The authors thank the Deutsches Zentrum f&#252;r Luft- und Raumfahrt (DLR), the Deutsche Forschungsgemeinschaft (DFG), and the TU Braunschweig for continual support of our laboratory activities. The low-velocity drop tower has been established under DLR grant 50WM0936, the high-velocity drop tower has been developed under DFG grant INST 186/959-1 as part of the CRC 963 &#8220;Astrophysical Flow Instabilities and Turbulence&#8221;.

CAUTION: Depending on the hazardousness of the used particles, which can be found in the corresponding Safety Data Sheets, mouth protection and safety gear must be worn by the person working with the dust. It is also recommended to use a suction system to keep the ambient air dust-free.
1. Preparation of cm-sized Dust Aggregate Samples
<ol>
	<li>Calculate the amount of required material by m = &#934;&#160;&#961;0 V, where m is the required mass, &#934;&#160;is the desired volume filling factor (volume filling factor = 1 - porosity), &#961;0 is the material density, and V is the volume of the sample. 77 g of irregular silicate dust (&#961;0 = 2.6 g/cm3) is required to achieve a sample porosity of 70% (volume filling factor = 0.3) for a cylindrical sample of 5 cm diameter and height, respectively. 
	Note: Formation of terrestrial planets starts with coagulation of micrometer-sized dust grains &#8211; predominantly composed of silicates &#8211; into cm-sized porous bodies. A well-studied and suitable laboratory analog material is SiO2, which is available as irregular-shaped powder with a size distribution ranging from 0.5 to 10 &#181;m as well as in the form of monodisperse spherical grains for better comparability to theoretical models (see Table 1 and Figure 1).</li>
</ol>
<table border="0" cellpadding="0" cellspacing="0" width="569">
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:161px;">SiO2-monomer grain type</td>
			<td style="width:96px;">Manufacturer</td>
			<td style="width:119px;">Particle diameter</td>
			<td style="width:95px;">Particle shape</td>
			<td style="width:100px;">Example figure</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Monodisperse</td>
			<td>Micromod</td>
			<td>1.52 &#177; 0.06 &#181;m</td>
			<td>Spherical</td>
			<td>Figure 1 (left)</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Polydisperse</td>
			<td>Sigma-Aldrich</td>
			<td>0.1 &#8211; 10 &#181;m</td>
			<td>Irregular</td>
			<td>Figure 1 (right)</td>
		</tr>
	</tbody>
</table>
Table 1. Characteristics of the SiO2 particles used in the dust-aggregate collision experiments.
<img alt="Figure 1" fo:content-width="5in" src="/files/ftp_upload/51541/51541fig1highres.jpg" width="500" /> 
Figure 1. Electron-microscopy images of the monodisperse (left) and polydisperse (right) SiO2 particles used for the production of macroscopic dust aggregates. <a href="https://www.jove.com/files/ftp_upload/51541/51541fig1highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="2">
	<li>Take a container with micrometer-sized SiO2 particles (see Table 1) and pour its contents onto a sieve with a mesh size of 0.5 mm. Sift a sufficient amount of material and fill the calculated mass into the mold. Compress the material in the mold by pushing in a piston by hand until the sample height is reached (e.g. 5 cm). Turn around the mold on the piston, open the base plate and gently push the sample out. 
	Note: Samples can be produced in several shapes (spherical and cylindrical), sizes (1 mm to 10 cm) and porosities (60 to 85%) (see Figure 3). Samples can then be used individually in the collision experiments or combined into clusters, which then collide with other aggregates or clusters.</li>
</ol>
<img alt="Figure 2" src="/files/ftp_upload/51541/51541fig2highres.jpg" fo:content-width="5in" width="500" /> 
Figure 2. Photograph of the variation of dust-aggregate sample sizes and shapes.&#160;The following samples are shown: dust cylinders with 1 cm, 2 cm, and 5 cm diameter (back row), dust spheres with 1 cm and 2 cm diameter (center row), and 2-3&#160;mm-sized Al2O3 spheres (front). <a href="https://www.jove.com/files/ftp_upload/51541/51541fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="3">
	<li>Characterize the samples with respect to porosity and homogeneity. If samples fall outside allowed margins, produce new samples.
	<ol>
		<li>In order to determine the porosity of the dust sample, determine its volume by measuring its dimensions and its mass by means of a precise balance.</li>
		<li>Use X-ray tomography (XRT)12 to gain information on the homogeneity and pore-size distribution of the produced sample. 
		Note: For the 5 cm-sized dust aggregates, we found deviations from the mean volume filling factor, i.e.&#160;the ratio of the mass density of the sample and the material density of the monomer-dust particles, of only about 1% within the bulk of the volume of the samples and a slightly larger increase of the volume filling factor by up to 8% towards the outer boundaries12. Figure 3 shows an XRT reconstruction of a cut through a cylindrical dust aggregate of 5 cm diameter and 5 cm height. We do not use XRT for each dust aggregate but examine the internal structure and homogeneity of random samples.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 3" src="/files/ftp_upload/51541/51541fig3highres.jpg" fo:content-width="5in" width="500" /> 
Figure 3. Reconstruction of the internal structure of a cylindrical dust-aggregate sample of 5 cm height and 5 cm diameter after XRT analysis. The gray scale denotes the volume filling factor, which is the ratio of the mass density of the sample and the material density of the monomer-dust particles. From the XRT reconstruction, it is clearly visible that this high-porosity sample was assembled using mm-sized dust aggregates. <a href="https://www.jove.com/files/ftp_upload/51541/51541fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
2. Principle of Drop Tower Setup
<ol>
	<li>Release mechanisms: 
	In the upper part of the drop tower two release mechanisms are attached one on top of the other. Each of them holds a sample and releases it into free fall. The time difference between the release of the upper and the lower particle determines the relative velocity of the collision. According to the morphology and the shape of the particles, appropriate release mechanisms are chosen. If a particle acceleration mechanism is used, only one release mechanism is required.
	<ol>
		<li>Particle-on-a-string release mechanism&#160;(spherical samples, upper particle): 
		This release mechanism consists of a linear solenoid magnet and a solid metal counter piece.
		<ol>
			<li>Attach the particle to be released to a string. Hold the string in place by clamping it between the solenoid magnet and the solid metal counter piece.</li>
			<li>To release the particle, apply an electric current to the solenoid magnet (see Movie 1).</li>
		</ol>
		</li>
		<li>Trap-door release mechanism (spherical samples, lower particle): 
		This release mechanism consists of a rotary solenoid magnet to which a particle holder is attached.
		<ol>
			<li>Place the particle into a semi-spherical mold, which is rotated downward by a rotational solenoid when an electric current is applied (see Movie 1).</li>
			<li>This mechanism can also be used for the release of particle clusters or aggregate clumps. In this latter case, mount two trap-door release mechanisms above one another (see Movie 2).</li>
		</ol>
		</li>
		<li>Scissor-type double release mechanism (cylindrical samples): 
		This release mechanism consists of two pairs of rotary solenoid magnets to which a metal rod is attached. The two solenoid magnets of each release mechanism are placed such that the two metal rods are parallel.
		<ol>
			<li>Position the two samples onto the two parallel rods each.</li>
			<li>Apply an electric current to the two rotational solenoids to release the particles into free fall. (see Movie 3).</li>
		</ol>
		</li>
		<li>Double-wing trap-door release mechanism (cylindrical samples, in combination with particle acceleration mechanism): 
		This release mechanism consists of two spring-loaded metal plates, which together form a v-shaped particle holder. The two metal plates are held in place by a metal rod, which is attached to a rotary solenoid magnet.
		<ol>
			<li>Place the cylindrical dust sample onto the closed trap-door.</li>
			<li>Unlock the trap-door by applying an electric current to the solenoid magnet. To avoid bouncing-back of the doors, eddy-current brakes stop them (see Movie 4). 
			Note: It is important to release the particles into free fall without initial velocity and rotation. For this purpose several release mechanisms have been developed (2.1.1 &#8211; 2.1.4).</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Particle acceleration mechanisms: 
	Accelerates the particles either by a pre-loaded spring or by an electro-magnetically driven linear stage. Both accelerators can be equipped with sample holders for differently shaped particles.</li>
	 
