DOI 10.1140/epje/i2004-10068-2 Eur. Phys. J. E 15, 287–291 (2004) T HE EUROPEAN P HYSICAL JOURNAL E Development of an optical trap for microparticle clouds in dilute gases J. Steinbach 1, a , J. Blum 2 , and M. Krause 1 1 Astrophysikalisches Institut, Friedrich-Schiller-Universit¨ at Jena, Schillerg¨ asschen 2-3, 07745 Jena, Germany 2 Institut f¨ ur Geophysik und Extraterrestrische Physik, Technische Universit¨ at zu Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany Received 26 July 2004 and Received in final form 6 September 2004 / Published online: 10 November 2004 – c  EDP Sciences / Societ` a Italiana di Fisica / Springer-Verlag 2004 Abstract. Long-duration experiments with clouds of microparticles are planned for the ICAPS facility on board the International Space Station ISS. The scientific objectives of such experiments are widespread and are ranging from the simulation of aerosol behaviour in Earth’s atmosphere to the formation of planets in the early solar system. It is, however, even under microgravity conditions, impossible to sustain a cloud of free-floating, microscopic particles for an extended period of time, due to thermal diffusion and due to unavoidable external accelerations. Therefore, a trap for dust clouds is required which prevents the diffusion of the particles, which provides a source of relative velocities between the dust grains and which can also concentrate the dust to higher number densities that are otherwise not achievable. We are planning to use the photophoretic effect for such a particle trap. First short-duration microgravity experiments on the photophoretic motion of microscopic particles show that such an optical particle-cloud trap is feasible. First tests of a two-dimensional trap were performed in the Bremen drop tower. PACS. 47.55.Kf Multiphase and particle-laden flows – 45.80.+r Control of mechanical systems – 45.50.-j Dynamics and kinematics of a particle and a system of particles – 44.35.+c Heat flow in multiphase systems 1 Introduction: principle of the optical trap This work is part of the ICAPS (Interactions in Cosmic and Atmospheric Particle Systems) project for the Inter- national Space Station ISS [1]. We are currently develop- ing a three-dimensional trap for clouds of microparticles. The idea of the optical trap is to counteract, under micro- gravity conditions, thermal diffusion and residual accelera- tions, such as thermophoresis, and to provide the experi- menters with basically unlimited research time for agglom- eration and light scattering experiments. The optical trap under consideration is based on the photophoretic effect, i.e. the light-induced motion of particles in gases. This mo- tion is due to a temperature gradient across the particles which results from differential absorption of light. Using strongly absorbing particles, only the illuminated hemi- spheres are heated and are therefore slightly warmer than the backsides. Molecules of the surrounding gas stochas- tically hit the particles’ surfaces which leads to the Brow- nian motion. A fraction of the molecules is not geometri- cally reflected off the particles but resides on their surfaces for some time. During this residence time the molecules on a e-mail: julia@astro.uni-jena.de the illuminated hemispheres acquire a slightly higher tem- perature 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. Neg- ative photophoresis, i.e. motion towards the light source, is also possible: Transparent particles focus the incident light such that a somewhat warmer backside results. For the design of an optical trap for an ensemble of particles embedded in a dilute gas, we utilise the positive photophoretic motion of the particles. The principle upon which our trap is based is shown in Figure 1 for one spatial dimension. The light from three pairs of opposing lamps (each pair oriented in one of the three spatial dimensions) is emitted isotropically from a square surface. The light- emitting surfaces are imaged such that their focal planes span the surface of a cube with a sidelength of 20 mm. Thus, any homogeneous particle for which a positive pho- tophoretic effect exists experiences a net force towards the centre of the trap. The relative light intensities of opposite light sources are adjusted such that the net inward pho- tophoretic motion of the particles inside the “light walls” is always larger than the thermal diffusion and any resid- ual motion of the dust grains.