On the pressure dependence of thermal rebound

Based on direct measurements of the critical velocity for nanoparticle rebound, vcr, the possibility of thermal rebound is revisited. The thermal velocity, vtherm, for Ag and NaCl particles in the size range between 10 nm and 100 nm is calculated and compared to the respective value of vcr. Thermal rebound is impossible for dense spherical silver particles and porous NaCl particles. For dense NaCl particles vtherm exceeds the measured values for vcr for particles between 20 nm and 60 nm in diameter, proving the possibility of thermal rebound in a size-range much larger than expected. This result is in disagreement with the work of other researchers that did not observe thermal rebound in this size-range for NaCl particles. As these studies were performed under ambient pressure, while the direct measurement of vcr was performed at reduced pressure around 10 mbar, thermal rebound must be pressure dependent for nanoparticles. This conclusion is confirmed with a simple model calculation, showing that at ambient pressure gas friction contributes significantly to the particle energy loss before the particle leaves a surface completely. This necessitates a redefinition of the term ‘critical velocity’.