	<li>Control electronics: 
	Set the timer and release electronics to the appropriate values to achieve the desired collision velocity and to operate the camera in a center-of-mass frame. 
	Note: The timing of the particle release, particle acceleration and camera release is performed by a set of electronic timers, whose functionality is explained in Movie 5.</li>
</ol>
3. Performing Experiments
<ol>
	<li>Low-velocity collisions (small drop tower):
	<ol>
		<li>Load samples into the scissor-type double release mechanism and close vacuum glass tube.</li>
		<li>Start evacuation and set timer parameters.</li>
		<li>Attach cameras to their magnetic release units. Start continuous camera recording. 
		Note: Due to the high intensity of the LED bright-field illumination, a sufficiently short exposure time of the high-speed camera can be chosen so that the particle motion during the exposure is negligible. On top of that, the f-stop of the camera objective must be set to high enough values to extend the depth-of-focus over the whole diameter of the drop tower.</li>
		<li>When the desired vacuum quality is reached, switch on lighting, press the start button and download the image sequences.</li>
	</ol>
	</li>
	<li>High-velocity collisions (large drop tower):
	<ol>
		<li>Load samples into the double-wing trap-door release mechanism and accelerator and close the vacuum glass tube.</li>
		<li>Start evacuation and set timer parameters.</li>
		<li>Start continuous camera recording. When the desired vacuum quality is reached, switch on lighting and press the start button. Download the image sequences. 
		Note: Due to the high intensity of the LED bright-field illumination, a sufficiently short exposure time of the high-speed camera can be chosen so that the particle motion during the exposure is negligible. On top of that, the f-stop of the camera objective must be set to high enough values to extend the depth-of-focus over the whole diameter of the drop tower.</li>
	</ol>
	</li>
</ol>
4. Example Experiments
<ol>
	<li>Load the samples carefully into the appropriate release mechanism.
	<ol>
		<li>Low-speed collisions (double release mechanism; 0.09 m/sec): 5&#160;cm vs. 5&#160;cm, bouncing. 
		Load the samples into two scissor-type release mechanisms. To achieve collision velocities of 0.09 m/sec, place the particles 7 mm apart and set the time delay of the release mechanisms to 9 msec. 
		Note: At this impact velocity, the dust samples bounce off each other after the collision. The image sequence is captured by a free-falling high-speed camera (see Movie 6).</li>
		 
