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 tripod in such a way that the baseplate is in the lowest quarter of the visual field and focus on the particle.</li>
		<li>Remove the particle using tweezers.</li>
	</ol>
	</li>
	<li>Experimental Procedure
	<ol>
		<li>Adjust the height of the particle chamber in such a way that the desired impact velocity of the particle is reached. Use the scale on the stick attached to the holding plate as an indicator of the height. Close the particle chamber with the tip of the pipette by pushing it down so that the circumference of the pipette touches the edge of the chamber&#39;s drill hole. Open the sleeve and lift the top cover of the vacuum chamber.</li>
		<li>Put one single sphere in the particle chamber with tweezers. The sphere can be solid or liquid-filled (as in Louge et al.14), depending on what kind of particles shall be analyzed. However, in this work only solid particles are examined. Place the top cover on the lower part of the vacuum chamber (cylinder) and connect the top cover and the lower part of the vacuum chamber with the sleeve.</li>
		<li>Evacuate the chamber with the vacuum pump until a level of 0.1 mbar (or any other desired value) is reached. Measure the pressure with a vacuum gauge. Close the valve at the side of the vacuum chamber and turn off the vacuum pump. Wear safety goggles when working under vacuum conditions.</li>
		<li>Apply a frame rate of 10,000 fps and adjust the camera settings (position/zoom) to obtain a resolution of 528 x 396 pixels. Start the recording of the high-speed camera and open the hole of the particle chamber to liberate the particle. Simultaneously pull and turn the stick attached to the tip of the pipette to avoid stick-slip problems due to high friction between stick and seal ring.</li>
		<li>Stop the recording of the camera directly after impact because only a limited amount of pictures can be saved and the first ones are overwritten when this limit is exceeded. Cut the movie around the instant of the impact at the screen and save it on the memory card.</li>
		<li>Repeat the experiment ten times to obtain statistically significant results. The results are statistically significant if after ten repetitions, the mean value does not alter anymore (this might be different for other materials depending on the homogeneity of the sample or other particle shapes).</li>
	</ol>
	</li>
	<li>Evaluation Procedure
	<ol>
		<li>Calibrate the software with the known size of a particle or another object using one frame of the video made in step 1.2.4 to obtain a conversion between pixels and distances. Use the horizontal diameter as it is not blurred due to the particle&#39;s motion.
		<ol>
			<li>Count the number of pixels of the horizontal diameter and then divide the known distance by the number of pixels to get the conversion factor &#39;distance per pixel&#39;. A picture of the calibration process is shown in Figure 4.</li>
		</ol>
		</li>
		<li>Set a reference point of motion on the top of the sphere ten frames before and one frame before the impact to calculate the impact velocity. Figure 5 presents the two reference points of motion. With the conversion factor from step 1.3.1, use the number of pixels between the two points to obtain the travelled distance. Divide the distance by the passed time (product of number of frames and time step) to obtain the impact velocity.</li>
		<li>Set a reference point of motion on the top of the sphere one frame after and ten frames after the impact to calculate the rebound velocity. Determine the rebound velocity analogously to step 1.3.2.</li>
		<li>Calculate the COR as ratio of rebound velocity to impact velocity.</li>
