The authors have no acknowledgements.

1. Experiments with Particles Coarser or Equal to 700 &#181;m
<ol>
	<li>Preparation of the experimental setup
	<ol>
		<li>Remove the sleeve and lift the top cover of the vacuum chamber. Place the baseplate consisting of the desired wall material in the vacuum chamber. Turn the lower part of the vacuum chamber sideways to slide in the plate carefully by hands.</li>
		<li>Place exactly one of the particles to be examined with tweezers in the center of the baseplate. Afterwards adjust the height of the camera with a tripo...

<ol>
	<li>Ennis, B. J., Green, J., Davies, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+legacy+of+neglect.">The legacy of neglect.</a> U.S. Chem. Eng. Prog. 90 (4), 32-43 (1994).</li><li>Nedderman, R. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Statics and Kinematics of Granular Materials. , (1992).</li><li>Seifried, R., Schiehlen, W., Eberhard, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Numerical+and+experimental+evaluation+of+the+coefficient+of+restitution+for+repeated+impacts.">Numerical and experimental evaluation of the coefficient of restitution for repeated impacts.</a> Int. J. Impact Eng. 32, 508-524 (2005).</li><li>Thornton, C., Ning, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+theoretical+model+for+the+stick/bounce+behaviour+of+adhesive,+elastic-plastic+spheres.">A theoretical model for the stick/bounce behaviour of adhesive, elastic-plastic spheres.</a> Powder Technol. 99, 154-162 (1998).</li><li>Antonyuk, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Energy+absorption+during+compression+and+impact+of+dry+elastic-plastic+spherical+granules.">Energy absorption during compression and impact of dry elastic-plastic spherical granules.</a> Granul. Matter. 12, 15-47 (2010).</li><li>G&#252;ttler, C., Hei&#223;elmann, D., Blum, J., Krijt, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Normal+Collisions+of+Spheres:+A+Literature+Survey+on+Available+Experiments.">Normal Collisions of Spheres: A Literature Survey on Available Experiments.</a> arXIV. , (2012).</li><li>Bharadwaj, R., Smith, C., Hancock, B. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+coefficient+of+restitution+of+some+pharmaceutical+tablets/compacts.">The coefficient of restitution of some pharmaceutical tablets/compacts.</a> Int. J. Pharm. 402, 50-56 (2010).</li><li>Fu, J., Adams, M. J., Reynolds, G. K., Salman, A. D., Hounslowa, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+deformation+and+rebound+of+wet+granules.">Impact deformation and rebound of wet granules.</a> Powder Technol. 140, 248-257 (2004).</li><li>Sommerfeld, M., Huber, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+analysis+and+modelling+of+particle-wall+collisions.">Experimental analysis and modelling of particle-wall collisions.</a> Int. J. Multiphas. Flow. 25, 1457-1489 (1999).</li><li>Foerster, S. F., Louge, M. Y., Chang, H., Allia, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+the+collision+properties+of+small+spheres.">Measurements of the collision properties of small spheres.</a> Phys. Fluids. 6 (3), 1108-1115 (1994).</li><li>Lorenz, A., Tuozzolo, C., Louge, M. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+Impact+Properties+of+Small,+Nearly+Spherical+Particles.">Measurements of Impact Properties of Small, Nearly Spherical Particles.</a> Exp. Mech. 37 (3), 292-298 (1997).</li><li>Fu, J., Adams, M. J., Reynolds, G. K., Salman, A. D., Hounslowa, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+deformation+and+rebound+of+wet+granules.">Impact deformation and rebound of wet granules.</a> Powder Technol. 140, 248-257 (2004).</li><li>Wong, C. X., Daniel, M. C., Rongong, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Energy+dissipation+prediction+of+particle+dampers.">Energy dissipation prediction of particle dampers.</a> J. Sound Vib. 319, 91-118 (2009).</li><li>Louge, M. Y., Tuozzolo, C., Lorenz, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+binary+impacts+of+small+liquid-filled+shells.">On binary impacts of small liquid-filled shells.</a> Phys. Fluids. 9, 3670-3677 (1997).</li></ol>

The authors have nothing to disclose.