		<li>High-speed collisions (electromagnetic accelerator; 7.4 m/sec): 2 cm vs. 2 cm, fragmentation. 
		Load one sample onto the double-wing trap-door release mechanism; place the other sample on the sample holder of the linear-stage accelerator. 
		Note: To achieve collision velocities of 7.4 m/sec, the lower dust aggregate is smoothly accelerated upward with 2 g, while simultaneously the upper dust aggregate falls. At a relative velocity of 7.4 m/sec, the dust samples fragment (see Movie 7).</li>
		 
		<li>High-speed collision of small aggregates on big aggregates: 0.5 cm vs. 5 cm, mass transfer. 
		Load the large sample onto a scissor-type release mechanism; place the smaller sample on the sample holder of the spring accelerator. 
		Note: To achieve collision velocities required for mass transfer, the lower dust aggregate is smoothly accelerated upward, while simultaneously the upper dust aggregate falls. At this relative velocity, the smaller sample fragments and transfers a small amount of mass onto the larger sample. As the camera falls along the upper (more massive) particle, the images taken by the high-speed camera give the impression of a large particle more or less at rest (see Movie 8), which is not true as seen from the outside of the drop tower.</li>
	</ol>
	</li>
	<li>Close the vacuum glass tube.</li>
	<li>Carefully open the vacuum valve to the pumps to start slow evacuation and set the timer parameters to the required time difference for the desired collision velocity.</li>
	<li>Attach cameras to their release units (if free-falling cameras are used). Start continuous camera recording and switch on lighting.</li>
	<li>When the desired vacuum quality is reached, press the release button to initiate the timer sequence.</li>
	<li>Download the image sequences recorded by the high-speed cameras to a computer.</li>
</ol>
5. Data Analysis
<ol>
	<li>Chose an appropriate threshold gray value between the background and the objects&#8217; gray value. Create a binary image based on this threshold by setting pixels with gray values above the threshold to white (binary value 1) and pixels with lower gray values to black (binary value 0).</li>
	<li>Determine the position of the particles&#8217; center of mass in each of the images. A good approximation to determine the center of mass for symmetrical particles is the center of the projected area. This is computed from binarized images.</li>
	<li>Use the relative position of the objects&#8217; centers of mass and the time information from the camera images to calculate the relative velocity (see Movie 9). The slopes of the position curve are shown on the right-hand-side of Movie 9.
	<ol>
		<li>In case of a rebounding collision, determine the relative velocities before and after contact. Calculate the coefficient of restitution, i.e. the ratio of the velocity after the collision and before. Plot the relative velocity against the coefficient of restitution. An example of this analysis is shown in Figure 4.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 4" fo:content-width="5in" src="/files/ftp_upload/51541/51541fig4highres.jpg" width="500" /> 
Figure 4.&#160;Example of the analysis of bouncing collisions. The coefficient of restitution, i.e. the ratio of the rebound velocity and the impact velocity, is plotted as a function of the collision velocity. Circles show data for spherical dust aggregates of 2 cm diameter13 (see Figure 2), triangles denote collisions between cylindrical dust aggregates of 5 cm diameter and 5 cm height (see Figure 2) and two different volume filling factors of 0.3 and 0.4, respectively12. The data show a trend of decreasing coefficient of restitution with increasing impact velocity. <a href="https://www.jove.com/files/ftp_upload/51541/51541fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="2">
	<li>If one or both particles fragment, determine the sizes of as many of the largest fragments as possible by measuring their respective projected areas and assuming appropriate shapes.
	<ol>
		<li>If fragmentation of only one particle occurs, it usually transfers a certain amount of its mass to the surviving particle. Determine the amount of transferred mass by measuring the accreted volume, assuming an appropriate shape and porosity to quantify the mass-transfer efficiency.</li>
	</ol>
	</li>
</ol>

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VII. The low-velocity collision behavior of large dust agglomerates.</a> The Astrophysical Journal. 758, 35 (2012).</li><li>Beitz, E., G&#252;ttler, C., Blum, J., Meisner, T., Teiser, J., Wurm, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Low-velocity+collisions+of+centimeter-sized+dust+aggregates.">Low-velocity collisions of centimeter-sized dust aggregates.</a> The Astrophysical Journal. 736, 34 (2011).</li><li>Beitz, E., Blum, J., Mathieu, R., Pack, A., Hezel, D. 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Preparation, critical rolling friction force, and specific surface energy.</a> Icarus. 214, 717-723 (2011).</li></ol>

We have nothing to disclose.

