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ICAPS

1. Development of a prototype instrument for the production of high-porosity agglomerates ( “dust cakes”)

The experimental setup consists of a cogwheel deagglomerator (Poppe et al. 1997) with which the powder sample is disintegrated into its monomer grains. The cogwheel deagglomerator is operated in rarefied air with a typical pressure of 100 Pa. The single monomer particles couple to the gas on a timescale of t f ~ 1 ms and are stopped within a distance of less than 100 mm after leaving the fast-spinning cogwheel. As the gas is laminarly streaming through the apparatus, the monomer grains are slowly transported away from the cogwheel and reach a gas-permeable substrate where they are deposited in a hit-and-stick behavior. Due to the low gas pressure, the particles are in the free molecular flow regime so that no hydrodynamic flow forms in the vicinity of the deposition point. Within a few hours of experiment time, a cylindrical high-porosity dust agglomerate with 25 mm diameter and a thickness of up to ~20 mm is formed. The dust samples can be easily transported and manipulated. It turned out that cutting the dust samples by means of a razor blade was achievable without the destruction or partial compaction of the sample. For the determination of the mechanical properties of the agglomerates, we produced cuboidal or cylindrical samples. The volume filling factor of the samples was determined by measuring volume and mass of each agglomerate. It turned out that the volume filling factor is 0.15, in quantitative agreement with the predictions from the numerical simulations. The close agreement between experimental and theoretical volume filling factors shows that a restructuring of the agglomerate due to gravity-induced rolling of "particle trees is insufficient to considerably change the volume filling factor of the agglomerate. Results on the formation and mechanical properties of high-porosity “dust cakes” are published in Blum & Schräpler 2004 and Blum 2004. In addition to that, experiments on the impacts of mm-sized high-porosity agglomerates into “dust cakes” have been performed.

 

Schematics of the experimental setup for the formation of “dust cakes”.

 

(a) An example of an agglomerate with a volume filling factor of 0.15. (b) Specimen of an agglomerate after manual cutting to 10 × 10 mm2. (c) Result of a Monte Carlo simulation of ballistic deposition. (d) High resolution scanning electron microscopy (SEM) image of the surface of an agglomerate consisting of SiO2 spheres with 1.5 µm diameter.

 

2. Development of a prototype Paul trap for IPE

Trapping principle of the Paul trap
A Paul trap is an electrical trap that traps charged particles in an AC field. An AC voltage is applied to the larger middle ring electrodes (see figure 1). Next to the apparent oscillatory motion, charged particles experience a second order motion towards the field-free centre of the trap.
The trapping speed scales linear with the chosen voltage and decreases exponentially as the ambient pressure increases as the oscillatory motion of the particles is increasingly damped. The choice of the charge of the trapped particles is mainly done by the choice of the driving frequency of the AC voltage. For a detailed discussion of Paul trap see [1] and the classical treatment in [2].

 

Figure 1: Paul trap
 

Experimental setup and results
Taken into account the particles size (1,2µm) and the required free molecular flow (Epstein regieme) and the desire to observe the particles' interaction predominantly due to their thermal energy and surface forces, it is neccessary that the particles are only slightly charged. This results in the neccessety to perform experiments on the paultrap under microgravity conditions as the charge-to-mass ration of the particles allows no electrostatic levitation under the given pressure conditions as the required voltage is by one to two orders of magnitude above the break-through voltage of any gas for the given pressures.

The experimental setup consists of a cogwheel des-agglomerator thas disintegrates the dust samples into its monomer grains. Upon des-agglomeration the particles are charged due to collisions with the cogwheel as well as among eachother. They are typcially charged with only a few electronic charged per micron-sized particle (see figure 2), so that the particles' interaction is dominated by the thermal energy of the particles as the Coulomb energy is by an order of magnitude lower that the thermal energy. Upon entry of the experimental chamber and start of microgravity conditions the trap switched on. This proved neccessary as otherwise, if the trap is operational already during injection under normal gravity conditions, it traps highly charged particles in an off-centre position and upon onset of microgravity this cloud of highly charged particles moves rapidly towards and through the centre of the trap and thus inducing unwanted hydrodynamic flows in the trap that destroy any injected dust cloud.
Further, in order to prevent neutralization of aggregates in the trap it is advisable to take care that predominantly only particles of the same sign of charge enter the trap. Given that the injected bear only charges of the same sign this allows for prolonged observation time of a dust cloud as external forces like thermophoreses are easily compensated by the trap.
Figure three shows an composite of five consecutive images taken about a second after injection. Clearly visible are the hyperbolic trajectories of the particles' oscillation.

Figure 2:
 
 
[1] W.Paul: "Electromagnetic traps for charged and neutral particles" Rev. of Modern Physics, Vol 62, No. 3, June 1990
[2] D.M.D. Leibfried et al.: "Quantum dynamics of single trapped ions", Rev. of Modern Physics, Bd. 75
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Created by: D. Langkowski, 23.03.2001
Last modified: 28.07.2005