		<li>Repeat the steps 1.3.1-1.3.4 for the evaluation of all recorded drop test videos.</li>
	</ol>
	</li>
</ol>
2. Experiments with Powders Finer or Equal to 400 &#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 an adequate reference object such as a particle with a known size in the center of the baseplate with tweezers. Afterwards adjust the height of the camera with a tripod in such a way that the baseplate is in the lowest quarter of the visual field and focus the reference object.</li>
		<li>Record a short video of the reference object when it is lying on the baseplate with exactly the same settings as in the following experiments.</li>
		<li>Remove the reference object using tweezers.</li>
	</ol>
	</li>
	<li>Experimental Procedure
	<ol>
		<li>Adjust the height of the particle chamber in such a way that the desired impact velocity of the particle is reached. Use the scale on the stick attached to the holding plate as an indicator of the height. Close the particle chamber with the tip of the pipette by pushing it down so that the circumference of the pipette touches the edge of the chamber&#39;s drill hole. Open the sleeve and lift the top cover of the vacuum chamber.</li>
		<li>Put 50 to 100 spheres in the particle chamber. To guide the spheres into the particle chamber, deposit them first on a folded sheet of paper. Use the folded paper as a groove to slide the particles into the chamber. Place the top cover on the lower part of the vacuum chamber (cylinder) and connect the top cover and the lower part of the vacuum chamber with the sleeve.</li>
		<li>Evacuate the chamber with the vacuum pump until a level of 0.1 mbar (or any other desired value) is reached. Measure the pressure with a vacuum gauge. Close the valve at the side of the vacuum chamber and turn off the vacuum pump. Wear safety goggles when working under vacuum conditions.</li>
		<li>Start the recording of the high-speed camera with 10,000 fps and a resolution of 528 x 396 pixels and open the hole of the particle chamber to liberate the particles. Simultaneously pull and turn the stick attached to the tip of the pipette to avoid stick-slip problems due to the high friction between stick and seal ring. Pull very slowly to prevent that all the particles drop at the same time.</li>
		<li>Stop the recording of the camera 5 to 6 sec after the impact of the first particle because only a limited amount of pictures can be saved and the first ones are overwritten when this limit is exceeded. Cut the movie at the screen in such a way that at least 10 clearly focused impacts of particles are visible and save it on the memory card.</li>
	</ol>
	</li>
	<li>Evaluation Procedure
	<ol>
		<li>Calibrate the software with the known size of the reference object from the video of step 2.1.3 to obtain a conversion between pixels and distances. Count the number of pixels of the size of the reference object and then divide the known distance by the number of pixels to get the conversion factor &#39;distance per pixel&#39;.</li>
		<li>Set a reference point of motion on the top of the first clearly focused sphere in the video ten frames before and one frame before the impact to calculate the impact velocity. Calculate the impact velocity analogously to step 1.3.2 together with the conversion factor from step 2.3.1.</li>
		<li>Set a reference point of motion on the top of the first clearly focused sphere one frame after and ten frames after the impact to calculate the rebound velocity. Calculate the rebound velocity analogously to step 2.3.2.</li>
		<li>Calculate the COR as ratio of rebound velocity to impact velocity.</li>
		<li>Repeat steps 2.3.2-2.3.3 for the evaluation of the impacts of another nine clearly focused spheres.</li>
	</ol>
	</li>
</ol>