To validate the functionality of the experimental setup in general, tests with similar material combinations as in other established setups (Antonyuk et al.5 and Wong et al.13) were performed. Since very similar results were obtained, the general procedure seems to work. Nevertheless, caution has to be taken towards the procedure and the analysis and further improvements are necessary.
The main limitation of the experimental setup is the quality of the v...

Powders and granules are everywhere around us. A life without them is impossible in modern societies. They appear in food and drinks as grains or even flour, sugar, coffee and cocoa. They are needed for daily used objects like the toner for laser printer. Also the plastic industry is not imaginable without them, because plastic is transported in granular form before it is melted and given a new shape. After Ennis et al.1 at least 40% of the value added to the consumer price index of the United States of America by the chemical industry (agriculture, food, pharmaceuticals, minerals, munitions) is connected to particle technology. Nedderman2 even stated that about 50% (weight) of the products and a minimum of 75% of the raw materials are granular solids in the chemical industry. He also declared that there occur many problems concerning storage and transportation of granular materials. One of these is that during transport and handling many collisions take place. To analyze, describe and predict the behavior of a particulate system, Discrete Element Method (DEM) simulations can be performed. For these simulations knowledge of the collision behavior of the particulate system is necessary. The parameter that describes this behavior in DEM simulations is the coefficient of restitution (COR) that has to be determined in experiments.
The COR is a number that characterizes the loss of kinetic energy during the impact as described by Seifried et al.3. They explained that this is caused by plastic deformations, wave propagation and viscoelastic phenomena. Thornton and Ning4 also mentioned that some energy might be dissipated by work due to interface adhesion. The COR depends on impact velocity, material behavior, particle size, shape, roughness, moisture content, adhesion properties and temperature as stated in Antonyuk et al.5. For a completely elastic impact all absorbed energy is returned after the collision so that the relative velocity between the contact partners is equal before and after the impact. This leads to a COR of e = 1. During a perfectly plastic impact all the initial kinetic energy is absorbed and the contact partners stick together which leads to a COR of e = 0. Furthermore, G&#252;ttler et al.6 explained that there are two types of collisions. On the one hand, there is the collision between two spheres which is also known as the particle-particle contact. On the other hand, there is the collision between a sphere and a plate that is also called particle-wall contact. With the data for the COR and other material properties like coefficient of friction, density, Poisson&#39;s ratio and shear modulus DEM simulations can be performed to determine the post-collisional velocities and orientations of the particles as explained by Bharadwaj et al.7. As shown in Antonyuk et al.5, the COR can be calculated with the ratio of rebound velocity to impact velocity.
Therefore an experimental setup for free-fall tests to examine the particle-wall contact of particles with a diameter from 0.1 mm to 4 mm was constructed. The advantage of free-fall experiments compared to accelerated experiments as in Fu et al.8 and Sommerfeld and Huber9 is that rotation might be eliminated. Hence, the transfer between rotational and translational kinetic energy which influences the COR can be avoided. Aspheric particles need to be marked as in Foerster et al.10 or Lorenz et al.11 to take rotation into account. As the COR is depending on the impact velocity, the impact velocities in the experiments have to match the ones in the real transport and handling processes. In free-fall experiments under atmospheric pressure, the impact velocity is limited by the drag force, having an increasing influence for a decreasing particle size. To overcome this drawback, the experimental setup works under vacuum conditions. A second challenge is to drop just one single particle since then it is possible to characterize all properties that influence the COR beforehand, for instance surface roughness and adhesion. With this knowledge, the COR can be determined according to the properties of the particle. For this, a new release mechanism was developed. Another issue is the adhesive forces of powders with a diameter inferior to 400 &#181;m. Therefore, a dry and ambient temperature environment is necessary to overcome adhesion.
