  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
micrometres and a few hundred micrometres. It is found that 95% 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.