<ol><li>Ennis, B. J., Green, J., Davies, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+legacy+of+neglect%5BTitle%5D)+AND+%22U.S.+Chem.+Eng.+Prog%22%5BJournal%5D)">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://scholar.google.com/scholar?hl=en&safe=off&q=author%3aNedderman%2C+RM+%22Statics+and+Kinematics+of+Granular+Materials%22">Statics and Kinematics of Granular Materials</a>. , Cambridge University Press. Cambridge. (1992).</li>
<li>Seifried, R., Schiehlen, W., Eberhard, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Numerical+and+experimental+evaluation+of+the+coefficient+of+restitution+for+repeated+impacts%5BTitle%5D)+AND+%22Int.+J.+Impact+Eng%22%5BJournal%5D)">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/pubmed?term=((A+theoretical+model+for+the+stick%2Fbounce+behaviour+of+adhesive%2C+elastic-plastic+spheres%5BTitle%5D)+AND+%22Powder+Technol%22%5BJournal%5D)">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/pubmed?term=((Energy+absorption+during+compression+and+impact+of+dry+elastic-plastic+spherical+granules%5BTitle%5D)+AND+%22Granul.+Matter%22%5BJournal%5D)">Energy absorption during compression and impact of dry elastic-plastic spherical granules.</a> Granul. Matter. 12, 15-47 (2010).</li>
<li>Güttler, C., Heißelmann, D., Blum, J., Krijt, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Normal+Collisions+of+Spheres%3A+A+Literature+Survey+on+Available+Experiments%5BTitle%5D)+AND+%22arXIV%22%5BJournal%5D)">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/pubmed?term=((The+coefficient+of+restitution+of+some+pharmaceutical+tablets%2Fcompacts%5BTitle%5D)+AND+%22Int.+J.+Pharm%22%5BJournal%5D)">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/pubmed?term=((Impact+deformation+and+rebound+of+wet+granules%5BTitle%5D)+AND+%22Powder+Technol%22%5BJournal%5D)">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/pubmed?term=((Experimental+analysis+and+modelling+of+particle-wall+collisions%5BTitle%5D)+AND+%22Int.+J.+Multiphas.+Flow%22%5BJournal%5D)">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/pubmed?term=((Measurements+of+the+collision+properties+of+small+spheres%5BTitle%5D)+AND+%22Phys.+Fluids%22%5BJournal%5D)">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/pubmed?term=((Measurements+of+Impact+Properties+of+Small%2C+Nearly+Spherical+Particles%5BTitle%5D)+AND+%22Exp.+Mech%22%5BJournal%5D)">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/pubmed?term=((Impact+deformation+and+rebound+of+wet+granules%5BTitle%5D)+AND+%22Powder+Technol%22%5BJournal%5D)">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/pubmed?term=((Energy+dissipation+prediction+of+particle+dampers%5BTitle%5D)+AND+%22J.+Sound+Vib%22%5BJournal%5D)">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/pubmed?term=((On+binary+impacts+of+small+liquid-filled+shells%5BTitle%5D)+AND+%22Phys.+Fluids%22%5BJournal%5D)">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 videos. On the one hand, the images can be blurred because the particle is not exactly in the visual field of the camera. Here, the particle impacts not perfectly centric due to instability of the particle chamber during particle release. On the other hand, the resolution is responsible for the accuracy of determining the velocities, because the number of pixels limits the number of nuances. Therefore it is necessary to test different frame rates since the higher the frame rate the lower the resolution. To avoid calibration problems of the conversion between pixels and distances as in the experiments with 700 &#181;m particles, a standardized calibration with a video of a reference object with an adequate, known size should be taken with exactly the same camera settings as in the experiments. A recommendation for such an object could be a ruler placed at the impact position of the particle. This object would also be preferable for experiments with larger particles since larger reference distances reduce the calibration error.
To minimize the instability of the particle chamber during the release, the setup will be automated and the stick connected to the tip of the pipette will be guided by bushings. A wire is connected to the top of the stick and then guided over one pulley aligned above the stick to another pulley connected to a motor that pulls the wire. Therefore also a frame is constructed. The future setup is shown in Figure 8.
To be able to release powders inferior to 20 &#181;m several improvements will be made. For these fine particles adhesive forces much higher than the gravitational ones occur so that particles stick together and to walls. Therefore a vibration of the particle chamber and the needle could be a means to overcome the adhesive forces and make the particle leave the chamber. Another difficulty is that these particles are too small to be visible during the experiment. Therefore a sensor is required that indicates the instant when the particle falls because the high speed camera can only save a limited amount of pictures and overwrites the beginning of the video when this is exceeded. Moreover, a microscope objective will be mounted on the camera to be able to analyze particles down to 10 &#181;m.
Also the particle-particle contact needs to be examined to be able to characterize all collisions in DEM simulation. One possibility to evaluate this could be to place many particles on the baseplate and to carry out the drop experiment for an impact on a particle then. Another idea to analyze the particle-particle contact would be to modify this experimental setup and apply a similar technique to the one of Foerster et al.10 and Lorenz et al.11. In their experiments two particles were liberated with different initial heights and collide before the impact on the ground. Afterwards the results for the particle-particle and the particle-wall contact need to be compared.