The experimental setup consists of several parts. An exterior view of the existing experimental setup is shown in Figure 1. First, there is the vacuum chamber that is made out of glass. It is composed of a lower part (cylinder), a top cover, a seal ring and a sleeve to connect the parts. The lower part has two openings for a connection with the vacuum pump and the vacuum gauge. The top cover has four openings. Two of them are necessary for the sticks of the release mechanism described below and also two that can be used for further improvements of the experiment. All these openings can be closed with seal rings and screw caps when working under vacuum conditions.
Moreover, a new release mechanism was developed since the use of a vacuum nozzle as in many other experiments documented in literature (for example Foerster et al.10, Lorenz et al.11, Fu et al.12 or Wong et al.13) is not possible in a vacuum environment. The mechanism is realized by a cylindrical chamber with a conical drill hole that is held by a plate. This is connected to a stick that fits in one of the seal rings of the top cover of the vacuum chamber and guarantees the adjustment of a variable initial height for the free-fall experiments. A scale is drawn on the stick for measuring the height. The closing of the particle chamber is implemented by a conical tip of a pipette that is again connected to a stick. The new release mechanism can be seen in Figure 2 and works as described here: in the initial state the pipette tip is pushed down so that the circumference of the tip touches the edge of the chamber&#39;s drill hole. The chamber is closed with the pipette tip such that there is no space for a particle to leave the chamber through the hole. To release the particle, the stick is pulled upwards very slowly together with the tip connected to it. As the diameter of the tip is getting smaller a gap between its circumference and the edge of the drill hole arises through which the particle can leave the chamber. Although one might expect a rotation of the particle with the newly developed release mechanism as the particle could &#39;roll&#39; out of the chamber, a different behavior appears in the experiments. Figure 3 shows the impact of an aspherical particle from 50 frames before to 50 frames after the impact in steps of 25 frames. From the shape of the particle no rotation is visible before the impact (1-3) whereas afterwards it obviously spins (4-5). Therefore the claimed non-rotational release is taking place with this release mechanism.
Another component of the experimental setup is the baseplate. In fact there are three different kinds of baseplates consisting of different materials. One is made of stainless steel, a second of aluminum and a third of polyvinyl chloride (PVC). These baseplates represent frequently used materials in process engineering for example in reactors and tubes.
To determine the impact and rebound velocities, a high-speed camera with 10,000 fps and a resolution of 528 x 396 pixels is used. This configuration is chosen as there is always one picture near the impact and also the resolution is still satisfactory. The camera is connected to a screen that shows the videos in the instant when they are recorded. This is necessary, because the high speed camera can only save a limited amount of pictures and overwrites the beginning of the video when this amount is exceeded. Furthermore, a strong light source for the illumination of the visual field of the high-speed camera is required. For illumination uniformity a sheet of technical drawing paper is glued on the backside of the vacuum chamber that spreads the light.
Finally, a two-stage rotary vane pump is used to establish a vacuum of 0.1 mbar and a vacuum gauge measures the vacuum to guarantee constant environmental conditions.
For the here presented work glass beads with different particle diameters (0.1-0.2, 0.2-0.3, 0.3-0.4, 0.700, 1.588, 2.381, 2.780, 3.680 and 4.000 mm) are used. The beads are made of soda lime glass and are spherical with a rather smooth surface.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>High-speed camera Olympus i-SPEED 3</td>
			<td>Olympus</td>
			<td></td>
			<td>High-speed camera to capture the particle impact</td>
		</tr>
		<tr>
			<td>Screen Olympus i-SPEED CDU</td>
			<td>Olympus</td>
			<td></td>
			<td>Screen to work with the high-speed camera</td>
		</tr>
		<tr>
			<td>Light source Olympus ILP-2</td>
			<td>Olympus</td>
			<td></td>
			<td>Light source necessary for taking videos at high frame rates</td>
		</tr>
		<tr>
			<td>Vacuum pump Alcatel Pascale 2005 D</td>
			<td>Alcatel</td>
			<td></td>
			<td>Vacuum pump to generate the vacuum during the experiments</td>
		</tr>
		<tr>
			<td>Vacuum gauge Alcatel CFA 212</td>
			<td>Alcatel</td>
			<td></td>
			<td>Vacuum gauge to measure the vacuum level</td>
		</tr>
		<tr>
			<td>i-SPEED Software Suite (Control version)</td>
			<td>Olympus</td>
			<td></td>
			<td>Software to evaluate the videos</td>
		</tr>
		<tr>
			<td>Glass beads</td>
			<td>Sigmund Lindner GmbH</td>
			<td></td>
			<td>SiLibeads Type P (0.700, 1.588, 2.381, 2.780, 3.680, 4.000 mm) 
			SiLibeads Type S (0.1-0.2, 0.2-0.3, 0.3-0.4 mm) 
			http://www.sigmund-lindner.com (see supplier&#39;s website for more information about the glass properties)</td>
		</tr>
		<tr>
			<td>Safety goggles</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