Due to the high mechanical precision, the failure rate of both drop towers is extremely low. This is of utmost importance, because the sample preparation may take up to several hours, depending on size, shape and porosity of the desired dust aggregates. It should be mentioned that large dust aggregates with very high porosities are extremely fragile and, thus, difficult to handle. It may occur that these dust aggregates break during extraction off the mold or transfer to the drop tower. In these cases, a new sample has to be prepared. Thus, it is important that the small drop tower allows reliable (and predictable) collision velocities down to 0.01 m/sec&#160;11,13. The lowest impact speed so far achieved was 0.004 m/sec. These small impact velocities can only be reached for free particles in a microgravity environment. The laboratory drop tower is a cheap and versatile realization of such a microgravity facility.
Alternative methods to achieve low impact speeds make use of levitation techniques14,15 (e.g.&#160;by electromagnetic or aerodynamic levitation) but generally induce a force between the colliding particles, which has to be taken into account in the analysis of the collisions. Moreover, levitation often induces rotational motion14, which, if unwanted, does not allow rotation-free collisions but, on the other hand, might even allow realistic simulations of collisions among rotating particles. In case of aerodynamic levitation, air-cushion effects during the collision may induce unwanted conditions that do not match those in protoplanetary disks. However, levitation allows for unlimited observation time and repeatable experiments so that it has to be considered an alternative to the drop tower if the time limitation is essential. All our efforts so far have been concentrated on SiO2 as a representative of the silicates in the terrestrial-planet formation region of young Solar Systems. As most of the mass of protoplanetary disks is concentrated beyond the condensation point of water ice, it is essential to also study the collision behavior of aggregates consisting of &#181;m-sized H2O-ice grains. We are currently setting up a cryo-vacuum drop tower for this purpose. It must be noted that the temperatures in such simulation experiments must be below ~150 K, which is the temperature of the so-called &#8220;snow line&#8221; in protoplanetary disks (the &#8220;snow line&#8221; divides the inner regions where water is in the vapor phase from the outer regions where it is found as solid water ice). We have shown that the formation of &#181;m-sized water-ice particles is feasible and that aggregates thereof can be produced16 so that we are optimistic to have first results on their collision behavior within the next 1-2 years.

It is generally accepted that planet formation starts with the non-gravitational accumulation of microscopically small dust grains into larger dust aggregates (see review by Blum &#38; Wurm)1. The dust particles collide within their protoplanetary disks due to Brownian motion, relative drift motions, and turbulence of the nebular gas (see review by Johansen et al.)2. If the collision velocities are sufficiently low, the dust particles stick together to form larger agglomerates. A wealth of laboratory measurements over the past years have led to a dust-aggregate collision model that predicts the outcome of a pair of dust aggregates with arbitrary masses and collision velocities3. The basic collisional results are sticking (in general for small aggregate masses and low collision velocities), bouncing, and fragmentation (for high impact speeds). However, the transitions between these phases are not sharp and there are other outcomes, like, e.g.&#160;mass transfer or erosion. Applying this model to a typical protoplanetary disk predicts the growth of cm-sized dust aggregates within a few thousand years4. The presence of cm-sized dust aggregates has been extensively investigated by astronomical observations over the past years and can now be considered as established (see review by Testi et al.)5 so that we conclude that the principle mechanism by which the first macroscopic bodies in young planetary systems form has been identified.
However, the further growth to bodies of at least kilometer sizes is not so clear. For the terrestrial-planet region, two hypotheses are currently discussed (see also the recent reviews on this matter by Johansen et al.2 and Testi et al.5): (i) concentration of the cm-sized dust aggregates by, e.g.&#160;the streaming instability6 and subsequent gravitational collapse7,8 and (ii) growth of a few &#8221;lucky winners&#8221; to larger sizes with subsequent mass accretion by the mass-transfer process9,10,11. In both models, cm-sized dust aggregates undergo an enormous number of mutual collisions at low to moderate velocities. It is unclear what the possible outcomes of these collisions (besides bouncing) are.
To improve the dust-aggregate collision model by G&#252;ttler et al.3 and to investigate in more detail the collisions among macroscopic dust aggregates in the relevant velocity regimes, we set up two drop towers in our laboratory, in which individual aggregate-aggregate collisions can be studied in great detail under vacuum and microgravity conditions. Both drop towers possess a free-fall height of 1.5 m, which limits the observation time to ~0.5 sec. Thus, we observe the collisions by high-speed cameras with megapixel format and up to 7,500 frames per second. For maximum contrast and high recording speeds, bright-field illumination is chosen. Lighting is thus provided by high-intensity LED panels and homogenized by diffuser screens. Thus, the high-speed cameras view the colliding dust aggregates as dark objects in front of an illuminated screen. To avoid flickering, the LEDs are DC powered.
To achieve low collision velocities, the two dust aggregates are placed above one another in a double release mechanism. Releasing the upper aggregate a time of t before the lower one results in a relative velocity of v = g t, with g&#160;= 9.81 m/sec2 being the gravitational acceleration of the Earth. The two high-speed cameras, which view the collision from two directions 90&#176; apart, are typically released in between the two dust aggregates (typically t/2 after the upper particle). The cameras run in continuous-recording mode, which is terminated by the impact of the camera holders into sand buckets. The maximum frame rate in this operational mode is 1,000 images per second at megapixel resolution. With this setup, velocities down to below 0.01 m/sec have been achieved. Due to limitations of the mechanical setup of the double release mechanism, the maximum relative collision speed is ~3 m/sec. Collisions involving dust aggregates with up to 5 cm in size have been investigated in this drop tower. For higher collision velocities up to ~10 m/sec, a second drop tower is used, which is equipped with an electromagnetic accelerator that is capable to smoothly accelerate dust aggregates up to 5 m/sec in a vertical upward direction. The other dust aggregate is held by a double-wing trap-door release mechanism and can be released rotation-free into free fall at any given time. Here, it does not make sense to use free-falling cameras. We rather use two stationary high-speed cameras with up to 7,500 frames per second and megapixel resolution. Due to the larger diameter of this drop tower, dust aggregates up to (and possibly above) 10 cm in size can be used.