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><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)&lt;br/&gt; SiLibeads Type S (0.1-0.2, 0.2-0.3, 0.3-0.4&amp;nbsp;mm)&lt;br/&gt; http://www.sigmund-lindner.com (see supplier&#039;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 particles greater or equal to 700 &#181;m independent of the air pressure.
For particles with a diameter less than 400 &#181;m the COR stays nearly constant with a value of e = 0.9 under vacuum conditions. Under atmospheric pressure the COR decreases with decreasing particle diameter. A reason for this could be that the air in front of the particle is compressed during the free-fall which results in kind of a cushion that damps the collision, absorbs kinetic energy and due to that leads to a lower COR. In both cases the deviations are higher than for coarser particles. An explanation for this might be that the fine particles only had the size of a few pixels in the videos. Therefore the error due to the choice of the pixels in a blurred picture is intense.
The results for the impact velocity depending on the particle size for atmospheric pressure and vacuum are presented in Figure 7. For the impact velocity the mean values, the maximum and the minimum are shown. The mean value of the impact velocity is evaluated with approximately vi = 2 msec-1 for particles greater than 700 &#181;m independent of the air pressure. An exception appears for a particle diameter of 700 &#181;m where the impact velocity is significantly lower under vacuum conditions and even slightly more under atmospheric pressure. For a decreasing particle diameter a decreasing impact velocity under atmospheric pressure was expected. In contrast to that, the impact velocity should stay the same under vacuum conditions. Having a closer look at the evaluation method it can be seen that for the particles with a diameter of 700 &#181;m the calibration for the conversion between pixels and distances is different to that for the coarser particles. The ratio of pixels per millimeter is significantly higher which results in lower velocities. A reason for the false calibration might be that the camera is not able to correctly recognize the shape of the finer particles. Using the same standardized calibration as for the coarser particles the impact velocities are still approximately in the same range and the outliers can be eliminated.
For powders with a diameter inferior to 400 &#181;m the impact velocity decreases significantly with a decreasing particle diameter under atmospheric pressure. The equilibrium of air friction force and gravitational force, and also the settling velocity, is reached earlier for finer particles. In contrast to that, the impact velocity under vacuum conditions is nearly constant also for the powders. This proves the theory of an infinitely accelerating particle, when there is no air that can result in a drag force and because of that an equilibrium of forces is never reached. It also shows the necessity of vacuum conditions and therefore also the newly developed release mechanism to reach high impact velocities with fine particles. In these experiments just a slight decrease of the impact velocity is recognizable that can be explained by the fact that only a vacuum of 0.1 mbar was reached which is not a perfect vacuum. The much higher deviations for particles with a median diameter of 0.113 mm occur as the influence of the error due to the choice of the pixels in a blurred picture is higher for lower velocities.
<img alt="Figure 1" src="/files/ftp_upload/53299/53299fig1.jpg" /> 
Figure 1. Exterior view of the vacuum chamber. This figure shows the vacuum chamber from the side. One can see the lower part with its two openings for a connection with the vacuum pump and the vacuum gauge. Moreover, the top cover with four openings with seal rings and screw caps are visible. The seal ring is between the lower part and the upper part. The sleeve was removed in this picture. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53299/53299fig2.jpg" /> 
Figure 2. Release mechanism with particle chamber and tip of a pipette. This figure describes the newly developed release mechanism for vacuum experiments. Firstly, the plate holding the cylindrical chamber with a conical drill hole can be seen. In addition, the two sticks for the adjustment of a variable initial height and the connection to the conical tip of a pipette are presented. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53299/53299fig3.jpg" /> 
Figure 3. Non-rotational release. This figure shows a series of pictures of an aspherical particle from 50 (1) and 25 frames (2) before the impact as well as at the impact (3) and at 25 (4) and 50 (5) frames after the impact. The identical shape of the particle up to the impact reveals the non-rotational release. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53299/53299fig4.jpg" /> 
Figure 4. Calibration of the software. This figure shows a particle from a video of a recorded free-fall experiment. The red line represents the size of the particle and embraces the number of pixels necessary for calculating the conversion factor. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53299/53299fig5.jpg" /> 
Figure 5. Reference point of motion. This figure presents a particle in a video of a recorded free-fall experiment. The two red crosses illustrate the two reference points of motion on the top of the sphere in the respective frame: the upper one at ten frames before the impact and the lower one at one frame prior to the impact. The distance between the two points is used to calculate the impact velocity of the particle. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/53299/53299fig6.jpg" /> 
Figure 6. Influence of particle size and air pressure on the COR. This figure shows the mean values as well as the maximum and minimum values with the error bars for the COR depending on the particle size. The blue diamonds represent results for experiments under atmospheric pressure whereas the orange circles show results for experiments under vacuum conditions. Glass particles were dropped on a stainless steel baseplate from an initial height of 200 mm. Each data point represents the mean value of ten repetitions of the experiment. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/53299/53299fig7.jpg" /> 
Figure 7. Influence of particle size and air pressure on the impact velocity. This figure shows the mean values for the impact velocity depending on the particle size. Moreover the maximum and minimum values depicted by the error bars are presented. The filled blue diamonds demonstrate results for experiments under atmospheric pressure whereas the filled orange circles display results for experiments under vacuum conditions. The empty diamond and the empty circle illustrate outliers because of calibration issues. In the experiments glass particles were dropped on a stainless steel baseplate from an initial height of 200 mm. Each data point represents the mean value of ten repetitions of the experiment. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" src="/files/ftp_upload/53299/53299fig8.jpg" /> 
Figure 8. The future experimental setup. This figure depicts the future experimental setup to minimize the instability of the particle chamber during the release. The automated setup with the stick guided by bushings, as well as the wire for the connection of the stick to the motor via two pulleys is shown. Also the frame is displayed. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53299/53299fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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 ...

