In a laboratory experiment, water ice aggregates are trapped in a vacuum chamber at a pressure of 2 mbar due to photophoresis and thermophoresis. The particles are located between a Peltier element at the bottom at 250 K and a reservoir of liquid nitrogen at the top at 77 K. Particle sizes vary between 20 μm and a few hundred μm. It is found that 95 per cent of all the particles rotate about their vertical axis. A qualitative model is developed which explains why particles should mainly align to and rotate around the vertical. The results imply that rotation does not decrease the vertical...

In a laboratory experiment, water ice aggregates are trapped in a vacuum chamber at a pressure of 2 mbar due to photophoresis and thermophoresis. The particles are located between a Peltier element at the bottom at 250 K and a reservoir of liquid nitrogen at the top at 77 K. Particle sizes vary between 20 μm and a few hundred μm. It is found that 95 per cent of all the particles rotate about their vertical axis. A qualitative model is developed which explains why particles should mainly align to and rotate around the vertical. The results imply that rotation does not decrease the vertical strength of photophoretically driven motion in, e.g., protoplanetary discs.