Size dependent polarities in tribocharged dust aggregates.

It is long known that particles of the same material but with different sizes charge with different polarities in mutual collisions. In most cases, the smaller grains become negative. Here, we study tribocharging of (sub-)mm dust aggregates in the course of microgravity experiments by determining the charges of particles through their motion within an electric field.

Violation of triboelectric charge conservation on colliding particles.

In microgravity experiments, we quantified the net charge on systems of two identical, 434-μm-diameter glass spheres before and after a collision. We find that charge conservation is significantly violated. Independent of the sign of the total charge, the systems regularly lose some of their net charges, that is, they slightly discharge.

Ice particles trapped by temperature gradients at mbar pressure.

In laboratory experiments we observe that ice particles (≤100 μm) entrained in a low pressure atmosphere (~1 mbar) get trapped by temperature gradients between three reservoirs at different temperature. Confining elements are a peltier element at 250 K (bottom), a liquid nitrogen reservoir at 77 K (top), and the surrounding vacuum chamber at 293 K. Particle levitation and trapping is modeled by an interplay of thermophoresis, photophoresis, and gravity.

Self-sustained levitation of dust aggregate ensembles by temperature-gradient-induced overpressures.

In laboratory experiments we observe dust aggregates from 100 mum to 1 cm in size composed of micrometer-sized grains levitating over a hot surface. Depending on the dust sample aggregates start to levitate at a temperature of 400 K. Levitation of dust aggregates is restricted to a pressure range between 1-40 mbar.

Dust eruptions by photophoresis and solid state greenhouse effects.

We carried out experiments that show a gas pressure dependent ability of light to eject particles from a dust bed. Dust eruptions also occur upon removal of the light source. This can be attributed to a solid state greenhouse effect and photophoretic forces.

Ejection of dust by elastic waves in collisions between millimeter- and centimeter-sized dust aggregates at 16.5 to 37.5 m/s impact velocities.

We report on experiments in which millimeter-sized Si O2 dust aggregates consisting of (sub)-micrometer-sized grains impact into centimeter-sized targets that consist of the same kind of dust particles. The porosity of the granular targets is between 74% and 88%. Impact speeds are between 16.

Experiments on light scattering and extinction by small, micrometer-sized aggregates of spheres.

We performed experiments to study the extinction, scattering, and polarization of light by ensembles of fractal dust aggregates that consist of spherical monomers large compared with the wavelength. Extinction was measured on a homogeneous dust cloud. Scattering and polarization were measured on a collimated dust beam.