development-an-experimental-setup-for-measurement-coefficient

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Linear Momentum, Impulse and Collisions

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8.1K Views.  University of Technology of Compiègne.  De restitutiecoëfficiënt is een parameter die het verlies van kinetische energie tijdens beschrijft botsing. Hier wordt een vrije val setup onder vacuüm ontwikkeld kunnen de restitutiecoëfficiënt parameter bepalen deeltjes micrometer range met hoge impact snelheden. 

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

Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids&#160;and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data.
The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.1&#160;Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

This paper demonstrates the experimental procedure to measure terminal settling velocities of spherical particles in surfactant-based shear thinning viscoelastic fluids. Fluids over a wide range of rheological properties are prepared and settling velocities are measured for a range of particle sizes in unbounded fluids and fluids between parallel walls.

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic ...

Experiments on Ultrasonic Lubrication Using a Piezoelectrically-assisted Tribometer and Optical Profilometer

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at a frequency above the acoustic range (20 kHz). As a solid-state technology, ultrasonic lubrication can be used where conventional lubricants are unfeasible or undesirable. Further, ultrasonic lubrication allows for electrical modulation of the effective friction coefficient between two sliding surfaces. This property enables adaptive systems that modify their frictional state and associated dynamic response as the operating conditions change. Surface wear can also be reduced through ultrasonic lubrication. We developed a protocol to investigate the dependence of friction force reduction and wear reduction on the linear sliding velocity between ultrasonically lubricated surfaces. A pin-on-disc tribometer was built which differs from commercial units in that a piezoelectric stack is used to vibrate the pin at 22 kHz normal to the rotating disc surface. Friction and wear metrics including effective friction force, volume loss, and surface roughness are measured without and with ultrasonic vibrations at a constant pressure of 1 to 4 MPa and three different sliding velocities: 20.3, 40.6, and 87 mm/sec. An optical profilometer is utilized to characterize the wear surfaces. The effective friction force is reduced by 62% at 20.3 mm/sec. Consistently with existing theories for ultrasonic lubrication, the percent reduction in friction force diminishes with increasing speed, down to 29% friction force reduction at 87 mm/sec. Wear reduction remains essentially constant (49%) at the three speeds considered.