development of an experimental setup for the measurement of the coefficient of restitution under vacuum conditions

The Discrete Element Method is used for the simulation of particulate systems to describe and analyze them, to predict and afterwards optimize their behavior for single stages of a process or even an entire process. For the simulation with occurring particle-particle and particle-wall contacts, the value of the coefficient of restitution is required. It can be determined experimentally. The coefficient of restitution depends on several parameters like the impact velocity. Especially for fine particles the impact velocity depends on the air pressure and under atmospheric pressure high impact velocities cannot be reached. For this, a new experimental setup for free-fall tests under vacuum conditions is developed. The coefficient of restitution is determined with the impact and rebound velocity which are detected by a high-speed camera. To not hinder the view, the vacuum chamber is made of glass. Also a new release mechanism to drop one single particle under vacuum conditions is constructed. Due to that, all properties of the particle can be characterized beforehand.

For the analysis glass particles with a diameter of 100 &#181;m to 4.0 mm were dropped from an initial height of 200 mm on a stainless steel baseplate with a thickness of 20 mm.
Figure 6 shows the mean values as well as the maximum and minimum values for the COR depending on the particle size for atmospheric pressure and vacuum. The mean value of the COR is found to be approximately e = 0.9 for particle...

Watch this Scientific Journal Video about Development of an Experimental Setup for the Measurement of the Coefficient of Restitution under Vacuum Conditions at JoVE.com

Development of an Experimental Setup for the Measurement of the Coefficient of Restitution under Vacuum Conditions

The coefficient of restitution is a parameter that describes the loss of kinetic energy during collision. Here, a free-fall setup under vacuum conditions is developed to be able to determine the coefficient of restitution parameter for particles in micrometer range with high impact velocities.

The Discrete Element Method is used for the simulation of particulate systems to describe and analyze them, to predict and afterwards optimize their ...

