Size dependence of the photophoretic force

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.