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

The overall goal of this procedure is to experimentally simulate collisions between dust aggregates in the early solar system and to understand the first steps of planet formation. This is accomplished by first preparing two porous dust aggregates representing the dust particles in the young solar system. The second step is to insert both dust aggregates into one of the laboratory drop towers, which has been conditioned to generate a collision between the dust aggregates under microgravity conditions.

Next, the experiment is performed and high speed cameras record the collision between the two dust aggregates. The final step is the analysis of the high speed recording of the collision with respect to energy loss, mass transfer, and fragmentation of the dust aggregates. Ultimately, these results are used to develop physical models of the dust aggregate collisions in the young solar system, which are required for the modeling of the first steps of planet formation.

This method can help answer key questions in the field of planet formation, such as the outcome of collisions of the building blocks of planets. We first had the idea for this method when we are looking for means to carry out a large number of very low velocity collisions between fragile dust aggregates. The main advantage of this technique of existing methods like commercial microgravity facilities, is that experiments can be performed more frequently and are cost efficient in the lab.

Visual demonstration of this method is critical as the sample preparation steps are difficult to learn because this prepared samples are very fragile. To begin, calculate the amount of material required for the desired shape using the following equation. Sift a sufficient amount of material through a 0.5 millimeter mesh C, and fill the calculated mass into a mold.

Then compress the material inside the mold by pushing a piston by hand until the sample has reached five centimeters. Finally, turn the mold upside down. Open the base plate and gently push the sample out with the piston.

For spherical upper particle samples, use the particle on a string release mechanism. In order to set up this type of release, attach the particle to be released to a string. This is done during the sample compression using a piston with a passage for a string.

Hold the string in place by clamping it between the solenoid magnet and the solid metal counter piece for spherical lower particle samples. Use the trap door release mechanism. Start by placing the particle into a semis spherical mold.

At the appropriate time, apply an electric current to the solenoid magnet of the upper release mechanism hereafter. Apply an electric current to rotate the trap door downward using a rotational solenoid when examining more complex arrangements such as clusters of spheres mount two trap door release mechanisms above one another. Once both trap door mechanisms have been loaded, apply an electrical current to release the samples at the correct times for cylindrical samples.

Mount a scissor type double release mechanism into place. Then gently set the cylindrical samples onto the release plates. As before, apply an electrical current to the solenoid magnets to release the samples.

If a sample will be accelerated instead of simply free falling, use a particle acceleration mechanism such as an electromagnetically driven linear stage. When testing cylindrical samples in combination with particle acceleration, use the double wing trap door release mechanism. Place the cylindrical dust sample onto the closed trap door.

Then unlock the trap door by applying an electric current to the solenoid magnet. Notice how the eddy current breaks prevent the door from bouncing back into the sample to achieve low speed collision. Velocities of 0.09 meters per second.

Place one of the one centimeter particles into the particle on a string mechanism and the other one centimeter particle seven millimeters away in the trap door release mechanism. Then close the vacuum glass tube. Carefully open the valve to the vacuum pumps to start slow evacuation of air in the tube.

Next, set the time delay of the release mechanisms to nine milliseconds. This means that first the upper dust aggregate will be released and nine milliseconds later the lower particle, the camera will be released in between next attach cameras to their release units and switch on the lighting. Then start the continuous recording.

When the desired vacuum of better than one millibar is reached, press the release button to initiate the timer sequence when the experiment complete, download the image sequences recorded by the high speed cameras to a computer. Prepare two cylindrical samples of five centimeter diameter and height for a high speed collision by first loading one sample onto the double wing trap door release mechanism, and the other on the sample holder of the linear stage accelerator. Once the samples are in place, prepare the chamber as before by sealing the glass tube and opening the vacuum valve to remove air from the tube.

Then set up the timing of the release mechanism and the linear accelerator according to the desired conditions. Next, turn on the appropriate lighting and start recording with the high speed camera. Once the desired vacuum of better than 0.01 millibar is reached, press the release button to initiate the collision upon completion.

Download the image sequences recorded by the cameras as before for aggregates of different sizes. Load the large sample onto a release mechanism, then place the smaller sample on the sample holder of the accelerator. These electron microscopy images of the mono disperse spherical and poly disperse irregular silica particles illustrate the physical characteristics for samples used in the laboratory drop tower experiments.

Dust aggregates can be made in various sample sizes and shapes, including dust cylinders in one centimeter, two centimeter, and five centimeter diameters and dust spheres in one centimeter and two centimeter diameters. In addition, clusters of two to three millimeter sized illuminous spheres can be used using x-ray tomography reconstruction of the internal structure of a cylindrical dust aggregate sample. It is clearly visible that this high porosity sample was assembled using millimeter sized to dust aggregates.

The gray scale denotes the volume filling factor, the ratio of the mass density of the sample, and the material density of the monomer dust particles bouncing collision velocities were analyzed by tracking the particle positions over time to derive the velocity after and before the collision. The ratio of these values is called the coefficient of restitution and describes the energy loss in the collision. The analyzed coefficients of restitution are plotted against the collision velocities.

Data for spherical dust, aggregates, and collisions between cylindrical dust aggregates show a trend of a decreasing coefficient of restitution with increasing impact velocity Once mastered. This technique can be done in one hour if it's performed properly. Working With micrometer sized particle can be extremely hazardous, therefore precautionary such as protective masks, gloves, and extractor hoods like this should be always taken during the procedure.