We present a protocol for using a piezoelectrically-assisted tribometer and optical profilometer to investigate the dependence of ultrasonic wear and friction reduction on linear velocity, contact pressure, and surface properties.

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at ...

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The system comprises a high-pressure rheometer which is connected to a circulation loop. The rheometer has a rotational flow-through measurement cell with two alternative geometries: coaxial cylinder and double gap. The circulation loop contains a mixer, to bring the crude oil sample into equilibrium with CO2, and a gear pump that transports the mixture from the mixer to the rheometer and recycles it back to the mixer. The CO2 and crude oil are brought to equilibrium by stirring and circulation and the rheology of the saturated mixture is measured by the rheometer. The system is used to measure the rheological properties of Zuata crude oil (and its toluene dilution) in equilibrium with CO2 at elevated pressures up to 220 bar and a temperature of 50 &#176;C. The results show that CO2 addition changes the oil rheology significantly, initially reducing the viscosity as the CO2 pressure is increased and then increasing the viscosity above a threshold pressure. The non-Newtonian response of the crude is also seen to change with the addition of CO2.

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A method for measuring the rheology of crude oil in equilibrium with carbon dioxide at reservoir conditions is presented.

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The ...

Environment

The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various characteristic length scales can result in pressure fluctuations over a wide frequency range. This property of turbulence requires sensing devices to have sufficient sensitivity over a wide range of frequencies. Furthermore, the small characteristic length scales of turbulent structures require small sensing areas and the ability to place the sensors in very close proximity to each other. The complex geometries of the solid bodies, often including large surface curvatures or discontinuities, require the probe to have the ability to be set up in very limited spaces. The development of a remote microphone probe, which is inexpensive, consistent, and repeatable, is described in the present communication. It allows for the measurement of pressure fluctuations with high spatial resolution and dynamic response over a wide range of frequencies. The probe is small enough to be placed within the interior of typical wind tunnel models. The remote microphone probe includes a small, rigid, and hollow tube that penetrates the model surface to form the sensing area. This tube is connected to a standard microphone, at some distance away from the surface, using a "T" junction. An experimental method is introduced to determine the dynamic response of the remote microphone probe. In addition, an analytical method for determining the dynamic response is described. The analytical method can be applied in the design stage to determine the dimensions and properties of the RMP components.

Here, we present a protocol to measure, with high spatial resolution, the unsteady surface pressure in turbulent flows. This method demonstrates the construction of a remote microphone probe (RMP) and the determination of its frequency-dependent, complex transfer function. An analytical determination of the dynamic response is presented and validated.

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various ...

How to Build a Vacuum Spring-transport Package for Spinning Rotor Gauges

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to 1.0 Pa. In this application, the SRGs are frequently transported to laboratories for calibration. Events can occur during transportation that change the rotor surface conditions, thus changing the calibration factor. To assure calibration stability, a spring-transport mechanism is often used to immobilize the rotor and keep it under vacuum during transport. It is also important to transport the spring-transport mechanism using packaging designed to minimize the risk of damage during shipping. In this manuscript, a detailed description is given on how to build a robust spring-transport mechanism and shipping container. Together these form a spring-transport package. The spring-transport package design was tested using drop-tests and the performance was found to be excellent. The present spring-transport mechanism design keeps the rotor immobilized when experiencing shocks of several hundred g (g = 9.8 m/sec2 and is the acceleration due to gravity), while the shipping container assures that the mechanism will not experience shocks greater than about 100 g during common shipping mishaps (as defined by industry standards).