<table><tbody><tr><td>Monodisperse SiO&lt;sub&gt;2&lt;/sub&gt; particles</td><td>Micromod</td><td>43-00-153</td><td>Particle diameter 1.52 &amp;plusmn; 0.06 &amp;micro;m; particle shape spherical</td></tr><tr><td>Polydisperse SiO&lt;sub&gt;2&lt;/sub&gt; particles</td><td>Sigma-Aldrich</td><td>S5631</td><td>Particle diameter 0.1 &amp;ndash; 10 &amp;micro;m; particle shape irregular</td></tr></tbody></table>

laboratory drop towers for the experimental simulation of dust-aggregate collisions in the early solar system

For the purpose of investigating the evolution of dust aggregates in the early Solar System, we developed two vacuum drop towers in which fragile dust aggregates with sizes up to ~10 cm and porosities up to 70% can be collided. One of the drop towers is primarily used for very low impact speeds down to below 0.01 m/sec and makes use of a double release mechanism. Collisions are recorded in stereo-view by two high-speed cameras, which fall along the glass vacuum tube in the center-of-mass frame of the two dust aggregates. The other free-fall tower makes use of an electromagnetic accelerator that is capable of gently accelerating dust aggregates to up to 5 m/sec. In combination with the release of another dust aggregate to free fall, collision speeds up to ~10 m/sec can be achieved. Here, two fixed high-speed cameras record the collision events. In both drop towers, the dust aggregates are in free fall during the collision so that they are weightless and match the conditions in the early Solar System.

Using the well-characterized dust-aggregate samples described in the protocol (see Figures 1-3), any collision observed in one of the laboratory drop towers will yield scientifically valuable information on the outcomes of similar collisions in protoplanetary disks. We have so far systematically investigated the collision outcomes of 2 cm sized spherical dust aggregates (with volume filling factors of 0.5) in the velocity range between 0.008 and 2.02 m/sec13 and of 5 cm sized cylindrical dust aggregates (with volume filling factors between 0.3 and 0.5) in the velocity range between 0.004 and 2 m/sec12. We found bouncing between the dust aggregates as the dominating outcome for velocities below ~0.4 m/sec for both types of dust aggregates (see Movie 6 for an example). In Figure 4, the coefficient of restitution of these bouncing collisions is shown. The circles denote the experiments with 2 cm sized spherical samples13 and the triangles represent results from collisions among 5 cm sized dust cylinders with two different packing densities12. Although the coefficients of restitution of individual experiments scatter widely, the average value of the coefficient of restitution decreases with increasing collision velocity.
Both dust aggregates typically fragment upon impact for velocities above ~1 m/sec (see Movie 7 for an example). For velocities between ~ 0.4 and ~1 m/sec, fragmentation of only one of the two colliding dust aggregates can occur. In this case, the non-fragmenting dust aggregate gains a few percent of mass by mass transfer13. The above-mentioned velocity limits are not sharp but denote approximately where the boundaries between the different regimes lie2,11. For collisions between dust aggregates of different sizes and moderate velocities, impacts will generally not lead to the fragmentation of the larger of the two dust aggregates. On the opposite, the larger bodies increase their mass by transfer of part of the mass of the smaller impactors (see Movie 8).
For the cases, in which the two dust aggregates bounce off one another, the transfer from the translational kinetic energy before the collision (mind that the dust aggregates do not rotate before the collision) into translational kinetic energy, rotational kinetic energy, and other (dissipative) energy channels (e.g.&#160;compaction of the dust aggregates) can be determined. We found that for central collisions (in which the rotational energy can be neglected) the relative amount of dissipated energy strongly increases with increasing velocity and is higher for lower volume filling factors of the dust aggregates12. This behavior can be modeled by molecular-dynamics simulations12.
<a href="https://www.jove.com/files/ftp_upload/51541/string_trapdoor.MP4" target="_blank">Movie 1</a>. High-speed movie (played back in slow motion) of the particle-on-a-string (top) and trap-door release mechanism (bottom).
<a href="https://www.jove.com/files/ftp_upload/51541/mold_cluster_cluster.MP4" target="_blank">Movie 2</a>. High-speed movie (played back in slow motion) of the double trap-door release mechanism. Both samples are clumps of Al2O3 particles of 2 mm diameter, which remain confined during free fall due to the extremely low disturbance during release.
<a href="https://www.jove.com/files/ftp_upload/51541/scissor.MP4" target="_blank">Movie 3</a>. High-speed movie (played back in slow motion) of the scissor-type double release mechanism.
<a href="https://www.jove.com/files/ftp_upload/51541/dw-trapdoor.MP4" target="_blank">Movie 4</a>. High-speed movie (played back in slow motion) of the double-wing trap-door release mechanism.
<a href="https://www.jove.com/files/ftp_upload/51541/trigger.wmv" target="_blank">Movie 5</a>. Animation of the timer electronics switching the upper and lower release mechanism as well as the camera release to free fall.
<a href="https://www.jove.com/files/ftp_upload/51541/9ms_2.MP4" target="_blank">Movie 6</a>. High-speed movie (played back in slow motion) of a bouncing collision between two 5 cm-sized dust-aggregate cylinders. The two dust aggregates are released by the scissor-type double release mechanism and collide with 0.09 m/sec velocity.
<a href="https://www.jove.com/files/ftp_upload/51541/fragmentation.MP4" target="_blank">Movie 7</a>. High-speed movie (played back in slow motion) of two 2 cm-sized cylindrical dust aggregates colliding at a relative velocity of 7.4 m/sec. Both aggregates fragment completely.
<a href="https://www.jove.com/files/ftp_upload/51541/mass-transfer.MP4" target="_blank">Movie 8</a>. High-speed movie (played back in slow motion) of a 5 mm-sized dust aggregate impacting a 5 cm-sized cylindrical solid target. As the impact velocity of 4.3 m/sec is above the fragmentation speed of the small dust aggregate, this breaks apart and transfers part of its mass to the target, which is clearly visible in the movie.
<a href="https://www.jove.com/files/ftp_upload/51541/tracking_new.mov" target="_blank">Movie 9</a>. Determination of the particle trajectories by a semi-automatic particle-tracking algorithm. Here, the collision between two 2 cm-sized spherical dust aggregates is shown.

