photophoretic motion of microparticles (Photophoresis)

abstract

To understand the formation of planets in the early solar system long-duration experiments with clouds of micrometer-sized particles are planned for the ICAPS facility on board the International Space Station ISS, for example light-scattering and agglomeration experiments. For the implementation of these experiments it is necessary to find a way to manipulate microparticles under microgravity conditions, e.g. to create different velocities between them or to "trap" them within a certain volume, counteracting thermal diffusion and unavoidable external accelerations which would otherwise lead to a dispersion of the particle cloud. We are planning to use the photophoretic effect to realize this kind of trapping and particle manipulation. Photophoresis is the light-induced motion of particles in gases which is due to a temperature gradient across the particles resulting from differential absorption of light. Using strongly absorbing particles, only the illuminated hemispheres are heated and are therefore slightly warmer than the backsides. Molecules of the surrounding gas stochastically hit the particles´ surface which leads to the Brownian motion. Thereby a fraction of the molecules resides on the particles´ surfaces for some time. During this residence time the molecules on the illuminated hemispheres acquire a slightly higher temperature and leave the particles with increased velocities. This leads to a higher momentum transfer and finally to the motion of the irradiated particle away from the light source. This effect is called positive photophoresis. Negative photophoresis, i.e. motion towards the light source, is also possible: Transparent particles focus the incident light such that a somewhat warmer backside results.

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experiment year: 2009
number of drops: 15

experiment year: 2009
number of drops: 15

experiment year: 2003
number of drops: 15