Here we describe how to build a robust spring-transport mechanism for a spinning rotor gauge. This device securely immobilizes the rotor and keeps it under vacuum during transportation. We also describe packaging that minimizes the risk of damage during transport. Tests show our design works for typical shocks during transport.

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to ...

Measurement of Outgassing Rates of Steels

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of steels meet the criterion of low outgassing required for high or ultrahigh vacuum applications. However, a given material can present different outgassing rates depending on its manufacturing process or the various pretreatment processes involved during the fabrication. Thus, the measurement of outgassing rates is highly desirable for a specific vacuum application. For this reason, the rate-of-pressure rise (RoR) method is often used to measure the outgassing of hydrogen after bakeout. In this article, a detailed description of the design and execution of the experimental protocol involved in the RoR method is provided. The RoR method uses a spinning rotor gauge to minimize errors that stem from outgassing or the pumping action of a vacuum gauge. The outgassing rates of two ordinary steels (stainless steel and mild steel) were measured. The measurements were made before and after the heat pretreatment of the steels. The heat pretreatment of steels was performed to reduce the outgassing. Extremely low rates of outgassing (on the order of 10&#8722;11 Pa m3 sec&#8722;1 m&#8722;2) can be routinely measured using relatively small samples.

A protocol for the measurement of outgassing rates of hydrogen from ordinary steel vacuum chambers using the rate-of-pressure rise method is presented.

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of ...

A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System

Injecting carbon dioxide (CO2) into a deep coal seam is of great significance for reducing the concentration of greenhouse gases in the atmosphere and increasing the recovery of coalbed methane. A visualized and constant-volume gas-solid coupling system is introduced here to investigate the influence of CO2 sorption on the physical and mechanical properties of coal. Being able to keep a constant volume and monitor the sample using a camera, this system offers the potential to improve instrument accuracy and analyze fracture evolution with a fractal geometry method. This paper provides all steps to perform a uniaxial compression experiment with a briquette sample in different CO2 pressures with the gas-solid coupling test system. A briquette, cold-pressed by raw coal and sodium humate cement, is loaded in high-pressure CO2, and its surface is monitored in real-time using a camera. However, the similarity between the briquette and the raw coal still needs improvement, and a flammable gas such as methane (CH4) cannot be injected for the test. The results show that CO2 sorption leads to peak strength and elastic modulus reduction of the briquette, and the fracture evolution of the briquette in a failure state indicates fractal characteristics. The strength, elastic modulus, and fractal dimension are all correlated with CO2 pressure but not with a linear correlation. The visualized and constant-volume gas-solid coupling test system can serve as a platform for experimental research about rock mechanics considering the multifield coupling effect.

A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System

This protocol demonstrates how to prepare a briquette sample and conduct a uniaxial compression experiment with a briquette in different CO2 pressures using a visualized and constant-volume gas-solid coupling test system. It also aims to investigate changes in terms of coal&#8217;s physical and mechanical properties induced by CO2 adsorption.

Injecting carbon dioxide (CO2) into a deep coal seam is of great significance for reducing the concentration of greenhouse gases in the atmosphere and ...

Calibration Procedures for Orthogonal Superposition Rheology

Orthogonal superposition (OSP) rheology is an advanced rheological technique that involves superimposing a small-amplitude oscillatory shear deformation orthogonal to a primary shear flow. This technique allows the measurement of structural dynamics of complex fluids under non-linear flow conditions, which is important for the understanding and prediction of the performance of a wide range of complex fluids. The OSP rheological technique has a long history of development since the 1960s, mainly through the custom-built devices that highlighted the power of this technique. The OSP technique is now commercially available to the rheology community. Given the complicated design of the OSP geometry and the non-ideal flow field, users should understand the magnitude and sources of measurement error. This study presents calibration procedures using Newtonian fluids that includes recommendations for best practices to reduce measurement errors. Specifically, detailed information on the end-effect factor determination method, sample filling procedure, and identification of the appropriate measurement range (e.g., shear rate, frequency, etc.) are provided.