Watch this Scientific Journal Video about Laboratory Drop Towers for the Experimental Simulation of Dust-aggregate Collisions in the Early Solar System at JoVE.com

Laboratory Drop Towers for the Experimental Simulation of Dust-aggregate Collisions in the Early Solar System

We present a technique to achieve low-velocity to intermediate-velocity collisions between fragile dust aggregates in the laboratory. For this purpose, two vacuum drop-tower setups have been developed that allow collision velocities between &#60;0.01 and ~10 m/sec. The collision events are recorded by high-speed imaging.

For the purpose of investigating the evolution of dust aggregates in the early Solar System, we developed two vacuum drop towers in which fragile dust ...

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Kinetics of a Particle: Impulse and Momentum

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12.7K Views.  Technische Universität Braunschweig.  Wij presenteren een techniek voor lage snelheid bereiken intermediaire snelheid botsingen tussen fragiele stof aggregaten in het laboratorium. Daartoe zijn twee vacuümdaaltijd-toren opstellingen ontwikkeld waarmee botsing snelheden tussen 

Video: Laboratory Drop Towers for the Experimental Simulation of Dust-aggregate Collisions in the Early Solar System

Simulation of the Planetary Interior Differentiation Processes in the Laboratory

A planetary interior is under high-pressure and high-temperature conditions and it has a layered structure. There are two important processes that led to that layered structure, (1) percolation of liquid metal in a solid silicate matrix by planet differentiation, and (2) inner core crystallization by subsequent planet cooling. We conduct high-pressure and high-temperature experiments to simulate both processes in the laboratory. Formation of percolative planetary core depends on the efficiency of melt percolation, which is controlled by the dihedral (wetting) angle. The percolation simulation includes heating the sample at high pressure to a target temperature at which iron-sulfur alloy is molten while the silicate remains solid, and then determining the true dihedral angle to evaluate the style of liquid migration in a crystalline matrix by 3D visualization. The 3D volume rendering is achieved by slicing the recovered sample with a focused ion beam (FIB) and taking SEM image of each slice with a FIB/SEM crossbeam instrument. The second set of experiments is designed to understand the inner core crystallization and element distribution between the liquid outer core and solid inner core by determining the melting temperature and element partitioning at high pressure. The melting experiments are conducted in the multi-anvil apparatus up to 27 GPa and extended to higher pressure in the diamond-anvil cell with laser-heating. We have developed techniques to recover small heated samples by precision FIB milling and obtain high-resolution images of the laser-heated spot that show melting texture at high pressure. By analyzing the chemical compositions of the coexisting liquid and solid phases, we precisely determine the liquidus curve, providing necessary data to understand the inner core crystallization process.

The high-pressure and high-temperature experiments described here mimic planet interior differentiation processes. The processes are visualized and better understood by high-resolution 3D imaging and quantitative chemical analysis.

A planetary interior is under high-pressure and high-temperature conditions and it has a layered structure. There are two important processes that led to ...

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.

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 ...

Experimental Protocol to Investigate Particle Aerosolization of a Product Under Abrasion and Under Environmental Weathering

The present article presents an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental weathering, which is a fundamental element to the approach of nanosafety-by-design of nanostructured products for their durable development. This approach is basically a preemptive one in which the focus is put on minimizing the emission of engineered nanomaterials' aerosols during the usage phase of the product's life cycle. This can be attained by altering its material properties during its design phase without compromising with any of its added benefits. In this article, an experimental protocol is presented to investigate the nanosafety-by-design of three commercial nanostructured products with respect to their mechanical solicitation and environmental weathering. The means chosen for applying the mechanical solicitation is an abrasion process and for the environmental weathering, it is an accelerated UV exposure in the presence of humidity and heat. The eventual emission of engineered nanomaterials is studied in terms of their number concentration, size distribution, morphology and chemical composition. The purpose of the protocol is to study the emission for test samples and experimental conditions which are corresponding to real life situations. It was found that the application of the mechanical stresses alone emits the engineered nanomaterials' aerosols in which the engineered nanomaterial is always embedded inside the product matrix, thus, a representative product element. In such a case, the emitted aerosols comprise of both nanoparticles as well as microparticles. But if the mechanical stresses are coupled with the environmental weathering, the experimental protocol reveals then the eventual deterioration of the product, after a certain weathering duration, may lead to the emission of the free engineered nanomaterial aerosols too.

In this article, an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental weathering is presented. Results on the emission of engineered nanomaterials, in the form of aerosols are presented. The specific experimental set-up is described in detail.

