The implications of particle rotation on the effect of photophoresis

TL;DR

This study investigates how particle rotation affects photophoresis in vacuum conditions, showing that most particles rotate vertically and that this rotation does not weaken photophoretic forces in space environments.

Contribution

The paper provides experimental evidence and a qualitative model explaining particle rotation behavior and its impact on photophoresis, especially in protoplanetary discs.

Findings

95% of particles rotate about their vertical axis

Rotation does not diminish the vertical photophoretic force

Particles tend to align and rotate vertically

Abstract

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.