The overall goal of this experimental setup is to measure the coefficient of restitution of colliding particles under vacuum conditions. This method can contribute to solve engineering problems since the coefficient of restitution is present in any engineering operation such as transportation, handling, storage of granular solids in the engineering industry. The main advantage of this method is that it can be applied to fine powders under vacuum conditions without accelerating the particle when it comes into the chamber.Sven Drucker, a grad student from my lab will be demonstrating the procedure. He works in the framework of a project called Pardem. For working with particles of any size, first remove the sleeve over the vacuum chamber and lift off the top cover.Then, select the base-plate with the desired wall material into the chamber. Next, turn the lower part of the vacuum chamber sideways and carefully slide the base-plate into position. Next, using tweezers, place exactly one particle of a known size at the center of the base-plate.Then, adjust the position of a high-speed video camera mounted to a tripod so that the base-plate is in the lowest quarter of the visual field and focus is on the particle. Set the high-speed camera to record 10, 000 frames per second at a resolution of 528 by 396 pixels. Then, take a brief video of the particle for reference purposes.Don't forget to remove the particle afterwards. When working with particles that are 700 microns or coarser, first adjust the height of the particle chamber so the desired impact velocity is reached. Measure the height using the scale attached to the holding plate.Then, close the particle chamber with the tip of a pipet. Now, lift the top cover of the vacuum chamber and put one single sphere inside of the particle chamber using tweezers. This example shows a solid sphere.However, liquid-filled spheres can also be tested. Place the top cover on the lower part of the vacuum chamber and connect the top cover and the lower part of the vacuum chamber with the sleeve. Now, put on safety goggles before generating the vacuum.Using the pump, evacuate the chamber until the desired pressure is obtained. The target pressure in this demonstration is 0.1 millibars. Then, close the valve on the side of the chamber and turn off the pump.At this point, start the video recording. While recording, open the hole of the particle chamber to liberate the particle, and at the same time pull and turn the stick attached to the tip of the pipet. This will prevent stick slip problems due to high friction between the stick and the sealed ring.After the impact occurs, immediately stop the recording. Only a limited number of frames can be saved, and the first are over-written as soon as the limit is exceeded. Cut the movie around the instant of the impact at the screen, and save it on the memory card.Anticipate repeating the experiment about 10 times for an accurate main value. When working with finer particles, use essentially the same procedure with a few alterations. Load 50 to 100 spheres onto a folded sheet of paper and load the particles into the chamber off the paper.When pulling the stick at the initiation of the experiement, pull very slowly to prevent all the particles from dropping at the same time. When cutting the movie, do it in such a way that at least 10 clearly focused impacts are visible. Before evaluating the data, it is first necessary to calibrate the software.Load a frame with the particle of known size obtained during the setup. Then, count the number of pixels of the horizontal diameter and divide the known distance by the number of pixels to get the conversion factor, distance per pixel. The horizontal is easier to use because of the contrast between the particle and white background.To calculate the impact velocity, set a reference point of motion on the top of the sphere 10 frames before the point of impact and a second reference point one frame before the impact. Use the number of pixels between the two points and the conversion factor to calculate the traveled distance. Then, divide the distance by the passed time between nine frames to obtain the impact velocity.To calculate the rebound velocity, use reference points on the top of the sphere, one frame after the impact and ten frames after the impact. Finally, calculate the coefficient of restitution, or COR, as the ratio of rebound velocity to impact velocity. Repeat all these steps to evaluate all the recorded drop test videos.Glass particles with diameters of 100 microns to four millimeters were dropped from an initial height of 200 millimeters onto a stainless steel base-plate with a thickness of 20 millimeters. The mean value of the COR was about 0.9 for particles 700 microns or larger. This result was independent of the air pressure.For particles with a diameter less than 400 microns, the COR was nearly constant with a value of 0.9 under vacuum conditions. Under atmospheric pressure, the coefficient of restitution decreased with particle diameter. The results for the impact velocity depended on the particle's size and atmospheric pressure.Fine powders were a lot slower under atmospheric pressure, whereas their impact velocity decreased only slightly under vacuum conditions. The data showed some exceptional outliers at 700 microns that may have been created by a false calibration. After watching this video, you should have a good understanding how to measure the coefficient of restitution in free fall experiments under vacuum conditions.Once mastered, this method can be done in one and a half hours for 10 particles if it is properly applied. Don't forget that working with a glass cylinder in vacuum conditions is extremely hazardous, and that precautions should always be taken.

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8.0K 조회수.  University of Technology of Compiègne. The overall goal of this experimental setup is to measure the coefficient of restitution of colliding particles under vacuum conditions. This method can contribute to solve engineering problems since the coefficient of restitution is present in any engineering operation such as transportation, handling, storage of granular solids in the engineering industry. The main advantage of this method is that it can be applied to fine powders under vacuum conditions without accelerating the particle wh...