The photophoretic force on a micron-sized sphere of glycerol is measured for a continuously changing size parameter (through evaporation) for excitation at 10.63 and 9.58 μ. Although the force at 9.58 μ is always forward ("positive" photophoresis), the force at 10.63 μ reverses in sign ("negative" photophoresis) and shows a periodic structure in size parameter (2πR/λ) with a periodicity of 0.77±0.03. It is shown that the point of reversal may be used in obtaining the imaginary component of the refractive index. The possibility of solar levitation of atmospheric aerosols is discussed.
ASJC Scopus subject areas
- Physics and Astronomy(all)