically idential prototype of our shaker and electronics to the ZARM engineers during our visit in Bremen in September. They considered the setup robust to any accelerations occuring during the catapult experiment.

A.2.3. Point load and distributed load

A.2.4. Centre of mass

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The centers of mass of the platforms are shown in the pictures below.

Figure 12: The green dots show the centers of mass of the platforms. left) platform 1 (lower platform), right) platform 2 (upper platform)