6 research outputs found
Laboratory studies of ice-particle collisions in Saturn's dense rings
In this work, we report on microgravity studies of particle ensembles
simulating ice-particle collisions in Saturn's dense main rings. We have
developed an experimental method to study the energy dissipation in a many-body
system consisting of approx. one hundred cm-sized glass spheres. The temporal
development of the mean particle velocity, ranging from ~10 cm/s (at the
beginning) to ~0.35 cm/s (after 9s of experiment duration), can be explained by
a constant coefficient of restitution of 0.64. A comparison to values obtained
for pure water-ice bodies shows that future cryogenic ice-collision experiments
can achieve collision velocities of ~0.1 cm/s, and thus will very well simulate
the conditions in Saturn's main rings.Comment: Accepted by "Proc. Powders and Grains 2009", Publisher AI
Towards a Dynamical Collision Model of Highly Porous Dust Aggregates
In the recent years we have performed various experiments on the collision
dynamics of highly porous dust aggregates and although we now have a
comprehensive picture of the micromechanics of those aggregates, the
macroscopic understanding is still lacking. We are therefore developing a
mechanical model to describe dust aggregate collisions with macroscopic
parameters like tensile strength, compressive strength and shear strength. For
one well defined dust sample material, the tensile and compressive strength
were measured in a static experiment and implemented in a Smoothed Particle
Hydrodynamics (SPH) code. A laboratory experiment was designed to compare the
laboratory results with the results of the SPH simulation. In this experiment,
a mm-sized glass bead is dropped into a cm-sized dust aggregate with the
previously measured strength parameters. We determine the deceleration of the
glass bead by high-speed imaging and the compression of the dust aggregate by
x-ray micro-tomography. The measured penetration depth, stopping time and
compaction under the glass bead are utilized to calibrate and test the SPH
code. We find that the statically measured compressive strength curve is only
applicable if we adjust it to the dynamic situation with a 'softness'
parameter. After determining this parameter, the SPH code is capable of
reproducing experimental results, which have not been used for the calibration
before.Comment: Accepted by "Proc. Powders and Grains 2009", Publisher AI
Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution
This work focuses on the control
of the polymerization process
for melon (“graphitic carbon nitride”), with the aim
of improving its photocatalytic activity intrinsically. We demonstrate
here that reduction of the synthesis temperature leads to a mixture
of the monomer melem and its higher condensates. We show that this
mixture can be separated and provide evidence that the higher condensates
are isolated oligomers of melem. On evaluating their photocatalytic
activity for hydrogen evolution, the oligomers were found to be the
most active species, having up to twice the activity of the monomer/oligomer
mixture of the as-synthesized material, which in turn has 3 times
the activity of the polymer melon, the literature benchmark. These
results highlight the role of “defects”, i.e., chain
terminations, in increasing the catalytic activity of carbon nitrides
and at the same time point to the ample potential of intrinsically
improving the photocatalytic activity of “carbon nitride”,
especially through the selective synthesis of the active phase