The present article presents an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental ...

Experimental Methods of Dust Charging and Mobilization on Surfaces with Exposure to Ultraviolet Radiation or Plasmas

Electrostatic dust transport has been hypothesized to explain a number of observations of unusual planetary phenomena. Here, it is demonstrated using three recently developed experiments in which dust particles are exposed to thermal plasma with beam electrons, beam electrons only, or ultraviolet (UV) radiation only. The UV light source has a narrow bandwidth in wavelength centered at 172 nm. The beam electrons with the energy of 120 eV are created with a negatively biased hot filament. When the vacuum chamber is filled with the argon gas, a thermal plasma is created in addition to the electron beam. Insulating dust particles of a few tens of microns in diameter are used in the experiments. Dust particles are recorded to be lofted to a height up to a few centimeters with a launch speed up to 1 m/s. These experiments demonstrate that photo and/or secondary electron emission from a dusty surface changes the charging mechanism of dust particles. According to the recently developed &#34;patched charge model&#34;, the emitted electrons can be re-absorbed inside microcavities between neighboring dust particles below the surface, causing the accumulation of enhanced negative charges on the surrounding dust particles. The repulsive forces between these negatively charged particles may be large enough to mobilize and lift them off the surface. These experiments present the advanced understanding of dust charging and transport on dusty surfaces, and laid a foundation for future investigations of its role in the surface evolution of airless planetary bodies.

Dust charging and mobilization is demonstrated in three experiments with exposure to thermal plasma with beam electrons, beam electrons only, or ultraviolet (UV) radiation only. These experiments present the advanced understanding of electrostatic dust transport and its role in shaping the surfaces of airless planetary bodies.

Electrostatic dust transport has been hypothesized to explain a number of observations of unusual planetary phenomena. Here, it is demonstrated using ...

Environment

Stress Distribution During Cold Compression of Rocks and Mineral Aggregates Using Synchrotron-based X-Ray Diffraction

We report detailed procedures for performing compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus (D-DIA) coupled with synchrotron X-radiation. A cube-shaped sample assembly is prepared and compressed, at room temperature, by a set of four X-ray transparent sintered diamond anvils and two tungsten carbide anvils, in the lateral and the vertical planes, respectively. All six anvils are housed within a 250-ton hydraulic press and driven inward simultaneously by two wedged guide blocks. A horizontal energy dispersive X-ray beam is projected through and diffracted by the sample assembly. The beam is commonly in the mode of either white or monochromatic X-ray. In the case of white X-ray, the diffracted X-rays are detected by a solid-state detector array that collects the resulting energy dispersive diffraction pattern. In the case of monochromatic X-ray, the diffracted pattern is recorded using a two-dimensional (2-D) detector, such as an imaging plate or a charge-coupled device (CCD) detector. The 2-D diffraction patterns are analyzed to derive lattice spacings. The elastic strains of the sample are derived from the atomic lattice spacing within grains. The stress is then calculated using the predetermined elastic modulus and the elastic strain. Furthermore, the stress distribution in two-dimensions allow for understanding how stress is distributed in different orientations. In addition, a scintillator in the X-ray path yields a visible light image of the sample environment, which allows for the precise measurement of sample length changes during the experiment, yielding a direct measurement of volume strain on the sample. This type of experiment can quantify the stress distribution within geomaterials, which can ultimately shed light on the mechanism responsible for compaction. Such knowledge has the potential to significantly improve our understanding of key processes in rock mechanics, geotechnical engineering, mineral physics, and material science applications where compactive processes are important.

We report detailed procedures for compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus coupled with synchrotron X-radiation. Such experiments allow quantification of the stress distribution within samples, that ultimately sheds light on compaction processes in geomaterials.

We report detailed procedures for performing compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus (D-DIA) ...

A System to Create Stable Nanoparticle Aerosols from Nanopowders

Nanoparticle aerosols released from nanopowders in workplaces are associated with human exposure and health risks. We developed a novel system, requiring minimal amounts of test materials (min. 200 mg), for studying powder aerosolization behavior and aerosol properties. The aerosolization procedure follows the concept of the fluidized-bed process, but occurs in the modified volume of a V-shaped aerosol generator. The airborne particle number concentration is adjustable by controlling the air flow rate. The system supplied stable aerosol generation rates and particle size distributions over long periods (0.5-2 hr and possibly longer), which are important, for example, to study aerosol behavior, but also for toxicological studies. Strict adherence to the operating procedures during the aerosolization experiments ensures the generation of reproducible test results. The critical steps in the standard protocol are the preparation of the material and setup, and the aerosolization operations themselves. The system can be used for experiments requiring stable aerosol concentrations and may also be an alternative method for testing dustiness. The controlled aerosolization made possible with this setup occurs using energy inputs (may be characterized by aerosolization air velocity) that are within the ranges commonly found in occupational environments where nanomaterial powders are handled. This setup and its operating protocol are thus helpful for human exposure and risk assessment.

We designed and developed an effective nanopowder aerosolization setup and operating protocol. The system generated nanoparticle aerosols with stable number concentrations and size distributions for long durations, requiring only small quantities of test material (min. 200 mg).

Nanoparticle aerosols released from nanopowders in workplaces are associated with human exposure and health risks. We developed a novel system, requiring ...