We present a detailed calibration protocol for a commercial orthogonal superposition rheology technique using Newtonian fluids including end-effect correction factor determination methods and recommendations for best practices to reduce experimental error.

Orthogonal superposition (OSP) rheology is an advanced rheological technique that involves superimposing a small-amplitude oscillatory shear deformation ...

A Friction Testing-Bioreactor Device for Study of Synovial Joint Biomechanics, Mechanobiology, and Physical Regulation

In primary osteoarthritis (OA), normal 'wear and tear' associated with aging inhibits the ability of cartilage to sustain its load-bearing and lubrication functions, fostering a deleterious physical environment. The frictional interactions of articular cartilage and synovium may influence joint homeostasis through tissue level wear and cellular mechanotransduction. To study these mechanical and mechanobiological processes, a device capable of replicating the motion of the joint is described. The friction testing device controls the delivery of reciprocal translating motion and normal load to two contacting biological counterfaces. This study adopts a synovium-on-cartilage configuration, and friction coefficient measurements are presented for tests performed in a phosphate-buffered saline (PBS) or synovial fluid (SF) bath. The testing was performed for a range of contact stresses, highlighting the lubricating properties of SF under high loads. This friction testing device can be used as a biomimetic bioreactor for studying the physical regulation of living joint tissues in response to applied physiologic loading associated with diarthrodial joint articulation.

The present protocol describes a friction testing device that applies simultaneous reciprocal sliding and normal load to two contacting biological counterfaces.

In primary osteoarthritis (OA), normal 'wear and tear' associated with aging inhibits the ability of cartilage to sustain its load-bearing and lubrication ...

Bioengineering

Experimental Multiscale Methodology for Predicting Material Fouling Resistance

The buildup of fouling deposits in energy systems degrades their ability to transfer heat, reducing efficiency and causing operational issues such as localized corrosion. Each application presents its own challenges. In nuclear power generation, the buildup of CRUD (Chalk River Unidentified Deposits) induces unique, negative effects ranging from reduced heat transfer, to axial power shifting, to CRUD-induced localized corrosion (CILC), the last of which can cause sudden fuel failure. Knowledge of why CRUD forms and how to prevent its adhesion to nuclear fuel rods represents a major step towards eliminating it. This paper demonstrates a methodology to ascertain which materials may resist CRUD adhesion, thereby preventing its initiation and growth. It presents experiments targeted at multiple length scales: pool boiling (macroscale), which simulates CRUD initiation processes in a nuclear reactor, and atomic force microscope (AFM) force spectroscopy (microscale), which yields quantitative measurements of the adhesion between CRUD constituents and candidate CRUD-resistant materials. Early data from both sets of experiments show some correlation, suggesting that AFM force spectroscopy (AFM-FS) may be used to pre-screen materials for fouling resistance. So far, it appears that CRUD constituents adhere well to oxides, while carbides show both greatly reduced adhesion and no adherent CRUD in pool boiling experiments. The relationship between the AFM-FS measurements and the growth of CRUD in the pool boiling experiments may provide insight into the possible factors affecting fouling in nuclear reactors, eventually leading to its elimination.

Fouling, or corrosion product deposition, plagues numerous fields of energy production. At its core is adhesion between dissimilar materials. Understanding and controlling adhesion could reduce or eliminate fouling. This paper demonstrates pool boiling experiments (macroscale) and atomic force microscope force-spectroscopy measurements (microscale), which when in agreement, indicate fouling-resistance.

The buildup of fouling deposits in energy systems degrades their ability to transfer heat, reducing efficiency and causing operational issues such as ...

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

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