Simulation of Early Earth Hydrothermal Chimneys in a Thermal Gradient Environment

Deep sea hydrothermal vents are self-organizing precipitates generated from geochemical disequilibria and have been proposed as a possible setting for the emergence of life. The growth of hydrothermal chimneys in a thermal gradient environment within an early Earth vent system was successfully simulated by using different hydrothermal simulants, such as sodium sulfide, which were injected into an early Earth ocean simulant containing dissolved ferrous iron. Moreover, an apparatus was developed to sufficiently cool the ocean simulant to near 0 &#176;C in a condenser vessel immersed in a cold water bath while injecting a sulfide solution at hot to room temperatures, effectively creating an artificial chimney structure in a temperature gradient environment over a period of a few hours. Such experiments with different chemistries and variable temperature&#160;gradients resulted in a variety of morphologies in the chimney structure. The use of ocean and hydrothermal fluid simulants at room temperature resulted in vertical chimneys, whereas the combination of a hot hydrothermal fluid and cold ocean simulant inhibited the formation of robust chimney structures. The customizable 3D printed condenser created for this study acts as a jacketed reaction vessel that can be easily modified and used by different researchers. It will allow the careful control of injection rate and chemical composition of vent and ocean simulants, which should help accurately simulate prebiotic reactions in chimney systems with thermal gradients similar to those of natural systems.

The goal of this protocol is to form simulated hydrothermal chimneys via chemical garden injection experiments and introduce a thermal gradient across the inorganic precipitate membrane, using a 3D printable condenser that can be reproduced for educational purposes.

Deep sea hydrothermal vents are self-organizing precipitates generated from geochemical disequilibria and have been proposed as a possible setting for the ...

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

Efficient solar-hydrogen production has recently been realized on functionalized semiconductor-electrocatalyst systems in a photoelectrochemical half-cell in microgravity environment at the Bremen Drop Tower. Here, we report the experimental procedures for manufacturing the semiconductor-electrocatalyst device, details of the experimental set-up in the drop capsule and the experimental sequence during free fall.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the ...

Chemistry

Impacts of Free-falling Spheres on a Deep Liquid Pool with Altered Fluid and Impactor Surface Conditions

Vertical impacts of spheres on clean water have been the subject of numerous water entry investigations characterizing cavity formation, splash crown ascension and Worthington jet stability. Here, we establish experimental protocols for examining splash dynamics when smooth free-falling spheres of varying wettability, mass, and diameter impact the free surface of a deep liquid pool modified by thin penetrable fabrics and liquid surfactants. Water entry investigations provide accessible, easily assembled and executed experiments for studying complex fluid mechanics. We present herein a tunable protocol for characterizing splash height, flow separation metrics, and impactor kinematics, and representative results which might be acquired if reproducing our approach. The methods are applicable when characteristic splash dimensions remain below approximately 0.5 m. However, this protocol may be adapted for greater impactor release heights and impact velocities, which augurs well for translating results to naval and industry applications.

This protocol demonstrates the basic experimental configuration for water entry experiments with free-falling spheres. Methods for the alteration of liquid surface with penetrable fabrics, the preparation of chemically non-wetting spheres, and steps for splash visualization and data extraction are discussed.

Vertical impacts of spheres on clean water have been the subject of numerous water entry investigations characterizing cavity formation, splash crown ...

Scattering And Absorption of Light in Planetary Regoliths

Theoretical, numerical, and experimental methods are presented for multiple scattering of light in macroscopic discrete random media of densely-packed microscopic particles. The theoretical and numerical methods constitute a framework of Radiative Transfer with Reciprocal Transactions (R2T2). The R2T2 framework entails Monte Carlo order-of-scattering tracing of interactions in the frequency space, assuming that the fundamental scatterers and absorbers are wavelength-scale volume elements composed of large numbers of randomly distributed particles. The discrete random media are fully packed with the volume elements. For spherical and nonspherical particles, the interactions within the volume elements are computed exactly using the Superposition T-Matrix Method (STMM) and the Volume Integral Equation Method (VIEM), respectively. For both particle types, the interactions between different volume elements are computed exactly using the STMM. As the tracing takes place within the discrete random media, incoherent electromagnetic fields are utilized, that is, the coherent field of the volume elements is removed from the interactions. The experimental methods are based on acoustic levitation of the samples for non-contact, non-destructive scattering measurements. The levitation entails full ultrasonic control of the sample position and orientation, that is, six degrees of freedom. The light source is a laser-driven white-light source with a monochromator and polarizer. The detector is a mini-photomultiplier tube on a rotating wheel, equipped with polarizers. The R2T2 is validated using measurements for a mm-scale spherical sample of densely-packed spherical silica particles. After validation, the methods are applied to interpret astronomical observations for asteroid (4) Vesta and comet 67P/Churyumov-Gerasimenko (Figure 1) recently visited by the NASA Dawn mission and the ESA Rosetta mission, respectively.

Numerical and experimental methods are presented for multiple scattering of light in discrete random media of densely-packed particles. The methods are utilized to interpret the observations of asteroid (4) Vesta and comet 67P/Churyumov-Gerasimenko.

Theoretical, numerical, and experimental methods are presented for multiple scattering of light in macroscopic discrete random media of densely-packed ...

Laboratory Drop Towers for the Experimental Simulation of Dust-aggregate Collisions in the Early Solar System

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